The Magnesium isotope budget of the modern oceans

We have measured the magnesium (Mg) isotope ratios in 45 rivers including 16 of the largest rivers in the world, covering a range of drainage basin environments. The range in riverine δ^{26}Mg is 2.5‰, half the variation in terrestrial rock. The rivers analsyed constitute 30% of the global riverine flux of Mg to the oceans and a flux weighted mean Mg isotope composition of global runoff has been estimated at -1.09‰. Even taking into account uncertinty, this is distinct from seawater at -0.82‰. This difference arises, either from the fractionation of Mg isotope ratios in the ocean, and/or a Mg isotope budget which is not at steady state. The difference is consistent with a fractionation by carbonate precipitation. In the simplest steady state scenario, where the coeanic mass balance is maintained by riverine input and hydrothermal and dolomite output, Mg isotope ratios imply a minimum dolomite Mg flux of 9% of the total Mg output flux. The implications of the distinct isotopic composition of the riverine flux from modern seawater, is that the Mg isotope ratio of sewater has probably varied outside of current analytical uncertainty. Mg isotope ratios may therefore provide a valuable new tracer in palaeo-oceanography.