Collaborative Research: Climate as a Principal Control on Monsoon-Dominated Deltas: Late Quaternary Records from the Ganges-Brahmaputra System

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Funds are requested for a two-year study to reconstruct a 40-ky stratigraphic record of climate impacts and fluviosedimentary responses in the Ganges-Brahmaputra (G-B) delta system. The study's main objectives are to determine how changing source areas, weathering patterns, and delivery of sediment have influenced development of the Bengal basin under the control of shifting monsoon regimes. Previous NSF-supported studies of the Bengal Basin by the PIs demonstrate that it contains a high-resolution record of Late Quaternary fluviosedimentary signals from the Himalayan catchment, presenting an unparalleled opportunity to understand how dramatic changes in South Asian climate have impacted the sediments and character of a major deltaic sequence. Key features of the G-B sediment dispersal system that make it particularly appropriate for this study include: (1) strong climatic responses and rapid signal propagation from the catchment to margin, (2) a suite of geochemical and mineralogical signatures that can be related to processes and regions within the Indo-Himalayan catchment, and (3) the preservation of thick, high-resolution, and diverse stratigraphic records at the margin.Furthermore, the G-B system will provide important insight for river system behavior andstratigraphic responses to climate change, especially for the seven major fluvial systems fed by the Himalayan/Tibetan uplift.The main approaches for this study involve: (1) geochemical, stratigraphic, andpaleoenvironmental studies of borehole samples, (2) conceptual model development for monsoon-affected sediment dispersal systems and, (3) testing of basin response to changing climate and sediment delivery using state-of-art numerical models. Borehole samples will be collected from areas dominated by Ganges, Brahmaputra, and mixed inputs, as well as those of fine- and coarse-grained textures, providing a variety of sedimentary signals and sequences. Strontium and neodymium isotopes, along with mineralogy, will be used to trace the production and dispersal of sediment over recent interglacial, glacial, and interstadial periods. Weathering patterns will be traced using clay mineralogy and stable O and H isotopes. Radiocarbon and optically-stimulated luminescence dating will be used to establish a chronology and determine changing accretion rates. These data will be further correlated with organic d13C values and a collaborative palynology study to infer shifting floral communities. Altogether, this multi-proxy approach will allow us to identify diagnostic geochemical and stratigraphic signatures in the Bengal Basin that can be related to changing climatic conditions during the Late Quaternary. Finally, numerical-modeling experiments using both hydrologic and stratigraphic models will allow us to test our conceptual ideas regarding the influence of Late Quaternary climate change on river behavior and basin stratigraphy.Broader societal impacts: Understanding the influence of climate on the delivery of river sediment to the Bengal margin will contribute to our ability to predict the environmental consequences of climate change in monsoon-influenced fluviodeltaic settings. In particular, the subsistence livelihood of Bangladesh's 130 million inhabitants depends largely on agricultural practices that are sensitive to changes in sediment delivery, flooding patterns, and erosion/accretion cycles of lowland and coastal areas. In addition, mediation of the country's current groundwater arsenic crisis may hinge on a more complete understanding of the mineralogy and geochemistry of subsurface aquifers, to which this study will contribute. Furthermore, substantial scientific exchanges between the PIs and scientists/students in the developing host country will broaden the local and international impact of our results. Collaborative arrangements with Dhaka University and the Geological Survey of Bangladesh will assure the full involvement of Bangladesh scientists and resource managers. Two graduate students from the U.S., one at William & Mary and one at Stony Brook, will participate in all aspects of this work, and their theses/dissertations will shape the outcome of this study. The project will also involve undergraduate students through independent and/or senior research studies. Finally, both PIs are teaching faculty at their respective institutions, and the progress and results of this study will be incorporated into relevant courses.