CONSTRAINING PALEOREDOX CONDITIONS DURING THE MIDDLE SILURIAN MULDE/LUNDGRENI EVENT, CAPE PHILLIPS FORMATION, ARCTIC CANADA

The middle Silurian biochemical event known as the Mulde/lundgreni event (or ‘Big crisis’) is primarily documented by the significant decline in graptolites, conodonts, and radiolarian communities on a global scale. It coincides with a double-peaked positive carbon isotope excursion (CIE), which indicates that this extinction event also is associated with a global perturbation of the carbon cycle. Despite the extinction event and CIE’s recognition in several settings around the world, most of our understanding is based on the Swedish carbonate succession of Gotland. The other significant gap in knowledge about this event is that the possible cause(s) are still debated. Recent studies attribute this biotic turnover primarily to a global sea-level regression and cooling, but there are some inconsistencies as well, such as the absence of middle Silurian glacial sediments. In Earth's history, several extinction events have coincided with a change in marine redox conditions, however this hypothesis to date has never tested thoroughly in connection with the Mulde event and CIE.

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. Here we utilized Fe speciation, trace metals, and sulfur isotopes as paleoredox proxies, to differentiate oxic, anoxic, and euxinic paleoenvironments at the local scale. Based on the results of this study, we can better understand the evolution of marine redox conditions in this deeper marine basin. Additionally, these sections have been previously well-studied for graptolites and radiolarian taxa with the lungreni extinction event well characterized. Our new local paleoredox datasets can be directly compared to the established trends in biodiversity of these important Paleozoic planktic groups and local sea level reconstructions throughout the extinction event and CIE. These correlations are critical to assessing the mechanistic hypotheses proposed for this middle Silurian biogeochemical event.