DETRITAL ZIRCON CHARACTERIZATION OF THE GANGES AND BRAHMAPUTRA DRAINAGE NETWORKS: IMPLICATIONS FOR PLIO-PLEISTOCENE SEDIMENT SOURCING TO THE BENGAL FAN

The Ganges-Brahmaputra river system and the Bengal Fan represent Earth’s largest source-to-sink sediment-routing system. Previous research demonstrated an increase in sediment delivery from the Brahmaputra drainage basin to the Bengal Fan at the Plio-Pleistocene transition (PPT, 2.6 Ma) based on detrital zircon (DZ) age distributions from samples collected by IODP Expedition 354. However, a detailed record from modern upstream sediment sources is lacking. We have collected modern river sediments from northern India and Tibet, and analyzed them using laser ablation DZ U-Pb geochronology to characterize the U-Pb signature of Ganges and Brahmaputra river sands. We also present new DZ results from IODP 354 cores to complement previously published data and more completely characterize the Bengal Fan’s U-Pb signature. Our results provide a first order model of modern sediment routing for the region. Quantitative comparison of DZ age distributions from the Ganges drainage basin shows variability in Himalayan sources along strike, particularly in the headwaters of the western Himalaya. Preliminary mixture modeling shows that western sources may be dominant contributors to the downstream reaches of the Ganges River. For the Brahmaputra, quantitative comparison of DZ age distributions within the upstream reaches in Tibet reveals three unique sediment sources: the Gangdese Mountains, the northern Himalaya, and the Eastern Syntaxis. Our mixture modeling remains inconclusive as we await further results, but quantitative comparison of DZ age distributions from the Bengal Fan reveals two distinct groups, before and after the PPT. Preliminary mixture modeling shows Himalayan sources in the Ganges drainage are dominant contributors before the PPT, but after the PPT these sources decrease in response to an increase in Brahmaputra-derived sediment. Samples from the Indian craton have a unique signature that differs significantly from that of the Himalaya, but Indian craton sources appear to be minor contributors to Bengal Fan strata. Collectively, preliminary results support hypotheses that invoke changes in sediment provenance during the PPT, as driven by the onset of glaciation in the northern hemisphere, which triggered high rates of erosion in the high elevation sediment source areas to the Bengal Fan.