142 / 2025-04-08 02:42:52

Spatial variations in lithium isotope composition of the Doushantuo cap dolostone: Implications for chemical weathering, reverse weathering, and deglaciation of the Marinoan Snowball Earth

Cryogenian Period,Ediacaran Period,Snowball Earth,Cap dolostone,Lithium Isotopes

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

Lithium isotope compositions of marine carbonate deposits and biominerals are an emerging and promising proxy for chemical weathering on the continent, reverse weathering on the seafloor, and their impact on the geochemical cycles. However, the application of this proxy in the study of Precambrian carbonates is limited. In this study, we analyzed the lithium isotope compositions of the basal Ediacaran cap dolostone (i.e., the ~635 Ma Doushantuo cap dolostone) in South China to test several predictions of the Snowball Earth hypothesis. According to the snowball Earth hypothesis, continental chemical weathering diminished substantially during the glaciation (thus reducing a ⁷Li-enriched source into the ocean), whereas reverse weathering on the seafloor remained strong but with limited fractionation (thus weakening a ⁷Li-depleted sink from the ocean), potentially leading to synglacial seawater with relatively low δ⁷Li values. Subsequent deglaciation of the snowball Earth would result in a plume of meltwater with high δ⁷Li values (due to chemical weathering of a large amount of rock flours generated during the glaciation). The plume of meltwater would sit above more dense seawater, creating both vertical and lateral salinity and δ⁷Li gradients that could have been persistent 10^3–4 years. Consistent with this prediction, our data shows an overall decreasing δ⁷Li trend of the Doushantuo cap dolostone from nearshore to offshore environments. The δ⁷Li data are consistent with key predictions of the snowball Earth hypothesis and calls for the further tests of these predictions using other geochemical proxies.