Lithium Isotope Evidence for Cryogenian Post-Glaciation Enhanced Weathering and CO2 Drawdown

Abstract:

The growth of continental ice sheets at equatorial sealevel during the Neoproterozoic Marinoan are of considerable interest, because they may have served as a filter for animal evolution and atmospheric oxygenation. Further, the hypothesised post-glacial extreme greenhouse state provides the opportunity to test climate system responses to rapid warming. In particular, the debate focuses on whether the ubiquitous post-glacial deposition of cap carbonates and associated negative δ¹³C excursion was caused by an enhanced weathering-driven delivery of atmospheric CO₂ to the oceans, or by the thermal destabilisation of marine methane hydrates.

Lithium isotopes are a relatively novel tracer of continental weathering. Li is almost entirely situated in silicates, rather than carbonates, and its isotopic fractionation in rivers is demonstrably due to the intensity of silicate weathering. In addition, Li isotope fractionation remains constant in marine carbonates, regardless of changes in temperature or type of skeletal calcite. Determination of Li isotope ratios through several well-characterised sections of the Otavi Group, NW Namibia, indicates similar trends, with δ⁷Li decreasing sharply by ~13‰, with the isotopic minimum occurring within the cap carbonates, and before the δ¹³C minimum. The δ⁷Li values are the lowest ever recorded in carbonate. These trends strongly suggest a significant increase in the intensity of silicate weathering during the deglaciation. Combining these data with a series of Earth system models coupled to δ¹³C allows a comprehensive interpretation of the changes in continental weathering and atmospheric pCO₂, and the link between the two, during the deposition of the cap carbonates. Together, this data-model approach helps elucidate climate system behaviour during this period of rapid and extreme climate warming.