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Sediment Storage and Recycling in the Supply of Sand to the Indus Submarine Fan, Arabian Sea


Sandy turbidites are a common constituent of the Indus Submarine Fan and are dominantly supplied by erosion of the bedrocks of the Karakoram and Western Himalaya, likely starting ca. 45 Ma. Zircon U-Pb dating of detrital grains from the Quaternary canyon and from the fan within the Laxmi Basin deposited since 11 Ma (IODP Expedition 355) now provide constraints on the degree of reworking and recycling from source to sink over geologic time scales. Comparison of Holocene sands from the delta and canyon show resolvable differences between the two suggesting that much of the sand in the Holocene river is sequestered near the river mouth. Volume estimates indicate that <1% of the sediment discharge enters the canyon during the highstand. However, short-term changes in the U-Pb age populations demonstrate that the river is not fully buffered by reworking from the flood plains and that pulses of silt and fine sand lasting <100 years are resolvable at least in the upper canyon. Turbidite sands on the fan itself have U-Pb age populations that are most similar to sands in the modern river and not that at the Last Glacial Maximum. Sands must largely be deposited on the fan during glacial era sealevel low stands although the bulk of the erosion appears to occur during interglacial times when the monsoon is strong and bedrock erosion in the mountains is fast. Thus, although some muddy sediment is delivered to deep water throughout the sealevel cycle most sediment is stored on the shelf and in the delta plain with highstands before being reworked into the fan. This implies a lag time of up to a complete glacial cycle, ca. 100 k.y.