Source and evolution of dissolved boron in rivers: Insights from boron isotope signatures of end-members and model of boron isotopes during weathering processes

The evolution of atmospheric CO2 and the pH of the ocean can be reconstructed by the boron isotopic composition (delta B-11) of marine carbonates, which is influenced by the delta B-11 of the seawater. Boron (B) in the ocean is primarily affected by continental weathering through rivers. Thus, it is essential to understand the behavior of B and B isotopes in rivers and the factors affecting riverine B, which require a better understanding of sources and processes of B in river systems. This review evaluates the inventories of B reservoirs contributing to rivers and investigates the processes regulating the B isotope geochemistry of rivers. B is widespread at the Earth's surface and shows a wide range of concentrations between reservoirs. Different reservoirs also exhibit significant variations in B isotopic compositions. Mixing and Rayleigh effects are mainly responsible for the variations in the delta B-11 values of meteoric precipitation, which result in marine (delta B-11 = +37 +/- 7 parts per thousand), anthropogenic (delta B-11 = +9 +/- 10 parts per thousand), and mixing types (delta B-11 = +17 +/- 13 parts per thousand) of meteoric precipitation. The contribution of B to rivers from carbonate dissolution is negligible. Marine and non-marine evaporites have distinct delta B-11 values (marine delta B-11: + 27 +/- 9.4 parts per thousand and non-marine delta B-11: -2 +/- 8.6 parts per thousand) that primarily reflect their different depositional environments. S-type granites that are tourmaline-free have an estimated delta B-11 value of -14.2 +/- 4.9 parts per thousand and a Na/B value of 140 +/- 34. Non-S-type granites have a delta B-11 value of -8.9 +/- 6.7 parts per thousand and a Na/B value of 1190 +/- 170. Intraplate basalts exhibit a delta B-11 value of -5.2 +/- 4.4 parts per thousand and a Na/B value of 3300 +/- 770. Subduction-related basalts have a delta B-11 value of + 0.3 +/- 7.3 parts per thousand and a Na/B value of 1060 +/- 830. Shale has high B contents of siliciclastic sedimentary rocks (104 92 ppm). The inferred delta B-11 values of marine and continental shales are -8 parts per thousand and -16 parts per thousand, respectively. The effects of metamorphism can vary widely depending on the geologic setting and type of protolith. The delta B-11 values of wastewater are investigated based on their industrial, agricultural, and urban sources. This inventory of B reservoirs can be useful for studies on rivers on a continental scale. In regolith and groundwater, B isotopic fractionation mainly occurs due to water-rock interactions and the biological cycle of B, whereas adsorption on sediments leads to minor B isotopic fractionation in rivers. In groundwater, the reactive transport model reveals that the delta B-11 value of river water is sensitive to hydrological conditions. In regolith, the steady-state mass balance model is used to predict the B isotope behavior of soil solution in different weathering regimes. In the supply-limited regime (where chemical weathering is limited by tectonic forcing), the precipitation of secondary minerals controls the variations in the delta B-11 values of soil solution, leading to an increase in the difference in the delta B-11 values between soil solution and parent rock (delta B-11(diss)-delta B-11(rock)) with lower denudation rates, whereas secondary mineral dissolution produces the opposite change in delta B-11. In the kinetically limited regime (where chemical weathering is limited by climate), the biological cycle controls the variations in the delta B-11 values of soil solution, and the delta B-11 values of soil solution generally become closer to those of parent rock with higher denudation rates. The relationship between the denudation rates and delta B-11(diss)-delta B-11(rock) is thus not monotonous, indicating that additional constraints are required to distinguish between the two regimes. Understanding of B isotope geochemistry of rivers can be improved by better constraints on B end-member estimates, investigation of the B isotopic fractionation caused by weathering and biological cycling in regolith, and assessment of atmospheric and biological sub-cycle.