Silicic acid leakage from the Southern Ocean: Opposing effects of nutrient uptake and oceanic circulation
(Geophysical Research Letters. vol. 34, n° 0094-8276, pp. n/a-n/a, 16/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Statistical process control in assessing production and dissolution rates of biogenic silica in marine environments
This paper provides pieces of advice on the practices of quality assurance and quality control in assessing production and dissolution rates of biogenic silica in marine waters with stable isotope techniques. The objective is to make a rigorous contribution to the interpretation of 30Si isotopic measurements including modelling and uncertainty analyses. The results are illustrated with real data taken from Beucher et al. [Beucher, C., Tréguer, P., Corvaisier, R., Hapette, A/-M., Elskens, M., 2004a. Production and dissolution of biogenic silica in a coastal ecosystem of western Europe. Marine Ecology Progress Series 267:57-69.]. Prior to the flux rate assessment, there are a number of analytical considerations required for screening between optimal and defective experimental conditions. Three indexes were proposed to check the relevance of underlying assumptions when dealing with 30Si tracer enrichment and dilution techniques. Afterwards for extracting rate values from measurements, it is necessary to postulate a model, and if required an optimization method. Various models and formulae were compared for their precision and accuracy. It was shown that oversimplified models risk bias when their underlying assumptions are violated, but overly complex models can misinterpret part of the random noise as relevant processes. Therefore, none of the solutions can a priori be rejected, but each should statistically be assessed with hypothesis testing. A weighted least squares regression strategy combining an analysis of the standardized residuals and cost function (sum of the weighted least squares residuals) was used to select optimal solution subsets corresponding to a given data set, i.e. the solution that uses the most relevant processes and which was tested for the presence of outliers (observations or measurements with undue influence in the flux rate assessment).
(Marine Chemistry. vol. 106, n° 0304-4203, pp. 272-286, 11/07/2007)
VUB, LEMAR, IRD, IFREMER, UBO EPE, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Messinian event in the black sea: Evidence of a Messinian erosional surface
In 1975, sediment cores from leg DSDP 42b (sites 380A and 381) revealed a thin sediment layer in the Black Sea basin which points to a shallow water environment at the Miocene-Pliocene boundary. With these facts and in the wake of hypothesis of the Messinian Salinity Crisis (MSC), it was proposed that the Black Sea, like the Mediterranean Sea, suffered a desiccation period at the end of the Messinian (Hsü, K.J. and Giovanoli, F., 1979. Messinian event in the Black Sea. Palaeogeography, Palaeoclimatology, Palaeoecology, 29: 75–93). Whereas the main topics of the MSC in the Mediterranean Sea is now widely accepted, the lack of evidence for a Messinian erosional surface in the Black Sea left the debate about the Messinian desiccation of this basin open until today. The analysis of high resolution multi-channel seismic data acquired during the BlaSON surveys brings important new elements for this scientific debate: (1) Down the slope offshore the Bosporus, we show a clear erosional surface correlated to the top of the Upper Miocene shallow water environment unit of site DSDP 381. The overlying Lower Zanclean unit inevitably dates this erosional surface of the Messinian event. (2) A wide intra-Pontian erosional surface (IPU) is evidenced on the Romanian shelf. The IPU is characterized by a sharp decrease in the incision rate from outer (deep canyons) to inner shelf (superficial incisions network). According to the most recent Paratethyan and Mediterranean stratigraphic scale correlations, the IPU erosional surface is considered as the analogue to the Messinian erosional surface described down the slope offshore the Bosporus. In addition to recently discovered inland erosional signature, the wide regional erosional surface we underline on the Western Black Sea margins validates the Black Sea Messinian desiccation hypothesis. We also demonstrate that evaporative draw-down of the Black Sea implied the installation in the basin of a negative hydrologic budget during the Messinian. The lack of a major Messinian Danube canyon on the Romanian shelf supports the hypothesis of a Messinian Danube trapped in the Dacic basin. However, the presence of a Messinian superficial incision network connecting the location of the modern Danube delta to deeply incised Messinian canyons on the outer shelf (IPU) makes the hypothesis of a Danube reaching the partially desiccated Black Sea still possible.
(Marine Geology. vol. 244, n° 0025-3227, pp. 142-165, 05/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GM, IFREMER, LDO, INSU - CNRS, UBO EPE, CNRS
Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O2, Mn, Fe and N-Species in Experimental Units
Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O2, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O2 from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn2+ and then Fe2+, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe2+. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO3−/NO2− rich horizon at the new oxic horizon. Over the experiment period, NO3− concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.
