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Sediment Flux through the Indus Submarine Canyon, Arabian Sea

Abstract

Transport of sediment through canyons on continental margins is one of the primary processes responsible for building the submarine fans in many deep sea basins. These fans are not only of economic interest as hydrocarbon reservoirs, but are potentially high-resolution archives of changing environmental conditions in the onshore drainage basins. Such sediment records could allow the relationships between climate, tectonics, erosion, and weathering to be investigated on a number of time scales if the sediment transport can be understood, in particular the degree of reworking. Earlier models for sediment transport through such canyons indicated a dominant role for sea level in controlling this flux, but this largely ignored the role of climatically modulated sediment delivery. A simple model only based on sea level did not also account for the periodic filling of the canyon with sediment and its incision to form terraces.

This project exploits a series of cores from along the axis and on the sides of the Indus Submarine Canyon in the Arabian Sea, in order to test what controls the flux of sediment through this conduit to the upper fan since the Last Glacial Maximum (~20 ka). The main hypotheses to be tested is that rising sea levels were eroding pre-existing shelf sediment into the canyon during the Holocene, rather than allowing new river sediment to reach the canyon. In particular, we use a series of geochemical, isotopic and thermochronological methods to fingerprint pulses of sediment delivery into the canyon, in order to see how they have propagated from shallow to deep water. It is now clear that sedimentation has been extremely rapid in the head of the canyon in the recent geological past. And, we now have evidence for continued sedimentation in the submarine canyon extending at least into the middle Holocene, through and beyond the time of fastest sea level rise. Sandy turbidite deposits are found not only within the canyon thalweg until the middle Holocene, but also in draping terraces higher up on the wall of the canyon that presumably represent periods of incision (some of which are much older). The highest terrace dates back to around 50 ka, although this feature was not draped by younger sediment. Nd isotope data indicates that sediments within the canyon are similar in composition to each other, and to those deposited at the same time within the delta. This suggests a uniform provenance and a relatively simple routing of sediment from the river mouth into the canyon. Sediment supply driven by the strong monsoon appears to have overwhelmed the sea level effect in supplying sediment into the canyon, but whether that is propagated into the submarine fan is presently unknown.