The diversity of ancient clay mineral morphotypes: case studies of lacustrine deposits before and after the evolution of land plants

The evolution of land plants is predicted to have impacted clay formation and continental weathering, but petrological evidence of a shift in Earth’s ‘clay mineral factory’ coeval with the greening of the continents has so far proved elusive. In this study we apply a novel microscale mineral identification technique in conjunction with newly defined morphological criteria to differentiate between physically weathered and authigenic clays in pre- and post-vegetation mudrocks. Two lacustrine successions from northern Scotland were targeted, the Neoproterozoic Diabaig Formation and Devonian Caithness Flagstone Group, because they share common depositional environments, parent material, and provenance but were deposited either side of the Paleozoic onset of land plants. Despite the depositional similarities, 3900 grain counts reveal a distinct contrast in their respective clay mineral assemblages. The pre-vegetation Diabaig Formation records a weathering environment exclusively dominated by the physical erosion of regolith or bedrock (‘Class I clays’). The syn-vegetation Caithness Flagstone Group exhibits greater clay volume and physical weathered material augmented with particle morphologies relatable to pedogenesis (‘Class II clays’). The novel Class II clays in the Devonian samples occupy large domains or present as distinct composites that account for up to 14 wt% (median, 5.6 wt%) of the detrital illite fraction. This greater diversity and volume of clay material is suggestive of an evolution-induced shift away from weathering environments dominated by physical erosion and highlights the potential importance of chemical weathering reactions once thicker and more stable vegetated soils became established.