Terrestrial records of weathering indicate three billion years of dynamic equilibrium

Abstract

Although continental weathering intensity has been invoked as a primary control on biogeochemistry, tectonics, and the carbon cycle throughout geologic history, it remains poorly quantified over Earth’s history. As a direct product of continental weathering, paleosols (fossil soils) offer unique insight into past weathering intensity, but they remain underused in efforts to constrain terrestrial weathering patterns over geologic time. Here, we compile the largest terrestrial weathering record to date, comprising 248 paleosol and weathering profiles that span three billion years. We analyze a suite of weathering indices to test common hypotheses around state-changes in terrestrial weathering intensity due to atmospheric changes and terrestrial biosphere expansion. Contrary to commonly invoked assumptions, we find that these weathering indices reflect consistent average terrestrial weathering intensity through time. No unidirectional state changes in average weathering intensity, as have previously been hypothesized, are detectable in the record. However, Phanerozoic paleosols preserve an increase in the total range of Chemical Index of Alteration (CIA) values, with the increased CIA range driven by the appearance of high-CaO paleosols. We compare the paleosol weathering record to weathering intensities recorded by select fluvial sandstones and diamictites.We interpret the overall stability of the continental weathering record as reflecting the baseline level of weathering from which the Earth system deviates during periods of perturbation (i.e., major climate transitions, rapid tectonic activity). With consistent weathering intensity over geologic timescales, the record supports subaerially-emerged continental area as a critical control on total potential erosional flux and nutrient flux to the oceans. The paleosol community should work to build an even more complete database of paleosol geochemistry to allow more nuanced analyses of terrestrial weathering through time.