STRONTIUM ISOTOPE PROFILE FROM A LOWER JURASSIC SHALLOW WATER CARBONATE PLATFORM IN THE DINARIDES, SLOVENIA: A HIGH-RESOLUTION PICTURE OF THE TOARCIAN OCEANIC ANOXIC EVENT

Oceanic Anoxic Events (OAEs) such as the Toarcian Oceanic Anoxic Event (T-OAE) are associated with some of the most significant climatic and paleoceanographic changes in the Phanerozoic. Traditional approaches to studying OAEs examine the relationships between volcanism, geochemical changes, anoxia, and black shale deposition in deep-water settings. Nevertheless, OAEs are also intervals of significant shallow-water biotic turnover. Despite their importance, shallow-water OAE records are difficult to characterize; the dearth of pelagic fossils (e.g., ammonites and belemnites) means there is often poor biostratigraphic control and necessitates bulk-rock analysis for high-resolution studies. Strontium isotope (⁸⁷Sr/⁸⁶Sr) curves can be used to tie loosely constrained strata to better-dated Jurassic sections, thereby enhancing temporal controls. More importantly, ⁸⁷Sr/⁸⁶Sr and carbon isotopes (δ¹³C) are linked through the global weathering cycle. For the T-OAE, paired ⁸⁷Sr/⁸⁶Sr and δ¹³C measurements can be used to test the hypothesis that an increase in global weathering was a contributing factor to the Toarcian biotic crisis.

Here we present ⁸⁷Sr/⁸⁶Sr data from a section of the Adriatic Carbonate Platform on the Trnovski Gozd Plateau in Slovenia. Our data show the negative carbon isotope excursion that demarcates the onset of the T-OAE is followed by a positive Sr excursion. We interpret the paired isotope excursion as the release of light volcanogenic CO₂, followed by an increase in crustal weathering due to climatic warming. Thus, an increase in shallow-water nutrient levels may have been a co-driver for biotic turnover. Additionally, we use supplementary stable isotope and trace element data from carbonates to evaluate the reliability of the ⁸⁷Sr/⁸⁶Sr curve generated from bulk-rock. This study demonstrates a multi-proxy approach to quantifying the shallow-water geochemical signature of OAEs and the consequent transitions in life.