Containing nearly 3 000 000 cubic kilometers of sand, silt, mud, and other sediment, the Bengal Fan is an enormous cone-shaped pile of sediment on the floor of the Bay of Bengal. As the final resting place for much of the eroded detritus from the Himalaya, this deposit provides us with a geological record of the rise of Earth’s tallest mountains and the changes in climate that accompanied their uplift.
The Ganges and Brahmaputra river systems carry sediment from the Himalaya out to the Bay of Bengal. When it reaches the deep sea, the dense, sediment-laden water flows through channels on the fan in roiling clouds called turbidity currents. Our work on the Bengal Fan uses the alignment of magnetic minerals in turbidity current deposits (turbidites) to trace the speed and direction of these currents over time. Ultimately, this will help us understand the links between sediment supply, past climate, and the growth of the Bengal Fan.
A variety of magnetic minerals occur in the deep-sea sediments of the Bengal Fan. Some of these minerals were eroded from rocks high in the Himalaya and tell a story of uplift and erosion high in the mountains; others grew in the ocean and reflect chemical and biological activity in the ocean or in pore spaces between sediment particles. Untangling the various stories told by the magnetic minerals requires careful analysis not only of magnetic properties of the Bengal Fan sediments, but information from other sources as well, including electron microscopy, diffuse reflectance spectroscopy, and particle size analysis.
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