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  • Other Sources  (445)
  • AGU (American Geophysical Union)  (401)
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  • 1
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    Non‐exchangeable stable hydrogen isotope ratios in clay minerals and soil clay fractions: A method test (2022)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2023-01-20
    Description: Stable hydrogen isotope ratios (δ2H values) in structural hydroxyl groups of pedogenic clay minerals are inherited from the surrounding water at the time of their formation. Only non‐exchangeable H preserves the environmental forensic and paleoclimate information (δ2Hn value). To measure δ2Hn values in structural H of clay minerals and soil clay fractions, we adapted a steam equilibration method by accounting for high hygroscopicity. Our δ2Hn values for USGS57 biotite (−95.3 ± SD 0.9‰) and USGS58 muscovite (30.7 ± 1.4‰) differed slightly but significantly from the reported δ2H values (−91.5 ± 2.4‰ and −28.4 ± 1.6‰), because the minerals contained 1.1%–4.4% of exchangeable H. The low SD of replicate measurements (n = 3) confirmed a high precision. The clay separation method including destruction of Fe oxides, carbonates and soil organic matter, and dispersion did not significantly change the δ2Hn values of five different clay minerals. However, we were unable to remove all organic matter from the soil clay fractions resulting in an estimated bias of 1‰ in two samples and 15‰ in the carbon‐richest sample. Our results demonstrate that δ2Hn values of structural H of clay minerals and soil clay fractions can be reliably measured without interference from atmospheric water and the method used to separate the soil clay fraction. Highlights We tested steam equilibration to determine stable isotope ratios of structural H in clay. Gas‐tight capsule sealing in Ar atmosphere was necessary to avoid remoistening. Our steam equilibration method showed a high accuracy and precision. The clay separation method did not change stable isotope ratios of structural H in clay.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:549 ; controlled isotope exchange technique ; deuterium ; montmorillonite ; soil clay separation ; soil organic matter removal ; steam equilibration ; structural H ; USGS57 biotite ; vermiculite ; δ2H
    Language: English
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  • 2
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    Potential erodibility of semi‐arid steppe soils derived from aggregate stability tests (2022)
    Koza, Moritz ; Pöhlitz, Julia ; Prays, Aleksey ; [et al.]
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2023-01-26
    Description: Erosion is a severe threat to the sustainable use of agricultural soils. However, the structural resistance of soil against the disruptive forces steppe soils experience under field conditions has not been investigated. Therefore, 132 topsoils under grass‐ and cropland covering a large range of physico‐chemical soil properties (sand: 2–76%, silt: 18–80%, clay: 6–30%, organic carbon: 7.3–64.2 g kg−1, inorganic carbon: 0.0–8.5 g kg−1, pH: 4.8–9.5, electrical conductivity: 32–946 μS cm−1) from northern Kazakhstan were assessed for their potential erodibility using several tests. An adjusted drop‐shatter method (low energy input of 60 Joule on a 250‐cm3 soil block) was used to estimate the stability of dry soil against weak mechanical forces, such as saltating particles striking the surface causing wind erosion. Three wetting treatments with various conditions and energies (fast wetting, slow wetting, and wet shaking) were applied to simulate different disruptive effects of water. Results indicate that aggregate stability was higher for grassland than cropland soils and declined with decreasing soil organic carbon content. The results of the drop‐shatter test suggested that 29% of the soils under cropland were at risk of wind erosion, but only 6% were at high risk (i.e. erodible fraction 〉60%). In contrast, the fast wetting treatment revealed that 54% of the samples were prone to become “very unstable” and 44% “unstable” during heavy rain or snowmelt events. Even under conditions comparable to light rain events or raindrop impact, 53–59% of the samples were “unstable.” Overall, cropland soils under semi‐arid conditions seem much more susceptible to water than wind erosion. Considering future projections of increasing precipitation in Kazakhstan, we conclude that the risk of water erosion is potentially underestimated and needs to be taken into account when developing sustainable land use strategies. Highlights Organic matter is the important binding agent enhancing aggregation in steppe topsoils. Tillage always declines aggregate stability even without soil organic carbon changes. All croplands soil are prone to wind or water erosion independent of their soil properties. Despite the semi‐arid conditions, erosion risk by water seems higher than by wind.
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Keywords: ddc:631.4 ; climate change ; land use ; soil organic carbon ; soil texture ; water erosion ; wind erosion
    Language: English
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  • 3
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    Vertical SOC distribution and aromatic carbon in centuries old charcoal‐rich Technosols (2022)
    Bonhage, Alexander ; Raab, Thomas ; Schneider, Anna ; [et al.]
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2023-01-21
    Description: Charcoal‐rich Technosols on century‐old relict charcoal hearths (RCHs) are the subject of ongoing research regarding potential legacy effects that result from historic charcoal production and subsequent charcoal amendments on forest soil properties and forest ecosystems today. RCHs consist mostly of Auh horizons that are substantially enriched in soil organic carbon (SOC), of which the largest part seems to be of pyrogenic origin (PyC). However, the reported range of SOC and PyC contents in RCH soil also suggests that they are enriched in nonpyrogenic SOC. RCH soils are discussed as potential benchmarks for the long‐term influence of biochar amendment and the post‐wildfire influences on soil properties. In this study, we utilised a large soil sample dataset (n = 1245) from 52 RCH sites in north‐western Connecticut, USA, to quantify SOC contents by total element analysis. The contents of condensed highly aromatic carbon as a proxy for black carbon (BC) were predicted by using a modified benzene polycarboxylated acid (BPCA) marker method in combination with diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy‐based partial least square regression (r2 = 0.89). A high vertical spatial sampling resolution allowed the identification of soil organic matter (SOM) enrichment and translocation processes. The results show an average 75% and 1862% increase in TOC and BPCA‐derived carbon, respectively, for technogenic Auh horizons compared to reference soils. In addition to an increase in aromatic properties, increased carboxylic properties of the RCH SOC suggest self‐humification effects of degrading charcoal and thereby the continuing formation of leachable aromatic carbon compounds, which could have effects on pedogenic processes in buried soils. Indeed, we show BPCA‐derived carbon concentrations in intermediate technogenic Cu horizons and buried top/subsoils that suggest vertical translocation of highly aromatic carbon originating in RCH Auh horizons. Topmost Auh horizons showed a gradual decrease in total organic carbon (TOC) contents with increasing depth, suggesting accumulation of recent, non‐pyrogenic SOM. Lower aliphatic absorptions in RCH soil spectra suggest different SOM turnover dynamics compared to reference soils. Furthermore, studied RCH soils featured additional TOC enrichment, which cannot be fully explained now. Highlights BC to TOC ratio and high resolution vertical SOC distribution in 52 RCH sites were studied. RCH soils non‐BC pool was potentially different to reference soils. RCH soils feature TOC accumulation in the topmost horizon. There is BC translocation into buried soils on RCH sites.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:631.4 ; benzene polycarboxylated acid marker (BPCA) ; black carbon ; charcoal degradation ; charcoal kiln ; pyrogenic carbon ; relict charcoal hearth ; biochar
    Language: English
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  • 4
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    Defining Hydrogeological Site Similarity with Hierarchical Agglomerative Clustering (2022)
    Blackwell Publishing Ltd | Malden, US
    Details
    Publication Date: 2024-02-28
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Hydrogeological information about an aquifer is difficult and costly to obtain, yet essential for the efficient management of groundwater resources. Transferring information from sampled sites to a specific site of interest can provide information when site‐specific data is lacking. Central to this approach is the notion of site similarity, which is necessary for determining relevant sites to include in the data transfer process. In this paper, we present a data‐driven method for defining site similarity. We apply this method to selecting groups of similar sites from which to derive prior distributions for the Bayesian estimation of hydraulic conductivity measurements at sites of interest. We conclude that there is now a unique opportunity to combine hydrogeological expertise with data‐driven methods to improve the predictive ability of stochastic hydrogeological models.〈/p〉
    Description: 〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉〈italic〉Article impact statement〈/italic〉: This article introduces hierarchical clustering as a method for defining a notion of site similarity; the aim of this method is to improve the derivation of prior distributions in Bayesian methods in hydrogeology.〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://github.com/GeoStat-Bayesian/geostatDB
    Description: https://github.com/GeoStat-Bayesian/exPrior
    Description: https://github.com/GeoStat-Bayesian/siteSimilarity
    Keywords: ddc:551.49 ; hydrogeological sites ; hydrogeological modeling
    Language: English
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  • 5
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    Toward an adequate level of detail in flood risk assessments (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-01-24
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Flood risk assessments require different disciplines to understand and model the underlying components hazard, exposure, and vulnerability. Many methods and data sets have been refined considerably to cover more details of spatial, temporal, or process information. We compile case studies indicating that refined methods and data have a considerable effect on the overall assessment of flood risk. But are these improvements worth the effort? The adequate level of detail is typically unknown and prioritization of improvements in a specific component is hampered by the lack of an overarching view on flood risk. Consequently, creating the dilemma of potentially being too greedy or too wasteful with the resources available for a risk assessment. A “sweet spot” between those two would use methods and data sets that cover all relevant known processes without using resources inefficiently. We provide three key questions as a qualitative guidance toward this “sweet spot.” For quantitative decision support, more overarching case studies in various contexts are needed to reveal the sensitivity of the overall flood risk to individual components. This could also support the anticipation of unforeseen events like the flood event in Germany and Belgium in 2021 and increase the reliability of flood risk assessments.〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: BMBF http://dx.doi.org/10.13039/501100002347
    Description: Federal Environment Agency http://dx.doi.org/10.13039/501100010809
    Description: http://howas21.gfz-potsdam.de/howas21/
    Description: https://www.umwelt.niedersachsen.de/startseite/themen/wasser/hochwasser_amp_kustenschutz/hochwasserrisikomanagement_richtlinie/hochwassergefahren_und_hochwasserrisikokarten/hochwasserkarten-121920.html
    Description: https://download.geofabrik.de/europe/germany.html
    Description: https://emergency.copernicus.eu/mapping/list-of-components/EMSN024
    Description: https://data.jrc.ec.europa.eu/collection/id-0054
    Description: https://oasishub.co/dataset/surface-water-flooding-footprinthurricane-harvey-august-2017-jba
    Description: https://www.wasser.sachsen.de/hochwassergefahrenkarte-11915.html
    Keywords: ddc:551.48 ; decision support ; extreme events ; integrated flood risk management ; risk assessment
    Language: English
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  • 6
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    Small lateral air pressure gradients generated by a large chamber system have a strong effect on CO2 transport in soil (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-02-09
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Gas transport in soils is usually assumed to be purely diffusive, although several studies have shown that non‐diffusive processes can significantly enhance soil gas transport. These processes include barometric air pressure changes, wind‐induced pressure pumping and static air pressure fields generated by wind interacting with obstacles. The associated pressure gradients in the soil can cause advective gas fluxes that are much larger than diffusive fluxes. However, the contributions of the respective transport processes are difficult to separate. We developed a large chamber system to simulate pressure fields and investigate their influence on soil gas transport. The chamber consists of four subspaces in which pressure is regulated by fans that blow air in or out of the chamber. With this setup, we conducted experiments with oscillating and static pressure fields. CO〈sub〉2〈/sub〉 concentrations were measured along two soil profiles beneath the chamber. We found a significant relationship between static lateral pressure gradients and the change in the CO〈sub〉2〈/sub〉 profiles (R〈sup〉2〈/sup〉 = 0.53; 〈italic toggle="no"〉p〈/italic〉‐value 〈2e‐16). Even small pressure gradients between −1 and 1 Pa relative to ambient pressure resulted in an increase or decrease in CO〈sub〉2〈/sub〉 concentrations of 8% on average in the upper soil, indicating advective flow of air in the pore space. Positive pressure gradients resulted in decreasing, negative pressure gradients in increasing CO〈sub〉2〈/sub〉 concentrations. The concentration changes were probably caused by an advective flow field in the soil beneath the chamber generated by the pressure gradients. No effect of oscillating pressure fields was observed in this study. The results indicate that static lateral pressure gradients have a substantial impact on soil gas transport and therefore are an important driver of gas exchange between soil and atmosphere. Lateral pressure gradients in a comparable range can be induced under windy conditions when wind interacts with terrain features. They can also be caused by chambers used for flux measurements at high wind speed or by fans used for head‐space mixing within the chambers, which yields biased flux estimates.〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:631.4 ; advective flux ; chamber flux measurements ; static air pressure fields ; wind‐induced pressure pumping
    Language: English
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  • 7
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    Redoxtrons – An experimental system to study redox processes within the capillary fringe (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2023-11-18
    Description: Spatiotemporal characterisation of the soil redox status within the capillary fringe (CF) is a challenging task. Air‐filled porosities (ε), oxygen concentration (O〈sub〉2〈/sub〉) and soil redox potential (EH) are interrelated soil variables within active biogeochemical domains such as the CF. We investigated the impact of water table (WT) rise and drainage in an undisturbed topsoil and subsoil sample taken from a Calcaric Gleysol for a period of 46 days. We merged 1D (EH and matric potential) and 2D (O〈sub〉2〈/sub〉) systems to monitor at high spatiotemporal resolution redox dynamics within self‐constructed redoxtron housings and complemented the data set by a 3D pore network characterization using X‐ray microtomography (X‐ray μCT). Depletion of O〈sub〉2〈/sub〉 was faster in the organic matter‐ and clay‐rich aggregated topsoil and the CF extended 〉10 cm above the artificial WT. The homogeneous and less‐aggregated subsoil extended only 4 cm above the WT as indicated by ε–O〈sub〉2〈/sub〉–EH data during saturation. After drainage, 2D O〈sub〉2〈/sub〉 imaging revealed a fast aeration towards the lower depths of the topsoil, which agrees with the connected ε derived by X‐ray μCT (ε〈sub〉CT_conn〈/sub〉) of 14.9% of the total porosity. However, small‐scaled anoxic domains with O〈sub〉2〈/sub〉 saturation 〈5% were apparent even after lowering the WT (down to 0.25 cm〈sup〉2〈/sup〉 in size) for 23 days. These domains remained a nucleus for reducing soil conditions (E〈sub〉H〈/sub〉 〈 −100 mV), which made it challenging to characterise the soil redox status in the CF. In contrast, the subsoil aeration reached O〈sub〉2〈/sub〉 saturation after 8 days for the complete soil volume. Values of ε〈sub〉CT_conn〈/sub〉 around zero in the subsoil highlighted that soil aeration was independent of this parameter suggesting that other variables such as microbial activity must be considered when predicting the soil redox status from ε alone. The use of redoxtrons in combination with localised redox‐measurements and image based pore space analysis resulted in a better 2D/3D characterisation of the pore system and related O〈sub〉2〈/sub〉 transport properties. This allowed us to analyse the distribution and activity of microbiological niches highly associated with the spatiotemporal variable redox dynamics in soil environments. Highlights: The time needed to turn from reducing to oxidising (period where all platinum electrodes feature E〈sub〉H〈/sub〉 〉 300 mV) condition differ for two samples with contrasting soil structure. The subsoil with presumably low O〈sub〉2〈/sub〉 consumption rates aerated considerably faster than the topsoil and exclusively by O〈sub〉2〈/sub〉 diffusion through medium‐ and fine‐sized pores. To derive the soil redox status based upon the triplet ε–O〈sub〉2〈/sub〉–E〈sub〉H〈/sub〉 is challenging at present in heterogeneous soil domains and larger soil volumes than 250 cm〈sup〉3〈/sup〉. Undisturbed soil sampling along with 2D/3D redox measurement systems (e.g., redoxtrons) improve our understanding of redox dynamics within the capillary fringe.
