A multi-proxy approach to constrain reducing conditions in the Baltic Basin during the late Silurian Lau carbon isotope excursion

The Silurian was a dynamic time characterized by significant climatic and sea level changes, biotic crises, and carbon cycle volatility. The largest magnitude perturbation to the Silurian global carbon cycle was the mid-Ludfordian carbon isotope excursion, termed the Lau CIE, which was coincident with the Lau/Kozlowskii extinction (LKE). Much of the published research on the late Silurian has described changes in the biotic record and global marine redox conditions. Limited work has been done, however, to elucidate the variability in local paleo-redox conditions. Here, we use a suite of paleo-redox proxies to examine a shallow shelf carbonate succession from Gotland, Sweden, and a deep shelf clastic sequence from Latvia. Low iodine-to‑calcium ratios from the carbonate succession suggest an anoxic water column, or significant exchange with nearby reducing water masses. Iron speciation data and moderate trace metal enrichments suggest that the sediments of the deeper shelf were deposited in an oxygen minimum zone with a denitrifying water column before and during the LKE with sulfidic conditions limited to the sediment porewaters. After the extinction, a relative depletion in trace metal concentrations (V, Hg, U, Mo) suggest that water column conditions were locally less reducing. The depletion in trace metal concentrations is coeval with previously reported global increases in anoxia and euxinia after the LKE and, thus, pontentially could have resulted from a global drawdown in trace metals overprinted upon the local record of paleo-redox change.