Chemical Weathering and Erosional Transport in an Ancient Shield Terrain

Abstract

This project investigates how isotope systems respond to changes in continental weathering processes and the consequences for the chemical composition of the oceans. Both experimental and natural data indicate that Li and Mg stable isotope systems preserve information on the mineral reactions controlling water chemistry. Dissolution experiments indicate that primary mineral dissolution has little effect on Li, but does fractionate the isotopes of Mg, whereas secondary mineral formation involves preferential uptake of the lighter isotopes of both Li and Mg. Glacial regions have low chemical weathering rates so their rivers should, in principal, have Li isotope compositions (δ7Li) that are similar to those of the underlying rock. In practise, glacial rivers in Greenland have δ7Li values that differ significantly from the source rock. Subglacial uptake of 6Li by iron oxyhydroxides appears to influence the riverine Li isotope composition. In contrast, the principal control on Mg isotopes in glacial rivers appears to be primary mineral weathering. Carbonate dissolution dominates the chemistry of glacial rivers, and this phase possesses a light Mg isotope composition imparting this signature to the Greenland rivers. Both the Li and Mg isotope compositions of glacial rivers are similar to their respective global riverine averages, suggesting that the impact of glaciation on the Li and Mg composition of seawater may be small. The Re-Os radiogenic isotope system is also a useful tracer of continental weathering, its composition in the oceans being sensitive to changes in the flux and composition of continental weathering. Laterites are widespread in tropical regions and are shown to contain high concentrations of highly unradiogenic Os. Consequently, their subsequent weathering and erosion has potential to significantly affect the Os isotope composition delivered to the oceans.