    Keywords: ddc:631.4 ; environmental monitoring ; incubation experiments ; redox processes ; soil reducing conditions ; undisturbed soil ; X‐ray microtomography
    Language: English
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  • 8
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    The dilemma of analytical method changes for soil organic carbon in long‐term experiments (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2023-11-17
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈sec xmlns:mml="http://www.w3.org/1998/Math/MathML" id="ejss13362-sec-1003" xml:lang="en"〉 〈p xml:lang="en"〉Long‐term experiments (LTEs) have provided data to modellers and agronomists to investigate changes and dynamics of soil organic carbon (SOC) under different cropping systems. As treatment changes have occurred due to agricultural advancements, so too have analytical soil methods. This may lead to method bias over time, which could affect the robust interpretation of data and conclusions drawn. This study aims to quantify differences in SOC due to changes in dry combustion methods over time, using soil samples of a LTE established in 1963 that focuses on mineral and organic fertilizer management in the temperate zone of Northeast Germany. For this purpose, 1059 soil samples, collected between 1976 and 2008, have been analysed twice, once with their historical laboratory method right after sampling, and a second time in 2016 when all samples were analysed using the same elementary analyser. In 9 of 11 soil sampling campaigns, a paired 〈italic toggle="no"〉t〈/italic〉‐test provided evidence for significant differences in the historical SOC values when compared with the re‐analysed concentrations of the same LTE sample. In the sampling years 1988 and 2004, the historical analysis obtained about 0.9 g kg〈sup〉−1〈/sup〉 lower SOC compared with the re‐analysed one. For 1990 and 1998, this difference was about 0.4 g kg〈sup〉−1〈/sup〉. Correction factors, an approach often used to correct for different analytical techniques, could only be applied for 5 of 11 sampling campaigns to account for constant and proportional systematic method error. For this particular LTE, the interpretation of SOC changes due to agronomic management (here fertilization) deviates depending on the analytical method used, which may weaken the explanatory power of the historical data. We demonstrate that analytical method changes over time present one of many challenges in the interpretation of time series data of SOC dynamics. Therefore, LTE site managers need to ensure providing all necessary protocols and data in order to retrace method changes and if necessary recalculate SOC.〈/p〉 〈/sec〉〈sec xmlns:mml="http://www.w3.org/1998/Math/MathML" id="ejss13362-sec-0003" xml:lang="en"〉 〈title〉Highlights〈/title〉 〈p xml:lang="en"〉〈list list-type="bullet" id="ejss13362-list-0001"〉 〈list-item id="ejss13362-li-0001"〉〈p〉A total of 1059 LTE soil samples taken between 1976 and 2008 were re‐analysed for SOC in 2016〈/p〉〈/list-item〉 〈list-item id="ejss13362-li-0002"〉〈p〉Several methodological changes for SOC determination led to significant different SOC concentration in the same sample〈/p〉〈/list-item〉 〈list-item id="ejss13362-li-0003"〉〈p〉Interpretation and time series of LTE soil data suffer from consideration of analytical method changes and poor documentation of the same〈/p〉〈/list-item〉 〈list-item id="ejss13362-li-0004"〉〈p〉Soil archive establishment, thorough method protocols and diligent proficiency testing after soil method changes ameliorate the dilemma〈/p〉〈/list-item〉 〈/list〉〈/p〉 〈/sec〉
    Description: Brandenburger Staatsministerium für Wissenschaft, Forschung und Kultur http://dx.doi.org/10.13039/501100004581
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100004937
    Description: https://doi.org/10.4228/zalf-acge-b683
    Keywords: ddc:631.4 ; Bland–Altman ; carbon stocks ; data trueness ; Deming regression ; method bias ; soil archive ; soil survey
    Language: English
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  • 9
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    Optimised use of data fusion and memory‐based learning with an Austrian soil library for predictions with infrared data (2023)
    Blackwell Publishing Ltd | Oxford, UK
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    Publication Date: 2023-12-12
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Infrared spectroscopy in the visible to near‐infrared (vis–NIR) and mid‐infrared (MIR) regions is a well‐established approach for the prediction of soil properties. Different data fusion and training approaches exist, and the optimal procedures are yet undefined and may depend on the heterogeneity present in the set and on the considered scale. The objectives were to test the usefulness of partial least squares regressions (PLSRs) for soil organic carbon (SOC), total carbon (C〈sub〉t〈/sub〉), total nitrogen (N〈sub〉t〈/sub〉) and pH using vis–NIR and MIR spectroscopy for an independent validation after standard calibration (use of a general PLSR model) or using memory‐based learning (MBL) with and without spiking for a national spectral database. Data fusion approaches were simple concatenation of spectra, outer product analysis (OPA) and model averaging. In total, 481 soils from an Austrian forest soil archive were measured in the vis–NIR and MIR regions, and regressions were calculated. Fivefold calibration‐validation approaches were carried out with a region‐related split of spectra to implement independent validations with n ranging from 47 to 99 soils in different folds. MIR predictions were generally superior over vis–NIR predictions. For all properties, optimal predictions were obtained with data fusion, with OPA and spectra concatenation outperforming model averaging. The greatest robustness of performance was found for OPA and MBL with spiking with 〈italic toggle="no"〉R〈/italic〉〈sup〉2〈/sup〉 ≥ 0.77 (N), 0.85 (SOC), 0.86 (pH) and 0.88 (C〈sub〉t〈/sub〉) in the validations of all folds. Overall, the results indicate that the combination of OPA for vis–NIR and MIR spectra with MBL and spiking has a high potential to accurately estimate properties when using large‐scale soil spectral libraries as reference data. However, the reduction of cost‐effectiveness using two spectrometers needs to be weighed against the potential increase in accuracy compared to a single MIR spectroscopy approach.〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:631.4 ; data fusion ; independent validation ; infrared spectroscopy ; MBL ; nitrogen ; outer product analysis ; pH ; soil organic carbon ; spiking ; total carbon
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  • 10
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    Importance of catchment hydrological processes and calibration of hydrological‐hydrodynamic rainfall‐runoff models in small rural catchments (2023)
    Blackwell Publishing Ltd | Oxford, UK
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    Publication Date: 2024-01-19
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉In recent years, many two‐dimensional (2D) hydrodynamic models have been extended to include the direct rainfall method (DRM). This allows their application as a hydrological‐hydrodynamic model for the determination of floodplains in one model system. In previous studies on DRM, the role of catchment hydrological processes (CaHyPro) and its interaction with the calibration process was not investigated in detail. In the present, case‐oriented study, the influence of the spatiotemporal distribution of the processes precipitation and runoff formation in combination with the 2D model HEC‐RAS is investigated. In a further step, a conceptual approach for event‐based interflow is integrated. The study is performed on the basis of a single storm event in a small rural catchment (low mountain range, 38 km〈sup〉2〈/sup〉) in Hesse (Germany). The model results are evaluated against six quality criteria and compared to a simplified baseline model. Finally, the calibrated improved model is contrasted with a calibrated baseline model. The results show the enhancement of the model results due to the integration of the CaHyPro and highlight its interplay with the calibrated model parameters.〈/p〉
    Keywords: ddc:551.48 ; 2D hydrodynamic modeling ; calibration ; direct rainfall modeling ; hydrological processes ; radar data ; runoff formation
    Language: English
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  • 11
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    BODIUM—A systemic approach to model the dynamics of soil functions (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-01-26
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉The increasing demand for biomass for food, animal feed, fibre and bioenergy requires optimization of soil productivity, while at the same time, protecting other soil functions such as nutrient cycling and buffering, carbon storage, habitat for biological activity and water filter and storage. Therefore, one of the main challenges for sustainable agriculture is to produce high yields while maintaining all the other soil functions. Mechanistic simulation models are an essential tool to fully understand and predict the complex interactions between physical, biological and chemical processes of soils that generate those functions. We developed a soil model to simulate the impact of various agricultural management options and climate change on soil functions by integrating the relevant processes mechanistically and in a systemic way. As a special feature, we include the dynamics of soil structure induced by tillage and biological activity, which is especially relevant in arable soils. The model operates on a 1D soil profile consisting of a number of discrete layers with dynamic thickness. We demonstrate the model performance by simulating crop growth, root growth, nutrient and water uptake, nitrogen cycling, soil organic matter turnover, microbial activity, water distribution and soil structure dynamics in a long‐term field experiment including different crops and different types and levels of fertilization. The model is able to capture essential features that are measured regularly including crop yield, soil organic carbon, and soil nitrogen. In this way, the plausibility of the implemented processes and their interactions is confirmed. Furthermore, we present the results of explorative simulations comparing scenarios with and without tillage events to analyse the effect of soil structure on soil functions. Since the model is process‐based, we are confident that the model can also be used to predict quantities that have not been measured or to estimate the effect of management measures and climate states not yet been observed. The model thus has the potential to predict the site‐specific impact of management decisions on soil functions, which is of great importance for the development of a sustainable agriculture that is currently also on the agenda of the ‘Green Deal’ at the European level.〈/p〉
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Description: https://git.ufz.de/bodium/bodium_v1.0
    Keywords: ddc:631.4 ; agriculture ; computational model ; simulation ; soil microbiology ; soil structure ; sustainable soil
    Language: English
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  • 12
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    Temperature and soil moisture change microbial allocation of pesticide‐derived carbon (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-03-18
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Temperature and soil moisture are known to control pesticide mineralization. Half‐life times (DT〈sub〉50〈/sub〉) derived from pesticide mineralization curves generally indicate longer residence times at low soil temperature and moisture but do not consider potential changes in the microbial allocation of pesticide‐derived carbon (C). We aimed to determine carbon use efficiency (CUE, formation of new biomass relative to total C uptake) to better understand microbial utilization of pesticide‐derived C under different environmental conditions and to support the conventional description of degradation dynamics based on mineralization. We performed a microcosm experiment at two MCPA (2‐methyl‐4‐chlorophenoxyacetic acid) concentrations (1 and 20 mg kg〈sup〉−1〈/sup〉) and defined 20°C/pF 1.8 as optimal and 10°C/pF 3.5 as limiting environmental conditions. After 4 weeks, 70% of the initially applied MCPA was mineralized under optimal conditions but MCPA mineralization reached less than 25% under limiting conditions. However, under limiting conditions, an increase in CUE was observed, indicating a shift towards anabolic utilization of MCPA‐derived C. In this case, increased C assimilation implied C storage or the formation of precursor compounds to support resistance mechanisms, rather than actual growth since we did not find an increase in the 〈italic toggle="no"〉tfdA〈/italic〉 gene relevant to MCPA degradation. We were able to confirm the assumption that under limiting conditions, C assimilation increases relative to mineralization and that C redistribution, may serve as an explanation for the difference between mineralization and MCPA dissipation‐derived degradation dynamics. In addition, by introducing CUE to the temperature‐ and moisture‐dependent degradation of pesticides, we can capture the underlying microbial constraints and adaptive mechanisms to changing environmental conditions.〈/p〉
    Description: 〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Changing environmental conditions alter the MCPA degradation dynamics and the allocation of pesticide‐derived carbon to anabolic or catabolic metabolism.〈boxed-text position="anchor" content-type="graphic" id="ejss13417-blkfxd-0001" xml:lang="en"〉 〈graphic position="anchor" id="jats-graphic-1" xlink:href="urn:x-wiley:13510754:media:ejss13417:ejss13417-toc-0001"〉 〈/graphic〉 〈/boxed-text〉〈/p〉
    Description: Collaborative Research Center 1253 CAMPOS (DFG)
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: DFG Priority Program 2322 “Soil System”
    Description: Ellrichshausen Foundation
    Description: Research Training Group “Integrated Hydrosystem modeling”
    Description: https://doi.org/10.5281/zenodo.5081655
    Keywords: ddc:631.4 ; anabolism ; carbon use efficiency ; catabolism ; effect of soil moisture and temperature ; gene‐centric process model ; MCPA biodegradation
    Language: English
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  • 13
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    Reducing tillage intensity benefits the soil micro‐ and mesofauna in a global meta‐analysis (2022)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-03-22
    Description: Soil fauna drives crucial processes of energy and nutrient cycling in agricultural systems, and influences the quality of crops and pest incidence. Soil tillage is the most influential agricultural manipulation of soil structure, and has a profound influence on soil biology and its provision of ecosystem services. The objective of this study was to quantify through meta‐analyses the effects of reducing tillage intensity on density and diversity of soil micro‐ and mesofaunal communities, and how these effects vary among different pedoclimatic conditions and interact with concurrent management practices. We present the results of a global meta‐analysis of available literature data on the effects of different tillage intensities on taxonomic and functional groups of soil micro‐ and mesofauna. We collected paired observations (conventional vs. reduced forms of tillage/no‐tillage) from 133 studies across 33 countries. Our results show that reduced tillage intensity or no‐tillage increases the total density of springtails (+35%), mites (+23%), and enchytraeids (+37%) compared to more intense tillage methods. The meta‐analyses for different nematode feeding groups, life‐forms of springtails, and taxonomic mite groups showed higher densities under reduced forms of tillage compared to conventional tillage on omnivorous nematodes (+53%), epedaphic (+81%) and hemiedaphic (+84%) springtails, oribatid (+43%) and mesostigmatid (+57%) mites. Furthermore, the effects of reduced forms of tillage on soil micro‐ and mesofauna varied with depth, climate and soil texture, as well as with tillage method, tillage frequency, concurrent fertilisation, and herbicide application. Our findings suggest that reducing tillage intensity can have positive effects on the density of micro‐ and mesofaunal communities in areas subjected to long‐term intensive cultivation practices. Our results will be useful to support decision making on the management of soil faunal communities and will facilitate modelling efforts of soil biology in global agroecosystems. HIGHLIGHTS Global meta‐analysis to estimate the effect of reducing tillage intensity on micro‐ and mesofauna Reduced tillage or no‐tillage has positive effects on springtail, mite and enchytraeid density Effects vary among nematode feeding groups, springtail life forms and mite suborders Effects vary with texture, climate and depth and depend on the tillage method and frequency
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Description: https://doi.org/10.20387/bonares-eh0f-hj28
    Keywords: ddc:631.4 ; agricultural land use ; conservation agriculture ; conventional agriculture ; soil biodiversity ; soil cultivation
    Language: English
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  • 14
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    Fifty years after deep‐ploughing: Effects on yield, roots, nutrient stocks and soil structure (2023)
    Blackwell Publishing Ltd | Oxford, UK
    Details
    Publication Date: 2024-05-30
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Deep‐ploughing far beyond the common depth of 30 cm was used more than 50 years ago in Northern Germany with the aim to break root‐restricting layers and thereby improve access to subsoil water and nutrient resources. We hypothesized that effects of this earlier intervention on soil properties and yields prevailed after 50 years. Hence, we sampled two sandy soils and one silty soil (Cambisols and a Luvisol) of which half of the field had been deep‐ploughed 50 years ago (soils then re‐classified as Treposols). The adjacent other half was not deep‐ploughed and thus served as the control. At all the three sites, both deep‐ploughed and control parts were then conventionally managed over the last 50 years. We assessed yields during the dry year 2019 and additionally in 2020, and rooting intensity at the year of sampling (2019), as well as changes in soil structure, carbon and nutrient stocks in that year. We found that deep‐ploughing improved yields in the dry spell of 2019 at the sandy sites, which was supported by a more general pattern of higher NDVI indices in deep‐ploughed parts for the period from 2016 to 2021 across varying weather conditions. Subsoil stocks of soil organic carbon and total plant‐available phosphorus were enhanced by 21%–199% in the different sites. Root biomass in the subsoil was reduced due to deep‐ploughing at the silty site and was increased or unaffected at the sandy sites. Overall, the effects of deep‐ploughing were site‐specific, with reduced bulk density in the buried topsoil stripes in the subsoil of the sandy sites, but with elevated subsoil density in the silty site. Hence, even 50 years after deep‐ploughing, changes in soil properties are still detectable, although effect size differed among sites.〈/p〉
    Description: BonaRes http://dx.doi.org/10.13039/501100022576
    Keywords: ddc:631.4 ; aggregates ; carbon sequestration ; deep‐ploughing ; macronutrients ; subsoil ; Treposol
    Language: English
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  • 15
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    The boundary element method for stress concentrations and fracture problems (1986)
    Institute of Physics
    In:  Professional Paper, Boundary Element Methods. Theory and Application, Bristol, Institute of Physics, vol. 9, no. 16, pp. 1-23, (ISBN 1-4020-1729-4)
    Details
    Publication Date: 1986
    Keywords: Stress ; Rock mechanics ; Stress intensity factor ; Boundary Element Method ; Fracture ; ENDNOTE?
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    BIBLIOGRAPHY SEISMOLOGY
  • 16
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    Boundary Element Methods. Theory and Application (1986)
    Institute of Physics
    In:  Bristol, Institute of Physics, vol. 8, no. Publ. No. 12, pp. 95-104, (ISBN 0-865-42078-5)
    Details
    Publication Date: 1986
    Keywords: Rock mechanics ; Fracture ; Boundary Element Method ; Elasticity ; Dynamic
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  • 17
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    AGU style : a guide for contributors (1986)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 67 (39). pp. 743-755.
    Details
    Publication Date: 2017-08-22
    Description: AGU considers only original scientific contributions that have not been accepted or published elsewhere and are not under consideration by another publisher. A contribution is considered previously published if its data and conclusions are offered for sale or are generally available in other ways to the public. Regardless of the original publication medium, including print, magnetic tape, or microform, such contributions are not eligible for republication in AGU journals or books.
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  • 18
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    The intrusion of water masses into the sea southwest of Taiwan (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Oceans, 94 (C12). pp. 18213-18226.
