Tracing Erosion in the Himalaya through chemical and isotopic composition of the Ganga sediments

Abstract

Erosion of continental rocks is the basic process that regulates the geochemical cycles of elements and the landscapes on the earth’s surface. Nature and intensity of denudation are often imprinted in the chemical, mineralogical and isotopic composition of the different riverine phases. Chemical erosion of the Himalayan rocks, specifically crystallines exerts significant control on the CO₂ budget of the atmosphere on long time scales (e.g., Raymo et al. 1988; Raymo & Ruddiman 1992; Edmond & Huh 1997; France-Lanord & Derry 1997; Ruddiman et al. 1997; Krishnaswami et al. 1999). Therefore the role of change in eroding silicate rocks in the Himalaya as a driver of climate change is a topic of debate. In this context, the uplift of the Himalaya in contributing to enhanced silicate erosion and hence its relation to stabilize the climate has been a subject of investigation. Among the rivers draining the Himalaya, Ganga and Brahmaputra Rivers serve as major pathway for transporting the denudational products to the Bay of Bengal. Tectonics, lithology, climate and relief of the mountains are important parameters that control the rate of erosion; however, their relative roles are debated (Burbank et al. 2003; Molnar 2003; Wobus et al. 2003). The Himalaya, one of the rapidly eroding landforms, is characterized by differential erosion (Burg et al. 1998; Leland et al. 1998; Singh 2006) and its significance in regional tectonics and sediment supply to the plains and to the Bay of Bengal is another topic of interest. The Ganga Plain is one of the largest alluvial tracts which has formed in response to the interaction of Himalayan orogeny and its denudation. Therefore, it archives the record of denudation of the Himalayan rocks in space and time. In this study an attempt has been made to address issues pertaining to the sources of sediments to the Ganga Plain using Sr and Nd isotopes and spatial variability in the present day physical erosion rates in its different sub-basins. Further, the chemical composition of the Ganga sediments have been used to estimate the long term silicate erosion fluxes to the Bay of Bengal and its potential for the sequestration of carbon from the atmosphere. Sr and Nd isotope compositions of silicate fractions of the Ganga sediments in the plain have been used as proxies to trace sediment sources and to determine the spatial variability in physical erosion among the various sub-basins of the Ganga system (Fig. 1). These studies reveal that more than two thirds of the sediments of the Ganga plain are derived from the Higher Himalaya (HH) and that the Gandak sub-basin contributes about half of the Ganga bank sediments at Rajmahal. The erosion rates in the Himalayan drainage of the different subbasins of the Ganga, calculated based on the sediment proportions derived in this study and available sediment flux data, range from 0.5-0.25 to 6.3 mm/y. The highest erosionrate is in the Himalayan drainage of the Gandak basin, ∼6 mm/y attributable to intense rainfall in its head water basins characterized by high relief. Results of this study along with those available in literature, suggest that, in general, the erosion rates in the HH are higher compared to other regions of the Himalaya, however, even within the HH, there are hotspots where physical erosion is very rapid, 6 to 14 mm/y. These regions are the gorges of the Brahmaputra, the Indus and the Gandak. These hotspots undergo mechanical erosion quite disproportionate to their aerial coverage and contribute ∼8% of sediment flux from rivers. Such focused erosion may also be contributing to regional tectonics. The high peaks of the Namche Barwa and Gyala Peri, Annapurna and Dhaulagiri and Nanga Parbat can arise due to isostatic rebound resulting from focused erosion. Chemical composition of the Ganga bank sediments and suspended sediments has been used to estimate the silicate erosion fluxes to the Bay of Bengal. Results on the elemental fluxes of silicate derived Na, Ca and Mg show that CO₂ consumption in the basin is (4.6-0.9) × 10⁵ moles km^-2 yr^-1.