SILICATE WEATHERING REGIMES BEFORE LAND PLANTS INFORMED BY PALEOSOL LITHIUM ISOTOPES

Terrestrial silicate weathering is the primary sink for atmospheric carbon dioxide on geologic timescales. Lithium isotopes (δ⁷Li) are a promising tool for tracking silicate weathering across Earth history because they strongly fractionate during the formation of clay weathering products. Most work on the proxy centers around reconstructing globally-integrated seawater δ⁷Li from ancient marine sedimentary rocks, but the proxy’s fidelity in ancient terrestrial settings remains underexplored. Here we report δ⁷Li from 12 well-characterized and widely-studied paleosols from the Archean to early Paleozoic, encompassing Earth’s oldest preserved terrestrial weathering environments. We report muted isotopic fractionations (Δ⁷Li_{paleosol-parent}) in these paleosols relative to analogous modern soils. Most paleosols in our dataset exbibit a similar degree of elemental Li loss as modern soils, suggesting weathering of underlying bedrock as a primary control on Li isotope fractionation. A minority of paleosols exhibit elemental Li enrichments caused by the addition of material with a distinct isotopic signature to the top of the profile, potentially dust or marine aerosol input by analogy to modern soils. Despite pervasive sericitization and chloritization replacing the primary clay mineralogies of most Precambrian paleosols, we find no evidence for diagenetic or low-grade metamorphic overprinting of δ⁷Li. If taken as primary, muted δ⁷Li fractionations suggest more congruent weathering prior to the origin of land plants, where physical erosion dominates and clay formation is limited. Limited clay formation in the weathering zone could explain the rarity of mudrock in the Precambrian continental sedimentary record. Reduced fractionation in Precambrian paleosols also supports the marine carbonate δ⁷Li record, which requires that terrestrial inputs of dissolved Li to the oceans must have been isotopically lighter (and closer to un-weathered crustal values) than they are today.