Tracing chemical weathering under climatic gradients with silicon isotopes

There is a feedback between chemical weathering of Ca-Mg silicate rocks and global climate, associated with changes in atmospheric CO2, temperature and hydrological cycle. Chemical weathering in the Critical Zone controls nutrient delivery to terrestrial ecosytems and to the ocean, and silicon stable isotopes are used to trace the contribution from weathering processes. But the impact of climate on Si isotope variations in soils and rivers remains poorly constrained. Here, we evaluate the relationship between climate (precipitation), elemental depletion in soils, mineralogy of secondary phases and Si isotope variations in soils from Guadeloupe. Our results highlight that soils derived from a similar parent andesite exposed to different rainfall conditions (Western and Eastern slopes of La Soufrière volcano) display larger Si isotope fractionations associated with extensive Si depletion in soils exposed to heavy rainfall. Then, we investigate the impact of glacial weathering on riverine Si isotope signatures in Iceland, including the effect of physical erosion on chemical weathering fluxes. The results show that in basaltic catchments, glacier-fed and direct runoff rivers display significantly different Si isotope compositions, with lighter values associated with high physical erosion rates in glacial rivers. These approaches support that both soils and rivers Si isotope signatures respond to change in chemical weathering under climatic gradients. This provides a promising insight to develop Si isotopes as a tool for paleo-environmental reconstruction.