Late quaternary variations of elemental ratios (C/Si and N/Si) in diatom-bound organic matter from the Southern Ocean
The carbon cycle in the Southern Ocean is considered as a major factor controlling past atmospheric CO 2 concentration variations. However, accumulation rates of biogenic opal are not linearly related to carbon burial rates. Here, we show that it is possible to measure the carbon and nitrogen content of diatom-bound organic matter (%C diat and %N diat , respectively) and that the signals recorded do not appear to be analytical artifacts. Analyses of two cores from the Atlantic and Indian sectors of the Southern Ocean show that %C diat and %N diat change on glacial-interglacial cycles by 30-40% and 120-175%, respectively. Accordingly, C/N ratios vary between 3 and 7 on glacial-interglacial timescales. If changes recorded in the occluded organic matter are representative of the changes in the diatom bulk organic matter, this provides a new tool to document the carbon cycle in the Southern Ocean and to determine its role on past atmospheric pCO 2 variations. Laboratory experiments on diatom cultures are needed to validate the use of diatom organic bound C and N as a tracer of diatom physiology and of carbon export from surface waters. r 2002 Elsevier Science Ltd. All rights reserved. Re´sume´LeRe´sume´ Re´sume´Le cycle du carbone de l'Oc! ean Austral est certainement un des facteurs majeurs contr # olant les concentrations atmosph! eriques pass! ees en CO 2. Cependant, les taux d'accumulation de l'opale biog" ene ne sont pas lin! eairement li! es aux taux d'accumulation du carbone organique et ne peuvent donc pas # etre directement utilis! es en tant que traceur de la pal! eoproductivit! e. Nous montrons ici qu'il est possible de mesurer les teneurs en carbone et azote dans la mati" ere organique intrins" eque des diatom! ees (%C diat and %N diat respectivement), et que les signaux fossiles ne sont pas le r! esultat d'artefacts analytiques. L'analyse de deux carottes provenant des secteurs Atlantique et Indien de l'Oc! ean Austral indique que les %C diat and %N diat ont respectivement vari! e de 30-40% et de 120-175% au cours des cycles glaciaires-interglaciaires. Les rapports C/N ont en cons! equence vari! e de 3 " a 7 au cours des cycles glaciaires-interglaciaires. Si les variations enregistr! ees dans la mati" ere organique intrins" eque sont repr! esentatives des changements du bulk des diatom! ees, nous aurons un nouvel outil pour documenter le cycle oc! eanique du carbone et pour d! eterminer son r # ole dans les variations pass! ees de pCO 2 atmosph! eriques. Des exp! eriences en laboratoire sur des cultures de diatom! ees sont cependant n! ecessaires afin de valider l'outil %C diat and %N diat comme traceur des conditions physiologiques des diatom! ees et des flux de carbone export! es depuis la couche euphotique.
(Deep Sea Research Part II: Topical Studies in Oceanography. vol. 49, n° 0967-0645, pp. 1939 - 1952, 26/05/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Winter dynamics on the northern Portuguese shelf. Part 2: bottom boundary layers and sediment dispersal
The northern Portuguese shelf, between 41°N and 42°N, is characterised by the presence of a mid-shelf mud deposit, the Douro Mud Patch (DMP). Observations conducted between July 1996 and June 1999 under the framework of project OMEX II-II, are used to examine the impact of the dynamic processes in the lower water column, particularly on the DMP. The typical wave conditions observed during the winter maintain a highly energetic environment, capable of promoting the erosion of fine sediments at mid-shelf depths. However, the bottom nepheloid layers generated beneath these waves only extend a few meters above the bottom and are contained within rocky outcrops with similar heights that fringe the outer shelf. Each year there are about ten storms, mostly associated with southerly winds that create downwelling conditions over the shelf. The waves associated with these storms produce shear velocities over 3 cm/s at mid-shelf and bottom nepheloid layers which extend a few tens of meters above the bottom and spread offshore, over the outer shelf and upper slope. A rough estimate suggests that these events account for an offshore export of about 20×10 6 kg of fine sediments each year (equivalent to 1-2% of sediments trapped at DMP).
