ANTHROPOGENIC IMPACTS ON SEDIMENT FLUX TO THE OCEAN (Invited Presentation)

On millennial timescales variations in climate have a major impact on the rate and composition of sediment delivered to the ocean. In Asia changes in monsoon intensity since the Last Glacial Maximum affect rates of chemical weathering, as well as sediment transport, resulting in coherent millennial scale variations in composition linked to the alteration state of the sediment. Although stronger monsoons tend to cause faster chemical weathering this does not always result in greater sediment alteration. Human settlement of the continent has had a similar effect in intensifying erosion of chemically weathered soils. In the Pearl River Delta of China an influx of strongly weathered material, rich in kaolinite and with low K/Al values after 2.5 ka is linked to the establishment of agriculture and dense settlement of the onshore basin. Likewise, in the Red River delta of Vietnam there is a clear shift towards more chemically weathered material (higher CIA) after ~1.8 ka, the end of the Chinese Han dynasty. New analyses of sediment from the Indus Submarine Canyon now shows an increase in smectite-rich sedimentation after 5 ka. Although this might be linked to intensified erosion of the strongly weathered floodplains in the context of a weakening monsoon the timing also coincides with the onset of widespread agriculture related to the Indus Valley Civilization. Significant changes in the zircon U-Pb age spectra over the same period area are also noted. In the Mississippi zircon U-Pb age spectra were relatively constant over hundreds of years in the pre-industrial state, but have been more variable in recent years, varying significantly on a monthly scale. Care should be taken when using modern river sediments as analogs for geological samples as modern river systems are often highly disturbed relative to the natural state.