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  • 1
    Publication Date: 2019-07-25
    Type: Article , NonPeerReviewed
    Format: text
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  • 2
    Publication Date: 2019-08-05
    Description: The climate of the last two millennia was characterised by decadal to multi‐centennial variations which were recorded in terrestrial records and had important societal impacts. The cause of these climatic events is still under debate but changes in the North Atlantic circulation have often been proposed to play an important role. In this review we compile available high‐resolution paleoceanographic datasets from the northern North Atlantic and Nordic Seas. The records are grouped into regions related to modern ocean conditions and their variability is discussed. We additionally discuss our current knowledge from modelling studies, with a specific focus on the dynamical changes that are not well inferred from the proxy records. An illustration is provided through the analysis of two climate model ensembles and an individual simulation of the last millennium. This review thereby provides an up‐to‐date paleo‐perspective on the North Atlantic multidecadal to multi‐centennial ocean variability across the last two millennia.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2019-09-25
    Description: Cruise M140 combined sampling of plankton, mineral dust and other particles in the water column with recovery of data and samples from long-term observational platforms (sediment traps and dust-collecting buoys). The aim of the cruise was to provide new observations to improve our understanding of the ecology of planktonic foraminifera as important carriers of paleoceanographic proxies and to investigate how mineral dust deposition and the production of marine snow and biogenic particle ballast vary in space and time and how they affect the marine biological pump. To this end, the cruise followed a transect in the central western Atlantic between oligotrophic waters of the subtropical gyre and the productive coastal waters off Mauretania affected by coastal upwelling. To characterise population dynamics, ecology and physiology of planktonic foraminifera, we obtained a series of fourteen vertically resolved plankton net profiles along the cruise track, together with profiles of physical and chemical properties of the ambient water masses. Live foraminifera extracted from these profiles were used to quantify photosynthetic activity of selected species and determine their photoadaptation. High-resolution spatial and temporal sampling of the upper 300 m over 24 hours was carried out at two locations (recovering 41 and 46 vertical profiles), allowing the characterisation of patchiness and daily vertical migration of planktonic foraminifera. Moorings with sediment traps monitoring the seasonal and short-term variability of particle fluxes and buoys monitoring atmospheric dust deposition in the region were successfully recovered in the central Atlantic (M3), south of Cabo Verde (M1) and off Mauretania (CB and CBi) and redeployed in the latter two regions to continue the monitoring. Short-term variability of sizes and types of sinking particles in the water column were characterised in each of the monitoring regions with drifting sediment traps and in the Cape Blanc region off Mauretania also with continuous vertical particle camera profile. All aims of the cruise have been met – the plankton sampling and particle characterization studies were carried out successfully and all moorings and buoys could be recovered and/or redeployed as planned.
    Type: Report , NonPeerReviewed
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  • 4
    Publication Date: 2020-01-02
    Description: Diurnal vertical migration (DVM) is a widespread phenomenon in the upper ocean, but it remains unclear to what degree it also involves passively transported micro- and meso-zooplankton. These organisms are difficult to monitor by in situ sensing and observations from discrete samples are often inconclusive. Prime examples of such ambiguity are planktonic foraminifera, where contradictory evidence for DVM continues to cast doubt on the stability of species vertical habitats, which introduces uncertainties in geochemical proxy interpretation. To provide a robust answer, we carried out highly replicated randomised sampling with 41 vertically resolved plankton net hauls taken within 26 hours in a confined area of 400 km2 in the tropical North Atlantic, where DVM in larger plankton occurs. Manual enumeration of planktonic foraminifera cell density consistently reveals the highest total cell concentrations in the surface mixed layer (top 50 m) and analysis of cell density in seven individual species representing different shell sizes, life strategies and presumed depth habitats reveals consistent vertical habitats not changing over the 26 hours sampling period. These observations robustly reject the existence of DVM in planktonic foraminifera in a setting where DVM occurs in other organisms.