(Progress in Oceanography. vol. 52, n° 0079-6611, pp. 155-170, 26/05/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Present day sedimentary processes on the northern Iberian shelf
The Galicia-Minho Shelf features two large mud patches, the Douro and the Galicia Mud patches. These are recent sediment bodies that have accumulated under a combination of conditions including: (1) abundant supplies of sediment; (2) morphological barriers that act as sediment traps; and (3) hydrographic conditions that favour the accumulation of fine sediment in these sinks. This paper describes the mechanisms controlling the deposition of the fine-grained sediment depositions and the processes that result in resuspension processes on the Galicia-Minho Shelf. Fine-grained sediments are provided from discharges from the river basins on the southern sector of the shelf, mainly the Douro and Minho rivers. Sediments are exported from river estuaries onto the shelf during episodic flood events. In contrast, most of the sediments originating from the Galician hinterland fail to contribute significantly to sedimentation on the shelf, because they are retained in the Galician Rías, which function as sediment traps. Sediments deposited on the shelf are frequently remobilized, particularly during southwesterly storms that coincide with downwelling conditions. Once in suspension the fine-grained sediments are transported northwards by the poleward flowing bottom currents and are eventually deposited on the Douro and Galicia Mud patches after a series of resuspension events. The locations of the two mud patches are strongly influenced by the shelf morphology. Fines already deposited on the mud patches are occasionally reintroduced into the system by large storm events. Some material from the Douro Mud patch and adjacent areas is re-deposited in the Galicia Mud patch. It is probable that sediments re-suspended from the Galicia Mud patch are carried off the shelf when storm events coincide with downwelling conditions.
(Progress in Oceanography. vol. 52, n° 0079-6611, pp. 249-259, 26/05/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Reconstitution paléocéanographique du signal d15N : modélisation couplée physique et biogéochimique d'un Upwelling côtier.
Afin de reconstruire le signal isotopique sédimentaire dans un contexte d'upwelling côtier, nous avons développé un modèle couplé physique et biogéochimique pouvant prendre en compte le fractionnement des isotopes de l'azote au sein d'un réseau trophique. Le modèle biologique est un modèle NPZD (Nutriments, Phytoplancton, Zooplancton, Détritus) qui prend en compte le fractionnement des isotopes de l'azote lors de l'assimilation des nutriments et lors de l'excrétion du zooplancton. Le modèle physique POM (Princeton Ocean Model) simule la circulation de l'upwelling côtier. Appliqué pour différentes situations de niveaux marins, ce modèle couplé nous permet de reproduire les signaux sédimentaires d15N et flux d'azote organique et d'en tirer des conclusions sur les scénarios paléocéanographiques. Au large du Cap Blanc, Mauritanie, l'effet de la montée du niveau marin, par l'immersion de la plate forme continentale, semble être le principal facteur responsable lors de la dernière déglaciation des variations de flux d'azote organique et du signal isotopique au niveau des sédiments. Ceci nous permet de proposer que le bilan d'azote océanique au cours de la dernière déglaciation ait pu être plus équilibré et stable qu'initialement prévu. Cet effet s'accompagne d'une période de fonctionnement de l'upwelling qui aurait pu être plus longue au Dernier Maximum Glaciaire, aux environs de 10 mois, contre 4-5 mois pour l'actuel. Ce modèle a aussi été appliqué au système du Benguela (Afrique du Sud Ouest) pour étudier certains processus de recyclage.
(20/12/2001)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UB
Downward particle fluxes within different productivity regimes off the Mauritanian upwelling zone (EUMELI program)
A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of ∼71–78% biogenic origin (i.e. consisting of CaCO3, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18°30′N, 21°00′W) to the remote, open-ocean, oligotrophic site (21°00′N, 31°00′W). This decrease largely reflects the difference in total primary production between the two sites, from ∼260 to ∼110 g organic C m−2 yr−1. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site.
(Deep Sea Research Part I: Oceanographic Research Papers. vol. 48, n° 0967-0637, pp. 2251-2282, 01/10/2001)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LDEO, LEGOS, IRD, UT3, Comue de Toulouse, INSU - CNRS, CNES, CNRS, LPCM, UPMC, CNRS, LEP, EEP, IFREMER, CEREGE, IRD, AMU, CdF (institution), INSU - CNRS, CNRS, INRAE, OCEANIS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
The “Pyla 2001” experiment: Flying the new RAMSES P-band SAR facility
(20/09/2001)
OASU, UB, INSU - CNRS, ULR, CNRS, INRAE, LPI, BRGM, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, ITI, UEB, IMT, IMT Atlantique - ITI, IMT Atlantique, IMT, UMR ISPA, INRA, Bordeaux Sciences Agro, CETP, UVSQ, INSU - CNRS, CNRS, UBM, IRAMAT-CRP2A, IRAMAT, UTBM, UO, UBM, CNRS, CDGA, M.E.N.E.S.R., INRA, IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS
Abrupt Cooling of Antarctic Surface Waters and Sea Ice Expansion in the South Atlantic Sector of the Southern Ocean at 5000 cal yr B.P.
