Fractionation of Boron isotopes during erosion processes: The example of Himalayan rivers

International audienceThe boron isotopic composition of 5 rain waters and 35 river waters running from the highHimalayas (5050 m) down to the Bay of Bengal were measured by ion microprobe to make a first survey ofthe boron isotopic fractionations present in a major erosion system. The rains have rather homogeneous Bconcentrations and isotopic compositions (0.09 6 0.09 mmol/l and 5.4 6 1.7‰) at the exception of the twoKathmandu samples which probably reflect a significant anthropic contamination. The river waters show alarge range of B concentrations (from 0.09 to 28.68 mmol/l) and of B isotopic compositions (d11B valuesranging from 27.0 to 129.4‰). No systematic correlation between the d11B values and the major ionchemistry of the river waters is present, at the exception of the most B and Cl-rich rivers which havesystematically the highest d11B values, likely controlled by evaporite dissolution.After correction for the evaporite contribution (from 9.4 to 64.4% of dissolved B with a systematic decreasesouthwards) the riverine B concentrations and d11B values are still highly variable (from 0.10 to 5.83 mmol/land from 25.8 to 24.1‰, respectively). This variation reflects mostly different conditions of silicateweathering (congruent and incongruent silicate dissolution), the contribution from the dissolution of carbonatesbeing quite negligible ('5% of the dissolved B) on the river B budget. At variance with congruent silicatedissolution, incongruent dissolution occurs with a large B isotope fractionation which depends on pH(Dclay2water ' 231.2‰ at acidic pH and '25.8‰ at higher pH). The B systematic suggests strongly thatthese fractionations are produced over a wide range of pH (from 7 to 9.5) in soils and don’t result fromreactions between the dissolved and suspended load in rivers. The net result of Himalayan erosion for theglobal B cycle is the input to the sea of waters with highly variable B concentrations and isotopic compositions(0.67 and 0.29 mmol/l, 15.0 and 121.0‰ for the Ganga and the Brahmaputra during the monsoon period,respectively)