Papers by Jacques Giraudeau
One of the achievements of the IPY was to foster an unprecedented amount of research initiatives ... more One of the achievements of the IPY was to foster an unprecedented amount of research initiatives aiming at studying recent (Holocene) changes in ocean circulation and climate in the subarctic and arctic domains. Paleo-investigations offer invaluable information on natural environmental changes at decadal to millennial scale, as well as on the processes driving them. We hereby present some recent results based on proxy records from circum Icelandic areas.
Global Change Biology, 2020
Climate warming is rapidly reshaping the Arctic cryosphere and ocean conditions, with consequence... more Climate warming is rapidly reshaping the Arctic cryosphere and ocean conditions, with consequences for sea ice and pelagic productivity patterns affecting the entire marine food web. To predict how ongoing changes will impact Arctic marine ecosystems, concerted effort from various disciplines is required. Here, we contribute multi‐decadal reconstructions of changes in diatom production and sea‐ice conditions in relation to Holocene climate and ocean conditions off northwest Greenland. Our multiproxy study includes diatoms, sea‐ice biomarkers (IP25 and HBI III) and geochemical tracers (TOC [total organic carbon], TOC:TN [total nitrogen], δ13C, δ15N) from a sediment core record spanning the last c. 9,000 years. Our results suggest that the balance between the outflow of polar water from the Arctic, and input of Atlantic water from the Irminger Current into the West Greenland Current is a key factor in controlling sea‐ice conditions, and both diatom phenology and production in northeas...
Marine Geology, 2020
Nares Strait is one of three channels that connect the Arctic Ocean to Baffin Bay. Unique sea-ice... more Nares Strait is one of three channels that connect the Arctic Ocean to Baffin Bay. Unique sea-ice conditions in the strait lead to the formation of landfast ice arches at its northern and southern ends. These ice arches regulate Arctic sea-ice and freshwater export through the strait and promote the opening of the North Water polynya. The present study addresses the paucity of pre-satellite records of environmental conditions in the Nares Strait area, and aims at reconstructing Holocene sea-ice conditions and ocean circulation in the strait. The investigation is based on a marine sediment core strategically retrieved from under the current ice arch in Kane Basin to the south of Nares Strait, and provides a continuous record spanning the past ca 9 kyrs. We use benthic foraminiferal assemblages and sea-ice biomarkers to infer changes in Holocene ocean circulation and sea-ice conditions in Kane Basin. The establishment of the modern ocean circulation in Kane Basin is related to ice sheet retreat and postglacial rebound, while changes in sea-ice cover concur with major shifts in the Arctic Oscillation (AO). Our results suggest that sea-ice cover in Kane Basin was highly variable between ca 9.0 and 8.3 cal. ka BP, before increasing, probably in link with the 8.2 cold event and the opening of Nares Strait. A short period of minimum sea-ice cover and maximum Atlantic bottom water influence occurred between ca 8.1 and 7.5 cal. ka BP, when Kane Basin was deeper than for the remaining of the Holocene. As atmospheric temperatures dropped, sea-ice cover intensified in Kane Basin between ca 7.5 and 5.5 cal. ka BP, but strong winds under prevailing positive-like AO conditions likely prevented the formation of ice arches in Nares Strait. During this time, our micropaleontological data show that Atlantic water was progressively excluded from Kane Basin by the postglacial isostatic rebound. Increasingly cooler atmospheric temperatures and a shift towards more negative phases of the AO may have promoted the establishment of ice arches in Nares Strait between ca 5.5 and 3.0 cal. ka BP. Instabilities in the Kane Basin ice arch ca 3.0 cal. ka BP coincide with a shift towards prevailing positive phases of the AO, while a brief recovery of the ice arch occurred during more negative-like AO conditions between ca 1.2 and 0.2 cal. ka BP.
