Éditeur(s) : Elsevier Science Bv
Résumé : For many trace elements, continental margins are the location of intense exchange processes between sediment and seawater, which control their distribution in the water column, but have yet to be fully understood. In this study, we have investigated the impact of fluid seepage at cold seeps on the marine cycle of neodymium. We determined dissolved and total dissolvable (TD) concentrations for REE and well-established tracers of fluid seepage (CH4, TDFe, TDMn), and Nd isotopic compositions in seawater samples collected above cold seeps and a reference site (i.e. away from any fluid venting area) from the Niger Delta margin. We also analyzed cold seep authigenic phases and various core-top sediment fractions (pore water, detrital component, easily leachable phases, uncleaned foraminifera) recovered near the hydrocast stations. Methane, TDFe and TDMn concentrations clearly indicate active fluid venting at the studied seeps, with plumes rising up to about 100 m above the seafloor. Depth profiles show pronounced REE enrichments in the non-filtered samples (TD concentrations) within plumes, whereas filtered samples (dissolved concentrations) exhibit slight REE depletion in plumes relative to the overlying water column and display typical seawater REE patterns. These results suggest that the net flux of REE emitted into seawater at cold seeps is controlled by the presence of particulate phases, most probably Fe-Mn oxyhydroxides associated to resuspended sediments. At the reference site, however, our data reveal significant enrichment for dissolved REE in bottom waters, that clearly relates to diffusive benthic fluxes from surface sediments. Neodymium isotopic ratios measured in the water column range from epsilon(Nd) similar to-15.7 to -10.4. Evidence that the epsilon(Nd) values for Antarctic Intermediate waters (AAIW) differed from those reported for the same water mass at open ocean settings shows that sediment/water interactions take place in the Gulf of Guinea. At each site, however, the bottom water epsilon(Nd) signature generally differs from that for cold seep minerals, easily leachable sediment phases, and detrital fractions from local sediments, ruling out the possibility that seepage of methane-rich fluids and sediment dissolution act as a substantial source of dissolved Nd to seawater in the Gulf of Guinea. Taken together, our data hence suggest that co-precipitation of Fe-Mn oxyhydroxide phases in sub-surface sediments leads to quantitative scavenging of dissolved REE at cold seeps, preventing their emission into bottom waters. Most probably, it is likely that diffusion from suboxic surface sediments dominates the exchange processes affecting the marine Nd cycle at the Niger Delta margin. (C) 2011 Elsevier B.V. All rights reserved.
Earth And Planetary Science Letters (0012-821X) (Elsevier Science Bv), 2011-12 , Vol. 312 , N. 3-4 , P. 443-452

Droits : 2011 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00066/17754/15571.pdf
DOI:10.1016/j.epsl.2011.10.008
http://archimer.ifremer.fr/doc/00066/17754/

Éditeur(s) : Copernicus Gesellschaft Mbh
Résumé : Boundary Exchange (BE - exchange of elements between continental margins and the open ocean) has been emphasized as a key process in the oceanic cycle of neodymium (Nd) (Lacan and Jeandel, 2005a). Here, we use a regional eddy-permitting resolution Ocean General Circulation Model (1/4A degrees) of the North Atlantic basin to simulate the distribution of the Nd isotopic composition, considering BE as the only source. Results show good agreement with the data, confirming previous results obtained using the same parameterization of the source in a coarse resolution global model (Arsouze et al., 2007), and therefore the major control played by the BE processes in the Nd cycle on the regional scale. We quantified the exchange rate of the BE, and found that the time needed for the continental margins to significantly imprint the chemical composition of the surrounding seawater (further referred as characteristic exchange time) is of the order of 0.2 years. However, the timescale of the BE may be subject to large variations as a very short exchange time (a few days) is needed to reproduce the highly negative values of surface waters in the Labrador Sea, whereas a longer one (up to 0.5 years) is required to simulate the radiogenic influence of basaltic margins and distinguish the negative isotopic signatures of North Atlantic Deep Water from the more radiogenic southern origin water masses. This likely represents geographical variations in erosion fluxes and the subsequent particle load onto the continental margins. Although the parameterization of the BE is the same in both configurations of the model, the characteristic exchange time in the eddy-permitting configuration is significantly lower than the previous evaluations using a low resolution configuration (6 months to 10 years), but however in agreement with the available seawater Nd isotope data. This results highlights the importance of the model dynamics in simulating the BE process.
Ocean Science (1812-0784) (Copernicus Gesellschaft Mbh), 2010 , Vol. 6 , N. 3 , P. 789-797

