RECONSTRUCTION OF LOCAL AND GLOBAL MARINE REDOX CONDITIONS DURING DEPOSITION OF LATE ORDOVICIAN AND EARLY SILURIAN ORGANIC-RICH MUDROCKS IN THE SILJAN RING DISTRICT, CENTRAL SWEDEN
The most euxinic shales provide the most relevant estimates of global redox conditions. As expected, the euxinic Fjäcka Shale yields the highest δ98Mo (~1.3‰) and δ238U (~0.1‰) of the studied units. High Mo/TOC ratios (>30 ppm/wt%) of the Fjäcka Shale indicate weak basin restriction and large amounts of Mo in the euxinic bottom waters, which could lead to Mo isotope fractionations between seawater and sediments due to incomplete formation/removal of tetrathiomolybdate. This interpretation is further supported by high U isotope composition in the Fjäcka Shale, which is only slightly lower than the modeled value of 0.2‰ for modern open ocean euxinic sediments. Expanded ocean anoxia should lead to deposition of ORMs with low δ238U (<0‰) as observed during the Cenomanian-Turonian OAE2. Hence, the relatively high δ238U coupled with high Mo, Re, and U enrichments and Mo/TOC ratios in the Fjäcka Shale suggest a more oxygenated ocean prior to the Hirnantian glaciation than previously thought, though the extent of oxygenation was less than today.