Baffin Bay is a semi-enclosed basin connecting the Arctic Ocean and the western North Atlantic, t... more Baffin Bay is a semi-enclosed basin connecting the Arctic Ocean and the western North Atlantic, thus making out a significant pathway for heat exchange. Here we reconstruct the alternating advection of relatively warmer and saline Atlantic waters versus the incursion of colder Arctic water masses entering Baffin Bay through the multiple gateways in the Canadian Arctic Archipelago and the Nares Strait during the Holocene. We carried out benthic foraminiferal assemblage analyses, X-ray fluorescence scanning, and radiocarbon dating of a 738 cm long marine sediment core retrieved from eastern Baffin Bay near Upernavik, Greenland (Core AMD14-204C; 987 m water depth). Results reveal that eastern Baffin Bay was subjected to several oceanographic changes during the last 9.2 kyr. Waning deglacial conditions with enhanced meltwater influxes and an extensive sea-ice cover prevailed in eastern Baffin Bay from 9.2 to 7.9 ka. A transition towards bottom water amelioration is recorded at 7.9 ka by increased advection of Atlantic water masses, encompassing the Holocene Thermal Maximum. A cold period with growing sea-ice cover at 6.7 ka interrupts the overall warm subsurface water conditions, promoted by a weaker northward flow of Atlantic waters. The onset of the neoglaciation at ca. 2.9 ka is marked by an abrupt transition towards a benthic fauna dominated by agglutinated species, likely in part explained by a reduction of the influx of Atlantic Water, allowing an increased influx of the cold, corrosive Baffin Bay Deep Water originating from the Arctic Ocean to enter Baffin Bay through the Nares Strait. These cold subsurface water conditions persisted throughout the Late Holocene, only interrupted by short-lived warmings superimposed on this cooling trend.
Quaternary Science Reviews, 2004
The North Iceland shelf bears essential components of the present surface and intermediate circul... more The North Iceland shelf bears essential components of the present surface and intermediate circulation of the northern North Atlantic. Instrumental and historical data give evidence of the sensitivity of this domain to broad, regional-scale oceanic and atmospheric anomalies. Our investigation of the paleohydrological variability off Northern Iceland throughout the last 10 000 cal yr suggests that atmospheric forcing alone, through combined changes in strength of the wind stress curl and sea-level atmospheric pressure pattern over the Nordic Seas, is sufficient to explain the recorded changes in origins and dynamics of surface and intermediate water masses. Our biotic proxies, coccoliths and benthic foraminifera, were extracted from a giant piston core (MD99-2269) collected in a shelf trough where sediment accumulated at an excess rate of 2 m/kyr. The mid-Holocene from 6.5 to 3.5 cal kyr BP was a time of peaked carbonate production and subsequent sedimentation, and strong water-column stratification with a thick layer of cold-fresh Arctic surface water overlapping an enhanced flow of Irminger/Atlantic Intermediate water. Applying conditions triggering present-time carbonate plankton blooms in the studied area, we infer that a lowered cyclonic activity associated with decreased winter storms and reduced production of Arctic Intermediate Water in the Iceland Sea were conductive of the recorded mid-Holocene water column structure. The opposite situation (warm Atlantic surface water, low vertically-integrated inflow of Irminger water, abutment of Arctic Intermediate water in deep shelf troughs) characterized the early Holocene as well as a shorter late Holocene period centred at 2 cal kyr BP. The Little Ice Age (ca. 0.2-0.6 cal kyr BP) and a short event at around 3 cal kyr BP stand as times of extreme advection of polar waters and extended sea-ice development. A comparison of the recorded long-term Holocene evolution of water column structure off Northern Iceland with climate and hydrological changes in the northeastern Atlantic suggests that the strength of Atlantic inflow into the Nordic Seas was subjected to a balance between the Irminger and the Norwegian branches. This balance is thought to be mostly related to changes in the intensity and location of westerly winds and associated atmospheric pressure gradients in the North Atlantic.