    Details
    Publication Date: 2017-07-20
    Description: Characteristics of water masses were analyzed to study the Kuroshio intrusion into the sea southwest of Taiwan. Hydrographic data were obtained from CTD (conductivity, temperature, and depth) casts during two cruises in May and August 1986. In May, remnants of water intruding from the Kuroshio were found on the continental slope south of the Penghu Channel. By August, these were replaced by water from the South China Sea. During this period, water from the Kuroshio also appeared near the southern tip of Taiwan. The intrusion current reached a depth of at least 500 m and was probably part of a cyclonic circulation in the northern South China Sea. The results support the hypothesis of a seasonal pattern of the intrusion process: intrusion of water from the Kuroshio begins in late summer, intensifies in winter, and ceases by late spring when South China Sea waters again enter this region.
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  • 19
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    Compositional heterogeneity of tephras from the 1980 eruptions of Mount St. Helens (1982)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 87 (B13). pp. 10861-10881.
    Details
    Publication Date: 2017-01-25
    Description: Samples collected at hourly intervals on May 18–19, 1980, at three sites 200 km downwind from Mount St. Helens, have made possible a detailed reconstruction of the conditions that contribute to the compositional heterogeneity of mineral and glass components observed in distal tephra layers. The air fall tephra deposited at the sites during the first 7 hours of the May 18 eruption is mostly coarse grained, microlite-rich, nonjuvenile glass and feldspar. Grain-size maxima in this initial tephra can be related to the cataclysmic blast at 0832 and a subsequent pulse of the eruption at 1200. Juvenile, microlite-free glass increases in relative abundance at the sampling sites beginning at about 1900. Such a change between nonjuvenile and juvenile tephra can be related to a 5-km increase in column height associated with the last major pulse of the eruption which occurred at 1700 at the volcano. Electron microprobe study of both microlite-rich and microlite-free pumice in the time series samples reveals significant compositional differences. Interstitial glass in nonjuvenile pumice deposited during the first few hours at the sampling sites is enriched in SiO2 and K2O and depleted in TiO2, FeO*, and MgO relative to juvenile glass. By comparison, major element composition of the least evolved juvenile glass sampled during the last several hours of the eruption displays a slight trend toward less evolved composition. Least squares calculations suggest that the more evolved character of the nonjuvenile glass can be explained by greater fractional crystallization brought about by enhanced cooling in a cryptodome prior to eruption, whereas the temporal changes observed in juvenile glass composition during the last several hours of the eruption suggest the presence of a small, slightly zoned magma chamber at depth. Electron microprobe study of glass-coated ilmenites, magnetites, and plagioclases provides the following estimates of the physical conditions in this reservoir: 865°±50°C, PH2O = 2.2 kbar and -log ƒO2 = 11.7. Analyses of bulk pumice, glass and selected mineral phases from May 25, June 12, July 22, and October 16–18 pumices erupted from Mount St. Helens indicate that the bulk pumice (magma) compositions have become slightly more andesitic with time, while mineral and co-existing glass compositions have changed significantly in post-May 18 eruptions with both being more highly evolved than those associated with the May 18 eruption. An application of the magnetite-ilmenite geothermometer to June 12 and July 22 samples indicates temperatures of 919°±30°C and 930°±50°C, respectively. Least squares calculations suggest that such evolved post-May 18 glass and mineral phases can be derived by fractional crystallization of a magma composition like bulk May 18 pumice into approximately 50% crystals and 50% residual liquid. Such partitioning between crystals and residual liquid appears to have occurred on the scale of centimeters and is interpreted as a consequence of accelerated crystallization under reduced water pressure.
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  • 20
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    The Relationship Between Volcanic Eruptions and Climate Change: Still a Conundrum? (1988)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 69 (6). pp. 74-86.
    Details
    Publication Date: 2017-01-31
    Description: What is the relationship between volcanic eruptions and climate change? More than 200 years after the connection was first proposed, it remains a thorny question. This article provides a brief historical overview of the problem and a review of the various data bases used in evaluating volcanic events and associated climatic change. We use the term “climate” to describe changes in the atmosphere over wide regions for periods of several months and longer. We use “weather” to describe shorter-term, variable atmospheric fluctuations experienced over more restricted areas. We appraise the present state of knowledge and highlight some pitfalls involved in using available information. Cautiously, we suggest future avenues for study, including the possibility of “volcanic winters,” or severe eruption-induced coolings.
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  • 21
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    Variations in the global carbon cycle during the cretaceous related to climate, volcanism, and changes in atmospheric CO2 (1985)
    AGU (American Geophysical Union)
    In:  In: The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. , ed. by Sundquist, E. T. and Broecker, W. S. Geophysical Monograph, 32 . AGU (American Geophysical Union), Boulder, pp. 504-529.
    Details
    Publication Date: 2017-01-30
    Description: The Stratigraphie record from both deep-sea and shallow-water depositional environments Indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaC02 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off-ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of C02 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric C02. A significant fraction of this atmospheric C02 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess C02 may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian-Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted from a warm global climate and increased marine transgression as well as by the higher C02.
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  • 22
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    Caldera development during the Minoan eruption, Thira, Cyclades, Greece (1984)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 89 (B10). pp. 8441-8462.
    Details
    Publication Date: 2017-05-04
    Description: The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large-scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.
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  • 23
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    The Mesozoic tectonic history of the Magellan Microplate in the western central Pacific (1988)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 93 (B4). pp. 2857-2874.
    Details
    Publication Date: 2017-07-07
    Description: Magnetic lineation mapping in the western central Pacific has revealed a pair of opposite-sensed, fanned lineation patterns that define the accretionary boundaries of the fossil Magellan microplate. This tectonic synthesis results from extensive magnetic mapping of two new lineation patterns over a large area and extended mapping of previously identified lineations. The entire evolutionary history of the Magellan microplate is well constrained to a 9-m.y. period in the Early Cretaceous by synchronous spreading patterns and associated geologic data. During this period the microplate grew and evolved as a generally rectangular structure to a final size of 700 km×600 km with spreading centers on two opposing sides and transform faults on the other two sides. The lifetime and size of the Magellan microplate are somewhat longer and larger, respectively, than presently active microplates on the East Pacific Rise. However, these modern structures are still evolving and growing, and the tectonic behavior of the modern and Cretaceous systems appears to be similar. Study of both active and fossilized microplates should provide additional insights on their common tectonic histories. In particular, we show that the Magellan Trough spreading center behaved as an asymmetric accretionary plate boundary that can be described with two separate poles of motion very close to this spreading center during much of its history. The Magellan Trough spreading center then failed as a result of a larger ridge reorganization at the triple junction of the Pacific, Farallon, and Phoenix plates at Ml0N time. Microplate activity ceased when the microplate became welded to the Pacific plate at M9 time.
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  • 24
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    Crustal structure of the Chugach Mountains, southern Alaska: A study of peg-leg multiples from a low-velocity zone (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 94 (B11). pp. 16023-16035.
    Details
    Publication Date: 2018-03-06
    Description: A seismic refraction profile recorded along the geologic strike of the Chugach Mountains in southern Alaska shows three upper crustal high-velocity layers (6.9, 7.2, and 7.6 km/s) and a unique pattern of strongly focussed echelon arrivals to a distance of 225 km. The group velocity of the ensemble of echelon arrivals is 6.4 km/s. Modeling of this profile with the reflectivity method reveals that the echelon pattern is due to peg-leg multiples generated from with a low-velocity zone between the second and third upper crustal high-velocity layers. The third high-velocity layer (7.6 km/s) is underlain at 18 km depth by a pronounced low-velocity zone that produces a seismic shadow wherein zone peg-leg multiples are seen as echelon arrivals. The interpretation of these echelon arrivals as multiples supersedes an earlier interpretation which attributed them to successive primary reflections arising from alternating high- and low-velocity layers. Synthetic seismogram modeling indicates that a low-velocity zone with transitional upper and lower boundaries generates peg-leg multiples as effectively as one with sharp boundaries. No PmP or Pn arrivals from the subducting oceanic Moho at 30 km depth beneath the western part of the line are observed on the long-offset (90-225 km) data. This may be due to a lower crustal waveguide whose top is the high-velocity (7.6 km/s) layer and whose base is the Moho. A deep (~54 km) reflector is not affected by the waveguide and has been identified in the data. Although peg-leg multiples have been interpreted on some long-range refraction profiles that sound to upper mantle depths, the Chugach Mountains profile is one of the few crustal refraction profiles where peg-leg multiples are clearly observed. This study indicates that multiple and converted phases may be more important in seismic refraction/wide-angle reflection profiles than previously recognized.
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  • 25
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    Pleistocene evolution: northern hemisphere ice sheets and North Atlantic Ocean (1989)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 4 (4). pp. 353-412.
    Details
    Publication Date: 2017-03-14
    Type: Article , PeerReviewed
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  • 26
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    Seasonal phytoplankton distribution along the Peruvian coast (1981)
    AGU (American Geophysical Union)
    In:  In: Coastal Upwelling. , ed. by Richards, F. A. Coastal and estuarine sciences, 1 . AGU (American Geophysical Union), Washington, USA, pp. 348-356.
    Details
    Publication Date: 2017-08-10
    Description: During a 10-year study more than 2,000 phytoplankton samples were collected from the entire coast of Peru and analyzed. In general, diatoms were the most abundant group of organisms in all seasons. Predominant species were Rhizosolenia delicatula, Skeletonema costatum Thalassiosira subtilis, Thalassionema nitzschioides and several species of the genus Chaetooeros. Dinoflagellates and flagellates were observed frequently during summer. The mean distribution of the phytoplankton concentration during the 10 years shows the existence of several centers with higher cell densities along the coast, coinciding with the areas of more intense and persistent upwelling. Four major centers have been identified: Pimentel (˜6°S), Chimbote (˜9°S), Callao (˜12°S), and Tambo de Mora-Pisco (˜15°S); and two minor centers, Talara (˜4°S) and Ilo (˜17°S). The relative importance of each center seems to change according to the season. The highest phytoplankton concentration tended to be in the northern part of the coast during fall and winter and in the south through spring and summer.
    Type: Book chapter , NonPeerReviewed
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  • 27
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    Near-vertical and intermediate offset seismic reflection data from west of the Whipple Mountains, SE California (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 94 (B1). pp. 625-636.
    Details
    Publication Date: 2018-03-06
    Description: During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3-km-wide and 15-km-long region in Vidal Valley, roughly parallel to a segment of one of the near-vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray-tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2-km depth near the Whipple Mountains to a depth of 5-km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near-vertical profiles in the Whipple Mountains. (2) On the near-vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near-surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near-vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi-three-dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near-vertical reflection profiles to be related to the mid-Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models.
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    Statistical methods for describing seafloor topography (1987)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 14 (10). pp. 1061-1064.
    Details
    Publication Date: 2017-07-25
    Description: We present a method for objectively characterizing a swath of digitally sampled seafloor topography. Our method analyzes the distribution of surface slopes by compiling surface-normal vectors into a two-dimensional histogram using an equal-area projection. The direction of maximum variance (first principal axis) of the histogram is used to determine the azimuth of lineations in the topography, and the variance is used as a measure of seafloor roughness. We apply the method to short sections of Sea Beam swath data and find that the histogram parameters are effective in describing the behavior of the topography. In particular, similar patterns are observed for a sequence of histograms derived from data collected over the Mendocino and the Surveyor fracture zones in the northeast Pacific. Because the method does not require any data modification and is suitable for irregularly-shaped sample regions, it lends itself to real-time analysis.
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  • 29
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    The mathematician paleoplate (1988)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 93 (B4). pp. 3025-3040.
    Details
    Publication Date: 2017-12-08
    Description: The Pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations. One of the clearest examples of such a reorganization is documented in the Mathematician Seamounts region, where a distinctive geomorphology and well-developed magnetic anomalies are present. This reorganization involved a short-lived microplate between the failing Mathematician Ridge and a new propagating spreading center: the East Pacific Rise. It produced a transfer of a fragment of lithosphere from the Farallon to the Pacific plate, and also created a number of landforms and magnetic patterns, within and on the margins of the captured fragment; these make up the Mathematician paleoplate. In many cases, two sides of a microplate are active spreading ridges. A microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge. We define a paleoplate as the area of the seafloor, from the axis of a failed rift to the boundary of resumed, full spreading. It includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading, prior to complete abandonment of the failed rift. The Mathematician paleoplate has the following boundaries and components from west to east: the axis of the Mathematician failed rift, the fragment of captured Farallon plate, a complex rift initiation site at the Moctezuma Trough, a zone of slow spreading, and an as yet ill-defined eastern boundary where dual spreading stopped and full spreading resumed. The northern boundary of the paleoplate is the Rivera fracture zone; its southeastern boundary a now-inactive transform fault, the West O'Gorman fracture zone. In this case, as well as in other more poorly documented ones, relict landforms and magnetic patterns are carried on the aging lithosphere, away from the spreading ridge, recording a former geometry.
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  • 30
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    Seasonal variations in ebullitive flux and carbon isotopic composition of methane in a tidal freshwater estuary (1988)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 2 (3). pp. 289-298.
    Details
    Publication Date: 2018-01-11
    Description: Methane carbon isotopic composition ranged from −76.9 to −62.6‰ in a tidal freshwater estuary (the White Oak River, North Carolina, United States) with site specific seasonal variations ranging from 6 to 10‰. During warmer months, tidally induced bubble ebullition actively transported this methane to the atmosphere. At two sites, these seasonally varying fluxes ranged from 1.2 ± 0.3 to 1.3 ± 0.3 mol CH4 m−2yr−1 (19.2 to 20.8 g CH4m−2yr−1), with flux-weighted average isotopic compositions at two sites of −66.3 ± 0.4 and −69.5 ± 0.6‰. The carbon isotopic composition of naturally released bubbles was shown to be indistinguishable from the sedimentary methane bubble reservoir at three sites, leading to the conclusion that isotopic fractionation did not occur during the ebullition of methane. The hypothesis was developed that ebullitive methane fluxes are depleted in 13CH4 relative to fluxes transported via molecular diffusion or through plants, as zones of 13C enriching microbial methane oxidation are bypassed.
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  • 31
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    Continental margin subsidence: A Comparison between the east and west coasts of Africa (1982)
    AGU (American Geophysical Union)
    In:  In: Dynamics of Passive Margins. , ed. by Scrutton, R. A. Geodynamics series, 6 . AGU (American Geophysical Union), Washington, DC, pp. 59-71.
    Details
    Publication Date: 2016-04-11
    Description: Sedimentation rates (corrected for compaction) from along the passive continental margin of Africa between the Equatorial Fracture Zone and Somalia are used to compare the rates of subsidence of the continental crust since early Mesozoic time. Three distinctive subsidence histories can be identified which correspond with basinal areas that have different structural styles: rifted (west coast), sheared (Equatorial and Agulhas fracture zones) and sunk (zones of vertical tectonics in eastern Africa). A comparison of subsidence rates with other tensional margins (NE USA and the North Sea) and a consideration of the plate tectonic history of the African margins leads to the proposal of a geo and thermodynamic model that takes cognizance of the worldwide mid-Cretaceous rheological discontinuity between taphrogenic and epeirogenic basin formation recognized by Kent, and the more generally accepted, purely plate tectonic driven model of margin subsidence. The new suggestion involves a lower Mesozoic worldwide rise in the geothermal gradient in the lithosphere which produces metamorphism of the base of the continental crust and initiates taphrogenesis along lineaments throughout Gondwanaland. A lowering of the geothermal gradient in the lower Cretaceous produces a switch to epeirogenic subsidence, driven solely by sediment loading and thermal contraction, by Aptian/Albian times. The thermal event facilitated continental separation, and sea floor spreading commenced locally at various times along the active taphrogenic belts. Local thermal and tectonic aberrations associated with this phenomenon over print onto the worldwide pattern of marginal basin subsidence. A further rise in the geothermal gradient may have been responsible for renewed taphrogenesis in eastern Africa in Tertiary times.
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  • 32
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    Renewal rates of East Atlantic deep water estimated by inversion of 14C data (1987)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Oceans, 92 (C3). pp. 2953-2969.
    Details
    Publication Date: 2016-04-14
    Description: The renewal of the deep water of the East Atlantic and its large-scale internal circulation are studied on the basis of the distributions of potential temperature, silicate, ΣCO2, and 14C. An isopycnal multibox model including advection, mixing, and sources and sinks is set up and described. Tracer data are input for the model, and balance equations for the various properties for the boxes of the model serve as constraints for the determination of water fluxes, mixing coefficients, and source parameters. Extremal values for various model parameters that are consistent with the tracer data (satisfy the balance equations within the estimated tolerances) are calculated by linear programming techniques. 14C data are seen to be valuable in determining absolute flow rates. Model results confirm the importance of the Romanche Facture Zone for the renewal of east Atlantic deep water. Eastward flows through the Romanche Fracture Zone were found to be between 2.6 and 5.1 Sv. Flows through the Vema Fracture Zone amount to at most 20% of the Romanche Fracture Zone inflow. Contributions of Antarctic Bottom Water at the southern end of the East Atlantic (Walvis Ridge) and of Iceland Scotland Overflow Water at the northern end are very small (〈 5% of equatorial inflow). Diapycnal mixing coefficients are between 1 and 10 cm2/s, and values for the dissolution rates of silicate and carbon are in the expected range.
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  • 33
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    Some observations of the Azores Current and the North Equatorial Current (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 94 (C3). p. 3181.
    Details
    Publication Date: 2018-04-17
    Description: The regions containing the two zonal currents of the subtropical gyre in the eastern North Atlantic, the Azores Current and the North Equatorial Current (NEC), have quite different physical characteristics. Associated with the Azores Current are strong horizontal thermohaline gradients that can be located easily both at the surface and at depth with temperature data alone, thus making satellite IR imagery and expendable bathythermograph profiles suitable for observing it. During winter, the surface expression of the Azores Current is often found to the north of the strongest subsurface gradients. In contrast to the Azores Current and to the central water mass boundary just to the south, the NEC has relatively weak horizontal temperature and salinity gradients, requiring density information in order to identify it. There is no clear surface manifestation found with the NEC. Common to both currents, though, is that each transports O(8 Sv) in the upper 800 m of the ocean near 27°W, with the largest velocities being in the upper 400 m.
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  • 34
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    The distribution of diurnal sea surface warming events in the western Sargasso Sea (1985)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 90 (C6). p. 11811.
    Details
    Publication Date: 2018-02-26
    Description: Large diurnal sea surface warming exceeding 1°C is common in the western North Atlantic Ocean and is often of large horizontal extent. These events correlate closely with very light winds and high insolation. In the area investigated, 17°–40°N and 55°–80°W, the largest warming is found in the western portion of the ridge associated with the Azores-Bermuda high, where the lowest wind speeds are observed. The distribution of warming events shows that the largest number occur between June and August, when insolation is highest and percent cloud cover and wind speed are low. The most probable latitude of warming events moves north from approximately 25°N in spring to near 30°N in summer, a shift similar to that seen in the minimum of the climatological winds. Local areas have a probability as high as 30% for diurnal warming in excess of 1°C in the summer. The net heat flux into the ocean, calculated by using monthly mean values for low latitudes in the summer, excluding diurnal warming events, is biased consistently high by as much as 5 W/m2 relative to the same values calculated with warming events included.
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  • 35
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    Electrical conductivity, temperatures, and fluids in the lower crust (1983)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 88 (B11). p. 9475.
    Details
    Publication Date: 2016-03-02
    Description: We have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles. Regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models; we avoided those data where purely conductive heat flow suggested temperatures more than about 1000°C, substantially higher than solidus temperatures and outside the range of validity of heat flow models. The resulting plots of log conductivity σ versus 1/T demonstrate that even low-conductivity layers (LCL) have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits. In addition, technically active regions show systematically higher conductivities than do shield areas. Because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions, these materials probably account for the relatively higher conductivities of rocks in situ in the crust; free water in amounts of 0.01–0.1% in fracture porosity could explain crustal conductivities. Other possibilities are graphite, hydrated minerals in rare instances, and sulfur in combination with other volatiles. As most of the temperatures are less than 700°C, partial melting seems likely only in regions of highest heat flow where the conductive temperature profiles are inappropriate. Another result is that at a given temperature, crustal high-conductivity layers (HCL) are more conductive by another order of magnitude and show more scatter than do LCL's. Because the differences between HCL's and LCL's are independent of temperature, we must invoke more than temperature increases as a cause for large conductivity increases; increased fluid concentration in situ seems a probable cause for enhanced conductivities in HCL's. From the point of view of these observations, it does not matter whether the fluids are in communication with the surface or trapped at lithostatic pressures.
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    Heat flow in southernmost California and the origin of the Salton Trough (1985)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 90 (B8). 6709-6736 .
    Details
    Publication Date: 2016-04-18
    Description: Heat flow in the Imperial Valley and adjacent crystalline rocks is very high (∼140 mW m−2). Gravity and seismic studies suggest the crust is about 23.5 km thick with the lower half composed of gabbro and the upper fourth composed of low-density sediments. Conduction through such a crust resting directly on asthenosphere would give the observed heat flow if there were no extension or sedimentation. However, both processes must have been active, as the Imperial Valley is part of the Salton Trough, a pull-apart sedimentary basin that evolved over the past 4 or 5 m.y. To investigate the interrelations of these factors, we consider a one-dimensional model of basin formation in which the lower crustal gabbro and upper crustal sediments accumulated simultaneously as the crust extended and sedimentation kept pace with isostatic subsidence. For parameters appropriate for the Salton Trough, increasing the extension rate has little effect on surface heat flow because it increases effects of heating by intrusion and cooling by sedimentation in a compensating manner; it does, however, result in progressively increasing lower crustal temperatures. Analytical results suggest that the average extensional strain rate during formation of the trough was ∼20–50%/m.y. (∼1014 s−1); slower rates are inadequate to account for the present composition of the crust, and faster rates would probably cause massive crustal melting. To achieve the differential velocities of the Pacific plate at one end of the trough and North American plate at the other with this strain rate, extension must have, on the average, been distributed (or shifted about) over a spreading region ∼150 km wide. This is about 10 times wider than the present zone of active seismicity, suggesting that the seismic pattern is ephemeral on the time scale for the trough's formation. Narrow spreading zones are typical where sustained spreading is compensated by basaltic intrusion to form the thin oceanic crust, but where such spreading occurs in thicker continental crust, broader zones of distributed extension (with smaller strain rates) may be required for heat balance. The Salton Trough model suggests that distributed extension can be associated with substantial magmatic additions to the crust; their effect on crustal buoyancy has important implications for the relation between crustal extension and subsidence.
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  • 37
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    Petrological and geochemical variations along Iceland's Neovolcanic Zones (1985)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 90 (B12). pp. 10043-10072.
    Details
    Publication Date: 2016-05-04
    Description: Petrological, geochemical, and geophysical gradients along the SE volcanic zone in Iceland imply systematic variations in melting and crystallization conditions and in magma supply and eruption rates. At the southern tip of the zone, in Vestmannaeyjar, alkali basalt magmas are generated by small degrees of melting under a thick lithosphere. Farther north, in the Hekla-Katla region, greater degrees of melting result in the generation of transitional basalt magmas. Magma supply rates exceed eruption rates, and melts begin to accumulate at the base of the crust, as indicated by magnetotelluric evidence. Uniform rare earth element patterns in the Hekla-Katla basalts may be explained by homogenization in the melt accumulation zone or by uniform melting conditions. Infrequent replenishment of magma reservoirs in this region leads to mixing of compositionally diverse magmas and, consequently, to basalts with diverse phenocryst compositions and textures. Even farther north, in central Iceland, the melting anomaly associated with the SE zone has developed to the same degree as it has beneath the SW axial rift zone, leading to similar magmatic conditions. High magma supply rates and low cooling rates inhibit fractionation and lead to the eruption of voluminous olivine tholeiites. In these areas a broad spectrum of melt compositions is generated by variable degrees of melting over a wide depth range. The compositional diversity, e.g., in large ion lithophile element enrichment, is masked somewhat by reequilibration and mixing of melts on ascent and in the melt accummulation zone. Compositional diversity may be preserved, however, in the melt accummulation zone in a lateral direction away from the rift axis since distal parts of the melt zone are fed only by melts segregating at greater depths. The variations in magmatic conditions along the SE zone, which are analogous to those inferred along propagating rifts, may be related to a mantle blob that ascended beneath central Iceland 2–3 m.y. ago, spread out laterally and triggered a southward propagating rift.
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  • 38
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    Processing and analysis of large volumes of satellite-derived thermal infrared data (1987)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 92 (C12). pp. 12993-13002.
    Details
    Publication Date: 2019-04-04
    Description: Reducing the large volume of TIROS-N series advanced very high resolution radiometer-derived data to a practical size for application to regional physcial oceanographic studies is a formidable task. Such data exist on a global basis for January 1979 to the present at approximately 4-km resolution (global area coverage data, ≈2 passes per day) and in selected areas at high resolution (local area coverage and high-resolution picture transmission data, at ≈1-km resolution) for the same period. An approach that has been successful for a number of studies off the east coast of the United States divided the processing into two procedures: preprocessing and data reduction. The preprocessing procedure can reduce the data volume per satellite pass by over 98% for full-resolution data or by ≈84% for the lower-resolution data while the number of passes remains unchanged. The output of the preprocessing procedure for the examples presented is a set of sea surface temperature (SST) fields of 512 × 1024 pixels covering a region of approximately 2000 × 4000 km. In the data reduction procedure the number of SST fields (beginning with one per satellite pass) is generally reduced to a number manageable from the analyst's perspective (of the order of one SST field per day). This is done in most of the applications presented by compositing the data into 1- or 2-day groups. The phenomena readily addressed by such procedures are the mean position of the Gulf Stream, the envelope of Gulf Stream meandering, cold core Gulf Stream ring trajectories, statistics on diurnal warming, and the region and period of 18°C water formation. The flexibility of this approach to regional oceanographic problems will certainly extend the list of applications quickly.
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  • 39
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    Constraints and interpretation of 87Sr/86Sr ratios in Cenozoic dolomites (1988)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 15 (4). pp. 385-388.
    Details
    Publication Date: 2015-06-10
    Description: Replacement dolomitization by seawater has been modeled in order to quantify the Sr-isotope signature in Cenozoic dolomites as a function of precursor mineralogy and 87Sr/86Sr ratio, reaction stoichiometry and 87Sr/86Sr ratio of the dolomitizing fluids. High Sr carbonates, such as aragonite, may introduce a significant precursor memory into an otherwise seawater dominated Sr-isotope signature if small quantities of seawater per unit volume of precursor carbonate are involved. Dolomitization of low Sr carbonates (i.e. low-Mg calcite) are shown to create an isotopic signature indistinguishable from that of the seawater involved in the reaction. Therefore, by comparison with the Sr-isotope evolution curve of seawater, the- 87Sr/86Sr ratios of the dolomites can be used to record the oldest possible age of dolomitization and the youngest age of deposition. The implications for this approach have been applied to data obtained from a dolomitized core from Little Bahama Bank, Bahamas. Two periods of dolomitization are recognized, one in the early Late Miocene involving Middle Miocene or older rocks, and a second one around 2.4 Ma ago affecting early Pliocene carbonates.
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  • 40
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    Lithium-to-calcium ratios in Modern, Cenozoic, and Paleozoic articulate brachiopod shells (1989)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 4 (6). pp. 681-691.
    Details
    Publication Date: 2015-09-01
    Description: Li/Ca ratios in modern brachiopod shells generally correlate inversely with growth temperature, ranging from ∼20 µmol/mol at 30°C to ∼50 µmol/mol at 0°C with no apparent interspecific offsets. Causes of the temperature effect on Li/Ca ratios are not yet understood. Cenozoic brachiopod Li/Ca ratios average ∼30 µmol/mol, similar to the average observed in modern brachiopods. Relatively constant Li/Ca ratios for Eocene to Pleistocene nonluminescent brachiopod shells, consistent with previous observations of Cenozoic planktonic foraminifera, support the conclusion of little variation in Cenozoic seawater Li/Ca. Nonluminescent portions of Permian and Carboniferous brachiopods have Li/Ca ratios substantially lower (generally 〈10 µmol/mol) than modern, Cenozoic, or Devonian samples. Mass balance considerations, constrained by δ18O of brachiopods, suggest that low Li concentrations in Permo-Carboniferous seawater could be the result of a lower flux of dissolved Li from the continents and/or a higher flux of Li from seawater to clastic marine sediments. Nonluminescent Devonian brachiopods from a single hand specimen have Li/Ca ratios around 70% of the modern average. These Li/Ca ratios can be explained by either somewhat higher temperature with constant seawater Li/Ca, somewhat lower seawater Li/Ca at constant temperature, or a combination of slightly elevated temperature and slightly lower seawater Li/Ca.
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  • 41
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    Submesoscale, coherent vortices in the ocean (1985)
    AGU (American Geophysical Union)
    In:  Reviews of Geophysics and Space Physics, 23 (2). pp. 165-182.
    Details
    Publication Date: 2016-01-11
    Description: A variety of observations of intense, long-lived oceanic vortices are interpreted as examples of a distinct phenomenon, which is given the name Submesoscale, Coherent Vortices (SCV's). The distinguishing characteristics of SCV's are defined and illustrated by example, and a survey is made of the different SCV types presently known. On the basis of extant theoretical and modeling solutions, interpretations are made of the dynamics associated with SCV existence, movement, endurance, interactions with other currents, generation, and contributions to the transport of chemical properties in the ocean.
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  • 42
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    Coexisting calcalkaline and high-niobium basalts from Turrialba Volcano, Costa Rica: Implications for residual titanates in arc magma sources (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 94 (B4). pp. 4619-4633.
    Details
    Publication Date: 2015-07-31
    Description: Turrialba volcano, the southeasternmost volcano in the Central American arc, is constructed of medium to high-K calcalkaline basalts, andesites, and dacites, plus rare basalts with unusually high Nb concentrations. The compositions of these high-Nb basalts are more similar to those of intraplate basalts than they are to typical calcalkaline or arc-tholeiitic basalts. The association of calcalkaline and high-Nb basalts is rare in arc front volcanoes, seemingly being restricted to volcanoes that overlie Oligocene or younger subducting crust or that overlie the edges of subducting plates. The calcalkaline and high-Nb basalts at Turrialba have generally similar major element, trace element, and isotopic compositions but differ significantly in their Ba/La and La/Nb ratios. The geochemical similarities imply that they were derived from similar ocean island basalt sources. Their geochemical differences suggest that residual rutile stabilized by a large ion lithophile element bearing slab-derived fluid was present during calcalkaline basalt genesis but not during high-Nb basalt genesis. To explain the stability of rutile in a calcalkaline melt with a relatively low TiO2 concentration, we use a model that involves two stages of melting for both basalt types. Silica saturated high degree melts with mid-ocean ridge basalt like incompatible element concentrations generated by upwelling mantle are used as mixing end-members for both the calcalkaline and the high-Nb basalts. The calcalkaline basalts represent mixtures of the high-degree melts and oxidized small-degree melts generated by amphibole breakdown in mantle overlying the subducting slab. This small-degree melt has high incompatible element concentrations and is saturated in rutile. Arc-related lamprophyric rocks have compositions that are appropriate for these small-degree melts. High-Nb basalts are mixtures of the high-degree melts and more reduced small-degree melts that are undersaturated in rutile. These reduced melts may migrate around or through the subducting slab into the wedge to become involved in arc magma genesis.
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  • 43
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    Atmospheric carbon dioxide, orbital forcing, and climate (1985)
    AGU (American Geophysical Union)
    In:  In: The carbon cycle and atmospheric CO2: Natural variations archean to present; Proceedings of the Chapman Conference on Natural Variations in Carbon Dioxide and the Carbon Cycle, Tarpon Springs, FL, January 9-13, 1984. AGU (American Geophysical Union), Washington, DC, pp. 303-317.
    Details
    Publication Date: 2015-08-03
    Description: A 340,000-year record of benthic and planktonic oxygen and carbon isotope measurements from an equatorial Pacific deep-sea core are analyzed. The data provide estimates of both global ice volume and atmospheric carbon dioxide concentration over this period. The frequencies characteristic of changes in the earth-sun orbital geometry dominate all the records. Examination of phase relationships shows that atmospheric carbon dioxide concentration leads ice volume over the orbital bandwidth, and is forced by orbital changes through a mechanism, at present not fully understood, with a short response time. Changes in atmospheric CO2 are not primarily caused by glacial-interglacial sea level changes, which had been hypothesized to affect atmospheric CO2 through the effect on ocean chemistry of changing sedimentation on the continental shelves. Instead, variations in atmospheric CO2 should be regarded as part of the forcing of ice volume changes.
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  • 44
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    Cyclonic cold-core eddy in the eastern North Atlantic. II. Nutients, phytoplankton and bacteriaplankton (1987)
    Inter Research
    In:  Marine Ecology Progress Series, 39 . pp. 153-167.
