EVIDENCE FOR GLACIATION ACROSS THE SILURIAN-DEVONIAN BOUNDARY: INSIGHTS FROM OSMIUM AND CLUMPED ISOTOPES

The Silurian-Devonian boundary (SDB; 419.2 Ma) records one of the largest carbon cycle perturbations of the Paleozoic, via a marked large positive carbon isotope excursion (2 to 6‰). Although the cause remains elusive, it has been postulated that orogenic changes coupled with increased primary production from the expansion of early vascular terrestrial vegetation, led to increased carbon burial, lowering atmospheric CO₂, which drove cooling across the SDB. Organic carbon burial was reduced as a result of sea-level fall. Subsequently, carbon-rich deposits eroded during the regression of the Early Devonian Sea and a geodynamically active tectonic setting led to organic matter reburial and oxidation causing an abrupt shift in carbon isotope ratios to return to background levels.

Another theory extends periodic end-Ordovician and early-mid Silurian glaciations through the Silurian to the SDB. The Hirnantian glaciation and the Ireviken Event are punctuated by carbon isotope excursions linked to major southern hemispheric glaciation. Despite a lack of evidence, Late Silurian carbon isotope excursions like the Lau Event or SDB could be related to major southern hemisphere glaciations induced by CO₂ drawdown from the Caledonian Orogeny.

Utilised here are osmium isotope (¹⁸⁷Os/¹⁸⁸Os) values of organic-rich sedimentary rocks from three globally distributed geological formations that span the SDB. We record an osmium isotope profile similar to that for the Hirnantian glaciation, which reflects changes in the weathering of radiogenic continental crust that occur in tandem with the SDB carbon isotope excursion. This data in combination with oxygen (δ¹⁸O) and clumped (Δ₄₇) isotope data - which can be used to determine the δ¹⁸O and temperature of seawater - may be the key to deciphering between glaciation and an exogenic system shift as causes for the SDB carbon cycle perturbation.