Proceedings of the Ocean Drilling Program 175 Initial Reports, 1998
Thirteen sites were occupied during Ocean Drilling Program Leg 175 along southwestern Africa, sta... more Thirteen sites were occupied during Ocean Drilling Program Leg 175 along southwestern Africa, starting with the Congo Fan in the north and ending off the Cape of Good Hope. North of the Walvis Ridge, penetration was limited because of safety concerns. Sediments from the Congo Fan are rich in opal. Maximum rates of sedimentation were found at holes drilled off Lobito (Angola), with rates as high as 60 cm/k.y. because of high terrigenous influx from a rising coast. On the Walvis Ridge and in the Walvis Bay, organic matter-rich calcareous clays were recovered. The highest organic content, the most vigorous development of methane and carbon dioxide, and abundant noxious gases were found at Site 1084, located off Lüderitz, Namibia. The sites occupied off the Cape of Good Hope are characterized by calcareous oozes. Organic matter abundance, diatom accumulation, and composition of benthic faunas, among other indicators, are the most important tools for the reconstruction of the productivity history of the Angola-Benguela upwelling system. Problems arising include (1) the origin of the Matuyama Opal Maximum, centered near 2.2 Ma, (2) the question of phase relationships between diatom productivity and organic matter supply, and (3) the role of precessional effects in the modulation of productivity as a function of latitude. Regarding the first problem, it emerged that the maximum supply of diatoms is linked to incursions of both warm pelagic water and Antarctic water during the early Matuyama, presumably resulting in the formation of chaotic frontal zones. Coastal upwelling dominates after the maximum. Thus, opal deposition goes through an optimum situation within the latest Pliocene. We hypothesize that increased silicate concentrations in subsurface waters made this optimum possible. As the Benguela Current became stronger in the Quaternary, a more vigorous flow of underlying Antarctic Intermediate Water increased ventilation of the thermocline off southwestern Africa, thereby helping to decrease the silicate content of subsurface waters. Observations during Leg 75 regarding glacial/interglacial cycles on the Walvis Ridge (that warming is accompanied by
Proceedings of the Ocean Drilling Program, 155 Scientific Results, 1997
Although the Amazon Fan is a complex stratigraphic sequence of mass-transport deposits (MTDs), le... more Although the Amazon Fan is a complex stratigraphic sequence of mass-transport deposits (MTDs), levee complexes, and hemipelagic oozes, the interpretation of micropaleontological and isotopic data from the Amazon Fan has provided excellent results. Compared to other mega deep-sea fans, the Amazon Fan is highly structured, thus providing an unusual opportunity to investigate the Quaternary evolution of a deep-sea fan. Biostratigraphic work has been carried out on two of the main sedimentological fan units: (1) the upper Pleistocene/ Holocene sediments that cap the entire fan area and are equivalent in age or younger than the paleomagnetic Lake Mungo Excursion of 32 ka; and (2) the interglacial deep-carbonate units that underlie the MTDs and are younger than the Pseudoemiliania lacunosa event of 475 ka. Age assignments are primarily based on biostratigraphic analyses of calcareous nannofossils and planktonic foraminifers. Nannofossils are common only in the Holocene calcareous muds and in the deep carbonate units. Foraminifers are common in the same units, but are also present in the glacial levee muds underlying the Holocene calcareous clays. Additional age constraints are provided by interpretation of the paleomagnetic record and accelerator mass-spectrometry 14 C dates. Stable oxygen-and carbon-isotope data are used to further refine the biostratigraphic and environmental interpretations. Biostratigraphy of the cores retrieved during Leg 155 has provided an overall stratigraphic framework for understanding the Quaternary evolution of the Amazon Fan. Biostratigraphic and paleomagnetic data are combined to provide estimates of sedimentation rates, ranging from 5 cm/k.y. during interglacial periods, to over 5000 cm/k.y. during glacial periods. A combination of biostratigraphy, seismic stratigraphy, magnetostratigraphy, and sedimentation rate constraints was used to date the top of both the near-surface MTDs and the deep MTDs. These data suggest that the deep MTDs were last active at ~33 ka and 45 ka, whereas the near-surface MTDs were active during Termination I (11−14 ka).
