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  • 551  (78)
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  • 1
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    Carbon release and transformation from coastal peat deposits controlled by submarine groundwater discharge: a column experiment study (2021)
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    Publikationsdatum: 2021-12-06
    Beschreibung: Abstract Although the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon-rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 ± 14 wt%), alternately supplied with oxygen-rich brackish water from above and oxygen-poor, low-saline GW from below. The low-saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ13CDOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO2), implying DOC mineralization. Oxygen respiration, sulfate (SO42−) reduction, and methane (CH4) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with SO42−-reducing bacteria clearly dominating methanogens. With decreasing salinity and SO42− concentrations, CH4 emission rates increased from 16.5 to 77.3 μmol m−2 d−1 during a 14-day, low-saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH4 and CO2 emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary.
    Schlagwort(e): 550.724 ; coastal peatlands ; coastal peatlands ; biogeochemical processes ; carbon release ; column experiments
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 2
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    Efficiency and Accuracy of Micro-Macro Models for Mineral Dissolution (2021)
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    Publikationsdatum: 2021-12-03
    Beschreibung: Micro-macro models for dissolution processes are derived from detailed pore-scale models applying upscaling techniques. They consist of flow and transport equations at the scale of the porous medium (macroscale). Both include averaged time- and space-dependent coefficient functions (permeability, porosity, reactive surface, and effective diffusion). These are in turn explicitly computed from the time- and space-dependent geometry of unit cells and by means of auxiliary cell problems defined therein (microscale). The explicit geometric structure is characterized by a level set. For its evolution, information from the transport equations solutions is taken into account (micro-macro scales). A numerical scheme is introduced, which is capable of evaluating such complex settings. For the level-set equation a second-order scheme is applied, which enables us to accurately determine the dynamic reactive surface. Local mesh refinement methods are applied to evaluate Stokes type cell problems using P2/P1 elements and a Uzawa type linear solver. Applications of our permeability solver to scenarios involving static and evolving geometries are presented. Furthermore, macroscopic flow and transport equations are solved applying mixed finite elements. Finally, adaptive strategies to overcome the computational burden are discussed. We apply our approach to the dissolution of an array of dolomite grains in the micro-macro context and validate our numerical scheme.
    Schlagwort(e): 551.49 ; 550.724 ; porous media ; reactive flow ; dissolution processes ; modeling
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 3
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    Origin of isolated seamounts in the Canary Basin (East Atlantic): The role of plume material in the origin of seamounts not associated with hotspot tracks (2021)
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    Publikationsdatum: 2021-12-03
    Beschreibung: In contrast to seamount chains, small solitary seamounts/seamount groups have rarely been sampled despite their large number and therefore their origins remain enigmatic. Here we present new 40Ar/39Ar, trace element and Nd-Hf-Pb isotope data from the solitary Demenitskoy Seamount, the isolated Tolkien seamount group and the Krylov Seamount and Ridge in the Canary Basin, Central Atlantic Ocean. Their chemical compositions range from intraplate ocean-island-basalt (Demenitskoy) to mid-ocean-ridge-basalt (Tolkien and Krylov) types. Lavas from all three seamount groups, however, show geochemical evidence for involvement of enriched Canary/Cape Verde plume material. Seismic tomography shows that large areas around these mantle plumes consist of dispersed low-velocity material, which could represent diffusely-upwelling plume mantle. Melts from such upwelling mantle could form isolated seamounts. Diffuse upwelling of plume material is likely to be extremely widespread but has been poorly studied to date. Significance Statement A fundamental question concerns the origin of the hundreds of thousands of solitary seamounts and small isolated clusters of such seamounts on the seafloor of the world's ocean basins. Most of them do not fit into any currently accepted models (e.g. they are not associated with a linear hotspot track or plate boundary processes). Their formation could therefore represent a new kind of intraplate volcanism that in fact could be extremely widespread but has been thus far largely neglected. In this manuscript, we report geochemical data from three isolated seamount sites in the Canary Basin and propose a provocative model for their formation that can also be applied to isolated seamounts elsewhere. Our study is therefore also a plea for the long overdue systematic investigation of small seamount volcanism in the world's ocean basins. I hereby confirm that all the data and interpretations are new and have not been published elsewhere. All co-authors have been actively involved in this work, have approved the manuscript and agreed to this submission.
    Schlagwort(e): 551 ; Canary Basin ; seamounts ; isotopic compositions
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 4
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    Stepwise Weakening of the Pliocene Leeuwin Current (2021)
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    Publikationsdatum: 2021-11-30
    Beschreibung: The Indonesian Throughflow (ITF) operates as an important link in global thermohaline circulation, and ITF variability probably modulated Pliocene climate change. Yet, whether ITF variability accounted for oceanographic change south of Northwest Cape remains controversial. Here, we present a multiproxy oceanographic reconstruction from the Perth Basin and reconstruct the Pliocene history of the Leeuwin Current (LC). We show that the LC was active throughout the Pliocene, albeit with fluctuations in intensity and scope. Three main factors controlled LC strength. First, a tectonic ITF reorganization caused an abrupt and permanent LC reduction at 3.7 Ma. On shorter timescales, eustatic sea level and direct orbital forcing of wind patterns hampered or promoted the LC. At 3.3 Ma, for instance, LC intensity plunged in response to a eustatic ITF restriction. Site U1459 then fell outside the extent of a weakened LC, and the latitudinal sea surface temperature gradient along West Australia doubled its steepness.
