Sr, Nd, and Os isotopic data are presented for sediments from diverse locations in the Bay of Bengal. These data allow the samples to be divided into three groups, related to their sedimentary ...contexts. The first group, mainly composed of sediments from the shelf off Bangladesh and the currently active fan, has Sr and Nd characteristics consistent with a dominantly Himalayan source. Their 187Os/188Os ratios (∼1.2‐1.5) show that the average detrital material delivered by the Ganga‐Brahmaputra (G‐B) river system is not unusually radiogenic. A large difference in 187Os/188Os ratio exists between these Bengal Fan sediments and Ganga bedloads (187Os/188Os ∼2.5, Pierson‐Wickmann et al. 2000). This difference mainly reflects addition of a less radiogenic Brahmaputra component, though mineralogical sorting and loss of radiogenic Os during transport may also play some role. The second sample group contains sediments from elsewhere in the Bay, particularly those located on the continental slope. They display Os isotopic compositions (0.99–1.11) similar to that of present seawater and higher Os and Re concentrations. These characteristics suggest the presence of a large hydrogenous contribution, consistent with the lower sedimentation rate of these samples. Sr and Nd ratios indicate that a significant fraction of these sediments is derived from erosion of non‐Himalayan sources, such as the Indo‐Burman range. These observations could be explained by the deflection of sediments from the G‐B river system by westward currents in the head of the Bay. The third group contains only one sample, but shows that in addition to a Himalayan source, sediment discharge from Sri Lanka may influence the detrital component in the distal part of the fan. The similarity between the isotopic compositions of the group I R/V Sonne samples and those of Ocean Drilling Program Leg 116 France‐Lanord et al., 1993; Reisberg et al., 1997 suggests that the material eroding in the Himalayas has been roughly constant since the Miocene. The high Os isotopic ratios of leachates of both Sonne group I and Miocene Leg 116 sediments imply that much of the leachable highly radiogenic Os component was conserved during transport through the estuary or interaction with seawater. In constrast, samples with lower, but still relatively high, sedimentation rates (Sonne groups II and III and Pliocene Leg 116) seem to have significantly adsorbed or exchanged Os and Re with seawater. This suggests that in some cases the Os isotopic ratios of leachates of detrital sediments can be used to constrain the ancient marine Os record, or conversely, to date unfossiliferous sediments.
The evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6Gyr ago) is controversial. Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis, which may have been ...eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5Gyr later, in the Cambrian explosion.
Despite the significant importance of dissolved organic carbon (DOC) in aquatics ecosystems, the processes controlling DOC delivery to stream waters at the catchment scale are still poorly ...understood, in particular with regards to the relative importance of riparian versus upland soils as DOC sources. In this respect, the stable carbon isotopic composition of DOC (δ13CDOC) appears to be a promising tool as different δ13CDOC values are anticipated between wetland and upland soil DOC, because of differences in soil oxygenation, soil humidity and soil organic matter degradation scheme However, care must be exercised because of the possible occurrence of seasonal variations in the δ13CDOC values of both riparian and upland DOC , and because also of the possible mixing of DOC coming from spatially distinct sources. The markedly different isotopic patterns obtained during high resolution monitoring ( 2 δ units), while others showed no, or much more restricted isotopic variations. A comparison of these results with previously published data revealed that this temporal variability of intra-storm δ13CDOC values is the exact transposition of the temporal variability of δ13CDOC values that was found in the riparian soil waters of this catchment during the same period. The latter variability has been shown to arise from the combined effect of changes in the production mechanisms and ultimate sources of riparian DOC and of the lateral input in the riparian domains of an isotopically heavier DOC component coming from more upland areas. Overall, results from this study confirm that upland domains may be significant contributors of stream DOC flux in headwater catchments. They also show that upland soils behave as a size-limited reservoir with respect to DOC production, whereas more highly productive soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that the isotopic composition of DOC is an extremely powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, demonstrating in the meantime that the use of this tool requires that the temporal and spatial variability of the isotopic signatures of all potential DOC sources in the catchment is known accurately. Providing that this condition is fulfilled, the isotopic tool can allow up to quantify the proportions of DOC - and of corresponding water flows - coming from different contributing areas which may be of great importance for better understanding and better modeling of DOC transfer and water routing through the landscape.