Journal of Marine Systems, 2016
The Barents Sea is particularly vulnerable to large-scale hydro-climatic changes associated with ... more The Barents Sea is particularly vulnerable to large-scale hydro-climatic changes associated with the polar amplification of climate change. Key oceanographical variables in this region are the seasonal development of sea-ice and the location and strength of physico-chemical gradients in the surface and subsurface water layers induced by the convergence of Arctic-and Atlantic-derived water masses. Remote sensing imagery have highlighted the increasing success of calcifying haptophytes (coccolithophores) in the summer phytoplankton production of the Barents Sea over the last 20 years, as a response to an overall larger contribution of Atlantic waters to surface and sub-surface waters, as well as to enhanced sea-ice melt-induced summer stratification of the photic layer. The present study provides a first thorough description of coccolithophore standing stocks and diversity over the shelf and slope of the western Barents Sea from two sets of surface and water column samples collected during August-September 2014 from northern Norway to southern Svalbard. The abundance and composition of coccolithophore cells and skeletal remains (coccoliths) are discussed in view of the physical-chemical-biological status of the surface waters and water column based on in-situ (temperature, salinity, fluorescence) and shorebased (microscope enumerations, chemotaxonomy) measurements, as well as satellite-derived data (Chl a and particulate inorganic carbon contents). The coccolithophore population is characterized by a low species diversity and the overwhelming dominance of Emiliania huxleyi. Coccolithophores are abundant both within the well stratified, Norwegian coastal waterinfluenced shallow mixed layer off northern Norway, as well as within well-mixed cool Atlantic water in close vicinity of the Polar Front. Bloom concentrations with standing stocks larger than 4 million cells/l are recorded in the latter area north of 75°N. Our limited set of chemotaxonomic data suggests that coccolithophores contribute substantially (ca. 20% of the total Chl a) to the summer phytoplankton community which is made essentially of small-sized algal groups. Excluding the bloom area, coccolith calcite accounts for an average of 20% to the bulk particulate inorganic carbon content in the surface waters, and explains to some extent the satellite-derived spatial distribution of this parameter. Deep water living coccolithophore species thriving below the pycnocline as well as populations present in well-mixed cool Atlantic water are rapidly transferred to depth in the form of intact coccospheres down to at least 200 m. High amplitude internal waves which, according to our observations, affect a wide range of water depth up to the lower photic zone, might strengthen the vertical transfer of this sinking population.
Marine Geology, 2016
This paper presents an analysis of the combined influence of along-slope sediment transport and o... more This paper presents an analysis of the combined influence of along-slope sediment transport and off-bank sediment export from the Little Bahama Bank (LBB) to the periplatform sediment wedge of the northwestern part of the slope over the last 424 ka. The LBB northwestern slope is divided in (i) a plateau-like structure (margin) at 40 m water depth over at least 4 km parallel to the edge of the LBB; (ii) the uppermost slope with a mean slope angle of~1.15°from 40 to 300 m water depth; (iii) the upper slope with slope angle of~0.7 from 300 to 650 m water depth, (iv) the middle slope with slope angle of~1.2, from 650 to 800 m water depth, and (v) the lower slope with slope angle of b~0.5, from 800 to 900 m water depth. The uppermost slope, the upper slope, and the middle slope of the northwestern LBB were characterized by periplatform oozes that became more diluted with pelagic sediment toward the distal part of the slope. This sediment distribution of the northwestern LBB slope varied significantly over times according to the flooded surface of the LBB. The major flooding periods are related to the highest Relative Sea Level (RSL) (N−6 m) that occurred during interglacial periods, the highest sedimentation rates (10-30 cm/ka) and the finest sediment facies were found on the slope. During interglacial periods when RSL b −6 m, LBB was emerged but bank margins were still flooded and correspond to intermediate sedimentation rates (a few to 10 cm/ka) on the slope. Finally, during glacial periods (RSL b − 90 m), LBB was emerged (including its margins), sedimentation rates on the slope dropped to a few mm/ka associated to coarser sediment facies. Off-bank-transported sediment is the main sediment supply during sea-level highstands, occurring preferentially during three major periods of LBB flooding over the last 424 ka: marine isotopic stages 1, 5e and 11. During sealevel lowstands, shallow carbonate production was very low but could develop over a 4 km-wide plateau-like structure when RSL was above −40 m. The regional Antilles Current affected the sea floor along the northwestern LBB slope and influenced coral mound distribution as well as sediment facies and sequences along the upper and middle slopes (300-800 m). During glacial periods, the stronger influence of the Antilles Current upon the along-slope sedimentation promoted diagenesis via the development of indurated nodules in the upper slope (~400 m water depth). It also encouraged bi-gradational sequences showing a coarsening-up unit followed by a fining-up unit along the middle slope (~800 m water depth) that is thoroughly bioturbated. The characteristics of these contourite sequences were similar to those described in siliciclastic environments, but in contrast were condensed with low sedimentation rates over long (glacial) periods.
