ASSESSING GLOBAL MARINE REDOX CHANGES DURING THE MID-SILURIAN MULDE/LUNDGRENI EXTINCTION EVENT

The Silurian Period, a dynamic interval within the Phanerozoic Era, experienced notable climate shifts, fluctuations in the global carbon cycle, and multiple extinctions, including the mid-Silurian Mulde/lundgreni event at ~428 Ma. This Mulde/lundgreni event caused significant declines in various marine organisms, notably graptolites (with a reduction of 95%), as well as radiolarians, acritarchs, conodonts, and fish. The extinction coincided with sea-level fluctuations and a distinct double positive carbon isotope excursion (CIE). Previous explanations suggest that the event was caused by cooling climate and falling sea levels but lack precise constraints, and little is known about how marine redox conditions fluctuated throughout middle Silurian oceans during this interval. In this study, we analyzed shale samples of the Cape Phillips Formation from two deeper-water basinal sections in Arctic Canada, Abbot River and Twilight Creek. The Mulde CIE has been previously documented at both localities, where the presence of local anoxic bottom waters has been established through analyses of Fe speciation, pyrite sulfur isotopes, and redox sensitive trace metals across the studied interval. Our new Tl isotopic datasets suggest that global oceans became more reducing near the onset of the Mulde CIE and remained reducing throughout the CIE interval. Furthermore, we compare these new Tl isotopes from Arctic Canada to another Tl isotope dataset from a different paleobasin, the Baltic Basin (Latvia), to provide a more holistic global perspective on marine redox conditions through this interval of the middle Silurian. This study presents new insights and constraints on paleoredox conditions during the middle Silurian and suggests a complex marine redox and climate history associated with the mid-Silurian Mulde/lundgreni event and associated CIE.