DETRITAL ZIRCON QUANTITATIVE ANALYSIS OF THE GANGES DRAINAGE NETWORK AND IMPLICATIONS FOR MIOCENE TO MODERN 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 at 2.6 Ma based on detrital zircon (DZ) age distributions from IODP Expedition 354 core samples. However, a detailed record from modern upstream sediment sources in the Ganges drainage basin is lacking. We collected modern river sediments from northern India and analyzed them using laser ablation U-Pb geochronology to characterize the signature of the Ganges River and major tributaries and to provide a first-order model of 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 Himalayas. Quantitative comparison of DZ U-Pb age distributions from the Bengal Fan reveals two distinct groups corresponding to before and after the Plio-Pleistocene transition, where previous research interpreted a change in sedimentation associated with the onset of northern hemisphere glaciation. Mixture modeling of the modern Ganges main trunk is highly variable, which may indicate incomplete upstream mixing compared to a well-mixed basin sink of the Bengal Fan, implying that time and distance are important factors in the sediment mixing process. Mixture modeling shows the Ganges drainage contributed ~40-90% of the total U-Pb signal before 3.9 Ma, followed by a progressive decrease in response to a continued increase of Brahmaputra-derived sediment in the late Pliocene and Pleistocene. Mixture modeling shows that Western Himalayan sources were dominant contributors to the downstream reaches of the Ganges River during the Miocene, accompanied by significant contributions from the Indian Craton sources. In contrast, contributions from Central Himalayan sources were less. Collectively, results support hypotheses that invoke changes in sediment provenance due to the expansion of the Yarlung-Brahmaputra river system and a progressive increase in sediment sourcing in northern Lhasa, in addition to the onset of northern hemisphere glaciation, both of which worked to increase the supply of Jurassic to Eocene DZ from the high elevation Gangdese Mountains.