Proceedings of the Ocean Drilling Program, 2001
Planktonic foraminiferal assemblages from the upper Pleistocene part of Hole 1087A (0 to 12.1 met... more Planktonic foraminiferal assemblages from the upper Pleistocene part of Hole 1087A (0 to 12.1 meters below seafloor) are investigated to assess the role of global and local climate changes on surface circulation in the southern Benguela region. The benthic stable isotope record indicates that the studied interval is representative of the last four climatic cycles, that is, down to marine isotope Stage (MIS) 12. The species assemblages bear a clear transitional to subpolar character, with Neogloboquadrina pachyderma (d), Globorotalia inflata, and Globigerina bulloides, in order of decreasing abundance, as the dominant taxa. This species association presently characterizes the mixing domain of old upwelled and open ocean waters, seaward of the Benguela upwelling cells. Abundance variation of the dominant foraminiferal species roughly follows a glacial-interglacial pattern down to MIS 8, suggesting an alternation of upwelling strength and associated seaward extension of the belt of upwelled water as a response to global climate changes. This pattern is interrupted from ~250 ka down to MIS 12, where the phase relationship with global climate is ill defined and might be interpreted as a local response of the southern Benguela region to the mid-Brunhes event. Of particular interest is a single pulse of newly upwelled waters at the location of Site 1087 during early MIS 9 as indicated by a peak abundance of sinistral N. pachyderma (s). Variable input of warm, salty Indian Ocean thermocline waters into the southeast Atlantic, a key component of the Atlantic heat conveyor, is indicated by abundance changes
Proceedings of the Ocean Drilling Program, 2001
High-resolution planktonic and benthic stable isotope records from Ocean Drilling Program Site 10... more High-resolution planktonic and benthic stable isotope records from Ocean Drilling Program Site 1087 off southeast Africa provide the basis for a detailed study of glacial-interglacial (G-IG) cycles during the last 500 k.y. This site is located in the Southern Cape Basin at the boundary of the coastal upwelling of Benguela and close to the gateway between the South Atlantic and the Indian Oceans. It therefore monitors variations of the hydrological fronts associated with the upwelling system and the Atlantic-Indian Ocean interconnections, in relation to global climate change. The coldest period of the last 500 k.y. corresponds to marine isotope Stage (MIS) 12, when surface water temperature was 4°C lower than during the last glacial maximum (LGM) as recorded by the surface-dwelling foraminifer Globigerinoides ruber. The warmest periods occurred during MISs 5 and 11, a situation slightly different to that observed at Site 704, which is close to the Polar Front Zone, where there is no significant difference between the interglacial stages for the past 450 k.y., except the long period of warmth during MIS 11. The planktonic and benthic carbon isotope records do not follow the G-IG cycles but show large oscillations related to major changes in the productivity regime. The largest positive δ 13 C excursion between 260 and 425 ka coincides with the global mid-Brunhes event of carbonate productivity. The oxygen and carbon isotopic gradients between surface and deep waters display long-term changes superimposed on rapid and high-frequency fluctuations that do not follow the regular G-IG pattern; these gradients indicate modifications of the temperature, salinity, and productivity gradients due to changes in the thermocline depth,
Climate of the Past Discussions, 2011
The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic arou... more The Agulhas Current transport of heat and salt from the Indian Ocean into the South Atlantic around South Africa (Agulhas leakage), can affect the Atlantic meridional overturning circulation (AMOC) and, thus, influence global climate. However, efforts to elucidate forcing mechanisms connecting the Agulhas leakage with the upstream dynamics of the current have been hampered by a lack of climate records extracted from the area where the Agulhas current originates. We determine 800-kyr sea surface temperature (SST) and salinity (SSS) records from the "precursor" region of the Agulhas current and show that these records contain strong 100-kyr and 41-kyr cycles. This latter obliquitydriven cycle is nearly in phase with changes in the annual mean insolation and air temperature at high southern latitudes. In contrast, our SST and SSS records did not reveal precession-driven cycles, which is surprising given the lowlatitude location of the upstream Agulhas current. Together, this indicates that the dynamics of the Agulhas current system is mainly controlled by high latitude obliquity through its influence on the position of the Southern Hemisphere subtropical front (STF) and its associated westerlies. Our study demonstrates that obliquity may drive an important part of the 100 kyr cycles observed in the system rather than precession. Our results also suggest that a stronger Agulhas current,
Continental Shelf Research, 2014