    Schlagwort(e): 551 ; Perth Basin ; sediment core ; multiproxy geochemical records ; Pliocene ; Leeuwin Current (LC)
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 5
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    Middle to Late Pleistocene environments based on stable organic carbon and nitrogen isotopes of loess-palaeosol sequences from the Carpathian Basin (2021)
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    Publikationsdatum: 2021-11-03
    Beschreibung: Stable organic carbon and nitrogen isotopes can be used to interpret past vegetation patterns and ecosystem qualities. Here we present these proxies for two loess-palaeosol sequences from the southern Carpathian Basin to reconstruct the palaeoenvironment during the past 350 ka and establish regional commonalities and differences. Until now, isotopic studies on loess sequences from this region were only conducted on deposits from the last glacial cycle. We conducted methodological tests concerning the complete decalcification of the samples prior to stable isotope analyses. Two decalcification methods (fumigation method and wet chemical acidification), different treatment times, and the reproducibility of carbon isotope analyses were tested. Obtained results indicate that the choice of the decalcification method is essential for organic carbon stable isotope analyses of loess-palaeosol sequences because ratios vary by more than 10‰ between the wet chemical and fumigation methods, due to incomplete carbonate removal by the latter. Therefore, we suggest avoiding the fumigation method for studies on loess-palaeosol sequences. In addition, our data show that samples with TOC content 〈0.2% bear increased potential for misinterpretation of their carbon isotope ratios. For our sites, C3-vegetation is predominant and no palaeoenvironmental shifts leading to a change of the dominant photosynthesis pathway can be detected during the Middle to Late Pleistocene. Furthermore, the potential for further stable nitrogen isotope studies is highlighted, since this proxy seems to reflect especially past precipitation patterns and reveals favourable conditions in the southern Carpathian Basin, especially during interstadials.
    Schlagwort(e): 551 ; southern Carpathian Basin ; loess-palaeosol sequences ; stable isotope analyses ; Pleistocene ecosystem reconstruction
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 6
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    Compound-Specific Radiocarbon Analysis of (Sub-)Antarctic Coastal Marine Sediments—Potential and Challenges for Chronologies (2021)
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    Publikationsdatum: 2021-11-03
    Beschreibung: In Antarctic and Subantarctic environments, 14C-based age determination is often challenging due to unknown reservoir effects, low organic carbon contents of sediments, and high contributions of petrogenic (14C-free) carbon in ice marginal settings. In this study, we evaluate possible benefits and challenges of compound-specific radiocarbon analysis (CSRA) as a tool for age determination of marine Antarctic and Subantarctic sediment sequences. We present a comprehensive data set of 14C ages obtained on bulk organic carbon, carbonates, and on fatty acids (FA) from three coastal marine sediment cores from Subantarctic South Georgia and East Antarctica. Low molecular weight (LMW) FA represent the least 14C-depleted fraction, indicating that the phytoplankton-derived compounds can be a means of dating sediments. In contrast, vascular plant-derived high molecular weight FA are systematically depleted in 14C relative to the low molecular weight homologues, reflecting processes such as soil formation/erosion in the catchment. Comparative age-depth models show significant differences, depending on the material used for the respective models. While the land plant-derived FA may lead to an overestimation of the actual sediment age, LMW FA reveal complex aquatic reservoir effects. Bulk sedimentary organic carbon 14C ages likely provide appropriate age estimates in settings with low petrogenic carbon input in the Antarctic, whereas CSRA has the potential to produce improved age control in settings with high contributions of petrogenic carbon.
    Schlagwort(e): 551 ; Antarctica ; marine sediments ; compound‐specific radiocarbon analysis (CSRA)
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 7
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    Salt-induced crestal faults control the formation of Quaternary tunnel valleys in the southern North Sea (2021)
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    Publikationsdatum: 2021-10-29
    Beschreibung: Tunnel valleys are major features of glaciated margins and they enable meltwater expulsion from underneath a thick ice cover. Their formation is related to the erosion of subglacial sediments by overpressured meltwater and direct glacial erosion. Yet, the impact of pre-existing structures on their formation and morphology remains poorly known. High-quality 3D seismic data allowed the mapping of a large tunnel valley that eroded underlying preglacial delta deposits in the southern North Sea. The valley follows the N–S strike of crestal faults related to a Zechstein salt wall. A change in downstream tunnel valley orientation towards the SE accompanies a change in the strike direction of salt-induced faults. Fault offsets indicate important activity of crestal faults during the deposition of preglacial deltaic sediments. We propose that crestal faults facilitated tunnel valley erosion by acting as high-permeability pathways and allowing subglacial meltwater to reach low-permeability sediments in the underlying Neogene deltaic sequences, ultimately resulting in meltwater overpressure build-up and tunnel valley excavation. Active faults probably also weakened the near-surface sediment to allow a more efficient erosion of the glacial substrate. This control of substrate structures on tunnel valley morphology is considered as a primary factor in subglacial drainage pattern development in the study area.