Droits : Author(s) 2010. This work is distributed, EGU
http://archimer.ifremer.fr/doc/00013/12412/9201.pdf
DOI:10.5194/os-6-789-2010
http://archimer.ifremer.fr/doc/00013/12412/

Éditeur(s) : Pergamon-elsevier Science Ltd
Résumé : We report rare earth element (REE) and neodymium (Nd) isotope data for the organic fraction of sediments collected from various depositional environments, i.e. rivers (n=25), estuaries (n=18), open-ocean settings (n=15), and cold seeps (n=12). Sedimentary Organic Matter (SOM) was extracted using a mixed hydrogen peroxide/nitric acid solution (20%-H2O2 – 0.02M-HNO3), after removal of carbonate and oxy-hydroxide phases with dilute hydrochloric acid (0.25M-HCl). A series of experimental tests indicate that extraction of sedimentary organic compounds using H202 may be complicated occasionally by partial dissolution of sulphide minerals and residual carbonates. However, this contamination is expected to be minor for REE because measured concentrations in H2O2 leachates are about two-orders of magnitude higher than in the above mentioned phases. The mean REE concentrations determined in the H2O2 leachates for samples from rivers, estuaries, coastal seas and open-ocean settings yield relatively similar levels, with ΣREE = 109 ± 86 ppm (mean ± s; n=58). The organic fractions leached from cold seep sediments display even higher concentration levels (285 ± 150 ppm; mean ± s; n=12). The H2O2 leachates for most sediments exhibit remarkably similar shale-normalized REE patterns, all characterized by a mid-REE enrichment compared to the other REE. This suggests that the distribution of REE in leached sedimentary organic phases is controlled primarily by biogeochemical processes, rather than by the composition of the source from which they derive (e.g. pore, river or sea- water). The Nd isotopic compositions for organic phases leached from river sediments are very similar to those for the corresponding detrital fractions. In contrast, the SOM extracted from marine sediments display εNd values that typically range between the εNd signatures for terrestrial organic matter (inferred from the analysis of the sedimentary detrital fractions) and marine organic matter (inferred from the analysis of local surface seawater). A notable exception is the case of organic matter (OM) fractions leached from cold seep sediment samples, which sometimes exhibit εNd values markedly different from both terrigenous and surface seawater signatures. This suggests that a significant fraction of organic compounds in these sediments may be derived from chemosynthetic processes, recycling pore water REE characterized by a distinct isotopic composition. Overall, our results confirm that organic matter probably plays an important role in the oceanic REE budget, through direct scavenging and remineralization within the water column. Both the high REE abundances and the shape of shale-normalized patterns for leached SOM also suggest that OM degradation in sub-surface marine sediments during early diagenesis could control, to a large extent, the distribution of REE in pore waters. Benthic fluxes of organic-bound REE could hence substantially contribute to the exchange processes between particulates and seawater that take place at ocean margins. Neodymium isotopes could provide useful information for tracing the origin (terrestrial versus marine) and geographical provenance of organic matter, with potential applications in paleoceanography. In particular, future studies should further investigate the potential of Nd isotopes in organic compounds preserved in sedimentary records for reconstructing past variations of surface ocean circulation.
Geochimica Et Cosmochimica Acta (0016-7037) (Pergamon-elsevier Science Ltd), 2014-09 , Vol. 140 , P. 177-198

