Descending into the “snowball”: Improving interpretations of Tonian palaeoenvironments with multi-proxy elemental and isotopic geochemistry

Virgo, Georgina1,2, Collins, Alan2, Farkas, Juraj3, Blades, Morgan2, Amos, Kathryn1, Lloyd, Jarred2

1Australian School of Petroleum and Energy Resources, the University of Adelaide, SA 5005, Australia, 2Tectonics and Earth Systems (TES) and Mineral Exploration CRC, Department of Earth Sciences, the University of Adelaide, SA 5005, Australia, 3Metal Isotope Group (MIG), Department of Earth Sciences, the University of Adelaide, SA 5005, Australia

The Tonian–Cryogenian transition represents a period of significant physiochemical change in Earth history. It involved variations in oceanic and atmospheric oxygenation, significant changes in the biosphere, tectonic reorganisation, and the onset of the global ‘Sturtian’ glaciation. Tonian and Cryogenian sedimentary rocks in the Adelaide Superbasin, South Australia (SA), represent some of the most well-exposed, continuous and thick sections of this interval globally, recording major environmental shifts through distinct variations in lithology and isotope chemistry. Although this transition is geologically significant, it remains enigmatic due to a distinct lack of comprehensive, contemporary Tonian–Cryogenian research in South Australia.

We present robust palaeoenvironmental interpretations for a complete pre- to post- Sturtian glacial succession near Copley in the northern Flinders Ranges, SA. During fieldwork, a ~3km sedimentary log was measured for facies and sequence stratigraphic analyses, and 350 samples were collected for elemental and isotopic geochemical analyses. Our study reveals multiple regressive-transgressive cycles, recorded by deltaic rippled and cross-stratified sandstones, through lagoonal intraclastic magnesite and stromatolitic carbonates, to subtidal laminated siltstone and platform carbonates. These pre-glacial formations are unconformably overlain by subglacial to ice contact pebbly diamictites with quarzitic and dolomitic interbeds, which grade into proglacial laminated mudstone and sandstone with dropstones. We suggest that these facies reflect glaciomarine conditions. The post-glacial formation consists of subtidal laminated shales and carbonates, reflecting widespread transgression after the glaciation.

Elemental chemistry, along with C- and Sr-isotope signatures were analysed to determine primary basin water chemistries or palaeo-seawater compositions, and to further constrain the depositional setting. Results demonstrate a nearshore/restricted, dysoxic setting with indications for moderate hydrothermal input, which supports the sedimentological data. Furthermore, there is an inverse relationship between 13C and 87Sr/86Sr data, ranging from 7.37‰ to -6.68‰ and 0.7088 to 0.7182, respectively. In addition, the studied carbonates also exhibited relatively light 88Sr values (≤0.211‰). These isotopic observations could reflect a drop in relative sea level, increased weathering of carbonates and sufficient input of continental material, which is consistent with sequence stratigraphic interpretations. Such settings might be analogous to the modern Coorong depositional environment, SA, where interaction of seawater with brackish continental waters facilitate precipitation of primary dolomite and magnesite. Insights into the water chemistry and isotope signatures of these primary Mg-rich carbonates will assist with interpretations of isotope data collected from the studied Tonian dolomites and magnesites. This multi-proxy study presents new palaeoenvironmental insights into a key Tonian–Cryogenian succession, which sheds light in our understanding of how the world descended into one of the most severe glaciations ever recorded.


Biography

Third year Phd student, research focused on sedimentology and geochemistry of Neoproterozoic sediments in South Australia

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