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, 24/08/2026)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores
The last glacial to interglacial transition was studied using down core records of stable isotopes in diatoms and foraminifera as well as surface water temperature, sea ice extent, and ice-rafted debris (IRD) concentrations from a piston core retrieved from the Atlantic sector of the Southern Ocean. Sea ice is the first variable to change during the last deglaciation, followed by nutrient proxies and sea surface temperature. This sequence of events is independent of the age model adopted for the core. The comparison of the marine records to Antarctic ice CO 2 variation depends on the age model as 14 C determinations cannot be obtained for the time interval of 29.5-14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO 2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO 2. If gas exchange played a major role in determining glacial to interglacial CO 2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO 2 change must be sought elsewhere, rather than in the Southern Ocean.
(Paleoceanography. vol. 17, n° 0883-8305, pp. 8-1-8-7, 24/08/2026)
UF, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UC San Diego, UC, LLNL
Sources of fine-sized organic matter in North Atlantic Heinrich Layers: δ 13C and δ 15N tracers
Organic carbon (OC) and total nitrogen (TN) concentrations and stable isotope ratios (δ 13C, δ 15N) of fine (<50 μm) size fractions of deep-sea sediments from the central North Atlantic were employed to identify changes in sources of organic matter over the past 50 ka BP. Ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs (averages ± 1σ: OC/TN = 7.6 ± 0.8; δ 13C = -22.8 ± 1.0‰; δ 15N = 5.5 ± 0.6‰). δ 13C, OC, and TN concentrations shift to higher values during the Holocene, indicating a gradual decrease of fine terrigenous supply to the North Atlantic. The unchanged δ 15N record between last glacial and Holocene stages indicates that the central North Atlantic region remained oligotrophic at least during the past 50 ka BP, but additional studies are required to support this result in terms of nitrogen oceanic budget. During the phases of enhanced ice-rafted detrital supply corresponding to prominent Heinrich events (HL 1, HL 2, HL 4, and HL 5), fine-sized sedimentary organic matter has lower OC and TN concentrations, contrasting sharply with those of ambient glacial sediments. Lower δ 13C (down to -28‰) and δ 15N (down to 1.6‰) values and high OC:TN ratios (up to 14.7 ± 1.1) are found for HL 1, HL 2, and with lesser extent for HL 4. These values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter-bearing rock sources of the underlying geological basement. During HL 5, only the δ 13C offset records the input of fine size ice-rafted organic matter. Gradually changing soil development conditions during the time interval covering HL 5 to HL 1 (marine isotope stages 5 to 2), as well as varying erosion levels, have been hypothesized on the basis of constant δ 13C, increasing OC/TN and decreasing δ 15N values.
(Geochimica et Cosmochimica Acta. vol. 66, n° 0016-7037, pp. 223-239, 24/08/2026)
iEES, INRA, UPMC, UPEC UP12, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Isotopic evidence of methane-related diagenesis in the mud volcanic sediments of the Barbados Accretionary Prism
Extensive sampling of chosen sectors of the Barbados Accretionary Prism during four French oceanographic cruises (1985-1993) evidenced the presence of abundant diagenetic carbonate deposits associated mostly with zones of active or ancient fluid venting and mud expulsion processes (mud volcanoes, mud diapirs, diapiric ridges). Diagenetic carbonates (low- and high-Mg calcite, aragonite and low-Mg dolomite) are found to bind together dead benthic communities associated to fluid venting and to cover extensive areas of the sea floor where complete lithification of both autochtonous deposits and mud volcanic products occurs. Extreme cases of early diagenesis occur on mud dome structures on the summit of the ridge where the expelled sediments have been entirely lithified for a thickness of several metres. Low δ13C values of most dolomites (down to -60.2‰) and of most calcites and aragonites (down to -53.07‰) show the major contribution of oxidized methane to the dissolved inorganic carbon in the water from which these minerals precipitated. In contrast, a number of authigenic carbonates have δ13C values close to 0‰ and possibly precipitated from bottom waters. Unusually high δ18O values of the methane-related authigenic carbonates (up to 7.66‰ in dolomites and up to 6.98‰ in calcites and aragonites) are interpreted as due to precipitation from 18O-rich diagenetic fluids.
(Continental Shelf Research. vol. 22, n° 0278-4343, pp. 2355-2372, 24/08/2026)
LODYC, IRD, UPMC, CNRS, MNHN, 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