Droits : 2014 Elsevier Ltd. All rights reserved.
http://archimer.ifremer.fr/doc/00191/30250/28682.pdf
DOI:10.1016/j.gca.2014.05.016
http://archimer.ifremer.fr/doc/00191/30250/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Multiproxy analyses were performed on core MS27PT recovered in hemipelagic sediments deposited on the Nile margin in order to reconstruct Nile River palaeohydrological fluctuations during the last 100,000 years. The strontium and neodymium isotope composition of the terrigenous fraction and the major element distribution reveal large and abrupt changes in source, oscillating between a dominant aeolian Saharan contribution during arid periods and a dominant Nile River contribution during pluvial periods. Iron content shows a strong correlation with strontium and neodymium isotopes. This allows the use of a high-resolution continuous Fe record as a proxy of Blue Nile sediment input over the last 100,000 years. The detailed Fe record, with approximately 10 years resolution during pluvial periods, is consistent with subtropical African records of well-dated lake level fluctuations and thus constitutes a first continuous high resolution record of the East African monsoon regime intensity over Ethiopia.;The detailed Fe record shows the two main known pluvial periods attributed to strengthening of the African monsoon over Ethiopia, the Nabtian period from 14 to 8 ka cal BP and the Saharan period from 98 to 72 ka BR For the first time, the last glacial period (Marine Isotope Stage (MIS) 2,3 and 4) is documented with a continuous record showing large oscillations between high and low East African palaeo-monsoon regimes.;The end of the Nabtian period occurred at 8 ka in core MS27PT, i.e. much earlier than on the East Equatorial African region where it ended around 5.5 ka. We interpret this as evidence that the southward shift of the rain belt occurred 3000 years earlier over the Eastern Ethiopian Highland and propose that the gradual southward migration of the rain belt was associated with highly variable intensity and longer rainy seasons from 8 to 5 ka. During the last glacial period, two wet periods are present around 60-50 ka BP and 38-30 ka BP. These two humid periods are in phase with the rise of atmospheric CH4 concentrations suggesting that wetland tropical African area was one of the sources of the atmospheric CH4 during the MIS 3. During the Last Glacial Maximum and MIS 4, high Saharan aeolian influxes in phase with records of aeolian dust deposited in East Antarctica are documented. This study highlights the importance of reconstructions of monsoon rainfall fluctuation at high temporal resolution to better understand the link between low- and high-latitude climate variability at millennial timescales.
ISSN: 0277-3791

hal-00496481
https://hal.archives-ouvertes.fr/hal-00496481
DOI : 10.1016/j.quascirev.2010.02.006

Éditeur(s) : Elsevier Science Bv
Résumé : The breakup of Pangea and onset of growth of the Pacific plate led to several paleoenvironmental feedbacks, which radically affected paleoclimate and ocean chemistry during the Jurassic. Overall, this period was characterized by intense volcanic degassing from large igneous provinces and circum-Panthalassan arcs, new oceanic circulation patterns, and changes in heat and humidity transports affecting continental weathering. Few studies, however, have attempted to unravel the global interactions linking these processes over the long-term. In this paper, we address this question by documenting the global changes in continental drainage and surface oceanic circulation for the whole Jurassic period. For this purpose, we present 53 new neodymium isotope values (εNd(t)) measured on well-dated fossil fish teeth, ichthyosaur bones, phosphatized nodules, phosphatized ooids, and clastic sediments from Europe, western Russia, and North America. Combined with an extensive compilation of published εNd(t) data, our results show that the continental sources of Nd were very heterogeneous across the world. Volcanic inputs from a Jurassic equivalent of the modern Pacific Ring of Fire contributed to radiogenic εNd(t) values (− 4 ε-units) in the Panthalassa Ocean. For the Tethyan Ocean, the average surface seawater signal was less radiogenic in the equatorial region (− 6.3), and gradually lower toward the epicontinental peri-Tethyan (− 7.4), western Russian (− 7.4) and Euro-Boreal seas (− 8.6). Different Nd sources contributed to this disparity, with radiogenic Nd influxes from westward Panthalassan currents or juvenile volcanic arcs in open oceanic domains, and substantial unradiogenic inputs from old Laurasian and Gondwanan shields for the NW Tethyan platforms. Overall, the εNd(t) values of Euro-Boreal, peri-Tethyan, and western Russian waters varied quite similarly through time, in response to regional changes in oceanic circulation, paleoclimate, continental drainage, and volcanism. Three positive shifts in εNd(t) values occurred successively in these epicontinental seas during the Pliensbachian, in the Aalenian-Bathonian interval, and in the mid-Oxfordian. The first and third events are interpreted as regional incursions of warm surface radiogenic currents from low latitudes. The Aalenian-Bathonian shift seems linked to volcanic outbursts in the NW Tethys and/or circulation of deep currents resulting from extensional events in the Hispanic Corridor and reduced influences of boreal currents crossing the Viking Corridor. In contrast, the εNd(t) signals decreased and remained very low (< − 8) during the global warming events of the Toarcian and Late Oxfordian - Early Tithonian intervals. In these greenhouse contexts, a latitudinal expansion of humid belts could have extended the drainage pathways toward boreal Nd sources of Precambrian age and increased the supply of very unradiogenic crustal-derived inputs to seawater. Finally, a brief negative εNd(t) excursion recorded in parallel with regional drops in seawater temperature suggests that southward circulation of cold unradiogenic Arctic waters occurred in the NW Tethys in the Callovian - Early Oxfordian. All these results show that changes in surface oceanic circulation resulting from the Pangean breakup could have regionally impacted the evolution of seawater temperatures in the NW Tethys.
Gondwana Research (1342-937X) (Elsevier Science Bv), 2015-06 , Vol. 27 , N. 4 , P. 1599-1615

