Yangtse River sediments and erosion rates from source to sink traced with cosmogenic 10 Be: Sediments from major rivers

Abstract

Estimates of regional erosion and sediment mixing from different sources in the Yangtse River system are presented, based on sand samples collected from major tributaries and the trunk stream, at 23 sites between western Sichuan and the Yangtse Delta. Mixing is estimated from concentrations of Mg, Ca, Sr, Ti, Mn and Fe, which are substantially higher in sand from major tributaries in the western Yangtse River catchment than from tributaries in the eastern catchment. Intermediate concentrations occur in sand from the Yangtse Delta, both for modern samples from the surface and for early Holocene samples from drill holes. Mixing ratios indicate that 35 ± 5% of sand in the delta came from eastern sources. A similar result was obtained using cosmogenic 10Be in quartz grains as a tracer of mixing. Regional erosion rate estimated from 10Be in sand grains from high mountain catchments of the western Yangtse River are mostly similar to rates based on sediment gauging but are sometimes higher, and range to over 700 m Ma- 1, while 10Be measured at upper Yangtse River tributaries on the northeast Tibetan plateau gave rates of 20-30 m Ma- 1. For the eastern catchments, 10Be measurements from quartz sand and sediment gauging both gave rates of 30-70 m Ma- 1. Eroding at this rate, the eastern catchments could not supply more than 20% of the sediment in the delta, in contrast with 35% estimated from geochemical fingerprints. The relative input from eastern sources may have been higher in Late Pleistocene times, under a different climatic regime, and reworking of Pleistocene deposits may still be in progress.