Chapter Eight Diatoms: From Micropaleontology to Isotope Geochemistry
(pp. 327-369, 15/06/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)
We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, varied in phase with water height and was highest at low tide. Stable isotope data of POC and DOC displayed large variations in both pools, and similarly followed the variations in water height. Although the variation of d13CDOC (-23.8 to -13.8‰) was higher than that of d13CPOC (-26.2 to -20.5‰), due to the different end-member pool sizes, the d13C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, however, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH4, and partial pressure of CO2 (pCO2) were similarly governed by tidal influence and were up to 7- and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. When assuming that the high dissolved inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, d13C data on DIC indicate that the organic matter source for mineralization had a signature of -22.4‰. Hence, imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data demonstrate how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter transport are fundamentally different.
(Biogeosciences. vol. 4, n° 1726-4170, pp. 311-322, 12/06/2007)
VUB, NIOO-KNAW, ULiège, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Deglacial laminated facies on the NW European continental margin: The hydrographic significance of British-Irish Ice Sheet deglaciation and Fleuve Manche paleoriver discharges
We have compiled results obtained from four high sedimentation rate hemipelagic sequences from the Celtic sector of the NW European margin (NE Atlantic) to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles. We focus on periods characteristic of deglacial transitions. We adopt a multiproxy sedimentological, geochemical, and micropaleontological approach, applying a sampling resolution down to ten microns for specific intervals. The investigation demonstrates the relationships between the Bay of Biscay hydrography and the glacial/deglacial history of both the proximal British-Irish Ice Sheet (BIIS) and the western European continent. We identify recurrent phases of laminae deposition concurrent with major BIIS deglacial episodes in all the studied cores. Evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale. We discuss their impact at a global scale considering the present and past key location of the Bay of Biscay versus the Atlantic Meridional Overturning Circulation (AMOC).
(Geochemistry, Geophysics, Geosystems. vol. 8, pp. n/a-n/a, 01/06/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BIAF, UA, IFREMER
Low-latitude hydrological cycle and rapid climate changes during the last deglaciation
Sea surface temperature and oxygen isotopic records from two well-dated Indian Ocean cores covering the last deglaciation show the occurrence of two periods of increased salinity along the route of warm surface water transport from the Indian to the Atlantic Ocean, one between 18 and 14.5 ka and the other during the Younger Dryas. Our results imply that during these periods, salt accumulated in the tropical Atlantic, creating favorable conditions for an abrupt resumption of the thermohaline circulation and abrupt northern hemisphere warming. Furthermore, we suggest that the observed pattern of millennial climate variability during the last glacial and deglaciation resulted from the interaction between the relatively slow rhythm of expansion and decay of the northern hemisphere ice sheets, and El Nino-Southern Oscillation variability, through changes in the position of the Intertropical Convergence Zone. This interaction generated an oscillator with millennial time response that operated at times of sufficient northern hemisphere ice sheets extent.
(Geochemistry, Geophysics, Geosystems. vol. 8, pp. Q05N12, 30/05/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEREGE, IRD, INRA, AMU, CdF (institution), INSU - CNRS, CNRS, CAM
Nature and rates of fine-sedimentation on a mid-shelf: ‘‘La Grande Vasie` re'' (Bay of Biscay, France)
The study area, ‘‘La Grande Vasie` re'' (LGV), stretches out on the French Atlantic continental shelf (at ca. 100m water depth), along 250km from the Gle´ nan Islands at the north to the southwest of Rochebonne at the south. Box-cores were sampled in this mid-shelf area during four cruises in June 1995, and in April, June and September 2002. They were investigated using sedimentological approaches (X-radiographs and grain-size analyses) and radionuclide studies (210Pb geochronology and excess 234Th). The main results are: (1) the surficial sediments are generally organized into a decimetrescale fining up sequence which can be the result of extreme storms; (2) an upper mixing layer of 7–20 cm reflects an important biological benthic activity and/or the impact of fishing (i.e. trawlers); (3) a thin (i.e. a few mm) surficial mud-rich layer is the result of the present-day sedimentation; (4) an apparent annual sedimentation rate of 1–3mm is recorded in several loci of the study area. Some seasonal variations appear, corresponding to the deposition of fine material from April to September, and to the reworking and the re-suspension during the winter. This fine material is the result of the decantation of estuarine plumes, mainly the Loire and the Vilaine rivers, over the study area. LGV lies (1) under the influence of a winter-to-spring thermo-haline wedge that acts as a filter for the transfer of fine river-borne material to the slope and the open sea, and (2) below water depths where the mean swell action permits sedimentation, mainly in summer. From the point of view of the nature of its sediments, LGV is not a mud-belt, but a heterolithic and patchy sandy area that is submitted to increasing silting with environmental changes, on a seasonal-time scale.
