Descent into the Cryogenian; secular trends in seawater chemistry offer insights into pre-Sturtian paleoenvironments

Abstract

The Tonian was a remarkable period in Earth’s history, experiencing dramatic changes in the Earth system that resulted in a series of global climatic catastrophes, ultimately leading to marked changes in the biosphere. The break-up of the supercontinent Rodinia is thought to have had a profound impact on the Earth system during the Tonian, ultimately resulting in the ~58 million-year deep freeze of the Sturtian glaciation. Increased continental margins and the weathering of vast continental flood basalts, promoting high levels of primary productivity, were large players in the drawdown of atmospheric CO2, the chemical implications of which resonate through Tonian ocean chemistry.

The laterally extensive Skillogalee and Myrtle Springs Formations, within the Burra Group sediments of South Australia, span a pre-Sturtian carbonate succession that offer further insights into late Tonian paleo-environments. High resolution geochemical trends throughout the Skillogalee and Myrtle Springs Formations display evidence of a shift from a restricted to a more open marine setting, that was met with a flux of hydrothermally influenced waters. A hydrothermal flux through underlying basalts likely accounts for the enigmatic widespread deposition of sedimentary magnesites throughout the Skillogalee Formation. The longevity and regional reproducibility of significant europium anomalies suggests that a mafic mantle flux endured throughout Burra Group sedimentation. Cerium anomalies, supported by Zn/Fe ratios, evince open marine seawaters were largely dysoxic throughout the late Tonian, constraining Neoproterozoic oxygenation to post-Sturtian. Neodymium isotopes in Burra Group marine waters display a rise towards primitive endmembers, reflecting a mafic weathering flux, induced, no doubt, by the break-up of Rodinia. High levels of organic carbon burial are coupled with pronounced mafic weathering, detailing a mass drawdown of CO2 throughout Burra Group sedimentation during the late Tonian, hereby also holding important ramifications for the instatement of the Cryogenian glaciations.