    Details
    Publication Date: 2018-05-04
    Description: A cyclonic cold-core eddy in the Northeast Atlantic of about 100 km in diameter at the sea surface was investigated in May 1985, approximately 3 wk after it had separated from the Polar Front. A strong thermocline, which was shallower but more pronounced than in the ambient water, separated a warm surface layer within the eddy from deeper cold water, while horizontal salinity gradients marked the boundary to the ambient water. The cold-core eddy could be distinguished from amblent Northeast Atlantic water in terms of its nutrient chemistry, phytoplankton species distribution and abundance, bacterial numbers and cell size. The surface layer of the eddy was distinct from deeper eddy water, and was characterized by high concentrations of chlorophyll a, total phytoplankton biomass, dinoflagellates and bacteria. At the eddy's margin diatoms were predominant. It is argued that the physical isolation of the eddy surface layer due to the formation of a shallow thermocline led to rapid utilisation of nutrients. This probably enabled the development of a dinoflagellate-dominated phytoplankton population and of organisms capable of heterotrophic regenerative processes.
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  • 45
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    Drilling deep into young oceanic crust, Hole 504B, Costa Rica Rift (1989)
    AGU (American Geophysical Union)
    In:  Reviews of Geophysics, 27 (1). pp. 79-102.
    Details
    Publication Date: 2016-05-30
    Description: Hole 504B is by far the deepest hole yet drilled into the oceanic crust in situ, and it therefore provides the most complete “ground truth” now available to test our models of the structure and evolution of the upper oceanic crust. Cored in the eastern equatorial Pacific Ocean in 5.9-m.y.-old crust that formed at the Costa Rica Rift, hole 504B now extends to a total depth of 1562.3 m below seafloor, penetrating 274.5 m of sediments and 1287.8 m of basalts. The site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust. As a result, the conductive heat flow approaches the value of about 200 mW/m² predicted by plate tectonic theory, and the in situ temperature at the total depth of the hole is about 165°C. The igneous section was continuously cored, but recovery was poor, averaging about 20%. The recovered core indicates that this section includes about 575 m of extrusive lavas, underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes; the latter have been sampled in situ only in hole 504B. The igneous section is composed predominantly of magnesium-rich olivine tholeiites with marked depletions in incompatible trace elements. Nearly all of the basalts have been altered to some degree, but the geochemistry of the freshest basalts is remarkably uniform throughout the hole. Successive stages of on-axis and off-axis alteration have produced three depth zones characterized by different assemblages of secondary minerals: (1) the upper 310 m of extrusives, characterized by oxidative “seafloor weathering“; (2) the lower extrusive section, characterized by smectite and pyrite; and (3) the combined transition zone and sheeted dikes, characterized by greenschist-facies minerals. A comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology, alteration, and porosity into three zones that are analogous to layers 2A, 2B, and 2C described by marine seismologists on the basis of characteristic seismic velocities. Many of the logs and experiments suggest the presence of a 100- to 200-m-thick layer 2A comprising the uppermost, rubbly pillow lavas, which is the only significantly permeable interval in the entire cored section. Layer 2B apparently corresponds to the lower section of extrusive lavas, in which original porosity is partially sealed as a result of alteration. Nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900–1000 m below seafloor, within the transition between extrusives and sheeted dikes, indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2B and 2C and confirming that layer 2C consists of intrusive sheeted dikes. A vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends—that between the sheeted dikes of seismic layer 2C and the gabbros of seismic layer 3, which has never been sampled in situ—may be within reach of the next drilling expedition to hole 504B. Therefore despite recent drilling problems deep in the hole, current plans now include revisiting hole 504B for further drilling and experiments when the Ocean Drilling Program returns to the eastern Pacific in 1991.
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  • 46
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    Radiocarbon-based chronology for the 18O/16O record for the Last Deglaciation (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (4). pp. 509-515.
    Details
    Publication Date: 2016-06-15
    Description: A radiocarbon-calibrated box model for today's ocean suggests that a lag of about 1750 years should exist between the arrival of the midpoint of the deglaciation 18O signal in the deep Atlantic Ocean and its arrival in the deep Pacific Ocean. In order to assess the actual lag, we have carried out accelerator radiocarbon measurements on two cores from the Atlantic Ocean and one core from the Pacific Ocean. Although the results are not definitive, there is a suggestion that the actual time lag was about 1000 years.
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  • 47
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    The cause of the glacial to interglacial atmospheric CO2 change: A Polar Alkalinity Hypothesis (1989)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 3 (3). pp. 215-239.
    Details
    Publication Date: 2016-06-16
    Description: In an attempt to create a scenario for the cause of the glacial to interglacial CO2 change recorded in air trapped in polar ice, we call on an increase in the alkalinity of polar surface waters. In this way we circumvent a major deficiency of the polar nutrient scenarios of Sarmiento and Toggweiler (1984), Siegenthaler and Wenk (1984) and Knox and McElroy (1984). Namely, our scenario does not require a drop in the nutrient content of polar surface waters in conformity with the demonstration by Boyle (1988a, b) that the cadmium content of planktonic foraminifera from polar regions did not decrease from late glacial to Holocene time. The rise in alkalinity required by our model is a natural consequence of the demise, during glacial time, of North Atlantic Deep Water as a major force in ocean circulation and of the nutrient maximum deepening of Boyle (1988b). Rather than being original, our hypothesis builds on the concept basic to the polar nutrient hypotheses, namely that the CO2 partial pressure in polar waters controls that for both the atmosphere and warm surface ocean. It also requires the alkalinity increase in surface waters produced by Boyle's nutrient deepening.
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  • 48
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    Marine geology of the Hess Rise: 2. Basement morphology, sediment thickness, and structural geology (1982)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 87 (B11). pp. 9259-9278.
    Details
    Publication Date: 2016-08-02
    Description: The basement morphology and sediment thickness of the Hess Rise, an oceanic plateau in the central North Pacific, have been mapped on the basis of seismic reflection profiles. The acoustic stratigraphy on and around the rise is correlated with the lithostratigraphy at Deep Sea Drilling Project sites 464, 310, 465, and 466. A total sediment isopach chart of the rise reveals small-scale departures from the expected sedimentary pattern (thick sediment in shallow areas; thin sediment in deep areas). Sediment-filled basement depressions result from mass transport; thin sediment (〈50 m) occurs on steep scarps, basement ridges, and areas affected by bottom currents. A pre-Senonian sediment isopach chart shows a thickening from less than 50 m to more than 250 m of sediment from the northeast to the southwest. This trend seems explainable only in terms of the time-transgressive nature of seafloor formed at a mid-ocean ridge. The axial trend of the rise (N30°W) parallels nearby Mesozoic magnetic lineations and seems to be isochronous as deduced from the Deep Sea Drilling Project data. The Hess Rise began developing in late Aptian time along a segment of the Pacific-Farallon Ridge. Important events in the history of the rise are late-stage volcanism on the southern margin of the rise along the Mendocino Fracture Zone, tectonism and volcanism about 85 Ma that resulted in a major regional unconformity (reflector C), and another period of tectonism and volcanism between 65 and 43 Ma that coincided with the formation of the Emperor Seamounts and created structural benches on the western side of the rise. A significant change in the paleoenvironment that apparently occurred around the Paleogene-Neogene boundary (∼25–20 Ma) caused pronounced changes in the depositional environment and resulted in another major regional unconformity (reflector A).
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  • 49
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    Benthic isotope evidence for changes of the Mediterranean outflow during the Late Quaternary (1987)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 2 (6). pp. 543-559.
    Details
    Publication Date: 2016-09-05
    Description: A suit of sediment cores close to and south of the Strait of Gibraltar (12°-36°N, 500–2800 m water depth) were analyzed for stable isotopes in epibenthic foraminifers Cibicidoides wuellerstorfi and Planulina ariminensis. During peak glacial times, the data exhibit higher δ13C values of up to 1.6‰ at intermediate depths near the Strait of Gibraltar (36°N). The values decrease to the south as evidenced by our data, but also to the north as revealed by data of intermediate depth cores north of 38°N (in Duplessy et al. [1987]). Thus, the distribution pattern of δ13C provides crucial evidence for an increased influence of nutrient depleted Mediterranean Outflow Water (MOW) on the glacial northeast Atlantic hydrography. During oxygen isotope Terminations I and II, the meridional carbon isotope gradient indicates a significantly decreased but still active MOW. As deduced from the δ18O fluctuations, the temperatures of the MOW in the Atlantic were lower during glacial times by as much as 5°C. During glacial times and during Termination I the maximum δ13C values of the MOW correlate with minimum values of the North Atlantic Deep Water (NADW) and vice versa. This inverse response to climatic change of the carbon isotope signals of both water masses indicates, that the supply of saline MOW to the north Atlantic may be less important for the formation of NADW than previously assumed.
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  • 50
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    Global variations of surface ocean productivity in low and mid latitudes: Influence on CO2 reservoirs of the deep ocean and atmosphere during the last 21,000 years (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (3). pp. 361-399.
    Details
    Publication Date: 2016-09-07
    Description: Based on detailed reconstructions of global distribution patterns, both paleoproductivity and the benthic δ13C record of CO2, which is dissolved in the deep ocean, strongly differed between the Last Glacial Maximum and the Holocene. With the onset of Termination I about 15,000 years ago, the new (export) production of low- and mid-latitude upwelling cells started to decline by more than 2-4 Gt carbon/year. This reduction is regarded as a main factor leading to both the simultaneous rise in atmospheric CO2 as recorded in ice cores and, with a slight delay of more than 1000 years, to a large-scale gradual CO2 depletion of the deep ocean by about 650 Gt C. This estimate is based on an average increase in benthic δ13C by 0.4–0.5‰. The decrease in new production also matches a clear 13C depletion of organic matter, possibly recording an end of extreme nutrient utilization in upwelling cells. As shown by Sarnthein et al., [1987], the productivity reversal appears to be triggered by a rapid reduction in the strength of meridional trades, which in turn was linked via a shrinking extent of sea ice to a massive increase in high-latitude insolation, i.e., to orbital forcing as primary cause.
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  • 51
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    Comparison of sinking and sedimentation rate measurements in a diatom winter/ spring bloom (1989)
    Inter Research
    In:  Marine Ecology Progress Series, 54 . pp. 109-119.
    Details
    Publication Date: 2018-05-04
    Description: Sinking and sedimentation rates of a natural phytoplankton community were simultaneously measured during the course of a diatom winterkpring bloom in a 13m3 experimental mesocosm. Sinking rate was determined directly in settling columns and was calculated from sediment trap catches. The 2 methods yielded significantly different results. Whole-community as well as speciesspecific sinking rates varied over time. These variations were related to changes of the environmental conditions. Over a 26d study period, a total of 7.5g cm-' was collected in the sediment traps. Viable phytoplankton cells were the primary component of the sedimented matter while zooplankton fecal pellets contributed on average less than 10 %. Assuming the Redfield atomic ratio for the collected material, the amount of carbon which sedimented during the winterkpring bloom could be predicted from pre-bloom nutrient concentrations. The daily sedimentation rate varied considerably over time and displayed a characterisbc pattern. This pattern is evidently a function of both suspended phytoplankton biomass and the temporal variation in whole-community sinking rate.
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  • 52
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    Macroplankton communities in Antarctic surface waters: spatial changes related to hydrography (1989)
    Inter Research
    In:  Marine Ecology Progress Series, 55 . pp. 251-259.
    Details
    Publication Date: 2018-05-07
    Description: During the Anglo-German Antarctic expedition in February 1982 macroplankton was collected in the waters off the Antarctic Peninsula. Macroplankton compos~tions at 36 stations were compared and degrees of similarity submitted to hierarchical cluster analysis. Results demonstrate strong spatial heterogeneity, which could be attributed to the different water masses in this region. These dissimilarities demarcate 4 provinces, each characterized by a distinct macroplankton community: (l) The 'Oceanic Community' comprises the stations influenced by the Westwind Drift; oceanic forms are typical (e.g. the hyperiid amphipod Vibilia antarctica, and the polychaete Vanadis antarctica). (2) The 'Bellingshausen Water Community' is influenced by Bellingshausen Sea water and oceanic specles are scarce; large numbers of Antarctic krill Euphausja superba occur but shallow water forms are also abundant. (3) The 'Nentic Community' consists of stations in the shelf water of the southern Bransfield Strait; it is charactenzed by postlarvae of several fish species, and meroplanktonic larvae of benthic forms. Large krill concentrations, however, are also encountered. (4) A 'Transitional Community' exists in environments where various water masses mingle (e.g. at shelf slopes). This community lacks typical forms.
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  • 53
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    On the Variability of Southern Ocean Front Locations Between Southern Brazil and the Antarctic Peninsula (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 94 (C4). pp. 4757-4762.
    Details
    Publication Date: 2019-02-27
    Description: A 4-year expendable bathythermograph data set (1984–1987) from the area between southern Brazil and the Antarctic Peninsula provides information on the interannual variability of front locations. Two boundaries of subtropical water at different depths are identified north and south of the Brazil Current-Falkland (Malvinas) Current confluence zone. The northern Subtropical Front is displaced over a large part of the Argentine Basin from one observational period to the other. The shallow southern Subtropical Front appears fixed to the Falkland Escarpment. The Polar Front and Subantarctic Front locations do not vary much, except for one case where a cold core eddy in the Polar Frontal Zone causes a large northward displacement of the Subantarctic Front.
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  • 54
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    Seasonal Changes in the North Atlantic Subtropical Gyre (1988)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 93 (C7). pp. 8111-8118.
    Details
    Publication Date: 2017-09-26
    Description: The eastern part of the North Atlantic subtropical gyre is found in the region between the Azores and the Cape Verde Islands. A study of the gyre structure in the area east of 35°W between 8°N and 41°N is presented. The geostrophic flow field determined from historical temperature-salinity data sets by objective analysis indicates seasonal variations in shape but no significant changes in the magnitude of volume transports. The eastern part of the gyre has a larger east-west and smaller north-south extension in summer compared with the winter season. The center shifts by about 2° latitude to the south from winter to summer. Long-term temperature time series (6.5 years) from a mooring near the Azores are consistent with these results, showing always a consistent temperature increase at the beginning of the year which is apparently due to the displacement of the northeastern part of the gyre. A comparison between the mean flow fields and fields obtained from individual zonal sections indicates large deviations north and south of the gyre but small deviations within the gyre.
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  • 55
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    On the Origin of the Azores Current (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 94 (C5). pp. 6159-6168.
    Details
    Publication Date: 2017-09-26
    Description: The Azores Current, south of the Azores Archipelago, is part of the subtropical North Atlantic gyre. Using an international hydrographic data set, we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the Azores Current in the western basin and seasonal changes in the related flow. Geostrophic currents are obtained by using the method applied by Stramma (1984) in the eastern basin. The Azores Current is found to originate in the area of the Southwest Newfoundland Rise (Figure 10). In winter an almost uniform current connects this region of origin with the Azores Current, while a branching into two current bands is observed in summer, with the southern band forming a marked cyclonic loop. Within the upper 800 m, all of the transport in the northern band and about 70% of the transport in the southern band recirculates in the eastern basin. Additionally, expendable bathythermograph data from the Azores Current region indicate an increase of eddy potential energy from winter to summer.
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  • 56
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    The response of drifting buoys to currents and wind (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 94 (C3). pp. 3201-3210.
    Details
    Publication Date: 2020-10-20
    Description: Two buoy types have been tested with respect to their drift performance under drogued and undrogued conditions. Additionally, forces acting on the buoys were measured directly. Quadratic drag laws have been confirmed for the drag in water and the combined drag of wind and waves. Stokes drift contributes about one half to the wind factor of 0.023, which is obtained for undrogued buoys in the Atlantic. The forces on a windowshade drogue are given by a linear relation between force and water velocity for speeds exceeding 10 cm/s. They have been extrapolated to speeds of less than 10 cm/s by both a linear and a quadratic relationship. Correlations between drift and wind speed in the Atlantic suggest that the linear law is a better approximation under realistic conditions. According to these measurements in the Atlantic the described buoy-drogue system with a windowshade drogue in 100-m depth is a good current-measuring device. Slippage is negligible for wind speeds of less than 15 m/s and is less than 2 cm/s under gale conditions. Undrogued buoys are strongly affected by wind and cannot be used for the analysis of currents without correction, even under light winds.
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  • 57
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    The chronology of the last Deglaciation: Implications to the cause of the Younger Dryas Event (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (1). pp. 1-19.
    Details
    Publication Date: 2016-06-16
    Description: It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep-sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.
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  • 58
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    Preliminary estimates for the radiocarbon age of deep water in the glacial ocean (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (6). pp. 659-669.
    Details
    Publication Date: 2016-06-15
    Description: Radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported. The differences for benthic-planktonic pairs suggest that the radiocarbon age for deep Atlantic water was somewhat larger than today's (i.e., 600±250, as opposed to 400 years) and that the radiocarbon age for deep Pacific water was also slightly larger than today's (2100±400, as opposed to 1600, years). Our results suggest that during glacial time, the deep Pacific was, as it is today, significantly depleted in radiocarbon relative to the deep Atlantic. As many questions remain unanswered regarding the reliability of this approach, these conclusions must be considered to be preliminary.