High sedimentation rates in fjords provide excellent possibilities for high resolution sedimentar... more High sedimentation rates in fjords provide excellent possibilities for high resolution sedimentary and geochemical records over the Holocene. As a baseline for an improved interpretation of geochemical data from fjord sediment cores, this study aims to investigate the inorganic/organic geochemistry of surface sediments and to identify geochemical proxies for terrestrial input and river discharge in the Trondheimsfjord, central Norway. Sixty evenly distributed surface sediment samples were analysed for their elemental composition, total organic carbon (C org), nitrogen (N org) and organic carbon stable isotopes (δ 13 C org), bulk mineral composition and grain size distribution. Our results indicate carbonate marine productivity to be the main CaCO 3 source. Also, a strong decreasing gradient of marinederived organic matter from the entrance towards the fjord inner part is consistent with modern primary production data. We show that the origin of the organic matter as well as the distribution of CaCO 3 in Trondheimsfjord sediments can be used as a proxy for the variable inflow of Atlantic water and changes in river runoff. Furthermore, the comparison of I-2 grain size independent Al-based trace element ratios with geochemical analysis from terrigenous sediments and bedrocks provides evidence that the distribution of K/Al, Ni/Al and K/Ni in the fjord sediments reflect regional sources of K and Ni in the northern and southern drainage basin of the Trondheimsfjord. Applying these findings to temporally wellconstrained sediment records will provide important insights into both the palaeoenvironmental changes of the hinterland and the palaeoceanographic modifications in the Norwegian Sea as response to rapid climate changes and associated feedback mechanisms.
The Holocene, 2014
During the last century, both earth surface temperature and moisture transport towards high latit... more During the last century, both earth surface temperature and moisture transport towards high latitudes have increased rapidly. The response of the sub-arctic region to these changes in terms of weathering, transport and delivery of terrigenous material towards the coastal and deep ocean is both complex and poorly understood. Sediments accumulating in fjords offer an excellent opportunity for studying such land–ocean interactions and may provide ultra-high-resolution records of environmental response to short-term climate variability. As a basis for Holocene climate change studies, modern sources, supply and distribution of particular sediment components in the Trondheimsfjord have been investigated and imply lithogenic elements as a promising proxy for terrigenous input and river discharge. To better understand the impact of atmospheric variability on central Norwegian environment, we examine instrumental time series and show that the dominant mode of the atmospheric circulation in t...
One of the achievements of the IPY was to foster an unprecedented amount of research initiatives ... more One of the achievements of the IPY was to foster an unprecedented amount of research initiatives aiming at studying recent (Holocene) changes in ocean circulation and climate in the subarctic and arctic domains. Paleo-investigations offer invaluable information on natural environmental changes at decadal to millennial scale, as well as on the processes driving them. We hereby present some recent results based on proxy records from circum Icelandic areas.
Biogeosciences Discussions, 2013
Extant coccolithophores and their relation to the governing oceanographic features in the norther... more Extant coccolithophores and their relation to the governing oceanographic features in the northern North Atlantic were investigated along two zonal transects of surface water sampling, both conducted during summer 2011 and fall 2007. The northern transects crossed Fram Strait and its two opposing boundary currents (West Spitsbergen
Proceedings of the National Academy of Sciences, 2012
Heat and salt transfer from the Indian Ocean to the Atlantic Ocean (Agulhas leakage) has an impor... more Heat and salt transfer from the Indian Ocean to the Atlantic Ocean (Agulhas leakage) has an important effect on the global thermohaline circulation and climate. The lack of long transfer record prevents elucidation of its role on climate changes throughout the Quaternary. Here, we present a 1,350-ka accumulation rate record of the planktic foraminiferal species Globorotalia menardii . We demonstrate that, according to previous assumptions, the presence and reseeding of this fauna in the subtropical southeast Atlantic was driven by interocean exchange south of Africa. The Agulhas transfer strengthened at glacial ice-volume maxima for every glacial-interglacial transition, with maximum reinforcements organized according to a 400-ka periodicity. The long-term dynamics of Agulhas leakage may have played a crucial role in regulating meridional overturning circulation and global climate changes during the Mid-Brunhes event and the Mid-Pleistocene transition, and could also play an importa...
