OCEANOGRAPHIC REDOX CHANGES ASSOCIATED WITH THE LATE SILURIAN LAU EXTINCTION EVENT: NEW GEOCHEMICAL EVIDENCE FROM THE PRIEKULE-20 DRILL CORE, LATVIA

The mid-Ludfordian (late Silurian) is characterized by global oceanographic and biotic changes associated with the Lau carbon isotope excursion (CIE). The Lau CIE is well documented in areas such as Sweden, Ukraine, Czech Republic, Australia, Latvia, and North America. This excursion is one of the largest positive carbon isotope excursions recorded in the Phanerozoic, +8‰ or greater in some localities. The associated Lau-Kozlowskii extinction event (LKE), was initially recognized in conodonts and graptolites, and is the most severe and widespread documented biotic event in the Silurian. Importantly, anachronistic sedimentary facies have also been associated with the LKE, which are comparable to those linked with larger mass extinctions such as the Permo-Triassic. The causes and consequences of the LKE and the related CIE are not well understood with most work hypothesizing changes in the global carbon cycle, redox conditions and/or eustatic sea level. Here we report a new, multi-proxy data set for the late Silurian in the Priekule-20 drill core from Latvia (Baltica), including δ¹³C_carb, δ¹³C_org, TOC, trace metal enrichments, Fe speciation, and ε²⁰⁵Tl analyses. These data will help to elucidate the temporal variations of the global carbon cycle during such a perturbation though the use of multiple isotope systematics and redox proxies. This study begins to test whether the Lau CIE was the result of the increased burial of organic carbon due to increased reducing conditions in the world’s oceans or the weathering of a subaerial carbonate platform due to eustatic sea level fall. Coeval positive excursions in both the δ¹³C_{carb & org} and δ³⁴S_pyr data suggest that these perturbations were caused by the enhanced burial of organic matter and pyrite under euxinic (anoxic and sulfidic) waters. An expansion of euxinic conditions globally would drive a δ³⁴S_pyr excursion and be detrimental to marine life, which could serve as a mechanistic cause for the extinction event that occurs during the rising limb of the Lau CIE. Other proxies, such as Fe speciation, trace metal concentrations and Tl isotopes, will help determine local vs. global redox conditions and the global extent of reducing conditions. The novel Tl isotope data produced in this study indicate a minimum in oxygenated waters during the rising limb of the Lau CIE coincident with the LKE.