RECONSTRUCTING THE LI ISOTOPIC COMPOSITION OF TERMINAL EDIACARAN SEAWATER

This study aims to use the lithium isotope geochemistry of marine carbonates to constrain terrestrial weathering processes during the terminal Ediacaran interval. In the Khatyspyt Formation, Olenek Uplift, arctic Siberia, δ⁷Li values of limestone samples range from 16.1‰ to 17.9‰, (average = 16.6‰, n = 4) and dolostone samples range from 19.0‰ to 23.5‰ (average = 21.0‰, n = 3). This suggests that the corrected δ⁷Li composition of terminal Ediacaran seawater was ~20.8‰ (with Δ_SW–Cal.=5‰ and Δ_SW–Dol.=0‰), significantly lower than the modern δ⁷Li (31‰) composition. A preliminary Li box model suggests that the δ⁷Li depletion in terminal Ediacaran seawater value can be best explained by a significant riverine influx of isotopically light Li to the ocean, indicating intensive congruent weathering on land. In the Precambrian, the lack of evidence for extensive Si biomineralization suggests that silica was saturated in seawater, leading to extensive reverse weathering and limited Li isotopic fractionation during clay formation. New discoveries, however, of sponge-grade animals in Cryogenian and Ediacaran successions suggest that there was a sink for silica in Neoproterozoic oceans. This sink may have alternatively decreased seawater Si saturation, thus leading to higher Li isotopic fractionation during reverse weathering. Despite these uncertainties, we assume that the fractionation of Li output was the same as the modern ocean (Δ_SW⁃SED = 16‰) in terminal Ediacaran seawater, and hydrothermal input was also the same as the modern ocean due to near-modern hydrothermal activity occurring since the Paleoproterozoic. Under this scenario, to reach a steady-state δ⁷Li value of 20.8‰, the δ⁷Li composition of river water must have been lower than 4.8‰. We suggest that the δ⁷Li value of river water was likely similar to primary continental rocks (~0‰), indicating extensive congruent weathering. This extensive chemical weathering is consistent with the contemporaneous Pan-African orogenies that exposed continental areas and, via chemical weathering, delivered dissolved inorganic carbon, alkalinity, and silica to oceans. In summary, we use marine carbonates from Siberia to estimate the δ⁷Li composition of late Ediacaran seawater, providing insight into weathering processes at the dawn of animal life.