The Life and Times of Snowball Earth: Organic and Inorganic Geochemical Signatures of Cryogenian Ecosystems and the Rise of Algae

Abstract

Understanding the transition from a microbial Precambrian world to one dominated by complex eukaryotes remains a central, unresolved question in science. This study examines this ecological transition, using organic and inorganic geochemical signatures preserved in Neoproterozoic marine sediments. Extreme climatic perturbations from the Cryogenian Snowball Earth glaciations are hypothesised to have played a deciding role in this transition, facilitating a fundamental shift at the base of global marine food webs, from dominant bacterial to eukaryotic primary productivity, the so called 'rise of algae'. However organic records spanning this interval are absent, and so these claims remain speculative. In this thesis, the evolutionary and ecological shifts coincident with the Cryogenian Snowball Earth glaciations (717-635 Ma) are explored. The GSWA Vines 1 drillcore provides an exceptionally well-preserved 1,700 m glacial succession preserving a nearly complete history of the Marinoan glaciation in the epeiric sea setting of the Centralian Superbasin. This glacial succession provides a unique opportunity to fill a critical gap in the temporal record of molecular fossils, or "biomarkers", from one of Earth's most exciting chapters. Combining detailed biomarker analysis with contemporaneous inorganic geochemical proxies, the first quantitative insights into marine ecosystems during Snowball Earth are presented, highlighting the resilience and ecological dynamics of early eukaryotic communities that endured the frigid icehouse conditions and prolonged environmental extremes imposed by global ice cover. These results document a demise in eukaryotic production during the Marinoan glaciation, with a eukaryotic assemblage reminiscent of primitive old-world communities, and no detectable evidence of green algae. This likely reflects the challenging conditions imposed by Snowball conditions, with paleosalinity and paleoredox indicators demonstrating dramatic environmental shifts in the Officer Basin's epeiric sea setting, directly associated with ice-sheet development. The Snowball Earth biota reflect an energetically limited ecosystem, likely hampered by low-light conditions from extensive ice-cover. Putting these results into a global context, a new timeline for the rise of algae emerges, documenting a two-step algal rise, with an initial emergence of red algae in the Tonian, followed by a larger expansion of green algae in the early Ediacaran. The new record's unidirectional pattern implies a stable and directional ecological response, reviving the possibility that intrinsic biological factors, such as evolutionary innovation or competitive advantage, may have played a central role in the rise of algae, which may have occurred independent of glaciation. An early Ediacaran rise of algae also places significant algal activity closer to the appearance of large animals, encouraging exploration of links between metazoan evolution and efficient food sources. Show more