The Magnesium Isotope Budget of the Modern Oceans

Magnesium (Mg) is the 8th most abundant element in the continental crust and the 4th most abundant species in seawater. Its transfer from the continents to the oceans, via rivers, and return to the solid Earth via hydrothermal exchange at mid ocean ridges constitutes one of the major chemical exchanges between the mantle and the hydrosphere. However, the oceanic cycle of Mg has proved difficult to quantify both in the modern day and in the past.

Mg has three stable isotopes and measurement of their ratios by MC-ICP-MS (expressed as δ²⁶Mg) are emerging as a powerful tool for understanding geochemical cycles. We have measured Mg isotope ratios in more than 100 rivers from different geologic, tectonic and climatic settings, including 16 of the largest rivers in the world, accounting for almost 50% of the riverine flux of Mg to the oceans.

The range in riverine δ²⁶Mg is 2.5‰, half the variation in terrestrial rock. The average of all rivers measured so far is -1.34±0.11 (2σ_mean) relative to the DSM3 standard. The best estimate of the Mg isotope composition of continental runoff is a flux weighted mean of the largest rivers in the world at -1.09‰. Even taking into account uncertainty, this average riverine δ²⁶Mg value is distinct from seawater at -0.82‰ This difference arises either from the fractionation of Mg isotope ratios in the ocean or a Mg budget which is not in steady state.

The difference is consistent with fractionation by carbonate precipitation. In the simplest steady state scenario, where the oceanic 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 output Mg flux. This is greater than some previous estimates of the modern dolomite flux.