    Type: Article , PeerReviewed
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  • 5
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    PANGAEA
    In:  Supplement to: Jonkers, Lukas; Kucera, Michal (2015): Global analysis of seasonality in the shell flux of extant planktonic Foraminifera. Biogeosciences, 12(7), 2207-2226, https://doi.org/10.5194/bg-12-2207-2015
    Publication Date: 2020-01-17
    Description: Shell fluxes of planktonic Foraminifera species vary intra-annually in a pattern that appears to follow the seasonal cycle. However, the variation in the timing and prominence of seasonal flux maxima in space and among species remains poorly constrained. Thus, although changing seasonality may result in a flux-weighted temperature offset of more than 5° C within a species, this effect is often ignored in the interpretation of Foraminifera-based paleoceanographic records. To address this issue we present an analysis of the intra-annual pattern of shell flux variability in 37 globally distributed time series. The existence of a seasonal component in flux variability was objectively characterised using periodic regression. This analysis yielded estimates of the number, timing and prominence of seasonal flux maxima. Over 80% of the flux series across all species showed a statistically significant periodic component, indicating that a considerable part of the intra-annual flux variability is predictable. Temperature appears to be a powerful predictor of flux seasonality, but its effect differs among species. Three different modes of seasonality are distinguishable. Tropical and subtropical species (Globigerinoides ruber (white and pink varieties), Neogloboquadrina dutertrei, Globigerinoides sacculifer, Orbulina universa, Globigerinella siphonifera, Pulleniatina obliquiloculata, Globorotalia menardii, Globoturborotalita rubescens, Globoturborotalita tenella and Globigerinoides conglobatus) appear to have a less predictable flux pattern, with random peak timing in warm waters. In colder waters, seasonality is more prevalent: peak fluxes occur shortly after summer temperature maxima and peak prominence increases. This tendency is stronger in species with a narrower temperature range, implying that warm-adapted species find it increasingly difficult to reproduce outside their optimum temperature range and that, with decreasing mean temperature, their flux is progressively more focussed in the warm season. The second group includes the temperate to cold-water species Globigerina bulloides, Globigerinita glutinata, Turborotalita quinqueloba, Neogloboquadrina incompta, Neogloboquadrina pachyderma, Globorotalia scitula, Globigerinella calida, Globigerina falconensis, Globorotalia theyeri and Globigerinita uvula. These species show a highly predictable seasonal pattern, with one to two peaks a year, which occur earlier in warmer waters. Peak prominence in this group is independent of temperature. The earlier-when-warmer pattern in this group is related to the timing of productivity maxima. Finally, the deep-dwelling Globorotalia truncatulinoides and Globorotalia inflata show a regular and pronounced peak in winter and spring. The remarkably low flux outside the main pulse may indicate a long reproductive cycle of these species. Overall, our analysis indicates that the seasonality of planktonic Foraminifera shell flux is predictable and reveals the existence of distinct modes of phenology among species. We evaluate the effect of changing seasonality on paleoceanographic reconstructions and find that, irrespective of the seasonality mode, the actual magnitude of environmental change will be underestimated. The observed constraints on flux seasonality can serve as the basis for predictive modelling of flux pattern. As long as the diversity of species seasonality is accounted for in such models, the results can be used to improve reconstructions of the magnitude of environmental change in paleoceanographic records.
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 6
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    PANGAEA
    In:  Supplement to: Jonkers, Lukas; Barker, Stephen; Hall, Ian R; Prins, Maarten A (2015): Correcting for the influence of ice-rafted detritus on grain size-based paleocurrent speed estimates. Paleoceanography, 30(10), 1347-1357, https://doi.org/10.1002/2015PA002830
    Publication Date: 2020-01-17
    Description: The grain size of deep-sea sediments provides an apparently simple proxy for current speed. However, grain size-based proxies may be ambiguous when the size distribution reflects a combination of processes, with current sorting only one of them. In particular, such sediment mixing hinders reconstruction of deep circulation changes associated with ice-rafting events in the glacial North Atlantic because variable ice-rafted detritus (IRD) input may falsely suggest current speed changes. Inverse modeling has been suggested as a way to overcome this problem. However, this approach requires high-precision size measurements that register small changes in the size distribution. Here we show that such data can be obtained using electrosensing and laser diffraction techniques, despite issues previously raised on the low precision of electrosensing methods and potential grain shape effects on laser diffraction. Down-core size patterns obtained from a sediment core from the North Atlantic are similar for both techniques, reinforcing the conclusion that both techniques yield comparable results. However, IRD input leads to a coarsening that spuriously suggests faster current speed. We show that this IRD influence can be accounted for using inverse modeling as long as wide size spectra are taken into account. This yields current speed variations that are in agreement with other proxies. Our experiments thus show that for current speed reconstruction, the choice of instrument is subordinate to a proper recognition of the various processes that determine the size distribution and that by using inverse modeling meaningful current speed reconstructions can be obtained from mixed sediments.
