When Life got Hard: An Environmental Driver for Metazoan Biomineralization

Given the template-directed mineralization of their funnel-in-funnel carbonate shells, Cloudina is considered the earliest complex multicellular metazoan. The tubular Ediacaran fossil has a global distribution, but is best known from the Nama Group in southern Namibia. We have recently re-discovered the fossil in a newly-recognized unconformity-bounded sequence above the Mara Member of the lower Nama Group in three locations south of Aus. Cloudina here occurs as individual cones floating in a micritic matrix, as strings of stacked cones on bedding surfaces, and as lithified micro-reefs in intimate association with microbial mats. The occurrence of Cloudina immediately above the Mara Member is critical insofar as this unit preserves evidence for the greatest negative carbon isotope anomaly in Earth history - the Shuram Excursion - when Ediacaran oceans were broadly ventilated and oxic conditions likely prevailed in shallow marine environments by the end of the biogeochemical perturbation. Given the global distribution of the Shuram Excursion, its stratigraphically cohesive pattern of carbon isotope change, and its preservation in a wide range of environments, the carbon isotope excursion may be understood in terms of the pulsed addition of ¹²C-rich alkalinity to the oceans. The driver of the biogeochemical anomaly and the onset of biomineralization is envisioned as tectonic in nature, and related to widespread Pan-African orogeny as East and West Gondwana were sutured. Erosion of uplifted Ediacaran terrains delivered thick piles of sediments to the oceans, as well as bio-limiting nutrients, especially N, P, and Fe, sulfate, and carbonate alkalinity produced through silicate weathering. The ¹²C-rich alkalinity may alternatively have built up in anoxic seawater through microbial sulfate reduction promoted by the weathering flux of sulfate to the oceans and a vast reservoir of dissolved organic carbon. Our regional chemostratigraphic analysis documents the final recovery from the Shuram Excursion within the newly-described sequence suggesting that shallow seawater had become oxidizing in this interval. The sudden appearance of biomineralized metazoans thus appears to reflect a rapid change in the redox state and the concentration of alkalinity in the immediate post-Shuram ocean.