Geothermal input significantly influences riverine and oceanic boron budgets

Abstract

The evolution of boron isotope compositions of seawater (δ11Bsw) over the Cenozoic has important implications for reconstructions of atmospheric CO2 and is tightly linked to boron input from the Himalaya-Tibetan Plateau. However, controls on evolution inδ11Bsw remain elusive. We report geochemical measurements of the Yarlung Tsangpo River draining the Tibetan Plateau and observe exceptionally high riverine boron concentrations and extremely low δ11B values. Calculation indicates that >50% of riverine boron is sourced from geothermal waters. Combined with global datasets, we show that global riverine boron input to the ocean is largely influenced by geothermal input. Mass-balance calculations indicate that an averaged 1.5-fold decrease in global geothermal inputs is sufficient to introduce 3‰ increase in Cenozoic δ11Bsw. Therefore, geothermal waters significantly affect global riverine and thus oceanic boron budgets. The increased δ11Bsw since the Cenozoic may be resulted partly from declining global geothermal activity.