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  • 59
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    The role of CaCO3 compensation in the glacial to interglacial atmospheric CO2 change (1987)
    AGU (American Geophysical Union)
    In:  Global Biogeochemical Cycles, 1 (1). pp. 15-29.
    Details
    Publication Date: 2016-06-15
    Description: The only viable explanations put forth to date for the glacial to interglacial change in atmospheric CO2 content suggested from measurements of the CO2 content of gas extracted from ice cores involve changes in the ocean's nutrient cycles. Any nutrient change capable of creating the 80 µatm changes in atmosphere CO2 pressure suggested by the ice core results also creates significant change in the deep ocean's CO3= content. Evidence from deep sea sediments suggests that these CO3= changes are compensated on the time scale of a few thousand years by reductions or increases in amount of CaCO3 accumulating in deep sea sediments. This compensation process has two important consequences. First, it significantly increases the magnitude of the CO2 change per unit of nutrient forcing. Second, it causes a delay in the response of the atmospheric CO2 change. While the first of these consequences is a boon to those seeking to explain the CO2 change, the second may prove to be a curse. The ice core CO2 record shows no evidence of a significant lag between the CO2 response and the polar warming. In any case it is important that we improve our knowledge of the magnitude and timing of the CaCO3 preservation events which mark the close of episodes of glaciation and of the dissolution events which mark the onset of these episodes.
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  • 60
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    Oligocene to Miocene carbon isotope cycles and abyssal circulation changes (1985)
    AGU (American Geophysical Union)
    In:  In: The Carbon Cycle and Atmospheric CO: Natural Variations Archean to Present. , ed. by Sundquist, E. T. and Broecker, W. S. Geophysical Monograph, 32 . AGU (American Geophysical Union), Washington, pp. 469-486.
    Details
    Publication Date: 2016-07-20
    Type: Book chapter , NonPeerReviewed
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  • 61
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    Distribution of thorium isotopes between dissolved and particulate forms in the deep sea (1982)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Oceans, 87 (C3). pp. 2045-2056.
    Details
    Publication Date: 2016-07-12
    Description: The distribution of 234Th, 230Th, and 228Th between dissolved and particulate forms was determined in 17 seawater samples from the Guatemala and Panama basins. Sampling was carried out in situ with battery-powered, submersible pumping systems in which the seawater first passed through a Nuclepore filter (1.0-μm pore size) and then through a cartridge packed with Nitex netting that was impregnated with MnO2 to scavenge the dissolved Th isotopes. Natural 234Th was used as the tracer for monitoring the efficiency of scavenging. For all three isotopes, most of the activity was found in the dissolved form. On the average 4% of the 234Th, 15% of the 228Th, and 17% of the 230Th occurred in the particulate form, though the percentages were found to be strongly dependent on particle concentration. These distributions are not consistent with chemical scavenging models that assume irreversible uptake of Th on particle surfaces. The results can be explained, however, if continuous exchange of Th isotopes between seawater and the particle surfaces is assumed. Vertical profiles of both particulate and dissolved 230Th show increasing concentrations with depth, as required by the assumption of reversible exchange. Some of the dissolved 230Th profiles, however, show a reversal of this trend near the bottom, indicating accelerated scavenging near the water/sediment interface. Kinetics of both adsorption and desorption can be examined if at least two Th isotopes are measured in the same samples. Results show that reaction times are short (a few months) compared to the residence time of suspended matter in the deep ocean (several years), indicating that particles suspended in the deep sea are close to equilibrium with respect to exchange of metals at their surfaces.
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  • 62
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    A quasi-synoptic survey of the thermocline circulation and water mass distribution within the Canary Basin (1986)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 91 (C8). pp. 9739-9748.
    Details
    Publication Date: 2019-04-04
    Description: Shipboard hydrographic measurements and moored current meters are used to infer both the large-scale and mesoscale water mass distribution and features of the general circulation in the Canary Basin. We found a convoluted current system dominated by the time-dependent meandering of the eastward flowing Azores Current and the formation of mesoscale eddies. At middepths, several distinctly different water masses are identified: Subpolar Mode and Labrador Sea Water are centered in the northwest, Subantarctic Intermediate Water is centered in the southeast, and the saltier, warmer Mediterranean tongue lies between them. Mesoscale structures of these water masses suggest the presence of middepth meanders and detached eddies which may be caused by fluctuations of the Azores Current.
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  • 63
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    Comparison between radiocarbon ages obtained on coexisting planktonic foraminifera (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (6). pp. 647-657.
    Details
    Publication Date: 2016-06-15
    Description: As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera. While ideally no differences should exist, we do find them. In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences.
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  • 64
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    Azimuth determination of and from horizontal ocean bottom seismic sensors (1987)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 92 (B5). pp. 3567-3572.
    Details
    Publication Date: 2016-09-15
    Description: A number of factors must be considered when using particle motion information to determine azimuths to source or azimuths of horizontal geophone axes on or below the ocean floor. These factors include anisotropy, unmatched sensor response, incorrect instrument location, tilted sensors or dipping structure, and poor coupling. Accurate determination of azimuths requires identification and understanding of these factors and their effects on the data.
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  • 65
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    Mass/age distribution and composition of sediments on the ocean floor and the global rate of sediment subduction (1988)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Solid Earth, 93 (B12). pp. 14933-14940.
    Details
    Publication Date: 2018-02-07
    Description: The total mass of sediments on the ocean floor is estimated to be 262 × 1021 g. The overall mass/age distribution is approximated by an exponential decay curve: (11.02 × 1021 g)e−0.0355t Ma. The mass/age distribution is a function of the area/age distribution of ocean crust, the supply of sediment to the deep sea, and submarine erosion and redeposition. About 140 × 1021 g of the sediment on the ocean floor is pelagic sediment, consisting of about 74% CaCO3, with the remainder opaline silica and red clay. Of the sediment on the ocean floor, 122 × 1021 g is detritus, mostly terrigenous, but a small portion (about 6 × 1021 g) is volcanic. Because very little pelagic sediment is obducted, virtually all of the pelagic sediment mass and some fraction of the terrigenous sediment is being subducted at a rate estimated to be about 1 × 1021 g per million years. The composition of sediment on the ocean floor differs significantly from that of average passive margin and continental sediment, so that the loss of ocean floor sediment through subduction may drive the composition of global sediment toward enrichment in silica, alumina, and potash and toward depletion in calcium.
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  • 66
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    Compartmental models of nitrogen cycling in tropical and temperate marine environments (1987)
    Inter Research
    In:  Marine Ecology Progress Series, 38 . pp. 137-149.
    Details
    Publication Date: 2018-05-04
    Description: Time-course measurements of 15N tracer kinetics in particulate organic and in N+4 pools from tropical and temperate regions were used to test several compartmental models describing the exchange of I5N tracer in microplankton communities. Several lines of evidence suggested the involvement of a third, dissolved pool, arbitrarily labelled dissolved organic nitrogen (DON). Although the kinetic patterns of tracer movement were different between the tropical statlons and the temperate one, the same 3-compartmental model in which PON and DON can exchange material only through the intermediate of NH: gave the best fit. Only the transfer coefficients were modified. Results show that compartmental analysis is useful for the estimation of compartmental transfer rates and for testing the assumptions implicit in any given model.
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  • 67
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    Metridium senile: dispersion and small scale colonization by the combined strategy of locomotion and asexual reproduction (laceration) (1985)
    Inter Research
    In:  Marine Ecology Progress Series, 26 . pp. 271-277.
    Details
    Publication Date: 2014-01-23
    Description: The process of biotope occupation by the anenome Metridium s en~l ein the absence of pelagic larvae has been investigated by an in situ monitoring experiment lasting from July 1981 to April 1982. This process is composed of several behavioral phases: dispersion of the anemones on the new substrate by h ~ g hlo comotory activity, colonization of the 'chosen' habltat by intensive asexual reproduction and finally, a stationary phase. One characteristic of the particular ecological situation in the Flensburg fjord (Western Baltic) seems to be the absence in M senile of sexual reproduction and larvae resulting in a mono- or oligoclonality of the Fjord population. This study shows that even if pelagic dispersal is missing, local dispersion and proliferation can be assured by locomotion and laceration.
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  • 68
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    Marine epibiosis. I. Fouling and antifouling: some basic aspects (1989)
    Inter Research
    In:  Marine Ecology Progress Series, 58 . pp. 175-189.
    Details
    Publication Date: 2014-01-23
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  • 69
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    Southward subsurface flow below the Somali Current (1983)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 88 (C10). pp. 5973-5979.
    Details
    Publication Date: 2018-12-03
    Description: The existence of a southward‐flowing current beneath the northern part of the seasonally reversing Somali Current is documented in a 2½‐year‐long time series of currents obtained at moored stations near 5°N about 30 km off the Somali coast. Its mean annual transport in the layer 150–600 m amounts to about 5×106 m3/s. The undercurrent has a pronounced seasonal cycle in phase with the near surface flow, suggesting a close coupling to the monsoonal wind forcing. With the spin‐up of the deepreaching northern Somali gyre after the onset of the southwest monsoon, the undercurrent is temporarily destroyed in the northern Somali Basin during June/July but is re‐established in August. The undercurrent does not reach 3°N but turns offshore north of that latitude.
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  • 70
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    Paleoenvironmental changes in the Norwegian Sea and the northeast Atlantic during the last 2.8 m.y.: Deep Sea Drilling Project/Ocean Drilling Program Sites 610, 642, 643 and 644 (1988)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (5). pp. 563-581.
    Details
    Publication Date: 2019-05-06
    Description: Sedimentological, isotopic and magnetostratigraphic investigations of Ocean Drilling Program and Deep Sea Drilling Project sites 642, 643, 644 and 610 document the oceanographic and climatic evolution of the Norwegian Sea and the northeastern Atlantic over the last 2.8 m.y.. The results show that a major expansion of the Scandinavian Ice Sheet to the coastal areas took place at about 2.56 Ma. Relatively severe glacials appeared until about 2 Ma. The period 2.6 ‐ 1.2 Ma experienced in general cold surface water conditions with only a weak influx of temperate Atlantic water as compared with late Quaternary interglacials. The Norwegian Sea was a sink of deep water through this period but deepwater ventilation was reduced and calcite dissolution was high compared with the Holocene. Deep water formed by other mechanisms than it does today. Between 2 and 1.2 Ma the glaciations in Scandinavia were small. A transition toward larger glacials took place during the period 1.2 to 0.6 Ma, corresponding to warmer interglacials and reduced calcite dissolution. Only during the last 0.6 m.y. has the oceanographic and climatic system of the Norwegian Sea varied in the manner described in previous studies of the late Quaternary. A strong thermal gradient was present between the Norwegian Sea and the northeastern Atlantic during the Matuyama (2.5–0.7 Ma). This is interpreted as a sign of a more zonal and less meridional climatic system over the region compared with the present situation.
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  • 71
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    Continental erosion and large-scale relief (1988)
    AGU (American Geophysical Union)
    In:  Tectonics, 7 (3). pp. 563-582.
    Details
    Publication Date: 2019-08-12
    Description: A worldwide investigation of continental erosion is carried out by the study of large drainage basins, on the basis of hydrological data, environmental factors, and basin relief distribution. Inside each basin, mean geochemical and mechanical denudation rates are defined. A multicorrelation analysis shows that the mechanical denudation rates Ds are uncorrelated with environmental factors and correlated with mean basin elevation H, while chemical denudation rates Dd are insensitive to relief but correlated with mean annual precipitation. Furthermore, two linear relationships between H and Ds are detected: (1) Ds (m/10³ yr) = 419×10−6 H (m) ‐ 0.245, with V (explained variance) = 95.1%; this law concerns basins related to orogenies younger than 250 Ma. The negative intercept is interpreted as a continental sedimentation rate of 245 m/m.y. An alternative model in which one invokes a critical elevation, separating erosion from sedimentation, is equally successful and leads to lower sedimentation rates (60–110 m/m.y.). For both models, one derives from the slope of the adjustments, erosion time constants on the order of 2.5 m.y. (2) Ds (m/10³ yr) = 61×10−6 H (m), with V = 86.5%; this law concerns basins related to older orogenies. The null intercept suggests the lack of continental storage. Because of the more important dispersion of the data, the erosion time constant is calculated separately for each basin; it ranges from 15 to 360 m.y. The tectonic implications of these results are discussed. In particular, the short time constant 2.5 m.y. agrees with orogenic uplift rates on the order of 1 mm/yr, observed in active mountain chains.
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  • 72
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    Chemical accumulation variations under the Peru Current during the past 130,000 years (1983)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 88 (C12). pp. 7667-7680.
    Details
    Publication Date: 2019-02-01
    Description: The titanium to aluminum ratio in core V19–29 is correlated with aluminosilicate accumulation rates. This correlation may be due to Pleistocene eolian transport fluctuations which alter the mean grain size of sedimented eolian material. The relation between aluminum accumulation rate and Ti/Al, established from accumulation rates integrated over 11,000–50,000 year intervals, can be inverted to compute a high‐resolution record of aluminosilicate and calcium carbonate accumulation rates over the past 130,000 years. Carbonate accumulation rates are closely related to the oxygen isotope record in the core, with a phase lag and damping constant that is compatible with the response time (shown to be only 6000 years) of calcium carbonate in the ocean. Carbonate sedimentation at this site responds to several processes independently correlated with climatic change. The relative importance of these processes for carbonate sedimentation at this site can be constrained by the record in this core and other lines of evidence: 15% of the increased carbonate deposition at this site during glacial periods may be due to diminished NADW (North Atlantic deep water) formation; 10% is due to carbonate productivity decreases in the North Atlantic; 25% may be due to a diminished shallow‐sea carbonate sink; and the residual 50% must be due to a local productivity increase. These assignments are consistent with observations on carbonate paleoceanography in the North Atlantic. Aluminosilicate accumulation rate variations correlate with the record of eolian quartz deposition near northwest Africa and, in a general way, with the climatic record. But in detail the record differs substantially from the oxygen isotope record and may provide independent evidence on the nature of climate dynamics.
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  • 73
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    Seismotectonics of New Guinea: A model for arc reversal following arc-continent collision (1987)
    AGU (American Geophysical Union)
    In:  Tectonics, 6 (1). pp. 53-67.
    Details
    Publication Date: 2019-06-11
    Description: The structure and evolution of the northern New Guinea collision zone is deduced from International Seismological Center (ISC) seismicity (1964–1985), new and previously published focal mechanisms and a reexamination of pertinent geological data. A tectonic model for the New Guinea margin is derived which illustrates the sequential stages in the collision and suturing of the Bewani‐Toricelli‐Adelbert‐Finisterre‐Huon‐New Britain arc to central New Guinea followed by subduction polarity reversal in the west. East of 149°E, the Solomon plate is being subducted both to the north and south; bringing the New Britain and Trobriand forearcs toward collision. West of 149°E the forearcs have collided, and together they override a fold in the doubly subducted Solomon plate lithosphere, which has an axis that is parallel to the strike of the Ramu‐Markham suture and that plunges westward at an angle of 5° beneath the coast ranges of northern New Guinea. Active volcanism off the north coast of New Guinea is related to subduction of the Solomon plate beneath the Bismarck plate. Active volcanism of the Papuan peninsula and Quaternary volcanism of the New Guinea highlands are related to slow subduction of the Solomon plate beneath the Indo‐Australian plate along the Trobriand Trough and the trough’s former extension to the west, respectively. From 144°–148°E, seismicity and focal mechanisms reveal that convergence between the sutured Bismarck and Indo‐Australian plates is accommodated by thrusting within the Finisterre and Adelbert ranges and compression of the New Guinea orogenic belt, together with basement‐involved foreland folding and thrusting to the south. The Finisterre block overthrusts the New Guinea orogenic belt, whereas the Adelbert block is sutured to New Guinea and overthrusts the oceanic lithosphere of the Bismarck Sea. Along the New Guinea Trench, west of 144°E, seismicity defines a southward dipping Wadati‐Benioif zone, and focal mechanisms indicate oblique subduction. Only this oldest, westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity.
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  • 74
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    Structure of the frontal part of the Andean Convergent Margin (1985)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 90 (B7). pp. 5429-5442.
