Investigating Sediment Transport Processes During Climate and Sea-Level Change in the Pleistocene-Holocene Indus System

Sediment deposited in deep water on continental margins is eroded from the adjacent landmasses and thus represents a record of the tectonic and climate history of that region. Decoding this sedimentary record is however not simple because a grain of sand eroded from the peaks of the Himalayas has a long and complex pathway to its eventual resting place in the Indian Ocean. Nonetheless, the rewards for understanding this sediment record are great, as they often provide the only evidence of how mountains now long eroded behaved. This is especially important in South Asia where it has been suggested that uplift of mountains, and especially the Tibetan Plateau, after the India-Asia collision has caused major climate changes, most notably the intensification of the Asian monsoon. However, proving the link between climate and tectonism has not yet been possible, though the sediments delivered to the sea by the Indus River likely hold out the best hope of reconstructing the series of tectonic and climate events that lead to the present day situation. Given that the monsoon now sustains 66% of mankind understanding its causes must be a high scientific priority.

In this project scientists who have previously been working on the river and delta systems of the Indus onshore in Pakistan now followed the sediment transport offshore across the shelf. Initial studies of shelf sediment show that this may not be derived from the river at all, but could be transported along the coast from the west. If so, where does the sediment in the river go to? Comparisons of 19th century and more recent charts, as well as generations of satellite images, show that the delta has been building out towards the top of the deep submarine canyon that supplies sediment into the deep sea. Does the sediment in the river bypass the shelf and run straight into the canyon? This seems hard to imagine when the coast was initially drowned by rising sea level caused by the end of the last ice age. Rising sea level would result in sediment being captured close to the mouth and an end to sedimentation in deep water. We mapped out where sediment has been accumulating in the recent past in order to see where sediment reaching the ocean from the Indus has been deposited, how quickly the deep sea started to receive sediment again after sea-level rise, and whether the types of sediment delivered to the deep sea changed with climate during deglaciation. If there was a long time gap in sedimentation in the deep sea caused by sea-level and climate change then this will affect how much erosion history we can reconstruct from those sediments. We surveyed the inner Pakistan Shelf, landward of previous surveys, with special attention to the region between the delta and the top of the Indus Canyon. We use dseismic reflection methods to map out sediment bodies and see how the delta began to build out to the top of the canyon after initial drowning. We used two styles of seismic survey, one providing a very detailed, but shallow record, and one providing greater penetration into the seafloor but with less detail. Coring of the sediments allows the age of the sediment to be determined by carbon dating of shell debris and other organic material or through the analysis of radioactive 210Pb. Furthermore, the sands and clays can be analyzed for Nd isotopes to constrain their sources (i.e., from the Indus or along the Makran coast). X-Ray analysis of clay minerals reveals changes in the nature of weathering in the sediment source regions, which can be matched to the known history of the monsoon at this time.