Antarctic surface waters were warm and ice free between 10,000 and 5000 cal yr B.P., as judged from ice-rafted debris and micro-fossils in a piston core at 53 • S in the South Atlantic. This evidence shows that about 5000 cal yr B.P., sea surface temperatures cooled, sea ice advanced, and the delivery of ice-rafted detri-tus (IRD) to the subantarctic South Atlantic increased abruptly. These changes mark the end of the Hypsithermal and onset of Neoglacial conditions. They coincide with an early Neoglacial advance of mountain glaciers in South America and New Zealand between 5400 and 4900 cal yr B.P., rapid middle Holocene climate changes inferred from the Taylor Dome Ice Core (Antarctica), cooling and increased IRD in the North Atlantic, and the end of the African humid period. The near synchrony and abruptness of all these climate changes suggest links among the tropics and both poles that involved nonlinear response to gradual changes in Northern Hemisphere insolation. Sea ice expansion in the Southern Ocean may have provided positive feedback that hastened the end of the Hypsithermal and African humid periods in the middle Holocene. C 2001 University of Washington.
(Quaternary Research. vol. 56, n° 0033-5894, pp. 191-198, 01/09/2001)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Climatic changes in the upwelling region off Cap Blanc, NW Africa, over the last 70 kyear: a multi-biomarker approach
(Organic Geochemistry. vol. 32, n° 0146-6380, pp. 981-990, 01/08/2001)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CEREGE, IRD, AMU, CdF (institution), INSU - CNRS, CNRS, INRAE, GEOTRAC, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
The timing of the last deglaciation in North Atlantic climate records
To determine the mechanisms governing the last deglaciation and the sequence of events that lead to deglaciation, it is important to obtain a temporal framework that applies to both continental and marine climate records. Radiocarbon dating has been widely used to derive calendar dates for marine sediments, but it rests on the assumption that the `apparent age' of surface water (the age of surface water relative to the atmosphere) has remained constant over time. Here we present new evidence for variation in the apparent age of surface water (or reservoir age) in the North Atlantic ocean north of 40°N over the past 20,000 years. In two cores we found apparent surface-water ages to be larger than those of today by 1,230 +/- 600 and 1,940 +/- 750 years at the end of the Heinrich 1 surge event (15,000 years BP) and by 820 +/- 430 to 1,010 +/- 340 years at the end of the Younger Dryas cold episode. During the warm Bølling-Allerød period, between these two periods of large reservoir ages, apparent surface-water ages were comparable to present values. Our results allow us to reconcile the chronologies from ice cores and the North Atlantic marine records over the entire deglaciation period. Moreover, the data imply that marine carbon dates from the North Atlantic north of 40°N will need to be corrected for these highly variable effects.
(Nature. vol. 412, n° 0028-0836, pp. 724-727, 01/08/2001)
PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CLIM, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Initiation of the European deglaciation as recorded in the northwestern Bay of Biscay slope environments (Meriadzek Terrace and Trevelyan Escarpment)
Three cores retrieved on the northwestern slope of the Bay of Biscay are described and discussed in the light of the European last deglaciation history. This integrated sedimentological and micropalaeontological study provides a detailed evolution scheme for the deep and sea-surface conditions of the Bay during the final deglacial step, with a direct link with the continental palaeoenvironments. As early as 15 ka 14C-BP, a European precursor melting event is recorded as a purge of the Channel and Irish Sea palaeoriver systems. ‘Pleni–Heinrich event conditions’ occurred in the Bay of Biscay between 14.4 and 13 ka 14C-BP with a typical Canadian signature only recorded at 14 ka 14C-BP, namely 1 ka later than the first evidence of melting of the British Ice-sheet. Our data demonstrate that, following Heinrich event 2, the Last Glacial Maximum was characterised by a gradual warming accompanied by, at least, two pulses of the North Atlantic Drift. These North Atlantic Drift/heat northward penetrations are supposed to have primarily forced the Heinrich event 1 collapse.
(Earth and Planetary Science Letters. vol. 188, n° 0012-821X, pp. 493-507, 01/06/2001)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS