Clay minerals record from Late Quaternary drill cores of the Ganga Plains and their implications for provenance and climate change in the Himalayan foreland

Abstract

This study documents the coupling of provenance and climate change over the last 100ka manifested in clay mineralogy of sediments from two cores (~50m deep) in the Ganga–Yamuna interfluve in the Himalayan Foreland Basin, India. Depth distribution of the texture and clay mineral assemblage in the two cores show notable differences on account of pedogenesis and sediment supply over the last 100ka. Core sediments from the northern part of the interfluve (IITK core) are micaceous and dominated by hydroxyl-interlayered dioctahedral low-charge smectitea (LCS) in fine clay fraction but by trioctahedral high-charge smectite (HCS) in silt and coarse clay fractions. In contrast, core sediments from the southern part of the interfluve (Bhognipur core) are poor in mica and both LCS and HCS are recorded in the upper 28m of the core while the lower part is dominantly LCS in all size fractions. The paleosols in the two cores formed in the sub-humid to semi-arid climatic conditions resulting in clay minerals such as 1.0–1.4nm minerals, vermiculite, HCS and also preserved the LCS, hydroxyl-interlayered vermiculite (HIV) and pseudo-chlorite (PCh), and kaolin that formed earlier in a humid climate. The preservation of LCS, HIV, kaolin and PCh is a clear indicator of climate shift from humid to semi-arid in the Ganga Plains as their formation does not represent contemporary pedogenesis in the alkaline chemical environment induced by the semi-arid climate. As the simultaneous formation of both HCS and LCS is not possible at the expense of mica, the abundance of LCS sediments from both the cores suggests the role of plagioclase weathering in the formation of LCS. In the upper 28m of the Bhognipur core, the presence of both HCS and LCS in the fine clays suggests a change in sediment provenance from cratonic to a dominantly Himalayan source during Holocene. The climatic records inferred from the typical clay mineral assemblages of the two interfluve cores are consistent with the Marine Isotope Stages (MIS). The humid interglacial stages (MIS 5, 3, and 1) are marked by dominance of HIV, PCh, and LCS whereas the dominance of HCS together with pedogenic carbonate (PC) is noted in semi-arid stages (MIS 4 and 2).