17:30: When Life Got Hard: An Environmental Driver for the Origin of Seashells
17:30 – 19:30
One of the greatest carbon cycle anomalies in Earth history – known as the Shuram Excursion – is preserved in middle Ediacaran Period strata worldwide. Given its global distribution and stratigraphically patterns of carbon, sulfur, strontium, and uranium isotope change across a variety of lithofacies, the event may be understood in terms of the addition of 12C-rich alkalinity to progressively ventilated oceans. Our petrologic and geochemical studies suggest that the carbon isotope trends resulted from the production of authigenic carbonate, which formed, in part, from the microbial oxidation of methane, while the sulfur and strontium isotope trends are likely associated with tectonic uplift and enhanced weathering fluxes of sediments, sulfate and other ions, and nutrients. Near the end of this environmental perturbation, carbonate shells of the early worm-like animal, Cloudina, were recently discovered in southern Namibia. Growth of the oceanic Ca2+ reservoir during the Shuram Excursion most likely promoted animal biomineralization in the marine environment insofar as phosphate-based metabolism necessitates the exclusion of calcium from animal cells and its storage in carbonate shells. Assuming the Shuram Excursion represents a global oceanographic event, evidence for animal biomineralization was predicted in equivalent strata from the Patom Uplift in southern Siberia along the Lena River. Field work there in September 2021 led to the discovery of carbonate-encrusted bioclasts of a completely different organism. The bioclasts stand out from the surrounding carbonate matrix by the presence of pyrite death masks that formed around organic remains interior to their thin exterior shells. These pyritiferous structures stand out in thin sections and in 3D X-ray tomography of fossiliferous cores conducted in the MizzouX Laboratory, and highlight the presence of pores and internal channels for fluid transport and filter feeding. The exquisite morphological details preserved in this unique taphonomic window suggest that these are the remains of a sponge-grade animal that biomineralized a carbonate shell during life, similar to Cloudina. Sheet-like structures extending beyond the shell may have served for the absorption of dissolved and particulate organic matter, or for gas exchange. If these bioclasts of sponge-grade organisms are related to the soft-bodied Ediacara metazoans, the presence of thin external carbonate shells may well explain why the soft-bodied biota at large was so clearly preserved as casts and molds in fine grained siliciclastic sediments. The coupling of a major biological innovation with the end of a profound carbon isotope perturbation supports the view that the Shuram Excursion was a tectonic and oceanographic phenomenon – as opposed to a global diagenetic conspiracy. The temporal concordance of these profound events as preserved in strata from southern Namibia and central Siberia may thus be considered as exceptional geological markers for the base of the Terminal Ediacaran System.
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