Biogeochemistry of Coccolithophore Blooms along the Continental Margin of the Northern Bay of Biscay.
Lei ChouJérôme HarlayCaroline De BodtNathalie RoevrosAlberto BorgesK. SuykensBruno DelilleKoen SabbeNicolas Van OostendeAnja EngelJudith PiontekNicole HändelS. GroomSabine Schmidt
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Coccolithophore
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High-resolution paleoproductivity variations have been reconstructed in a productive cell in the Alboran Sea for the Holocene. Fossil coccolithophore assemblages have been studied along with the [Formula: see text]-estimated sea-surface temperature (SST) and other paleoenvironmental proxies. The appearance of this cell is suggested at 7.7 ka cal. BP and was linked to the establishment of the western anti-cyclonic gyre. From that time until the present, the nannofossil accumulation rate of Florisphaera profunda has revealed successive episodes of weakening and strengthening of upwelling conditions in the Alboran Sea that have been simultaneous to changes in Western Mediterranean Deep Water (WMDW) formation in the Gulf of Lions. A two-phase scenario operating at millennial–centennial time scale is proposed to explain this climatic and oceanographic variability: (1) coeval with more arid climate conditions, weaker northerlies or north-westerlies blowing over the Gulf of Lions would have triggered a slackening of WMDW formation. This together with a minor Atlantic Jet (AJ) inflowing into the Alboran Sea would have led to less vertical mixing and, hence, a more stable water column in the study area; (2) wetter climate conditions would have prevailed in the region, while stronger northerlies or north-westerlies would have enabled WMDW reinforcement in the Gulf of Lions simultaneous to an intensification of the AJ that migrated southward. This would have increased vertical mixing, intensifying upwelling conditions in the study area. Here, the winter North Atlantic Oscillation (NAO) is considered to be an important forcing mechanism for this variability, influencing WMDW formation, which in turn has been linked to short-term productivity variations during the last 7.7 ka in the Alboran Sea.
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A mid- to late-Holocene paleohydrological reconstruction from the northeast Atlantic is proposed through the study of a high-resolution sedimentary record from the northern continental shelf of the Bay of Biscay (BoB). Three foraminiferal species dominate the assemblages with Rosalina globularis showing an overall decrease in absolute and relative abundances from ~7 to 0.4 cal. ka BP, whereas the opposite trend is observed for Cibicides refulgens and Lobatula. These long-term patterns are interpreted as a response to the overall cooling trend and/or the progressive deepening of the water column because of the relative sea-level (RSL) rise. Foraminiferal δ 18 O and grain-size analyses show a significant shift around 3.5–2.5 cal. ka BP toward a heavier isotopic signature and finer sediments. We mainly link this change to enhanced contribution of continental freshwaters and fine sediments after the near-stabilization of the RSL rise. By reducing coastal accommodation spaces, this led to a better channelization of river outflows and probably to the formation of the modern winter thermohaline front. Superimposed on these long-term patterns, our data highlight strong millennial-scale variability (1250-year peak). Such cyclicity is consistent with several records tracing changes in rainfall and storminess regimes in northern Europe, and the dynamics of the subpolar gyre (SPG). We suggest a millennial time-scale control of a NAO-like (North Atlantic Oscillation) climatic process modulating continental humidity (and the associated river discharges) and SPG dynamics through wind stress. Spectral analyses reveal an additional 500-year frequency peak implying a possible solar forcing.
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of Part B.- Surface Sediment Distributions.- Some unique sedimentological and geochemical features of deposits in coastal upwelling regions.- Sedimentation of organic matter in upwelling regimes.- Biogenic sediments on the South West African (Namibian) continental margin.- The modern upwelling record off northwest Africa.- Biogenic sedimentary structures in a modern upwelling region: Northwest African continental margin.- Upwelling records in recent sediments from southern Portugal: A reconnaissance survey.- Environmental controls on sediment texture and composition in low oxygen zones off Peru and Oregon.- Coastal upwelling, its influence on the geotechnical properties and stability characteristics of submarine deposits.- Upwelling along the western Indian continental margin and its geological record: A summary.- High Resolution Holocene Time Scales.- Stable isotope record of upwelling and climate from Santa Barbara Basin, California.- Decadal variation of upwelling in the Central Gulf of California.- Oxygen isotope composition of diatom silica and silico-flagellate assemblage changes in the Gulf of California: A 700-year upwelling study.- Stable isotopes of foraminifers off Peru recording high fertility and changes in upwelling history.- Pleistocene Time Scales.- Spatial and temporal patterns of organic matter accumulation on the Peru continental margin.- Variability of upwelling regimes (northwest Africa, south Arabia) during the latest Pleistocene: A comparison.- Glacial-interglacial cycles in oceanic productivity inferred from organic carbon contents in eastern North Atlantic sediment cores.- Differentiation of high oceanic fertility in marine sediments caused by coastal upwelling and/or river discharge off northwest Africa during the late Quaternary.- Distribution of organic carbon in the Gulf of Alaska Neogene and Holocene sedimentary record.- Pre-Pleistocene Phanerozoic Time Scales.- Organic geochemistry of sediments recovered by DSDP/IPOD Leg 75 from under the Benguela Current.- Potential deep-sea petroleum source beds related to coastal upwelling.- Cretaceous upwelling off northwest Africa: A summary.- Facies patterns of a Cretaceous/Tertiary subtropical upwelling system (Great Syrian desert) and an Aptian/Albian boreal upwelling system (NW Germany).- Indications of upwelling in the Lower Ordovician of Scandinavia.- Upwelling in the Paleozoic era.- Paleozoic black shales in relation to continental margin upwelling.- Participants.
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