Mineralogy of the Ganges and Brahmaputra Rivers: implications for river switching and Late Quaternary climate change

Abstract

Late Quaternary sediments of the Bengal basin contain a history of river switching and climate change as revealed from sand- and clay-size mineralogy of boreholes and modern riverbed grabs. Epidote to garnet ratios (E/G) in sand fraction sediments are diagnostic of source, with high (>1) E/G indicating Brahmaputra provenance and low (<1) E/G indicating Ganges provenance. In the clay fraction of surficial sediments smectite is diagnostic, with high values (∼39%) in the Ganges and low values (∼3%) in the Brahmaputra. In contrast, the Brahmaputra contains more kaolinite (29% vs. 18%), more illite (63% vs. 41%), and more chlorite (3% vs. 1%) than the Ganges. Analysis of mineralogic and stratigraphic data indicates that the two rivers have changed position several times during the Holocene. Extended periods of mixed river inputs appear to be isolated to the Early Holocene, suggesting rapidly migrating braided channels during sea level lowstand. Tectonically driven accommodation in the Sylhet Basin may have contributed to the favored easterly course of the Brahmaputra during much of the Holocene. Relative abundances of illite and chlorite (IC) vs. smectite and kaolinite (SK) record varying degrees of physical and chemical weathering, respectively. High IC values in early post-glacial deposits suggest a dominance of physical weathering at that time. However, a general increase in SK concentrations throughout the Holocene appears to reflect enhanced chemical weathering under increasingly warmer and more humid conditions. Notably, a peak in SK concentrations that corresponds to an Early Holocene warm period (∼10–7 ka) suggests that weathering patterns in the catchment respond quickly to climatic shifts.