C and O isotope compositions of modem fresh-water mollusc shells and river waters from the Himalaya and Ganga plain

The aim of this paper is to unfold the relationship between the O and C isotope compositions of modem fresh-water mollusc shells and water in order to refine the basis of interpretation for paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases. The O isotopic compositions of river waters in the Himalaya and Ganga plain have a large range, from -18 parts per thousand in the north of the high range up to -8 parts per thousand to -4 parts per thousand in the Ganga plain. delta O-18 of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for the Main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between -15 parts per thousand and -5 parts per thousand. Average delta O-18(Ara) values are consistent with precipitation at equilibrium with host waters at a temperature range of 20-25 degrees C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along the Main Himalayan rivers have delta O-18 values uniformly distributed within -11 parts per thousand and -6 parts per thousand reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4 parts per thousand. This shows that, depending on the type of river where the molluscs grow, the information in term of delta O-18 amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in O-18 due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0 parts per thousand to -10 parts per thousand reflecting input of soil derived alkalinity and plant productivity in the river. delta C-13 of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell delta C-13 are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year. (c) 2006 Elsevier B.V. All rights reserved.