    Details
    Publication Date: 2019-05-14
    Description: A multichannel seismic reflection record across the central Peru margin and trench was improved by processing 24 rather than the 12 channels previously processed and by thorough migrating to reject the strong diffractions that obscured weaker primary reflections. The increased resolution clarifies the structure of the 15‐km‐wide frontal accretionary complex and the adjacent truncated continental framework against which the trench sediment was imbricated. Resolved are individual thrust slices and packets adjacent to the trench axis, subducting sediment‐filled graben in the ocean crust beneath the lower slope, and a Tertiary stratigraphic section of the upper slope Yaquina Basin which is cut by normal faults. The resolution in this multichannel record provides criteria for reinterpreting single‐channel data off Chile with increased confidence. The previously proposed truncation of the South American continent along much of the Peru‐Chile Trench is confirmed and the development of an accretionary complex in front of the truncated continental crust appears to vary with the amount of sediment seen in the trench axis. The Andean margin frontal structure is similar to that off Central America, the Aleutian Trench, and the Japan Trench which suggests common truncation as well as accretion at the front of convergent margins.
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  • 75
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    A large tsunamogenic landslide and debris flow along the Peru Trench (1989)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 94 (B2). pp. 1703-1714.
    Details
    Publication Date: 2019-05-13
    Description: Along the continental margin of northern Peru, Sea Beam bathymetry and seismic reflection records reveal features from mass wasting in the middle and lower slope areas. A curved scarp cuts the middle slope and marks a slip surface seaward of which a 20 by 33 km block was displaced 800 m and back rotated 5° as it moved downslope. The front of that block is itself marked by a 1‐km‐high curved scar where the block failed and created a ∼30‐km debris flow avalanche. The avalanche morphology of closed highs and lows without directional fabric covers the lower slope and trench axis seaward of the detached block. If the slip was catastrophic, a local ∼50‐m‐high tsunami was generated. This example documents the type of slope failure commonly inferred as a source of destructive tsunamis. Since the block was detached along faults that displace beds 3–5 km deep, it represents more than a superficial slide where sediment was locally oversteepened. The detached block includes rocks that were part of the continental margin since at least Eocene time in front of which a 15‐km‐wide accretionary complex subsequently developed. Apparently, only low levels of horizontal compression could be transmitted into the upper plate because of weak coupling across the Benioff zone. This may have permitted detachment and mass movement of the block despite the long history of plate convergence here. Tsunamogenic slides and mass wasting at trench depths are difficult to detect without modern high‐resolution techniques.
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  • 76
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    Ocean Carbon Pumps: Analysis of Relative Strengths and Efficiencies in Ocean-Driven Atmospheric CO2 Changes (1985)
    AGU (American Geophysical Union)
    In:  In: The Carbon Cycle and Atmospheric CO2 : Natural Variations Archean to Present. , ed. by Sundquist, E. T. Geophysical monograph series, 32 . AGU (American Geophysical Union), Washington, D.C., pp. 99-110. ISBN 0-87590-060-7
    Details
    Publication Date: 2019-06-25
    Description: An ocean carbon pump is defined as a process that depletes the ocean surface of σCO2 relative to the deep‐water σCO2. Three pumps are recognized: a carbonate pump, a soft‐tissue pump, and a solubility pump. The first two result from the biological flux of organic and CaCO3 detritus from the ocean's surface. The third results from the increased CO2 solubility in downwelling cold water and is demonstrated by a one‐dimensional upwelling‐diffusion model of an abiotic ocean. In the soft‐tissue and solubility pumps, working strengths are defined in terms of the ΔσCO2 each creates between surface and deep‐water. Efficiencies of each pump are quantified as a ratio of working strength to potential maximum strength. Using alkalinity, nitrate, and σCO2 to remove the carbonate pump signal from ocean or model data, the individual working strengths of the soft‐tissue and solubility pumps can be calculated by scaling the soft‐tissue's ΔσCO2 to the surface‐to‐deep ΔPO4. This technique is applied to a three‐box ocean model known to demonstrate high‐latitude control of atmospheric CO2 through a variety of circulation and biological changes. Considering each pump separately reveals that the various changes which lower pCO2atm in the model are caused primarily by an increased solubility pump. Analysis of global ocean data indicates a positive solubility pump signal, subject to uncertainties in the C:P Redfield ratio and in the preindustrial pCO2atm. If C:P = 105 and pCO2atm = 270 μatm, the efficiency of the solubility pump is about 0.5. We suggest that this type of analysis of relative carbon pump strengths will be an effective method for inter‐model and intra‐model comparison and diagnosis of underlying oceanic mechanisms for pCO2atm changes.
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  • 77
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    Downstream Dilution of a Lahar: Transition From Debris Flow to Hyperconcentrated Streamflow (1985)
    AGU (American Geophysical Union)
    In:  Water Resources Research, 21 (10). pp. 1511-1524.
    Details
    Publication Date: 2019-04-03
    Description: Nearly instantaneous melting of snow and ice by the March 19, 1982, eruption of Mount St. Helens released a 4 × 106 m3 flood of water from the crater that was converted to a lahar (volcanic debris flow) through erosion and incorporation of sediment by the time it reached the base of the volcano. Over the next 81 km that it traveled down the Toutle River, the flood wave was progressively diluted through several mechanisms. A transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater, when the total sediment concentration had decreased to about 78% by weight (57% by volume). The hyperconcentrated lahar‐runout flood wave, transporting immense quantities of sand in suspension, continued to experience progressive downstream dilution. Although turbulence was significantly dampened by the extremely high suspended load, very large standing waves and antidune waves were observed. The hyperconcentrated lahar‐runout flow deposited an unusual, faintly stratified, coarse sand which locally contained small, isolated gravel lenses. Very similar deposits in the Quaternary stratigraphy of Mount St. Helens and other Cascades volcanoes suggest that lahars may be more frequent than previously recognized.
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  • 78
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    Seismic evidence for an ancient rift beneath the Cumberland Plateau, Tennessee: A detailed analysis of broadband teleseismic P waveforms (1984)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 89 (B9). pp. 7783-7795.
    Details
    Publication Date: 2020-06-19
    Description: Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source‐equalized horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat‐lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high‐velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust‐mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zone between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.
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  • 79
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    A parametric model for constitutive properties governing multiphase flow in porous media (1987)
    AGU (American Geophysical Union)
    In:  Water Resources Research, 23 (4). pp. 618-624.
    Details
    Publication Date: 2020-07-20
    Description: A parametric model is developed to describe relative permeability‐saturation‐fluid pressure functional relationships in two‐ or three‐fluid phase porous media systems subject to monotonic saturation paths. All functions are obtained as simple closed‐form expressions convenient for implementation in numerical multiphase flow models. Model calibration requires only relatively simple determinations of saturation‐pressure relations in two‐phase systems. A scaling procedure is employed to simplify the description of two‐phase saturation‐capillary head relations for arbitrary fluid pairs and experimental results for two porous media are presented to demonstrate its applicability. Extension of two‐phase relations to three‐ phase systems is obtained under the assumption that fluid wettability follows the sequence water 〉 nonaqueous phase liquid 〉 air. Expressions for fluid relative permeabilities are derived from the scaled saturation‐capillary head function using a flow channel distribution model to estimate effective mean fluid‐conducting pore dimensions. Constraints on model application are discussed.
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  • 80
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    Transform seismicity at the intersection of the oceanographer fracture zone and the Mid-Atlantic Ridge (1986)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 91 (B5). pp. 4839-4853.
    Details
    Publication Date: 2020-12-17
    Description: A seismic array consisting of nine Hawaii Institute of Geophysics (HIG) ocean bottom seismometers (OBSs) was deployed at the eastern intersection of the Oceanographer Fracture Zone (OFZ) and the Mid‐Atlantic Ridge (MAR). The 12‐day experiment was designed to relate low‐magnitude earthquakes to the structure and tectonics of the MAR‐OFZ intersection. An average of 10 locatable events with duration‐based magnitudes between −1.0 and 2.0 were recorded per day. Excellent hypocentral locations of 112 events were obtained. Earthquake locations based on more than eight observations generally show 50% confidence volume constraints within OBS location errors. The earthquake locations cover a broad swath across the corner of the intersection zone. Magnitude‐weighted earthquake location likelihood maps suggest a decline in magnitudes near the intersection bathymetry low. Composite focal plane solutions suggest source mechanisms which indicate that the region is dominated by extensional tectonics. Alternative source solutions indicating translational movement are presented but are inconsistent with apparent bathymetric trends. The transition from the diverging (MAR) to translational (OFZ) plate margin occurs in the context of reduced magma genesis and crustal thinning due to the influence of the adjacent older lithosphere. The region may be described in terms of semirigid plate tectonics accompanying transform valley genesis.
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  • 81
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    Convergence at the Caroline-Pacific plate boundary: Collision and subduction (1983)
    AGU (American Geophysical Union)
    In:  In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands: Part 2. , ed. by Hayes, D. E. Geophysical Monograph Series, 27 . AGU (American Geophysical Union), Washington, DC, pp. 326-348, 23 pp.
    Details
    Publication Date: 2021-03-29
    Description: The eastern boundary of the Caroline plate, in the western equatorial Pacific, is composed of three structural provinces distinguished primarily on the basis of morphology. Each province shows evidence for convergence between the Caroline and Pacific plates though the structural style varies considerably between each province. Most notably, the sense of underthrusting appears to change along the boundary at about 3°N. To the south, at the Mussau System, Caroline lithosphere underthrusts beneath the Mussau Ridge (which is part of the Pacific plate), while to the north the Caroline plate appears to overthrust the Pacific plate. Recently collected seismic reflection profiles across each province documents the structural changes along and across strike of the Caroline-Pacific plate boundary. With this information, we estimate that a minimum of approximately 4 km of crustal shortening has occurred at about 5°N due to convergence of the two plates. Further to the south (about 2°N), simple gravity models suggest that about 10 km of Caroline lithosphere lies beneath the present-day Pacific plate. Using a previously determined pole of rotation describing Caroline-Pacific relative motion (Weissel and Anderson, 1978), we grossly estimate the duration of the convergence between these two plates at about one million years. It is suggested that variation in the convergence rate along the plate boundary provides the primary control on the variation of structural deformation observed between provinces; however, favorable thermal conditions are factors that are considered. If the eastern boundary of the Caroline plate is a region of incipient though perhaps transient subduction, as we postulate, then the geophysical and geological evidence presented can constrain models on the initiation of subduction.
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  • 82
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    Transform fault earthquakes in the North Atlantic: Source mechanisms and depth of faulting (1988)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 93 (B8). pp. 9027-9057.
    Details
    Publication Date: 2021-06-01
    Description: We have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern Mid-Atlantic Ridge from an inversion of long-period body waveforms. The earthquakes occurred on the Gibbs, Oceanographer, Hayes, Kane, 15°20′, and Vema transforms. We have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width. For five of the transforms, earthquake centroid depths lie in the range 7–10 km beneath the seafloor, and the maximum depth of seismic faulting is 14–20 km. On the basis of a comparison with a simple thermal model for transform faults, this maximum depth of seismic behavior corresponds to a nominal temperature of 900° ± 100°C. In contrast, the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike-slip mechanisms is 700° ± 100°C. The difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments. Three large earthquakes on the 15°20′ transform have shallower centroid depths of 4–5 km and a maximum depth of seismic faulting of 10 km, corresponding to a limiting temperature of 600°C. The shallower extent of seismic behavior along the 15°20′ transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among North American, South American, and African plates to its present position near the transform. The source mechanisms for all events in this study display the strike-slip motion expected for transform fault earthquakes; slip vector azimuths agree to within 2°–3° of the local strike of the zone of active faulting. The only anomalies in mechanism were for two earthquakes near the western end of the Vema transform which occurred on significantly nonvertical fault planes. Secondary faulting, occurring either precursory to or near the end of the main episode of strike-slip rupture, was observed for five of the 12 earthquakes. For three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform. In all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone. We find no evidence to support the conclusions of Engeln, Wiens, and Stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere.
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  • 83
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    Fluid pressures in deforming porous rocks (1989)
    AGU (American Geophysical Union)
    In:  Water Resources Research, 25 (2). pp. 203-213.
    Details
    Publication Date: 2020-10-07
    Description: The constitutive relations describing the fluid pressure response of a porous medium to changes in stress and temperature must reflect the microscopic processes that are operative over the time scale allowed for the deformation. Short‐duration deformations are readily described by undrained moduli, and intermediate duration deformations by drained moduli, both of which are formulated through linear elastic theory. Long‐term deformations that operate over geologic time are normally dominated by irreversible processes and result in considerably larger deformations, for the same applied stress conditions, than would be expected from their elastic counterparts. Model constitutive equations are developed for both elastic and irreversible processes and the magnitude and interpretation of the relevant material properties examined. Although the theory is presented in general terms, a sample calculation shows that for sandstone the inelastic deformation is one and one half orders of magnitude greater than the elastic deformation at the same applied stress. This difference in magnitude has a significant effect on the effective hydraulic diffusivity, various pore pressure coefficients, and the prospective fluid pressure development of the sediment.
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  • 84
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    Giant protistan parasites on the gills of cephalopods (Mollusca) (1987)
    Inter Research
    In:  Diseases of Aquatic Organisms, 3 . pp. 119-125.
    Details
    Publication Date: 2021-06-21
    Description: Large Protista of unknown taxonomic affinities are described from 3 species of coleoid squids, and are reported from many other species of cephalopods. The white to yellow-orange, ovoid cyst-like parasites are partially embedded within small pockets on the surface of the gills, often in large numbers. Except for a holdfast region on one side of the large end, the surface of the parasite is elaborated into low triangular plates separated by grooves. The parasites are uninucleate; their cytoplasm bears lipid droplets and presumed paraglycogen granules. Trichocysts, present in a layer beneath the cytoplasmic surface, were found by transmission electron microscopy to be of the dinoflagellate type. Further studies are needed to clarify the taxonomic position of these protists.
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  • 85
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    Hochbergia moroteuthensis gen. et sp. nov., a giant protistan parasite from the giant squid Moroteuthis robusta (Mollusca: Cephalopoda) (1989)
    Inter Research
    In:  Diseases of Aquatic Organisms, 6 . pp. 197-200.
    Details
    Publication Date: 2021-06-21
    Description: A new species of giant unicellular parasite inhabiting the gills of squid hloroteuthis robusta is described from specimens collected from the northeastern Pacific Ocean. Hochbergia moroteuthensis gen. et sp. nov. belongs to a recently reported group of protistans whose taxonomic affinities are as yet unknown. Members of the group parasitize cephalopods and have a characteristic aspect: most of the body wall is raised into close-fitting triangular plates and there is a holdfast region, devoid of plates, by which the parasites attach to the host. H. moroteuthensis is the first species of the group to be named. The general form of the body, the precise number and arrangement of plates, and the morphology of the holdfast reglon are described.
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  • 86
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    A dislocation model of strain accumulation and release at a subduction zone (1983)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 88 (B6). pp. 4984-4996.
    Details
    Publication Date: 2018-08-28
    Description: Strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface. This process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone, consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip. Because steady state subduction does not contribute to the deformation at the free surface, deformation observed there is completely equivalent to that produced by the slip cycle alone. The response to that slip is simply the response of a particular earth model to embedded dislocations. For a purely elastic earth model, the deformation cycle consists of a coseismic offset followed by a linear‐in‐time recovery to the initial value during the interval between earthquakes. For an elastic‐viscoelastic earth model (elastic lithosphere over a viscoelastic asthenosphere), the postearthquake recovery is not linear in time. Records of local uplift as a function of time indicate that the long‐term postseismic recovery is approximately linear, suggesting that elastic earth models are adequate to describe the deformation cycle. However, the deformation predicted for a simple elastic half‐space earth model does not reproduce the deformation observed along the subduction zones in Japan at all well if stick slip is restricted to the main thrust zone. As recognized earlier by Shimazaki, Seno, and Kato, the uplift profiles could be explained if stick slip were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km, but independent evidence suggests that stick slip at such depths is unlikely.
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  • 87
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    Variations in the Global Carbon Cycle During the Cretaceous Related to Climate, Volcanism, and Changes in Atmospheric CO2 (1985)
    AGU (American Geophysical Union)
    In:  In: The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. , ed. by Sundquist, E. T. and Broecker, W. S. Geophysical Monograph, 32 . AGU (American Geophysical Union), Washington, D.C., pp. 504-529.
    Details
    Publication Date: 2018-09-04
    Description: The stratigraphic record from both deep‐sea and shallow‐water depositional environments indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaCO3 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off‐ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of CO2 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric CO2. A significant fraction of this atmospheric CO2 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess CO2 may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian‐Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted from a warm global climate and increased marine transgression as well as by the higher CO2.
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  • 88
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    Water mass formation in the Gulf of California (1988)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Oceans, 93 (C8). pp. 9223-9240.
