"Neoproterozoic glaciation, oxygenation, and the emergence of animals". By Francis Macdonald (Harvard University, EPS department)

The Cryogenian Snowball Earth events represent the largest episodes of climate change in the geological record, yet we do not know how or why runaway glaciation was initiated. These glaciations have been linked to a putative oxygenation event and the emergence of animals, but the relationships between these events have remained poorly constrained. I will report new geological, geochronological, geochemical, and climate modeling results that inspire new models for both the initiation of glaciation and the nature of environmental change in the glacial aftermath. Particularly, I propose that the proximal trigger for the Sturtian glaciation was the emission of volcanic aerosols associated with the Franklin large igneous province (LIP) and a sudden increase in planetary albedo. This hypothesis was motivated by geochronological constraints on the onset of the Sturtian glaciation and Franklin LIP, which show that they were coincident within analytical error of less than million years at ca. 716.3 Ma. Moreover, the Franklin LIP was the largest Neoproterozoic volcanic event and was emplaced into a sulfur evaporite basin at the equator. Geochronological data demonstrate that the Sturtian glaciation lasted ~57 million years. A compilation a paleontological data reveal a turnover in morphotypes across the Cyrogenian glaciations. Weathering proxy data suggest extreme weathering in the Snowball aftermath fertilized the ocean, promoting organic carbon burial in newly formed basins and the concomitant release of free oxygen. Uranium isotope data from carbonates record a temporary oxygenation event following the Sturtian glaciation, but anoxic conditions returned after the Taishir carbon isotope excursion. Thus, the large variability in the Cryogenian carbon cycle, recorded in the carbon isotope record, corresponds with evidence for fluctuating oxygenation, which may have stimulated Neoproterozoic micro-eukaryote turnover, but limited animals to small, sessile, soft-bodied forms. [Background reading]