Geothermal input significantly influences riverine and oceanic boron budgets

The evolution of boron isotope compositions of seawater (delta B-11(sw)) 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 in delta B-11(sw) 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 delta B-11 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%o increase in Cenozoic delta B-11(sw). Therefore, geothermal waters significantly affect global riverine and thus oceanic boron budgets. The increased delta B-11(sw) since the Cenozoic may be resulted partly from declining global geothermal activity. (c) 2023 Elsevier B.V. All rights reserved.