A 3 b.y. record of a biotic influence on terrestrial weathering

Abstract

Abstract Organic acids secreted by plants and microorganisms are ubiquitous in modern soils. These acids possess a particularly strong binding affinity for aluminum, accelerating its release from mineral surfaces and driving its preferential loss relative to immobile elements such as titanium. Aluminum-titanium (Al-Ti) decoupling in ancient soils may therefore serve as a tracer of a biotic influence on terrestrial weathering in Earth’s past. To explore this idea, we used a mass balance approach to quantify Al mobility in 33 definitive and chronologically well-constrained paleosols spanning the Archean to Miocene. We estimated expected Al release for a given weathering intensity under abiotic conditions using previously established experimental relationships between the relative losses of Al and magnesium (Mg), a mobile element with a much lower organic acid binding affinity. We report Al release likely attributable to organic acid weathering in all paleosols, with net loss in 13, net gain in 16, and a balance between loss and gain in four. This provides a new line of support for a significant terrestrial biosphere as far back as the mid-Archean. Interestingly, there is no statistically significant change in Al mobility across major transitions such as the Great Oxidation Event or the Paleozoic spread of vascular plants. This controversially suggests that localized organic acid weathering, at least within the surface environments captured by the paleosol record, may have been as common on early Earth as it is today.