Optical and geochemical study of organic matter in present oxic sediments (Equatorial North Pacific Ocean Nixo area)

Optical (palynofacies) and geochemical (C, N, amino acids) study of surface sediments from the Equatorial North Pacific Ocean, which are deposited in an oxic environment and characterized by ferromanganese nodule occurrence, revealed figured remains of pelagic, benthic and terrestrial organic matter. These organic remains consist of a residual organic fraction, related to alteration phenomena. The scarcity of amorphous organic matter contrasts markedly with anoxic sediments. Amino acid comparison between hand-picked figured organic elements and bulk sediments indicates that coarse components have distinct compositions, some being biologically active sites; the preserved amino acid content of bulk sediments being mostly related to fine fractions as clays

Relationships between heavy metals distribution and organic matter cycling in mangrove sediments (Conception Bay, New Caledonia)

Mangroves of New Caledonia act as a buffer between a lagoon of more than 20,000 km(2) and the Island, which suffers intense processes of erosion resulting from urbanization and natural resources exploitation. This preliminary study aims at determining how heavy metals are distributed in mangrove sediments and pore-waters in relationship with their organic content. To reach our goal, a series of 50-cm deep cores were collected in the mangrove of Conception Bay. The various coring sites are representative of live forests (Avicennia marina. Rhizophora stylosa), dead forest, and intertidal unvegetated area. The ranges of concentrations in sediments were the following (mu mol g(-1)): Cu (0.08 to 0.51), Co (0.01 to 0.38), Ni (0.03 to 3.55), Cr (0.36 to 3.11), Zn (0.68 to 2.36), Mn (1.13 to 5.0) and Fe (22.64 to 721.69). Heavy metals distribution within sediments and pore-water appear to result from diagenetic processes linked to OM decomposition. Beneath the dead Avicennia forest and in the unvegetated sediments, the higher the organic content, the higher the metal concentration in the solid phase. Beneath living mangrove stands, despite a higher organic content, as well as higher sulphur content, heavy metals concentrations were not higher than those measured in the other sampling sites. Beneath these forests, redox conditions were mainly controlled by the length of waterlogging and the activity of root system. We suggest that, because of the specificity of the Avicennia root system and its position in the intertidal zone, heavy metals are more bioavailable and potentially more mobile than beneath Rhizophora stand

The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)

In New Caledonia, one third of the Island is composed of ultramafic rocks, and lateritic soils enriched in Fe, Ni and Cr. Open-cast mining occurs all around the Island, and processes of erosion and sedimentation, which occur naturally along the coastline, are strongly amplified by mining activities. Due to their position, at the interface between land and sea. mangroves receive extensive amounts of particles emanating from rivers through estuaries. The purpose of this study is to understand the distribution and partitioning of some transitional metals (Fe, Mn, Ni, Cr) in sediments and pore-waters in a mangrove swamp, which is situated downstream of a catchment characterized by lateritic soils that were exploited a century ago. Quantitative analyses on bulk and after selective extraction, were carried out on cores collected along a transect within the intertidal zone, i.e. beneath a Rhizophora stylosa stand, an Avicennia marina stand, and within a salt flat. Mean metal concentrations were (mu mol g(-1)): Fe (1997) > Ni (44.2) > Cr (31.9) > Mn (8.8). Thus, Ni, Cr and Fe concentrations in this study are substantially higher than mangrove world average. In addition. Ni concentrations are 10 to 100 times higher than in other New Caledonian mangrove developing downstream of a catchment not composed of ultramafic rocks. The studied mangrove is characterized by gradients of water and organic contents with depth and along the intertidal zone, which induced different redox conditions, and thus different metals partitioning. Transitional metals are deposited in the mangrove mainly as oxides and/or oxy-hydroxides, that are subsequently dissolved by bacteria for the decomposition of organic matter, and which leads to a strong increase of metals in the dissolved phase. Then, dissolved metals were precipitated with organic and sulfide compounds. To conclude, organic diagenesis in mangrove sediments leads to the transfer of transitional metals from oxide form to organic and sulfide forms

Fossil Polymetallic Concretions from Deep Sea Drilling Project Leg 81: Mineralogical, Geochemical, and Statistical Studies

International audienc

Organic-rich sediments in ventilated deep-sea environments: Relationship to climate, sea level, and trophic changes

Sediments on the Namibian Margin in the SE Atlantic between water depths of similar to1000 and similar to3600 m are highly enriched in hydrocarbon-prone organic matter. Such sedimentation has occurred for more than 2 million years and is geographically distributed over hundreds of kilometers along the margin, so that the sediments of this region contain a huge concentrated stock of organic carbon. It is shown here that most of the variability in organic content is due to relative dilution by buried carbonates. This reflects both export productivity and diagenetic dissolution, not differences in either water column or bottom water anoxia and related enhanced preservation of organic matter. These observations offer a new mechanism for the formation of potential source rocks in a well-ventilated open ocean, in this case the South Atlantic. The organic richness is discussed in terms of a suite of probable controls including local wind-driven productivity (upwelling), trophic conditions, transfer efficiency, diagenetic processes, and climate-related sea level and deep circulation. The probability of past occurrences of such organic-rich facies in equivalent oceanographic settings at the edge of large oceanic basins should be carefully considered in deep offshore exploration

The glacial ocean productivity hypothesis: The importance of regional temporal and spatial studies

International audienceHigher ocean productivity has often been proposed to explain lower atmospheric carbon dioxide during the last glacial episodes. But recent consideration of marine cores from different areas show that higher local productivity can be postulated for interglacials as well as for glacial periods. Based on the detailed study of two piston cores from the northwest Africa upwelling system, the results presented here, including delta(18)O stratigraphy, organic carbon contents and fluxes, Ti/Al ratios and grain size measurements, clearly indicate that the two cases of sedimentary records can even co-exist within a single upwelling system. This regional heterogeneity is presumably attributed to combined wind stress and sea-level changes that would induce different sedimentary records in the northern and in the southern part of the system. These results emphasize the importance to understand and to model the response of the main kinds of highly productive oceanographic regional systems which are spatially heterogeneous due to complex continent-ocean interactions, or to the presence of mobile hydrodynamic heterogeneities. For such an understanding it is not necessary to acquire a huge amount of core data throughout the world ocean, but to increase the density of cores as well as the regional-scale modelling efforts in systems such as coastal and equatorial upwelling areas, and the migration areas of the southern polar front