Tracing paleoerosion in the Ganga Basin

Abstract

The Ganga River supplies about ∼500-1000 tons of sediments (Galy and France-Lanord 2001; Hay 1998; Milliman and Syvitski 1992), from the Himalaya, predominantly the Higher Himalaya (Singh et al., 2008) to the Ganga plain and finally to the Bay of Bengal.Records of erosional process in the Himalaya over millennia time scales are preserved in sediments of the Ganga plain and can serve as an archive to study paleoerosion in the region. In this study an effort is made to track the variations in the provenance of the sediments in the Ganga Plain, through analysis of silicate fraction of sediments from a 50 m long core for Sr, Nd concentration and isotopic composition. The core was raised from IIT Kanpur campus (26030'77"N and 80014'1.87"E). The Sr and Nd isotope measurements were made following conventional procedures and using Isoprobe-T Thermal Ionisation Mass Spectrometer. The chronology of this core is available based on the OSL dating of feldspars. Sr concentration in the silicate phase from different depths of the core varies from 77 ppm to 225 ppm with 87Sr/86Sr ranges from 0.72701 to 0.76708 (Fig. 1). The Nd content of these sediments varies from 18 ppm to 42 ppm with ∈Nd values in the range of -16.6 to -14.4. The Nd and Sr isotopic composition of these sediments are determined by the mixing proportion of material derived from the Higher Himalaya (HH) and the Lesser Himalaya (LH), the two major sources of sediments to the Ganga Plain. The Higher Himalayan source,which includes the Tethyan Sedimentary Sequences (TSS) and the Higher Himalayan Crystallines (HHC), is characterized by lower ⁸⁷Sr/⁸⁶Sr, 0.71 to 0.79 and enriched ∈Nd, -16 to -12. The Lesser Himalaya consists of lithologies with more radiogenic Sr and depleted Nd isotope composition, in the range of 0.72 – 0.95 and -25 to -23 respectively (Singh et al. 2008). The shaded bands represent major excursion in ⁸⁷Sr/⁸⁶Sr and ∈Nd. The Sr and Nd isotope profiles of the sediment core show four excursions at ∼6.6, 17.4, 34 and 41m corresponding to the ages of ∼20, 35, 70 and 85 Ka (Fig.1). The trend in ⁸⁷Sr/⁸⁶Sr excursion is opposite to that of ∈Nd, i.e., increase in ⁸⁷Sr/⁸⁶Sr coincides with decrease in ∈Nd. These excursions can be interpreted in terms of variations in mixing proportion of HHLH derived sediments. Such variations in relative contribution of HH-LH can occur in response to climate/tectonic changes. The observation that two of the excursions, ∼20 Ka and ∼70 Ka, coincide with the two glacial maxima is an indication of the climate control on the sediment supply (Owen et al. 2002; Prell and Kutzbach 1987; Sharma and Owen 1996). Glacial periods being characterized by low precipitation and more extensive ice cover over the HH decreases the contribution of the sediments from the HH. Thus increases in the proportion of LH sediment supply to the plain. The LH, being high ⁸⁷Sr/⁸⁶Sr and depleted in ∈Nd, caused high ⁸⁷Sr/⁸⁶Sr and depleted ∈Nd in the sediment. This study underscores the importance of climate in controlling erosion in the Himalaya over 100 Ka time scale.