Seminar

How quickly do particles of rock travel from source to sink, and how much reworking occurs on the way. Answering this question is fundamental if we are to use the deepwater marine sedimentary record to reconstruct changing erosion and weathering patterns in basins worldwide. In particular, the Indus river represents the only major drainage in the Western Himalayas and holds out the opportunity of allowing us to test the degree to which the Asian monsoon controls exhumation and the development of large structures in his mountain belt. New surveys from the rain shadow of the Himalayas demonstrate that storage of sediment lasts from millennia up to more than >200,000 years although the total volumes may be modest.  Greater accumulations are known in the valleys of the Karakoram and around Nanga Parbat, as well as on the floodplains of northern Pakistan and northwest India. Variations in the monsoon appears to be the critical control governing the storage and release of sediment in these areas. This type of buffering may account for as much as 50% of growth of the delta since the end of the last glacial period. Nonetheless, coherent variation in provenance indicators after 14 ka suggests that the river is not entirely buffered, in contrast to most predictions based on numerical models. Coherent variations are also seen in the submarine canyon where a maximum buffering time of around 8000 years can be demonstrated and probably much less in reality. There does however appear to be a difference between the sand and the muddy sediment discharged from the river with the sand preferentially captured near the river mouth. New zircon U-Pb data suggests that much of the submarine fan itself is built by sediment eroded when the monsoon is strong i.e. during interglacial times. The sediment may be stored on the shelf or in the delta and then reworked during sea level falls. Although the submarine fan may be effective at million year timescales it’s use in correlating erosion and weathering on millennial scales is probably limited.