INSIGHTS INTO THE GLOBAL REDOX STATE OF THE EARLY SILURIAN OCEANS FROM NEW THALLIUM ISOTOPE RECORDS WITHIN BALTICA

After the Late Ordovician Mass Extinction, marine fauna experienced a prolonged recovery period in the early Silurian (Llandovery Epoch), with highest rates of speciation in what has been termed the Rhuddanian-Aeronian Radiation Event. This was followed by a significant marine extinction (Sandvika/sedgwickii event) and a positive 2-6 ‰ carbon isotope excursion (CIE) in the late Aeronian, ~ 439 Ma. Currently, the causal factors surrounding the late Aeronian extinction and associated CIE are understudied relative to numerous other extinction events/CIE’s in Earth history. Many extinction/turnover events in the Paleozoic and Mesozoic have been shown to be driven, at least in part, by changes in redox conditions within global oceans. Here we test the hypothesis that expansion of marine reducing conditions was a causal mechanism for the late Aeronian Sandvika/sedgwickii extinction event. Specifically, utilizing the novel thallium isotope paleoredox proxy can bring to light the major changes in Mn-oxide burial rates within the global oceans. On geologically short-time scales this is controlled by redox conditions of marine bottom waters on a global-scale.

In this study we have analyzed black shale samples from the paleocontinent of Baltica in two basinal successions, the Röstånga-1 (southern Sweden) and Aizpute-41 (Latvia) drill cores. The novel global marine redox proxy, thallium isotopes, will be utilized on both cores where previous iron speciation and trace metal geochemistry have indicated these environments were locally anoxic to euxinic. Preliminary data suggests dynamic marine redox conditions from the Rhuddanian throughout the Aeronian. New insights are gained from coupling these geochemical results with eustatic sea level and records of early Silurian glaciation. Thus, these results suggest a more complex marine redox and climate history throughout the early Silurian Sandvika/sedgwickii event and associated CIE.