Multi-proxy geochemical approach to interpreting sedimentary deposits of a complex margin system: The Ganges-Brahmaputra Basin

When tracing sediments from their mountainous sources to their ultimate sink in the world oceans, the intermediate regions of the path cannot be overlooked. Storage, sorting, reworking and weathering of sediments occurs in many ways and sometimes in multiple basins along a river path. A clear understanding of the processes is important when calculating sediment budgets or tracing oceanic sediments to their original sources. However it can be complicated to unravel the complete story with so many processes occurring. Geochemical tools are an excellent means of understanding the changes occurring to sediments in a system, but often have their own drawbacks or limitations. Increasingly it is being found that multi-proxy geochemical studies are needed in order to best characterize and trace the history of sedimentary deposits. We present here an example involving the Ganges-Brahmaputra delta. The Ganges-Brahmaputra system is large and complex, containing the highest topography in the world, the largest sediment loads in the world, and the largest submarine fan system. The system was relatively dry during the last glacial maximum experiencing alpine glaciation, while in modern times it is dominated by a tropical monsoon system. The system has been shown to respond rapidly to climate change. The sharp seasonality of the modern climate along with some of the highest precipitation rates in the world make this system an excellent choice in trying to further understanding of complex sedimentary environments. Our work using Sr isotopes on the Ganges- Brahmaputra delta reveal that each river has a unique isotopic signal that persists throughout the Holocene. Sr isotopes also reveal a more radiogenic signal during times of increased SW monsoon intensity as well as at the end of the Pleistocene. Nd isotopes suggest that some of these changes may be due to a change in weathering intensity of the sediments while others may indicate different sourcing of fine and coarse grained sediments in the same deposit. Our data on the delta during the Holocene and latest Pleistocene agree to some extent with data published on the Northern Bay of Bengal sediments, but show differing trends in other instances. The work we present combined with the work of others involving clay mineralogy and Os isotopes all underscore the need for several tools to unravel the sedimentary story in such a complex system.