Mesoproterozoic lacustrine clay-mineral record of weathering prior to greening of the continents
While the evolution of land plants has long been considered a step-change to the continental weathering regime, the bulk composition of sedimentary sequences has remained largely unchanged since the emergence of the first continents. However, a key shortcoming of conventional characterisation techniques is the inability to differentiate sedimentary constituents and geochemical signals that are products of weathering versus post-depositional alteration. This research investigates alluvial and lacustrine mudrocks from the Precambrian Wester Ross Supergroup (NW Scotland), which record a snapshot of weathering prior to the greening of the continents. I employ a novel petrographic approach, combining SEM-EDS mineral mapping and TEM imaging, permitting the quantitative differentiation of detrital phases, overcoming the limitations of previous studies. I find that detrital clays (illite, chlorite) are consistently of physical weathering origin, anomalously abundant μm-size feldspar grains, and an absence of chemically weathered clays. This indicates subaerial weathering was controlled by physical mechanisms at this time, in contrast to mudstones deposited in vegetated Phanerozoic environments, arguing for a weak chemical weathering regime in the Proterozoic. I conclude that the lack of a plant weathering signal in bulk rock studies is due to the pervasive post-depositional alteration of chemically immature sediments during ‘subsurface weathering'.