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WHEN LIFE GOT HARD: AN ENVIRONMENTAL DRIVER FOR THE ORIGIN OF SEASHELLS

FRIDAY OCTOBER 25 – 11 AM

A. JAY KAUFMAN
DEPT. OF GEOLOGY, UNIVERSITY OF MARYLAND

ABSTRACT: 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.

BIO: A. Jay Kaufman's research has focused on the determination of changes in the isotopic composition of the oceans through time, by the analysis of stragraphic suites of little-altered carbonate rocks. Thus far, most of these studies have centered around Neoproterozoic (ca. 1000-544 million-year-old) sedimentary successions in Svalbard/East Greenland, Namibia, arctic Canada and Alaska, India, and the western USA. Temporal variations in C and Sr isotopes can be used as stratigraphic tools within and between basins, and through detailed correlations allow us to order key tectonic, biogeochemical, and paleoenvironmental events in Earth history.

Jay Kaufman research