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
2015 (English)In: Geological Society of America Abstracts with Programs. Vol. 47, No. 7,, 2015, Vol. 47, 698- p.Conference paper, Abstract (Other academic)
The Siljan ring district in central Sweden was created by a bolide impact at 377±2 Ma that triggered oil generation from organic-richmudrocks (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 constraintson the global ocean redox conditions before and after the Late Ordovician Hirnantian glaciation using the U and Mo isotopepaleoredox proxies. Here, we analyzed δ238U (relative to standard CRM145 = 0‰) and δ98Mo (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 ofRe, Mo, and U enrichment, Re/Mo and U/Mo ratios, and Fe speciation indicate euxinic and oxygenated bottom water conditions duringdeposition of the Fjäcka Shale and Nederberga Formation, respectively. The same proxies suggest that the Kallholn Formation wasdeposited 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 yieldsthe highest δ98Mo (~1.3‰) and δ238U (~0.1‰) of the studied units. High Mo/TOC ratios (>30 ppm/wt%) of the Fjäcka Shale indicateweak basin restriction and large amounts of Mo in the euxinic bottom waters, which could lead to Mo isotope fractionations betweenseawater and sediments due to incomplete formation/removal of tetrathiomolybdate. This interpretation is further supported by high Uisotope composition in the Fjäcka Shale, which is only slightly lower than the modeled value of 0.2‰ for modern open ocean euxinicsediments. 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 Shalesuggest a more oxygenated ocean prior to the Hirnantian glaciation than previously thought, though the extent of oxygenation was lessthan today.
Place, publisher, year, edition, pages
2015. Vol. 47, 698- p.
Isotopes, Ordovician, Silurian, Siljan region, Swede
Other Earth and Related Environmental Sciences
Research subject Earth Science with specialization in Historical Geology and Palaeontology
IdentifiersURN: urn:nbn:se:uu:diva-270309OAI: oai:DiVA.org:uu-270309DiVA: diva2:889552
GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)