Provenance and Weathering of Holocene Ganges-Brahmaputra Sediments: Applications of Strontium Geochemistry to Late Quaternary Delta Evolution.

摘要

The Ganges and Brahmaputra drainage systems make up one of the largest source-to-sink systems in the world. Together, the rivers carry >1Gt of sediment annually from the Himalayas to the continental margin where approximately two-thirds of this enormous load is presently sequestered to the Ganges-Brahmaputra delta (GBD), with the remaining third deposited on the Bengal Fan via the Swatch of No Ground canyon. There has been extensive work to characterize Himalayan lithologies and the sediments of the Bengal Fan, but surprisingly little focus on the geochemistry of deltaic deposits where the majority of sediments have been deposited in the Holocene. Here we demonstrate the use of geochemical signatures of provenance and weathering in Holocene sediments to understand how internal and external forcing mechanisms such as short-term climate change and river migration and avulsions have contributed to the shaping of the GBD. This study uses 87Sr/86Sr and Sr concentrations ([Sr]) of bulk sediments to track sediment provenance and major elemental and clay assemblages to characterize alteration of sediments due to weathering. We find that unique 87Sr/86Sr and [Sr] signatures persist throughout the Holocene and allow for tracing of individual river deposition patterns. Further, the Brahmaputra is responsible for the bulk of sediments preserved on the delta, dominating eastern and central delta deposition and mixing with Ganges sediments only in the western part of the delta. In the modern, the two rivers carry roughly equal sediment loads which raises the question of whether this proportion has remained stable through the Late Quaternary or if there is preferential sequestration of Brahmaputra sediments on the delta, which could affect the erosional signals preserved on the fan. With regard to weathering, deltaic deposits preserve weathering signals embedded in the sediments from processes in the catchment, but undergo very little post-depositional alteration during the Holocene due to rapid burial. There are no trends in weathering or provenance that can be definitively linked to climate change such as variations in summer monsoon intensity, suggesting that these signals are either not preserved in the delta, or are dwarfed by the magnitude of fluvial processes. This study is one step in unraveling the complex history of sedimentation in this large and dynamic system.