Lartiges Bruno

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Lartiges
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Bruno
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  • Preprint
    Loading and fate of particulate organic carbon from the Himalaya to the Ganga–Brahmaputra delta
    ( 2008-01-26) Galy, Valier ; France-Lanord, Christian ; Lartiges, Bruno
    We use the evolution of river sediment characteristics and sedimentary Corg from the Himalayan range to the delta to study the transport of Corg in the Ganga-Brahmaputra system and especially its fate during floodplain transit. A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and Corg content in order to evaluate the controls on Corg loading. Contributions of C3 and C4 plants are estimated from Corg stable isotopic composition (δ13Corg). We use the evolution of δ13Corg values from the Himalayan range to the delta in order to study the fate of Corg during floodplain transit. Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4 to 5 times higher specific area, Ganga sediments have similar Corg content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on Corg loading. The general correlation between the total Corg content and Al/Si ratio indicates that Corg loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river and, (2) the ability of mineral particles to form organo-mineral aggregates. Bed and suspended sediments have distinct δ13Corg values. In bed sediments, δ13Corg values are compatible with a dominant proportion of fossil Corg derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ13Corg values (-24.8 ‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ13Corg values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ13Corg values (-22.4 to -20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic Corg coming from the Himalaya is oxidised and replaced by floodplain Corg. This renewal process likely occurs 40 during successive deposition-erosion cycles and river course avulsions in the plain.
  • Article
    A Rouse-based method to integrate the chemical composition of river sediments : application to the Ganga basin
    (American Geophysical Union, 2011-11-01) Lupker, Maarten ; France-Lanord, Christian ; Lave, Jerome ; Bouchez, Julien ; Galy, Valier ; Metivier, Francois ; Gaillardet, Jerome ; Lartiges, Bruno ; Mugnier, Jean-Louis
    The Ganga River is one of the main conveyors of sediments produced by Himalayan erosion. Determining the flux of elements transported through the system is essential to understand the dynamics of the basin. This is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions. Using Acoustic Doppler Current Profiler (ADCP) acquisitions with sediment depth profile sampling of the Ganga in Bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments. The model shows that ca. 390 (±30) Mt of sediments are transported on average each year through the Ganga at Haring Bridge (Bangladesh). Modeled average sediment grain size parameters D50 and D84 are 27 (±4) and 123 (±9) μm, respectively. Grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation. The integrated sediment flux is characterized by low Al/Si and Fe/Si ratios that are close to those inferred for the Himalayan crust. This implies that only limited sequestration occurs in the Gangetic floodplain. The stored sediment flux is estimated to c.a. 10% of the initial Himalayan sediment flux by geochemical mass balance. The associated, globally averaged sedimentation rates in the floodplain are found to be ca. 0.08 mm/yr and yield average Himalayan erosion rate of ca. 0.9 mm/yr. This study stresses the need to carefully address the average composition of river sediments before solving large-scale geochemical budgets.