7. Boron in the Weathering Environment

This chapter reviews the state of art of the use of boron isotopes to understand water-rock interaction in the Critical Zone, the thin and reactive layer at the Earth’s surface. Because boron isotopes are largely fractionated by adsorption, coprecipitation and evaporation-condensation processes, boron isotopes are well adapted to trace the main processes that convert rocks into soils and sediments on terrestrial surfaces. The difference in affinity of boron isotopes between trigonal and tetrahedral species is the main cause of isotope fractionation of boron at the Earth’s surface. Due to the competition between the speciation of boron in solution and the speciation on boron onto or into solids or gas, large isotopic variations are predicted and observed. Measured boron isotopic composition in the weathering environment varies over a considerable range of about 70‰. Precipitation, rivers and biomass are usually enriched in ¹¹B, while a complementary depletion in ¹¹B (enrichment in ¹⁰B) is observed in clay minerals and on organic or inorganic surfaces. At the ecosystem scale, boron appears to behave as a micronutrient with a major flux of boron associated with biological recycling. The inputs of boron to ecosystems by chemical weathering or from the atmosphere are minor. When the residence time of water in the critical zone is high, such as in groundwater systems, boron contents increase and are much more dominated by a weathering signal. Boron is mainly added to the ocean by rivers, while the most important sink of boron is adsorption on clay minerals. This makes boron a particularly good tracer of the weathering/erosion balance of terrestrial surfaces in addition to its capacity for tracing the pH and ancient seawater. A lot remains to be done to better understand the behavior of boron and boron isotopes at the Earth’s surface and on the secular evolution of boron isotopes in the ocean but our review of the available literature shows that this tracer has a great potential at a local (ecosystem) and global (ocean) scale.