Monsoon and Anthropogenic Controls on Sediment Flux to Asian Marginal Seas
The production of sediment in Asia is controlled over a very long time periods (>10 million years) by the uplift of large-scale topography, most notably the Himalayas and Tibetan plateau during the Cenozoic. However, the rate of delivery at any one particular time is also a function of the climate. In general we see more erosion and faster transport of sediment to the marginal seas of Asia during periods of intense monsoon when erosion is focused along the margins of the Tibetan plateau and Himalaya. There is significant buffering of sediment between its initial erosion in the mountains and delivery to the coast, let alone to the deep water submarine fans of the Indian Ocean or South China Sea. This storage and release of sediment is also controlled both by sea level and also by monsoon intensity. There is not a linear relationship between monsoon intensity and sediment storage. In many examples sediment is stored in floodplains and in mountain valley terraces during times of strong monsoon when the rivers are full of sediment. The transport capacity is then exceeded. Conversely when the monsoon is weaker and less sediment has been derived from the sources then the river systems can cannibalise their own terraces and floodplains. This is more apparent in Southwest Asia and less important in the eastern parts of the Himalayan foreland. The buffering and release of sediment in floodplains is also controlled by anthropogenic activities. Damming has acted to reduce flux to the ocean, while agriculture has broken down old soil profiles and accentuated erosion. In many examples we see the delivery of more sediment and more weathered sediment to the deltas since the onset of agriculture across Asia. This trend has reduced in recent times as damming became more widespread across the continent. Future intensification of the monsoon may have different impacts in different parts of Asia, although the continued delivery of sediment to the coast will be important in helping to conserve land in these regions and combat the stress of rising sea level.
