Paper No. 277-11
Presentation Time: 10:50 AM
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
LU, Xinze, Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada, KENDALL, Brian, Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, Canada, LI, Chao, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, 430074, China, STEIN, Holly J., AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482 USA, Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, 0316, Norway, HANNAH, Judith L., AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482, USA, Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, 0316, Norway, GORDON, Gwyneth W., School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ 85287-1404 and EBBESTAD, Jan Ove R., Museum of Evolution, Uppsala University, Norbyvägen 22, Uppsala, SE-75236, Sweden, xlv@uwaterloo.ca
The Siljan ring district in central Sweden was created by a bolide impact at 377±2 Ma that triggered oil generation from organic-rich mudrocks (ORM) of the Late Ordovician (Katian) Fjäcka Shale and/or the Early Silurian (Rhuddanian-Telychian) Kallholn Formation. New drill cores obtained by Swedish private company IGRENE AB in 2011 provide an opportunity to significantly improve constraints on the global ocean redox conditions before and after the Late Ordovician Hirnantian glaciation using the U and Mo isotope paleoredox proxies. Here, we analyzed δ
238U
(relative to standard CRM145 = 0‰) and δ
98Mo
(relative to standard NIST SRM 3134 = +0.25‰) of 26 ORM samples from the Fjäcka Shale, Kallholn Formation, and latter deposited Nederberga Formation. The extent of Re, Mo, and U enrichment, Re/Mo and U/Mo ratios, and Fe speciation indicate euxinic and oxygenated bottom water conditions during deposition of the Fjäcka Shale and Nederberga Formation, respectively. The same proxies suggest that the Kallholn Formation was deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface.
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.
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