A new look at saponite formation and its implications for early animal records in the Ediacaran of South China

Abstract

Acanthomorphic acritarch fossils, including some interpreted to be the fossils of the earliest animal embryos, first appear in the lower Doushantuo Formation of the Yangtze Gorges area (YGA). Further, the complete paleontological and geochemical record for the YGA has played a central role in defining the global biological and geochemical backdrop that presaged and witnessed the dawn of diverse animal life. Despite the importance of the YGA in our understanding of Neoproterozoic Earth history, basic aspects about its depositional history remain debated. Foremost among the controversies, extensively studied sections in the YGA were recently tied to deposition in an alkaline lake, casting new but contentious light on the environments of early animal evolution and the broader significance of geochemical records from the YGA. Arguments for a lacustrine setting hinged on the presence of trioctahedral clays (saponite-corrensite). However, this clay type commonly forms in other environments, including the weathering profiles of mafic and ultramafic volcanics. Using a coupled geochemical and sedimentological approach, we argue that the trioctahedral clays in the lower Doushantuo of the YGA are better explained as weathering products from a regional mafic-to-ultramafic hinterland delivered by rivers to a shelf or lagoon in the Yangtze Gorges Basin. These novel provenance relationships for YGA sediments and associated clays are consistent with a marine setting for the early animal records and must factor in our current understanding of the broader geochemical fabric of the Doushantuo Formation.