Sr and Nd isotope compositions of alluvial sediments from the Ganga Basin and their use as potential proxies for source identification and apportionment

The Sr (87Sr/86Sr) and Nd (εNd values) isotopic composition of sediments transported by the Ganga fluvial system and deposited within the Ganga Basin have been used by other studies to identify and characterize sediment provenance. Isotopic data have also been used for the purpose of quantitative source apportionment. Furthermore, isotopic signatures of sediments imply the Higher Himalaya (HHS) source to be the major contributor of sediments to the Ganga Basin and Bay of Bengal, contrary to the few studies from the adjacent Indus Basin to the west that inferred higher erosion and sediment contribution from the Lesser Himalaya (LHS) source in recent times. In the present study, we test the reliability of using 87Sr/86Sr and/or εNd of river sediments as proxies for source(s) characterization and their apportionment. First, we compiled all available Sr and Nd isotope ratios of the Ganga basin sediments as well as their probable sources reported in earlier studies. After subjecting the data set to statistical scrutiny and removing outliers, we carefully evaluated spatial variability in the isotopic composition vis-à-vis the composition of lithologies drained by a complex network of both the Himalayan- and the Peninsular-sourced tributaries in the Ganga fluvial system. Our analysis shows a large (isotope) compositional overlap among probable sources, which suggests that application of isotopic composition of the Ganga Basin sediments as a proxy for provenance is not straight forward and is fraught with complications. Following the general notion that the Ganga Basin sediments are primarily derived from the HHS and LHS sources, we performed Monte Carlo simulations aimed to model the isotopic composition of sediment mixture. These results show greater uncertainty associated with quantitative source apportionment estimates given the available set of isotopic constraints. Compared to the 87Sr/86Sr ratios that can be modified during fluvial transport, the εNd values serve as more reliable provenance indicator although the use of this parameter for source apportionment may still have a large amount of uncertainty. Nevertheless, a Kernel density estimate (KDE) plot of εNd of river sediments suggests the influence of various source lithologies on the εNd pattern of the Ganga main channel sediments and indicates presence of dominantly HHS- and LHS-sourced sediments. The average εNd of sediments along the Ganga main channel shows distinct change at places in the plains due to local influence of contributing tributaries, most likely Peninsular-sourced tributaries. At present, virtually no isotopic data is available on the sediments of Peninsular-sourced tributaries, which makes it difficult to assess the contribution of cratonic sources to the isotopic budget of the Ganga Basin sediments, particularly in the Ganga-Yamuna interfluve regions.