Enhanced Weathering: The effect of water-rock ratio on secondary mineral formation – evidence from lithium isotopes

Silicate weathering is assumed to serve as a long-term climate stabilizing process by drawing down CO₂ from the atmosphere. Weathering also has the potential to act as a negative emissions technology (enhanced weathering). A particular aspect of weathering that affects CO₂ drawdown is clay formation, which hinders the sequestration of carbon. The Lithium (Li) isotope system is considered to trace silicate weathering processes and secondary mineral formation, but the exact mechanisms that control fractionation are still not fully understood, especially aspects like adsorption vs. incorporation, and the controls by secondary mineralogy. A further aspect that is currently unknown is the effect of the water-rock ratio, given that in principle, more water relative to rock should result in less supersaturation (and hence precipitation) of secondary minerals.

In this project, we present data from a series of closed batch silicate weathering experiments, conducted at different temperatures, and with different rock types and different water-rock ratios. We let basalt, granite and sandstone react with a characterized water for ~70 days under different conditions to simulate weathering of powdered silicate rocks in different climatic settings.

Our results show that the pH of the solution decreases by 0.1-0.2 for sandstone and granite, and 0.4-0.5 for basalt. At room temperature, Li concentrations in the sandstone solution linearly decrease from ~40µg/l to ~37µg/l while granite solution Li concentrations decrease from 275µg/l to 247µg/l, suggesting varying amounts of clay formation in the different experiments. At 6°C Li concentrations in the sandstone solution stay constant between 33µg/l and 36µg/l. At the same temperature the granite solution shows an increase in Li concentration from 234µg/l to 280µg/l. Different water/rock ratios provided no systematic change in pH, but element concentrations were clearly affected after ~70 days of reaction time. Lithium isotope ratios increase as secondary minerals form, with variations according to water-rock ratio and lithology. Overall, the data shows that the water-rock ratio has a significant effect on both rock dissolution and secondary mineral formation, with implications for both natural and enhanced weathering.