    Schlagwort(e): 551 ; southern North Sea ; Quarternary ; tunnel valley formation ; salt-induced faults
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 8
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    Cretaceous Evolution of the Central Asian Proto-Paratethys Sea: Tectonic, Eustatic, and Climatic Controls (2021)
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    Publikationsdatum: 2021-10-29
    Beschreibung: The timing and mechanisms of the Cretaceous sea incursions into Central Asia are still poorly constrained. We provide a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy together with detailed paleoenvironmental analyses of Cretaceous records of the proto-Paratethys Sea fluctuations in the Tajik and Tarim basins. The Early Cretaceous marine incursion in the western Tajik Basin was followed by major marine incursions during the Cenomanian (ca. 100 Ma) and Santonian (ca. 86 Ma) that reached far into the eastern Tajik and Tarim basins. These marine incursions were separated by a Turonian-Coniacian (ca. 92–86 Ma) regression. Basin-wide tectonic subsidence analyses imply that the Early Cretaceous sea incursion into the Tajik Basin was related to increased Pamir tectonism. We find that thrusting along the northern edge of the Pamir at ca. 130–90 Ma resulted in increased subsidence in a retro-arc basin setting. This tectonic event and coeval eustatic highstand resulted in the maximum observed geographic extent of the sea during the Cenomanian (ca. 100 Ma). The following Turonian-Coniacian (ca. 92–86 Ma) major regression, driven by eustasy, coincides with a sharp slowdown in tectonic subsidence during the late orogenic unloading period with limited thrusting. The Santonian (ca. 86 Ma) major sea incursion was likely controlled by eustasy as evidenced by the coeval fluctuations in the west Siberian Basin. An early Maastrichtian cooling (ca. 71–70 Ma), potentially connected to global Late Cretaceous trends, is inferred from the replacement of mollusk-rich limestones by bryozoan- and echinoderm-rich limestones.
    Schlagwort(e): 551 ; Tajik Basin ; Tarim Basin ; Cretaceous sea incursions ; tectonic subsidence ; proto‐Paratethys Sea
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 9
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    The Permeability of Columnar Jointed Lava (2021)
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    Publikationsdatum: 2021-10-29
    Beschreibung: Columnar jointed lava is an important facies in many geothermal reservoir systems. The permeability of jointed lavas is dominated by the contribution from fracture networks. We use a scaling for the permeability of a set of fractures in a solid or porous mass and extend this to arrays of hexagonal intercolumn fractures. To validate our analytical results, we create numerical domains with relevant geometries and extract system-scale permeability using the LBflow lattice-Boltzmann fluid flow simulation tool. Finally, we model the cooling contraction of columns to extend our results so that they predict the permeability with time after lava emplacement. Importantly, we use these results to estimate the range of permeabilities typical of columnar joints that form during cooling from high temperature and are preserved in the crust at moderate to low temperatures.
    Schlagwort(e): 551 ; fractured lavas ; permeability ; prediction
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
  • 10
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    Radiocarbon Evidence for the Contribution of the Southern Indian Ocean to the Evolution of Atmospheric CO2 Over the Last 32,000 Years (2021)
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    Publikationsdatum: 2021-10-28
    Beschreibung: It is widely assumed that the ventilation of the Southern Ocean played a crucial role in driving glacial-interglacial atmospheric CO2 levels. So far, however, ventilation records from the Indian sector of the Southern Ocean are widely missing. Here we present reconstructions of water residence times (depicted as ΔΔ14C and Δδ13C) for the last 32,000 years on sediment records from the Kerguelen Plateau and the Conrad Rise (~570- to 2,500-m water depth), along with simulated changes in ocean stratification from a transient climate model experiment. Our data indicate that Circumpolar Deep Waters in the Indian Ocean were part of the glacial carbon pool. At our sites, close to or bathed by upwelling deep waters, we find two pulses of decreasing ΔΔ14C and δ13C values (~21–17 ka; ~15–12 ka). Both transient pulses precede a similar pattern in downstream intermediate waters in the tropical Indian Ocean as well as rising atmospheric CO2 values. These findings suggest that 14C-depleted, CO2-rich Circumpolar Deep Water from the Indian Ocean contributed to the rise in atmospheric CO2 during Heinrich Stadial 1 and also the Younger Dryas and that the southern Indian Ocean acted as a gateway for sequestered carbon to the atmosphere and tropical intermediate waters.
    Schlagwort(e): 551 ; radiocarbon ; ventilation ; Southern Ocean ; Younger Dryas ; carbon cycle ; Indian Ocean
    Sprache: Englisch
    Materialart: map
    Standort Signatur Erwartet Verfügbarkeit
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    CITATION GEO-LEO
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