(Continental Shelf Research. vol. 27, n° 0278-4343, pp. 2099–2115, 16/05/2007)
M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GM, IFREMER
Variations of the ACC-CDW during MIS3 traced by magnetic grain deposition in midlatitude South Indian Ocean cores: Connections with the northern hemisphere and with central Antarctica
[1] We examine the magnetic mineral deposition in three cores located at midlatitude sites in the South Indian Ocean, in an area where sediment eroded from the Kerguelen-Crozet plateau and transported by the marine currents, principally the Antarctic Circum Current, accumulates at a high sedimentation rate. We focus on Marine Isotopic Stage 3, characterized by large climatic fluctuations at northern latitudes, and compare the obtained records to the climatic records at Byrd (Antarctica) and Summit (GISP2, Greenland) and to the North Atlantic Deep Water variations in the North Atlantic. Magnetic mineral deposition at the studied sites exhibits a profile with maxima at the time of Heinrich events H4 and H5, which suggests a strong Antarctic Circum Current when the North Atlantic Deep Water was reduced at northern latitude during these events. We show that an interhemispheric seesaw, characterized by temporary surface warmings in the southern hemisphere, is also marked by a strong Antarctic Circum Current at the time of the northern Heinrich events. The rapid Dansgaard-Oeschger oscillations evidenced in the northern records after interstadials IS12 and IS8 are not visible, suggesting a limited counterpart of these events in the midlatitude southern Indian Ocean.
(Geochemistry, Geophysics, Geosystems. vol. 8, pp. n/a-n/a, 01/05/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CLIMAG, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Effect of natural iron fertilization on carbon sequestration in the Southern Ocean
The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles1,2,3,4,5. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments6,7. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales8. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments7. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic9,10 and future climate change scenarios11—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.
(Nature. vol. 446, n° 0028-0836, pp. 1070-1075, 26/04/2007)
LOB, INSU - CNRS, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, ICOS-RAMCES, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, OCEANIS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, ACE-CRC, ACROSS, UTAS, LOCEAN, IPSL, ENS-PSL, PSL, UVSQ, UPMC, CEA, INSU - CNRS, X, IP Paris, CNES, CNRS, MNHN, IRD, UPMC, INSU - CNRS, CNRS, NIOZ, ELICO, ULCO, CNRS, ULCO, COM, INSU - CNRS, CNRS, CSIRO, CSIRO, LMGEM, CNRS, INSU - CNRS, VUB, LEGOS, IRD, UT3, Comue de Toulouse, INSU - CNRS, CNES, CNRS, LOV, OOVM, UPMC, INSU - CNRS, CNRS, UPMC, INSU - CNRS, CNRS, LOBB, OOB, UPMC, CNRS, UPMC, INSU - CNRS, CNRS, LEMAR, IRD, IFREMER, UBO EPE, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CNRS, LGE, UPD7, IPG Paris
Common Pattern of Gene Expression in Response to Hypoxia or Cadmium in the Gills of the European Glass Eel ( Anguilla anguilla )
European eel (Anguilla anguilla) populations are in decline. Glass eel recruitment has fallen 10-fold since the early 1980s. Estuaries play a fundamental role in the life history of eels because glass eels must pass through them to reach freshwater ecosystems. Unfortunately, because of their geographical position at the upstream basin slopes, estuaries accumulate metals like cadmium and are important sites of hypoxia events. In this context, we studied the effect of the oxygen level on the ventilation of the glass eel. In parallel, glass eels were submitted to different dissolved cadmium concentrations (0, 2, and 10 microg L(-1)) under two oxygen levels (normoxia PO2 = 21 kPa and Hypoxia PO2 = 6 kPa). The expression level of various genes involved in the mitochondrial respiratory chain, in the cellular response to metal and oxidative stresses, was investigated. Our results showed that hypoxia enhances (1) ventilation of the postlarval stage and (2) Cd accumulation in gills only at the lowest metal water concentration tested (2 microg Cd L(-1)). At the gene level, Cd exposure mimics the effect of hypoxia since we observed a decrease in expression of genes involved in the respiratory chain and in the defense against oxidative stress.
