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Continental weathering, Lithium isotopes, Atmospheric oxygenation, Weathering front
Abstract:
The Li isotopic composition of seawater (δ7LiSW) has been recognized as a promising tracer for deep-time geochemical cycles, reflecting water–rock reactions linked to the interactions between lithosphere, hydrosphere, biosphere and atmosphere. A major challenge in interpreting δ7LiSW is the significantly lower δ7LiSW values observed during many intervals of the geological history compared to the present-day level. Specifically, a δ7LiSW lower than ∼22 ‰ generally requires a globally riverine input with an average δ7Li much lower than that of the hydrothermal input (∼8.3 ‰) while keeping the net Li isotopic fractionation between seawater and Li sinks unchanged. However, riverine δ7Li values lower than ∼8.3 ‰ are relatively uncommon in modern Earth surface environments, whose spatial variability likely resembles a wide range of environments in the geological past. This study suggests that the low δ7LiSW of the Precambrian to early Paleozoic ocean can be attributed to the low efficiency of continental silicate weathering, driven by the low partial pressure of atmospheric oxygen (pO2) that results in shallow weathering fronts and thus thin regolith. A numerical model shows that a two-order-of-magnitude increase in pO2 from the Neoproterozoic to the Paleozoic could induce a ∼22 ‰ rise in δ7LiSW, fully accounting for the observed rise of δ7LiSW during this period. This is because enhanced continental weathering not only increases riverine Li input to the ocean with high δ7Li, but also provides more degraded weathering products that serve as precursors for authigenic clay formation in marine sediments, thus leading to a higher net Li isotopic fractionation between seawater and Li sinks.