Droits : 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/00179/29015/27443.pdf
DOI:10.1016/j.gr.2014.02.006
http://archimer.ifremer.fr/doc/00179/29015/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Multi-proxy analysis of two marine sediment cores (MS27PT and MD04-2726) from the Nile continental slope provides evidence of changes in Nile sediment discharge related to changes in Ethiopian African Monsoon (EAM) precipitation, and allows us to reconstruct changes in Nile River runoff, vegetation and erosion in the Nile headwaters. Sediment element composition and neodymium isotopic composition reveal significant changes in clastic sediment provenance, with sources oscillating between a Saharan aeolian contribution during the Last Glacial Maximum/deglacial transition and during the Late Holocene, and a Blue/Atbara Nile fluvial contribution during the African Humid Period (AHP). This study provides a new understanding of past environmental changes. Between 14.6 and 14.13 ka there was a major input of sediments from the Ethiopian Highlands, consistent with a stronger EAM at that time. Climate in the Nile basin was wetter between 14.8 and 8.4 ka, with a corresponding increase in Blue Nile water and sediment discharge via the main Nile into the Eastern Mediterranean. The gradual climatic transition from the AHP to the present-day dry climate was reflected in a decrease in Blue Nile sediment deposition and flood discharge between 8.4 and 3.7 ka, with aridity at a maximum between 3.7 and 2.6 ka. The onset of drier conditions in the Blue Nile basin seems to have begun before the 8.2 ka cooling event in the North Atlantic. We speculate that the climatic change from the wet AHP to the dry late Holocene may have been a result of a break in the low latitude dynamic equilibrium between climate, vegetation and erosion, which may in turn have affected the climate in higher latitudes. Reduced Nile flow may also have had an impact on Levantine Intermediate Water originating in the Eastern Mediterranean through an increase in intermediate water formation.
ISSN: 0277-3791

hal-01277950
https://hal.archives-ouvertes.fr/hal-01277950
DOI : 10.1016/j.quascirev.2015.10.030

Éditeur(s) : Elsevier
Résumé : The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (epsilon(Nd)similar to - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by epsilon(Hf) values (from -1.1 to +13) far more radiogenic than associated sediments (from -7.1 to -12.0) and turbidite sands (from -27.2 to -31.6). epsilon(Hf) values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between epsilon(Hf) of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous rocks and the,seawater array', which we refer to as the 'zircon-free sediment array' (epsilon(Hf)=0-91 epsilon(Nd)+3.10). Finally, we show that the Hf-Nd arrays for seawater, unweathered igneous rocks, zircon-free and zircon-bearing sediments (epsilon(Hf)= 1.80 epsilon(Nd)+2.35) can all be reconciled, using Monte Carlo simulations, with a simple weathering model of the continental crust. (C) 2008 Elsevier B.V. All rights reserved.
Earth and Planetary Science Letters (0012-821X) (Elsevier), 2009 , Vol. 277 , N. 3-4 , P. 318-326

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-6451.pdf
DOI:10.1016/j.epsl.2008.10.028
http://archimer.ifremer.fr/doc/00000/6451/