Impact of monsoon, vegetation, and landscape on pedogenesis: A case study using organic and inorganic tracers from the Himalayan foreland sediments

Abstract

The physical and chemical properties of paleosols are widely used to explain their genesis. However, the physical attributes of paleosols subjected to burial cannot be used for paleoenvironmental reconstructions. In contrast, selective pedogenic tracers such as rare earth (REE) and trace elements, and organic proxies such as pristane/phytane (Pr/Ph) ratios, and D/H and 13C/12C ratios of n-alkanes that are resistant to post-depositional processes can provide insight into pedogenic conditions. Although the upper Miocene Siwalik paleosols from the Himalayan Foreland Basin (HFB) have been used for past vegetation and climate reconstructions, little emphasis has been given to understand their formation. In this regard, the Jabbarkhad Siwaliks of the Kangra sub-basin, identified as distal alluvial fan deposits with records of temporal variability in rainfall and vegetation composition, have been used to investigate the combined impact of depositional environment, topography, climate and vegetation composition on paleosol formation.Prominent negative Eu-anomalies in CI-normalised REE patterns along with uniform ratios of refractory elements suggest a dominant and consistent source of sediments from K-rich granitic upper continental crust. A wide variation in the abundance of REE indicates the modification of sediments by pedogenetic processes. In the Jabbarkhad region, oxic (higher Pr/Ph) and well-drained soils favoured the proliferation of grasses. The high root density and organic turnover of grasses facilitated the development of a mature, strongly leached soil (high Ba/Sr) under high rainfall and humid conditions (lower δ2Η values). Less mature (lower Ba/Sr) paleosols were developed in low-lying anoxic (Pr/Ph <1) floodplain areas dominated by C3 trees near the active channel. Comparison of soil maturity from the Gangetic floodplain, a modern analogue of the Siwalik sediments, suggests that the spatial variability of paleosol maturity was controlled by the landscape mosaic of the Siwalik floodplain. Since pedogenesis and vegetation distribution are controlled by floodplain morphology, sampling of paleosols with similar pedogenic and geomorphic characteristics is essential for evaluation of vegetation distribution in temporally separated paleosols.