Radiogenic and stable Sr isotopes constrain weathering processes in rapidly eroding Taiwan catchments

Small mountainous rivers (SMRs) in subtropical Taiwan supply a disproportionately large quantities of suspended and dissolved flux to the ocean due to extraordinarily high denudation rates in the world. We present major elements, Sr-87/Sr-86, and delta(88)/Sr-86 data of river water and sediment from two typical Taiwan SMRs, Zhuoshui (Choshui) and Liwu Rivers. High contents of dissolved Ca and Mg are generally accompanied by high Sr-87/Sr-86 ratios in waters of the Zhuoshui and Liwu Rivers (0.710721-0.715551 and 0.710461-0.711618). Sequential extraction experiments of river sediments reveal that Sr mainly resides in the residual silicate and acid-soluble fractions. High Sr-87/Sr-86 ratios of 0.709908-0.714168 in the acid-soluble fraction of the Zhuoshui River sediments indicate that disseminated calcite is present in silicate rocks. Rapid weathering of disseminated calcite in response to high denudation rate determines the high Sr-87/Sr-86 ratios in the upper Zhuoshui River water. Its downstream decreasing of Sr-87/Sr-86 ratios across major lithology could be explained by isotopic exchange between river water and sediment leaching fractions. However, all the Sr-87/Sr-86 ratios of the sequential leaching sediments from the Liwu River largely overlap along its course. The relatively low Sr-87/Sr-86 ratios of 0.708747-0.711070 in the acid-soluble fraction of the Liwu River sediments could be sourced from disseminated calcite and/or Tailuko marble, thus supplying low Sr-87/Sr-86 ratios to river waters. Spatial variations in river water delta Sr-88/86 values mainly occur between the mainstram and the tributary. The relation between delta Sr-88/86 and Sr-87/Sr-86 in both rivers conservatively traces water-mass mixing, while the stable Sr isotope fractionation processes (e.g., biological uptake, clay reactions, and calcite precipitation) during transport in streams is negligible. Therefore, it is suggested that combining radiogenic and stable Sr isotopes in river water and/or sequential extracted sediment is an effective means to constrain chemical weathering as well as fractionation processes in a multi-lithological mountainous catchment with high denudation rate and wet climate. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Small mountainous rivers in subtropical Taiwan supply large quantities of suspended and dissolved materials to the ocean, reflecting the high denudation rates in their elemental and isotopic compositions of river water and sediment.