    Details
    Publication Date: 2018-09-04
    Description: The Gulf of California is a long, narrow marginal sea lying between the Baja California peninsula and the mainland of Mexico. Air‐sea fluxes of heat and moisture in the gulf are enhanced because of geographical isolation from the Pacific provided by the mountainous Baja California peninsula. In the northern gulf, annual evaporation rates are about 1 m y−1. Unlike most evaporative basins, however, the gulf gains heat from the atmosphere at an annual average rate of 20 to 80 W m−2 (Bray, 1988). Given the unusual air‐sea forcing of the gulf, what form or forms should water mass formation take? The annual moisture loss and heat gain require that high‐salinity surface water be transported downward to an intermediate depth and that cold, fresh inflow be transported upward to an intermediate depth. This is accomplished through several mechanisms. (1) Winter convection: this occurs only in a limited geographical region, the Wagner Basin of the far northern gulf, except in El Niño–Southern Oscillation years, when convection appears to be more widespread. (2) Dispersion of convected water in small eddylike features: this occurs within a large‐scale southward transport possibly driven by the large‐scale density gradient associated with atmospheric fluxes. (3) An anticyclonic circulation in the northern gulf: this is found south of the convection region and transports high‐salinity water off the shallow shelves and to substantial depths, where it may mix with water of central gulf origin. (4) Tidal mixing: most of the energy available for mixing in the northern gulf derives from the tides. In particular, tidal mixing over the sill in the island region is responsible for the substantial reduction in salinity of northern gulf waters as they enter the central gulf.
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  • 89
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    Sedimentation of spring phytodetritus to the deep-sea floor (1988)
    AGU (American Geophysical Union)
    In:  Eos, Transactions American Geophysical Union, 69 .
    Details
    Publication Date: 2016-12-22
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  • 90
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    A link between paleoceanography, early diagenetic cementation, and shear strength maxima in Late Quaternary deep-sea sediments? (1989)
    AGU (American Geophysical Union)
    In:  Paleoceanography, 4 (3). pp. 253-269.
    Details
    Publication Date: 2018-10-02
    Description: Near-surface sediments from the equatorial east Atlantic and the Norwegian Sea exhibit pronounced shear strength maxima in profiles from the peak Holocene and Pleistocene. These semi-indurated layers start to occur at 8–102 cm below the sediment surface and can be explained neither by the modal composition nor by the effective overburden pressure of the sediments. However, scanning electron microscope and microprobe data exhibit micritic crusts and crystal carpets, which are clearly restricted to (undisturbed) samples from indurated layers and form a manifest explanation for their origin. The minerals precipitated comprise calcite, aragonite, and in samples more proximal to the African continent SiO2 needles, and needles of as yet unidentified K-Mg-Fe-Al silicates, crusts of which dominate the indurated layers in the Norwegian Sea. By their stratigraphic position in deep-sea sediments the carbonate-based shear strength maxima are tentatively ascribed to dissolved adjacent pteropod layers from the early Holocene and hence to short-lived no-analogue events of early diagenesis. Possibly, they have been controlled by a reduced organic carbon flux, leading to increased aragonite preservation in the deep sea.
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  • 91
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    Food and feeding ecology of the sowrdfish Xiphias gladius in the western North Atlantic Ocean with estimates of daily ration (1985)
    Inter Research
    In:  Marine Ecology Progress Series, 22 . pp. 239-247.
    Details
    Publication Date: 2020-11-05
    Description: Analysis of stomach contents and predator-prey relations of 168 swordfish from the western North Atlantic Ocean is presented. The predominant food of swordfish is squid (82% by frequency of occurence) followed by fish (53%) consisting primarily of gladis, scombrids, butterfish, bluefish, and sand lance. Average food volume in stomach is 1% of average body weight. Estimates of daily ration range from 0.94% to 1,6% of avergae body weight, with yearly consumption ranging from 3.4 to 5.8 times average body weight per year. The prey biomass consumed by the sowrdfish population in the Georges Bank area is estimated to range from 2,100 to 11,000 mt over a residency time of 5 mo.
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  • 92
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    Satellite observations of sea surface cooling by hurricanes (1986)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research - Oceans, 91 (C4). p. 5031.
    Details
    Publication Date: 2016-04-19
    Description: Sea surface cooling associated with 13 hurricanes in the western North Atlantic between September 1981 and December 1984 is examined using satellite-derived sea surface temperature fields. Some surface cooling is observed in all cases; however, because of cloud cover and the fairly weak signal in some cases, we see pronounced cooling along an extensive and continuous portion of the storm path for only three strong hurricanes. The persistence of cooling following the passage of a hurricane varies from a few days to at least 16 days. The amplitude of cooling is moderately well correlated with hurricane strength and is as large as 3.5°C. When the hurricanes move rapidly, the maximum cooling occurs well to the right of the track (approximately 70 km), whereas for slowly moving hurricanes the maximum cooling occurs near or on the track. Because western North Atlantic hurricanes are often found in close proximity to high pressure systems, daytime satellite images must be made with some care because of diurnal warming.
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  • 93
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    Ocean carbon pumps: analysis of relative strengths and efficiencies in ocean‐driven atmospheric CO2 changes (1985)
    AGU (American Geophysical Union)
    In:  In: The carbon cycle and atmospheric CO2 [CO] : natural variations archean to present. Geophysical Monograph, 32 . AGU (American Geophysical Union), Washington, DC, pp. 99-110. ISBN 0-87590-060-7
    Details
    Publication Date: 2018-11-13
    Description: An ocean carbon pump is defined as a process that depletes the ocean surface of σCO2 relative to the deep‐water σCO2. Three pumps are recognized: a carbonate pump, a soft‐tissue pump, and a solubility pump. The first two result from the biological flux of organic and CaCO3 detritus from the ocean's surface. The third results from the increased CO2 solubility in downwelling cold water and is demonstrated by a one‐dimensional upwelling‐diffusion model of an abiotic ocean. In the soft‐tissue and solubility pumps, working strengths are defined in terms of the ΔσCO2 each creates between surface and deep‐water. Efficiencies of each pump are quantified as a ratio of working strength to potential maximum strength. Using alkalinity, nitrate, and σCO2 to remove the carbonate pump signal from ocean or model data, the individual working strengths of the soft‐tissue and solubility pumps can be calculated by scaling the soft‐tissue's ΔσCO2 to the surface‐to‐deep ΔPO4. This technique is applied to a three‐box ocean model known to demonstrate high‐latitude control of atmospheric CO2 through a variety of circulation and biological changes. Considering each pump separately reveals that the various changes which lower pCO2atm in the model are caused primarily by an increased solubility pump. Analysis of global ocean data indicates a positive solubility pump signal, subject to uncertainties in the C:P Redfield ratio and in the preindustrial pCO2atm. If C:P = 105 and pCO2atm = 270 μatm, the efficiency of the solubility pump is about 0.5. We suggest that this type of analysis of relative carbon pump strengths will be an effective method for inter‐model and intra‐model comparison and diagnosis of underlying oceanic mechanisms for pCO2atm changes.
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  • 94
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    Mechanisms of sediment accretion in the Middle America Trench off Mexico (1988)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 93 (B8). pp. 8911-8927.
    Details
    Publication Date: 2021-12-13
    Description: High-resolution seismic reflection and Sea Beam bathymetric data provide insights into the processes of sediment offscraping and accretion in the Middle America Trench off southern Mexico. Thick terrigenous sediments that are transported down Ometepec Canyon and accumulate along the trench floor are scraped off the oceanic plate and accreted in thrust packets to the lower trench slope. The packets offscraped represent most of the trench strata. Underlying hemipelagic deposits that accumulate on the seafloor seaward of the trench are subducted landward of the toe of the slope. Horizontal displacement on the thrust is less than 1 km. Leading edge folds are the surface expressions of the thrusts and strike subparallel to the base of the trench slope. The folds are continuous for as much as 10 km and have amplitudes as high as 200 m and wavelengths of 0.5 to 2 km. Folds are best developed along sections of the trench with interbedded silty turbidite and mud deposits. Fold are absent where thick coarse-grained fan deposits occur. Thickening of the thrust packets occurs by large-scale thrust duplication, by layer-parallel shortening, and by deposition of material that slumps off the leading edge of older upslope thrust blocks.
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  • 95
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    Seismicity surveys with ocean bottom seismographs off western Canada (1981)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 86 (B5). pp. 3867-3880.
    Details
    Publication Date: 2021-11-03
    Description: Three arrays of ocean bottom seismographs have been deployed to study the seismicity at the northern end of the Juan de Fuca ridge system off western Canada. Nearly 100 events were located with estimated accuracies generally better than ±10 km, all lying on or near the en echalon ridge-transform fault plate boundaries as defined in this area by the magnetic anomalies, the seafloor morphology and by other geophysical data. The depths of 12 events were determined to lie between 2 and 6 km below the top of the crust. The seismograms exhibit clear P and S wave arrivals along with phases that involve P to S and sometimes S to P conversion probably at the base of the sediments beneath the instruments. The event magnitudes have been estimated from signal duration using four calibration events that were well recorded by a land station. The magnitude estimates permit the determination of rough magnitude-frequency of occurrence relations over the magnitude range of 1 to 3 that are in surprisingly good agreement with the recurrence relations for the area at larger magnitudes from 75 years of land station data. The mean P wave velocity in the uppermost mantle from the earthquake data recorded by the sea floor arrays is 7.6 km s-1 and the mean Vp/Vs ratio is 1.71 or a Poisson's ratio of 0.24.
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  • 96
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    The Extreme North African Haboob in October 2008: High‐Resolution Simulation of Organized Moist Convection in the Lee of the Atlas, Dust Recirculation and Poleward Transport (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-01-04
    Description: This study investigates the mesoscale dynamics involved in the 8–11 October 2008 unseasonably strong African dust episode, during which dust was transported to the Iberian Peninsula (IP). We employ observational datasets and a high-resolution Weather Research and Forecasting model coupled with Chemistry simulations. The analysis shows that during 0900–1200 UTC 9 October, a mesoscale convective system developed over the Atlas Mountains and resulted in a southwestward propagating convective cold pool outflow on the southern foothills of the Anti-Atlas, which lifted dust from the source region. Between 1200 and 1800 UTC 9 October, new moist convection was enhanced over the Atlas Mountains due to intensifying confluence among a heat low, moist southwesterly Atlantic sea-breeze front, and northeasterly flow associated with the convective cold pool near western Algeria. This new moist convection intensified the strength of the convective cold pool outflow and haboob, both of which continued propagating southwestward. At 1200 UTC 10 October, the low-pressure system migrated poleward on the southern slopes of the Anti-Atlas Mountains in association with a mountain-plains solenoidal circulation due to the daytime differential heating between the southern slopes of the Anti-Atlas and nearby atmosphere. The deepening low-pressure and strengthening Atlantic sea-breeze redirected an equatorward advancing dust plume into the poleward direction. The dust plume ultimately crossed the Saharan Atlas Mountains on 11 October and finally impacted the IP. Key Points: - WRF-Chem simulation of an unseasonably strong haboob on the southern slopes of the Atlas Mountains - The equatorward-advancing dust plume was recirculated in the poleward direction by an Atlantic sea-breeze front - The Atlantic sea-breeze front and an intensified upper-level cutoff vortex are instrumental for dust transport over the Iberian Peninsula
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    Uncertainty in Aerosol Optical Depth From Modern Aerosol‐Climate Models, Reanalyses, and Satellite Products (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-01-24
    Description: Despite the implication of aerosols for the radiation budget, there are persistent differences in data for the aerosol optical depth (τ) for 1998–2019. This study presents a comprehensive evaluation of the large-scale spatio-temporal patterns of mid-visible τ from modern data sets. In total, we assessed 94 different global data sets from eight satellite retrievals, four aerosol-climate model ensembles, one operational ensemble product, two reanalyses, one climatology and one merged satellite product. We include the new satellite data SLSTR and aerosol-climate simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Aerosol Comparisons between Observations and Models Phase 3 (AeroCom-III). Our intercomparison highlights model differences and observational uncertainty. Spatial mean τ for 60°N – 60°S ranges from 0.124 to 0.164 for individual satellites, with a mean of 0.14. Averaged τ from aerosol-climate model ensembles fall within this satellite range, but individual models do not. Our assessment suggests no systematic improvement compared to CMIP5 and AeroCom-I. Although some regional biases have been reduced, τ from both CMIP6 and AeroCom-III are for instance substantially larger along extra-tropical storm tracks compared to the satellite products. The considerable uncertainty in observed τ implies that a model evaluation based on a single satellite product might draw biased conclusions. This underlines the need for continued efforts to improve both model and satellite estimates of τ, for example, through measurement campaigns in areas of particularly uncertain satellite estimates identified in this study, to facilitate a better understanding of aerosol effects in the Earth system. Key Points: - Present-day patterns in aerosol optical depth differ substantially between 94 modern global data sets - The range in spatial means from individual satellites is −11% to +17% of the multi-satellite mean - Spatial means from climate model intercomparison projects fall within the satellite range but strong regional differences are identified
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  • 98
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    Observations and Implications of Diurnal Climatology and Trends in Direct and Diffuse Solar Radiation Over China (2022)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-01-24
    Description: Solar radiation received at the Earth's surface (Rs) is comprised of two components, the direct radiation (Rd) and the diffuse radiation (Rf). Rd, the direct beam from the sun, is essential for concentrated solar power generation. Rf, scattered by atmospheric molecules, aerosols, or cloud droplets, has a fertilization effect on plant photosynthesis. But how Rd and Rf change diurnally is largely unknown owing to the lack of long-term measurements. Taking advantage of 22 years of homogeneous hourly surface observations over China, this study documents the climatological means and evolutions in the diurnal cycles of Rd and Rf since 1993, with an emphasis on their implications for solar power and agricultural production. Over the solar energy resource region, we observe a loss of Rd which is relatively large near sunrise and sunset at low solar elevation angles when the sunrays pass through the atmosphere on a longer pathway. However, the concentrated Rd energy covering an average 10-hr period around noon during a day is relatively unaffected. Over the agricultural crop resource region, the large amounts of clouds and aerosols scattering more of the incoming light result in Rf taking the main proportion of Rs during the whole day. Rf resources and their fertilization effect in the main crop region of China further enhances since 1993 over almost all hours of the day. Key Points: - The loss of direct radiation over China since 1993 is relatively large at sunrise and sunset with little effect on solar power generation - The diffuse component dominates solar radiation normally near sunrise and sunset, but for the whole day over the main sown area of China - The diffuse fraction is further enhanced in the main sown area of China over almost all hours of the day since 1993
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    Comparative Assessment of Climate Engineering Scenarios in the Presence of Parametric Uncertainty (2020)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-02-08
    Description: Climate engineering (CE) measures are increasingly discussed when dealing with the adverse impacts of climate change. While much research has focused on individual methods, few studies attempt to compare and rank the effectiveness of these measures. Furthermore, model uncertainties are seldom acknowledged and lesser still, estimated when CE scenarios are assessed. In this work, we quantify the variance in outcomes due to poorly constrained model parameters under several idealized CE scenarios. The four scenarios considered are (1) warming under the high emission scenario Representative Concentration Pathway 8.5 without CE applied and the same emission scenario with (2) afforestation,(3) solar radiation management, and (4) artificial ocean alkalinization. By considering the parametric uncertainty in model outputs, we demonstrate the problems with comparing these scenarios using a single parameter setting. Using statistical emulation, we estimate the probability distributions of several model outcomes. Based on such distributions, we suggest an approach to ranking the effectiveness of the scenarios considered according to their probability of avoiding climate thresholds.
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    A New Improved Estimation of Agulhas Leakage Using Observations and Simulations of Lagrangian Floats and Drifters (2020)
    AGU (American Geophysical Union) | Wiley
    Details
    Publication Date: 2023-02-08
    Description: A new estimate of Agulhas leakage transport is calculated using profiling floats and drifters. Since Richardson's seminal estimate of 15 Sv in 2007, the number of floats and drifters passing through the Agulhas Current has quadrupled. Within uncertainties we find the same leakage percentages as Richardson, with 34% of drifters leaking at the surface and 21% of floats leaking at 1,000 m depth. We find that the drifters tend to follow a northward leakage pathway via the Benguela Current compared to the northwestward leakage pathway of the floats along the Agulhas Ring corridor. We simulate the isobaric and profiling behavior of the floats and drifters using two high resolution models and two offline Lagrangian tracking tools, quantifying for the first time the sampling biases associated with the observations. We find that the isobaric bias cannot be robustly simulated but likely causes an underestimate of observed leakage by one or two Sverdrups. The profiling behavior of the floats causes no significant bias in the leakage. Fitting a simulated vertical leakage profile to the observed leakage percentages from the floats and drifters and using the mean Agulhas transport observed by a moored array at 34°S we find an improved Agulhas leakage transport of 21.3 Sv, with an estimated error of 4.7 Sv. Our new leakage transport is higher primarily because we account for leakage at depths down to 2,000 m, while Richardson considered only the top 1,000 m of the water column.
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