Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle

Analysis of river water, rock, travertine and soil from the high altitude, negligible vegetation setting of the Southern Tibetan Plateau demonstrates that Ca and Mg isotope ratios are fractionated during weathering. Dissolved Ca in the two rivers studied is derived primarily by limestone dissolution. δ44/42Ca in the rivers averages 0.43‰ and is statistically distinct from limestone at 0.31‰. The range in δ44/42Ca in these small rivers is 0.43‰, equivalent to the entire range in δ44/42Ca recorded in marine carbonate over the last 80 Ma. Precipitation of isotopically light travertine with a δ44/42Ca of 0.21‰ enriches solute Ca in heavy isotopes. The Mg isotope composition of the rivers is intermediate between limestone and silicate rock averaging − 1.5‰. Silicate soil has a δ26Mg of − 0.03‰, heavier than silicate rock by 0.5‰. These fractionations in the soil create a companion groundwater reservoir of heavy Ca and light Mg. Seasonal variations in Ca and Mg isotope ratios in the dissolved load are small, but define an array which can be modelled as a mixture between a fractionated groundwater reservoir and surface runoff (reflecting the isotopic composition of the lithology). Fractionation of Ca during the weathering of the continents is of importance to the global cycle of Ca. The riverine input of Ca to the oceans (dominated by carbonate weathering) is controlled not only by the composition of the primary continental crust but also by the size and composition of a fractionated reservoir on the continents. The impact on the oceanic cycle of Ca depends on the relative residence times of dissolved Ca in the ocean and the storage time of fractionated Ca.