(Aquatic Geochemistry. vol. 13, n° 1380-6165, pp. 157-172, 04/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins
The deep, northern, part of the Sea of Marmara (northwestern Turkey) is composed of several aligned, actively subsiding, basins, which are the direct structural and morphological expression of the North Anatolian Fault's northern branch. The last 20 kyr of their sedimentary fill (lacustrine before 12 kyr BP) have been investigated through giant piston coring onboard R/V MARION-DUFRESNE (MARMACORE Cruise, 2001) and chirp subbottom profiler onboard R/V ATALANTE during MARMARASCARPS Cruise (2002). Especially during the lacustrine stage, the infilling of the deep basins (Tekirda?, Central, Kumburgaz, and Çinarcic Basins; up to 1250 m depth) was dominated by turbidites (with coarse mixed siliciclastic and bioclastic basal part), intercalated in “hemipelagic-type” finegrained calcareous and slightly siliceous clays. Often – especially in the thickest ones – the turbidites show strong segregation and a sharp boundary between coarse part and suspendedload part. In the Central Basin, 8 m of a unique sedimentary event include a 5 to 8m-thick “homogenite” well imaged on seismic profiles. The latter is interpreted as related to a major – possibly triggered - tsunami effect, as described in the Eastern Mediterranean by Kastens and Cita (1981). In the marine (Holocene) upper part of the sedimentary fill, repeated to-and-from structures, affecting silt or fine sand, are evidencing seiche-like effects and, thus, earthquake triggering. Detailed correlations between two deep coring sites (1250 and 1200 m) indicate more than 100 % overthickening in the deepest one; this implies specific processes of distribution of terrigenous input by dense hyperpycnal currents (high kinetic energy, seiche effect, complex reflections on steep slopes). The peculiar sedimentary infilling of the Sea of Marmara's Central Basin (and, by extrapolation, of the whole set) is tentatively interpreted as a direct consequence of the strong seismic activity; the imprint of the latter is more obvious prior to the base of the Holocene, as environmental conditions favoured marginal accumulation (especially on the southern shelf) of large amounts of erosion products available for mass wasting.
(Sedimentary Geology. vol. 199, n° 0037-0738, pp. 69-85, 01/07/2007)
LGCA, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, CNRS, GEOAZUR 6526, IRD, UPMC, UNS, INSU - CNRS, UniCA, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GEOLOGY AND MINING DEPARTMENT, ITÜ, LDEO, CEREGE, IRD, INRA, AMU, CdF (institution), INSU - CNRS, CNRS, IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA
A New Morphodynamic Modelling Platform: Application to Characteristic Sandy systems of the Aquitanian Coast, France
Along coasts, waves and wave-induced currents are the main factors of morphological evolution. A morphodynamic model is constructed to take into account tide changes, wind conditions and waves in the computation of the induced currents and morphological evolution. The spectral wave model SWAN, the shallow-water model MARS and a sedimentary module are coupled to create the morphodynamic model. First, we validate the hydrodynamics of the model on two characteristic complex bathymetries: an idealised subtidal crescentic bar and an intertidal ridge and runnel system. The crescentic bar induces wave energy focalisation zones which could give rise to transverse bars. Thus, we investigate the morphology evolution of the intertidal area. Simulations appear to show the formation of inner bars that connect the subtidal bar with the intertidal area
(Journal of Coastal Research. vol. 50, n° 0749-0208, 30/06/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM, DYNECO, IFREMER
Anomalous flow below 2700 m in the EPICA Dome C ice core detected using d18O of atmospheric oxygen measurements
While there are no indications of mixing back to 800 000 years in the EPICA Dome C ice core record, comparison with marine sediment records shows significant differences in the timing and duration of events prior to stage 11 (~430 ka, thousands of years before 1950). A relationship between the isotopic composition of atmospheric oxygen (d18O of O2, noted d18Oatm) and daily northern hemisphere summer insolation has been observed for the youngest four climate cycles. Here we use this relationship with new d18O of O2 measurements to show that anomalous flow in the bottom 500 m of the core distorts the duration of events by up to a factor of 2. By tuning d18Oatm to orbital precession we derive a corrected thinning function and present a revised age scale for the interval corresponding to Marine Isotope Stages 11–20 in the EPICA Dome C ice core. Uncertainty in the phasing of d18Oatm with respect to insolation variations in the precession band limits the accuracy of this new agescale to ±6 kyr (thousand of years). The previously reported ~30 kyr duration of interglacial stage 11 is unchanged. In contrast, the duration of stage 15.1 is reduced by a factor of 2, from 31 to 16 kyr.
(Climate of the Past. vol. 3, n° 1814-9324, pp. 341-353, 21/06/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LGGE, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, INSU - CNRS, CNRS, NBI, UCPH, GLACCIOS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, UNIBE, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEP, UNIBE, OCCR, UNIBE
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