Despite the significant importance of dissolved organic carbon (DOC) in aquatics ecosystems, the processes controlling DOC delivery to stream waters at the catchment scale are still poorly ...understood, in particular with regards to the relative importance of riparian versus upland soils as DOC sources. In this respect, the stable carbon isotopic composition of DOC (δ13CDOC) appears to be a promising tool as different δ13CDOC values are anticipated between wetland and upland soil DOC, because of differences in soil oxygenation, soil humidity and soil organic matter degradation scheme However, care must be exercised because of the possible occurrence of seasonal variations in the δ13CDOC values of both riparian and upland DOC , and because also of the possible mixing of DOC coming from spatially distinct sources. The markedly different isotopic patterns obtained during high resolution monitoring ( 2 δ units), while others showed no, or much more restricted isotopic variations. A comparison of these results with previously published data revealed that this temporal variability of intra-storm δ13CDOC values is the exact transposition of the temporal variability of δ13CDOC values that was found in the riparian soil waters of this catchment during the same period. The latter variability has been shown to arise from the combined effect of changes in the production mechanisms and ultimate sources of riparian DOC and of the lateral input in the riparian domains of an isotopically heavier DOC component coming from more upland areas. Overall, results from this study confirm that upland domains may be significant contributors of stream DOC flux in headwater catchments. They also show that upland soils behave as a size-limited reservoir with respect to DOC production, whereas more highly productive soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that the isotopic composition of DOC is an extremely powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, demonstrating in the meantime that the use of this tool requires that the temporal and spatial variability of the isotopic signatures of all potential DOC sources in the catchment is known accurately. Providing that this condition is fulfilled, the isotopic tool can allow up to quantify the proportions of DOC - and of corresponding water flows - coming from different contributing areas which may be of great importance for better understanding and better modeling of DOC transfer and water routing through the landscape.
Monitoring the isotopic composition (delta C-13(DOC)) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative ...contribution to stream DOC flux. High-resolution (< hourly basis) delta C-13(DOC) data were obtained during six successive storm events occurring during the high-flow period in a small headwater catchment in western France. Intra-storm delta C-13(DOC) values exhibit a marked temporal variability, with some storms showing large variations (> 2 parts per thousand), and others yielding a very restricted range of values (< 1 parts per thousand). Comparison of these results with previously published data shows that the range of intra-storm delta C-13(DOC) values closely reflects the temporal and spatial variation in delta C-13(DOC) observed in the riparian soils of this catchment during the same period. Using delta C-13(DOC) data in conjunction with hydrometric monitoring and an end-member mixing approach (EMMA), we show that (i) > 80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the riparian soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its delta C-13 fingerprint, we find that the upland DOC contribution decreases from ca.similar to 30% of the stream DOC flux at the beginning of the high-flow period to < 10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a size-limited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that delta C-13(DOC) provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, having a high-resolution assessment of temporal and spatial variability.
Monitoring the isotopic composition (delta C-13(DOC)) of dissolved organic carbon (DOC) during flood events can be helpful for locating DOC sources in catchments and quantifying their relative ...contribution to stream DOC flux. High-resolution (< hourly basis) delta C-13(DOC) data were obtained during six successive storm events occurring during the high-flow period in a small headwater catchment in western France. Intra-storm delta C-13(DOC) values exhibit a marked temporal variability, with some storms showing large variations (> 2 parts per thousand), and others yielding a very restricted range of values (< 1 parts per thousand). Comparison of these results with previously published data shows that the range of intra-storm delta C-13(DOC) values closely reflects the temporal and spatial variation in delta C-13(DOC) observed in the riparian soils of this catchment during the same period. Using delta C-13(DOC) data in conjunction with hydrometric monitoring and an end-member mixing approach (EMMA), we show that (i) > 80% of the stream DOC flux flows through the most superficial soil horizons of the riparian domain and (ii) the riparian soil DOC flux is comprised of DOC coming ultimately from both riparian and upland domains. Based on its delta C-13 fingerprint, we find that the upland DOC contribution decreases from ca.similar to 30% of the stream DOC flux at the beginning of the high-flow period to < 10% later in this period. Overall, upland domains contribute significantly to stream DOC export, but act as a size-limited reservoir, whereas soils in the wetland domains act as a near-infinite reservoir. Through this study, we show that delta C-13(DOC) provides a powerful tool for tracing DOC sources and DOC transport mechanisms in headwater catchments, having a high-resolution assessment of temporal and spatial variability.
The Makran accretionary prism developed in the north-western part of the Indian Ocean as a consequence of the subduction of the Arabian Sea since Late Cretaceous times. It extends from southern Iran ...to the Baluchistan region of Pakistan where it joins the Chaman-Ornach-Nal left-lateral strike-slip fault systems to the north and the Owen Fracture Zone-Murray Ridge transtensional (right-lateral) system to the south in a complex triple junction near the city of Karachi. In September to October of 2004, we surveyed most of the accretionary complex off Pakistan with R/V Marion Dufresne. We achieved a nearly continuous bathymetric mapping of the prism and the subduction trench from 62°30′E to the triple junction near 62°30′E together with nearly 1000 km of seismic reflection (13 lines) and we took 18 piston cores in different geological settings. One of the main results is that the frontal part of the Makran accretionary prism is less two-dimensional than previously expected. We interpret the along-strike tectonic variation as a consequence of lateral variations in sediment deposition as well as a consequence of the under-thrusting of a series of basement highs and finally of the vicinity to the triple junction.
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