HR: 1340h
AN: GP43B-1215    [Abstracts]
TI: An Indian Monsoon Story As Told By The Bengal Fan Sediments
AU: * Abrajevitch, A
EM: alexabra@umich.edu
AF: Department of Geological Sciences, University of Michigan, 1100 North University, Ann Arbor, MI 48109-1005,
AU: Van der Voo, R
EM: voo@umich.edu
AF: Department of Geological Sciences, University of Michigan, 1100 North University, Ann Arbor, MI 48109-1005,
AU: Rea, D K
EM: davidrea@umich.edu
AF: Department of Geological Sciences, University of Michigan, 1100 North University, Ann Arbor, MI 48109-1005,
AB: Various climatic proxies indicate that significant change in the Indian monsoon system occurred at ~ 7-8 Ma. The exact nature of this change is debated and involves either the intensification of the monsoon or increased aridity in the region. We conducted a rock magnetic study of the Bengal Fan sediments to estimate the variation in the monsoon intensity, linked to precipitation in the source area of the fan – the Himalayan Foreland. Different magnetic minerals in the Bengal Fan sediments were identified based on their coercivity and low temperature properties; the contents of these minerals were estimated based on their saturation IRM values. Our results indicate the presence of several magnetic phases in the sediments, including goethite, hematite, a detrital component (likely a mixture of titano-magnetite and/or maghemite) as well as a very distinctive component that we interpret as biogenic magnetite. Variations in relative abundance of goethite and hematite in the studied section suggest that the Himalayan foreland had experienced an arid period that had started at approximate 7 Ma and lasted for about 600 kyr. The onset of aridity on the continent coincides with the disappearance of "biogenic magnetite" from the marine sediments, indicating a change in the oxidation state of bottom waters.
DE: 1505 Biogenic magnetic minerals
DE: 1512 Environmental magnetism
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting