Another continental pool in the terrestrial silicon cycle

Silicon is the second most abundant element on Earth and is involved in important terrestrial and marine biogeochemical processes. However, its terrestrial cycle is not well quantified and there is a ‘missing’ reservoir in the budget of silicon isotopes. Water in seas and rivers is enriched in silicon-30 compared with igneous rocks, suggesting that there is an unknown continental reservoir that is depleted in silicon-30. Now, with the help of a new-generation mass spectrometer, Basile-Doelsch et al. have obtained in situ measurements of silicon isotopes in quartz from sandstones in southern France. The results, showing greater silicon-30 depletion than in any previously reported terrestrial samples, suggest that silicified rocks formed from quartz precipitates in soils and groundwaters can account for the missing reservoir.