Boron isotopes geochemistry of the Changjiang basin rivers

Abstract

We report analyses of B isotopic compositions in water and suspended particulate matter collected in the Changjiang and its main tributaries. We showed that four sources control the dissolved boron budget; namely atmospheric deposition, evaporite dissolution, anthropogenic inputs and silicate weathering. The contribution of silicate weathering to the dissolved B load ranges from 40% to 50% for the Changjiang main channel and from 45% to 88% for the main tributaries. The isotopic composition of dissolved boron derived from silicate weathering range from −3‰ up to +9‰ suggesting that isotopic fractionation occurs during silicate weathering. The boron isotopic composition of suspended particulate matter range from −11.4‰ to −6‰. Boron derived from silicate weathering is preferentially carried out by the dissolved load which accounts for 30–96% of the total boron. We show that the isotopic compositions of both the dissolved load and suspended particulate matter are controlled by the competition between boron leaching and boron uptake into secondary phases. The first process is characterized by a loss of boron relative to the bedrock without apparent isotopic fractionation whereas the last one is associated to a large isotopic fractionation which enriches the dissolved boron in heavy isotope.