    Type: Dataset
    Format: application/zip, 7 datasets
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  • 7
    Publication Date: 2020-01-17
    Description: Research expedition M140 took place from August 11th, 2017 (Mindelo, Cabo Verde) to September 5th, 2017 (Las Palmas, Spain). CTD data for 16 stations with a total of 37 individual casts along the cruise track were recorded using a Sea & Sun Technology CTD90M (SN 979) down to depths of 700m. The CTD was equipped with the following sensors: Temperature sensor Pt100 model 1509 (Thermal Developments International), Conductivity sensor 7-pole platinum coated electrode cell in quartz glass (Sea & Sun Technology), Seapoint Chlorophyll Fluorometer (Seapoint) and dissolved oxygen sensor (DO522M18, Clark type, OxyGuard). Additionally data for two stations were also recorded with Seabird Electronics (SBE) 9 plus CTD (SN 979) mounted in a SBE water sampler rosette. The data files contain the data for temperature, salinity, density, chlorophyll a concentration and dissolved oxygen concentration; as raw data and processed and flagged according to the recommendations for real-time data processing of EuroGOOS and GTSPP, as well as the outlier detection method CoTeDe (https://github.com/castelao/CoTeDe). Biogeographic regions are determined according to Spalding et al. (2012). TEOS-10 unit conversions have been performed with the GSW Oceanographic Toolbox (McDougall and Barker, 2011).
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 8
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    Copernicus Publications (EGU)
    In:  Climate of the Past, 13 (6). pp. 573-586.
    Publication Date: 2020-01-17
    Description: The composition of planktonic foraminiferal (PF) calcite is routinely used to reconstruct climate variability. However, PF ecology leaves a large imprint on the proxy signal: seasonal and vertical habitats of PF species vary spatially, causing variable offsets from annual mean surface conditions recorded by sedimentary assemblages. PF seasonality changes with temperature in a way that minimises the environmental change that individual species experience and it is not unlikely that changes in depth habitat also result from such habitat tracking. While this behaviour could lead to an underestimation of spatial or temporal trends as well as of variability in proxy records, most palaeoceanographic studies are (implicitly) based on the assumption of a constant habitat. Up to now, the effect of habitat tracking on foraminifera proxy records has not yet been formally quantified on a global scale. Here we attempt to characterise this effect on the amplitude of environmental change recorded in sedimentary PF using core top δ18O data from six species. We find that the offset from mean annual near-surface δ18O values varies with temperature, with PF δ18O indicating warmer than mean conditions in colder waters (on average by −0.1 ‰ (equivalent to 0.4 °C) per °C), thus providing a first-order quantification of the degree of underestimation due to habitat tracking. We use an empirical model to estimate the contribution of seasonality to the observed difference between PF and annual mean δ18O and use the residual Δδ18O to assess trends in calcification depth. Our analysis indicates that given an observation-based model parametrisation calcification depth increases with temperature in all species and sensitivity analysis suggests that a temperature-related seasonal habitat adjustment is essential to explain the observed isotope signal. Habitat tracking can thus lead to a significant reduction in the amplitude of recorded environmental change. However, we show that this behaviour is predictable. This allows accounting for habitat tracking, enabling more meaningful reconstructions and improved data–model comparison.
    Type: Article , PeerReviewed
    Format: text
    Format: archive
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  • 9
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 1571 data points
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  • 10
    Publication Date: 2020-01-17
    Type: Dataset
    Format: text/tab-separated-values, 26 data points
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