Paleoceanography, 2005
In the light of new surface sediment d 15 N data collected over the Namibian shelf and slope, we ... more In the light of new surface sediment d 15 N data collected over the Namibian shelf and slope, we examined glacial-interglacial variations of N inventory in the area of Lüderitz (25°6S) by deciphering d 15 N signals of three cores distributed from the upper to the lower continental slope. The lower slope cores display low d 15 N during cold periods and high d 15 N during climatic optima, akin to many other records from the world ocean, whereas the upper slope core displays a high-frequency low-amplitude d 15 N signal without obvious glacial-interglacial variability. This dissimilarity results from the segregation of the upwelling structure in two cells, decoupling nutrient dynamics of the shelf from those beyond the shelf-edge. The d 15 N signal of the coastal cell is relatively constant irrespective of wind strength variations and shows that nitrate was never depleted in the surface water. For the deeper cores, comparisons between N isotopic signals and indicators of paleoproductivity (total organic carbon) and upwelling intensity (sea surface temperature and dust grain size) reveal that, over Milankovitch cycles, nitrate delivery to the photic zone was driven by the nutrient richness of the South Atlantic Central Water (depending, in turn, on Aghulas water inflow and denitrification at a global scale) rather than by atmospheric forcing. We propose that the d 15 N signals of the deeper cores do not only mirror changes in relative nitrate utilization, as it seems the case over annual timescales, but are arguably influenced by global ocean changes in middepth nitrate d 15 N.
Paleoceanography, 2006
Holocene records of sea surface conditions in the western Nordic seas were obtained from quantita... more Holocene records of sea surface conditions in the western Nordic seas were obtained from quantitative reconstructions based on dinoflagellate cyst assemblages. Two sediment cores from the east Greenland and the north Iceland shelves provide a detailed account of the long‐ and short‐term dynamics of the opposing flows of Arctic versus Atlantic waters. Both marker species and quantitative reconstructions depict an overall trend toward warmer winter temperatures, saltier surface waters, and decreased sea ice extent. The latter is supported by a close relationship between relative abundances of an Atlantic dinocyst species, Nematosphaeropsis labyrinthus, and ice‐rafted debris (IRD) records. We propose that the late Holocene increased IRD delivery in the Denmark Strait region was primarily induced by a combination of less extensive sea ice cover under increased Atlantic water inflow and sustained iceberg calving tied to the readvance of the Greenland ice sheet. Our records thus suggest d...
Marine Micropaleontology, 2008
Coccolith and dinoflagellate cyst (or dinocyst) population counts were compiled from existing sur... more Coccolith and dinoflagellate cyst (or dinocyst) population counts were compiled from existing surface sediment databases as well as new counts in order to establish an 87-sample database for which assemblages of both microfossil groups are known. This database allowed a direct comparison of the distribution of coccolith and dinocyst assemblages in the subtropical to subpolar North Atlantic. In addition, the relationship between these assemblages and sea surface environmental parameters was addressed, in order to identify possible differences in the ecology of the two plankton groups. The comparison highlights an excellent correspondence between dinocyst assemblages, coccolith assemblages and the distribution of the surface water masses represented in our database, notably in the subtropical and temperate domains. In the subpolar domain, coccolith assemblages are much less diversified than dinocyst assemblages in terms of species. As a result, the discrimination between the subpolar water masses based on coccolith assemblages is not as clear as in the subtropical/temperate regions, whereas dinocyst assemblages show a distribution pattern closely related with surface hydrography. Canonical correspondence analyses performed on coccolith and dinocyst assemblages show that sea surface temperature is the primary environmental parameter influencing the distribution of both groups. Dinocyst assemblages also seem to respond to the distance to the coast, and may therefore bring additional information compared with coccolith assemblages. Other significant environmental factors include sea surface salinity and productivity, but their relative importance changes depending on the inclusion of samples from extreme environmental settings in the database. Results suggest that the complexity of the interrelationships between the various environmental parameters makes it difficult to adequately bring to light all the different environments and their associated coccolith/ dinocyst assemblages in multivariate analyses. However, each surface water mass represented in our database is characterized by a unique combination of environmental parameters as well as by distinct associations of coccolith and dinocyst assemblages, thus showing that these microfossil groups closely relate to sea surface conditions, including temperature, salinity and productivity.
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Papers by Jacques Giraudeau