(Environmental Science and Technology. vol. 41, n° 0013-936X, pp. 3005-3011, 01/04/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Biogeochemical and contaminant cycling in sediments from a human-impacted coastal lagoon â Introduction and summary
From 2001 to 2003, the Microbent project (ââBiogeochemical processes at the water sediment interface in eutrophic environmentââ) was carried out within the framework of the Programme National Environnement CoËtier, the French contribution to LandeOcean Interaction in the Coastal Zone (LOICZ). The Microbent programme was focused on the study of sediment biogeochemical cycles of carbon, oxygen, sulphur, iron, nitrogen, and phosphorus in relation to the faunal activity in the sediment and their relation with the mobility of metallic contaminants at the sedimentewater interface (SWI) in a Mediterranean coastal lagoon (Thau lagoon, France; Fig. 1). The aim of Microbent was to set up an interdisciplinary study bringing together geochemists, sedimentologists, and biologists in order to understand and quantify the main reaction pathways, and the fluxes of contaminants at the SWI, including those related to benthic fauna. Work was focused on the processes which generate contaminant fluxes: (1) early diagenetic processes, which generate the chemical conditions of the environment; (2) processes leading to the transfer of contaminants from particles toward biofilms, water column, and organisms; and (3) processes of sediment mixing by organisms and sediment accumulation.
(Estuarine, Coastal and Shelf Science. vol. vol. 72, n° 0272-7714, pp. pp. 387-392, 01/04/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, OCEANIS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LCABIE, UPPA, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LMGEM, CNRS, BE, IFREMER, LGE, UPD7, IPG Paris
The origin of the 1500-year climate cycles in Holocene North-Atlantic records
Since the first suggestion of 1500-year cycles in the advance and retreat of glaciers (Denton and Karlen, 1973), many studies have uncovered evidence of repeated climate oscillations of 2500, 1500, and 1000 years. During last glacial period, natural climate cycles of 1500 years appear to be persistent (Bond and Lotti, 1995) and remarkably regular (Mayewski et al., 1997; Rahmstorf, 2003), yet the origin of this pacing during the Holocene remains a mystery (Rahmstorf, 2003), making it one of the outstanding puzzles of climate variability. Solar variability is often considered likely to be responsible for such cyclicities, but the evidence for solar forcing is difficult to evaluate within available data series due to the shortcomings of conventional time-series analyses. However, the wavelets analysis method is appropriate when considering non-stationary variability. Here we show by the use of wavelets analysis that it is possible to distinguish solar forcing of 1000- and 2500- year oscillations from oceanic forcing of 1500-year cycles. Using this method, the relative contribution of solar-related and ocean-related climate influences can be distinguished throughout the 10 000 Holocene intervals since the last ice age. These results reveal that the mysteriously regular 1,500-year climate cycles are linked with the oceanic circulation and not with variations in solar output as previously argued (Bond et al., 2001). In this light, previously studied marine sediment (Bianchi and McCave, 1999; Giraudeau et al., 2000), ice core (O'Brien et al., 1995) and dust records (Jackson et al., 2005) can be seen to contain the evidence of combined forcing mechanisms, whose relative influences varied during the course of the Holocene. Circum-Atlantic climate records cannot be explained by solar forcing, but require changes in ocean circulation, as suggested previously (Broecker et al., 2001; McManus et al., 1999).
(Climate of the Past Discussions [Climate of the Past Preprints]. vol. 3, n° 1814-9340, pp. 679-692, 26/03/2007)
LGGE, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, INSU - CNRS, CNRS, PBDS, INSU - CNRS, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, EDYTEM, USMB [Université de Savoie] [Université de Chambéry], CNRS, Fédération OSUG, WHOI, M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS