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

Lead Research Organisation: University of Aberdeen
Department Name: School of Geosciences

Abstract

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 propose to follow 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. In this project we propose to map 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 shall survey 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 shall use seismic 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 shall use 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 in eight chosen locations will allow 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), noting if this changes over short time spans and whether changes on the shelf correspond to those known from the delta. X-Ray analysis of clay minerals will be used to record 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. Particle size analysis will be performed on a selection of 200 samples in order to constrain the depositional processes and the power of the currents active on the shelf.
 
Description We have found that sediment entering the Indian Ocean in the Indus offshore has mostly been advected towards the east into the submarine canyon and onto the eastern continental shelf. This contrasts with the developments on the western shelf where longshore currents have brought material from the Makran to the Indus Delta region. We demonstrate that sedimentation continued in the deep water despite rising sea levels and that the sediment delivery into the canyon was directly from the River mouth and not from reworking of pre-existing deposits. We argue that the dominance of sediment supply over sealevel reflects the influence of the Asian monsoon.
Exploitation Route We now plan the new research cruise as well as further exploitation of the core material now stored in Southampton. In particular we wish to know why sedimentation on terraces in the canyon has been continuous and what causes those terraces to form and at what part of the sea level cycle. We are also examining the role that the desert plays in modulating sediment flux into the deepwater.
Sectors Electronics,Energy

URL http://www.geol.lsu.edu/pclift/pclift/The_Indus.html