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Van Tuyl Lecture: Mike Blum, University of Kansas
March 1, 2018 @ 5:00 pm - 6:00 pm
Mike Blum, University of Kansas
Van Tuyl Lecture, BE 241, 4 PM
Topic: Record of Sediment Transport from the Himalayan-Sourced Ganges-Brahmaputra Rivers to the Deep-Sea Bengal Fan: 18 Myr of Fan Deposition from Detrital Zircons
Mike Blum, Department of Geology, University of Kansas
Kimberly Rogers, Institute for Arctic and Alpine Research, University of Colorado
James Gleason, Department of Earth and Environmental Sciences, University of Michigan
Yani Najman, Lancaster Environment Centre, Lancaster University
Abstract: The Himalayan-sourced Ganges-Brahmaputra river system and the deep-sea Bengal Fan represent Earth’s largest sediment-dispersal system. This presentation summarizes a detrital zircon U-Pb (DZ) provenance record from the Bengal Fan from cores collected during IODP Expedition 354, with coring sites located 1350 km basinward of the shelf margin. Samples were collected from medium- to fine-grained turbidite sand and, based on shipboard biostratigraphic analyses, our samples are Early Miocene to late Pleistocene in age. Each sample was analyzed by LA-ICPMS at the Arizona Laserchron Center, with an average of n=270 concordant U-Pb ages per sample. We use these data to evaluate the influence of allogenic controls vs. autogenic processes on signal propagation from source-to-sink.
Large-scale sediment transfer to the Bengal Fan clearly records the strong tectonic and climatic forcing associated with and inherent to the Himalayas and Ganges-Brahmaputra system: even after up to 2500 km of river transport, and >1350 km of transport by turbidity currents, the DZ record faithfully represents Himalayan source terrains. However, the sand-rich turbidite part of the record is biased towards glacial periods when rivers extended across the shelf in response to climate-forced sea-level fall, and connected directly with slope canyons and the shelf margin. Moreover, specific U-Pb populations record the rapid transfer of signals from key events, most notably the Miocene integration of the Brahmaputra with the Asian plate based on the presence of Mesozoic Gangdese arc zircons in the Bengal Fan samples throughout the period of record, and the rapid Plio-Pleistocene incision through the eastern Himalayan syntaxis, which resulted in exhumation of zircons from decompression melts with U-Pb ages <10 Ma: grains with U-Pb ages of 4-10 Ma were exhumed and appear in the Bengal Fan by ca. 2.4 Ma, hence signal transfer over distances of >4000 km was very rapid. Last, only part of the Bengal Fan DZ record represents either a Ganges or Brahmaputra provenance. Instead, most samples represent varying degrees of mixing of sediments from the two systems: this mixing, or the lack thereof, represents the fingerprint of autogenic avulsions on the Ganges-Brahmaputra delta plain, which results in these two giant rivers delivering sediment separately to the shelf margin, or merging together as they do today.