Abstract
Knowledge of river sediment recycling provides important constraints on continent weathering and earth surface processes. In this study, we estimate the “comminution age” of sediments from the Changjiang (Yangtze River) and two small mountainous rivers in Taiwan based on their lithogenic (234U/238U) ratio. The (234U/238U) distributions in the Changjiang catchment are overall related to sediment grain size and chemical weathering regime, while (234U/238U) ratios in Taiwan rivers mainly depend on erosion/denudation processes. The comminution age constrains the time scale of sediment source-to-sink processes in catchments from sediment weathering/denudation to transportation, and finally deposition. Our results indicate that the comminution ages vary from 250 to 600kyr for the Changjiang sediments and ~110kyr for the Taiwan sediments. Different comminution ages are associated with contrasting erosion and weathering regimes and diverse topography between the large Changjiang catchment and small mountainous Taiwan basins. The longer comminution age of the Changjiang sediment is an interacting effect of a longer erosion/weathering history and sediment trapping effect (and thus slow transfer rate) created by broad floodplains and lakes in the middle and lower reaches. The shorter comminution age of the Taiwan sediment results from fast sediment denudation and transport associated with strong tectonic uplift, typhoon climate and steep topography. As these two major river systems dominate the sedimentology in East Asia continent margin, the distinct geological and topographical settings between the Changjiang and Taiwan river systems result in different sediment “source to sink” transport processes. This work presents a systematic and quantitative constraint on the time-scale of river sediment transfer process in East Asia, and also provides new insight into weathering regimes and sediment transport in monsoon climate-dominated continent and Island.
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