Magnesium isotopic composition of rivers draining karst-dominated regions in Southwest China

Abstract

Magnesium isotopic composition (δ26Mg) could constrain sources and processes of riverine Mg, thus providing new perspectives on budgeting the global Mg cycle. Carbonate lithology displaying karst covers about one-fifth of the Earth's ice-free continental area, however, Mg isotopic composition and associated fractionation processes in rivers draining carbonate are not well constrained. This study focused on tributaries of the mid-reaches of the Xijiang River, a carbonate dominated river in southwest China. The δ26Mg value of river water shows a large range of −2.23‰ to −0.61‰. The clay formation results in a maximum δ26Mg variation of ~0.3‰ and the influence of secondary carbonate precipitation on river δ26Mg is not observed. Lithology is the primary control of Mg isotopic composition of river water. The relations of Sr/Ca ratio and 87Sr/86Sr, and Mg/Ca ratio and δ26Mg value are suggestive of silicate, limestone, and dolomite weathering sources in the river. However, in some tributaries, the solute sources indicated by Sr and Mg isotopes are different, this could be due to the heterogeneous distribution of lateritic soil and weathering of reactive minerals with distinct Sr and Mg content. Compiling data of large rivers over the world, a relationship between the solute proportion of carbonate weathering and δ26Mg of river water is observed, suggesting that isotopic composition of carbonate weathering Mg flux is −2.3‰ at the continental scale. The result might provide links between riverine δ26Mg and chemical weathering flux in geological history.