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

Abstract

The aim of this paper is to unfold the relationship between the O and C isotope compositions of modern 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‰ in the north of the high range up to − 8‰ to − 4‰ in the Ganga plain. δ 18O 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‰ and − 5‰. Average δ 18O Ara values are consistent with precipitation at equilibrium with host waters at a temperature range of 20–25 °C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along the Main Himalayan rivers have δ 18O values uniformly distributed within − 11‰ and − 6‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of δ 18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in 18O due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0‰ to − 10‰ reflecting input of soil derived alkalinity and plant productivity in the river. δ 13C 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 δ 13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year.