Soil-geochemistry of the Ganga basin along a 2000 km transect: Implications for chemical weathering during pedogenesis influenced by source, climate, and neotectonics

Abstract

The Ganga Basin (GB), occupying a unique position in front of the rising Himalayas, is one of the largest fluvial sedimentary basins in the world. Over the last 13.5 ka, the overbank sediments of this basin were modified by pedogenesis. Previous studies of GB soils have established soil geomorphology, micromorphology, and clay mineralogy. However, the information on geochemical evolution and chemical weathering during the pedogenesis of these sediments is unknown. This study provides the geochemical characteristics of 31 pedons representing the entire GB along a 2000 km west-east transect to understand silicate weathering during pedogenesis. The absence of lithological discontinuity and the Ti/Al ratio values confirmed that chemical weathering in these pedons is due to pedogenesis. The depth functions (<50 cm to >50 cm) of the major oxides, weathering indices, and age relationship of the soils show that the Western Yamuna Plains (WYP: Pedons 1–10), Yamuna-Ganga Interfluve (YGI: Pedons 11–15), and Ganga–Ghaghara Interfluve (GGI: Pedons 16–25) are characterised by weak, intermediate, and strong chemical weathering during the pedogenesis of the alluvium derived from the Himalayas. In contrast, the deltaic plains (DP: Pedons 26–31) show weak to intermediate chemical weathering during the pedogenesis, with a major input of the cratonic flux. The major cations show enrichment as Ca > Mg > Mn > Fe > Na > Al > K > P, with a minor depletion of silica and no change in Ti during pedogenesis in GB soils. The rate of chemical weathering shows an increase of the weathering indices (CIA, CIA-K, CALMAG) by 1.5%–1.7% in the Bw, Bt, and Bss horizons over 1000 years of pedogenesis in the Ganga Basin.