Investigation of Changes in Paleoceanographic Redox State as a Driver for Early Silurian Extinction Events Using Multiple Geochemical Proxies in the Baltic Basin

• Other Authors: Benayoun, Emily (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Geology
• Online Access: http://purl.flvc.org/fsu/fd/2019_Spring_Benayoun_fsu_0071N_15112

Two mudstone-dominated early Silurian drill cores from the Baltic Basin were analyzed for pyrite sulfur isotopes (δ34Spy), organic matter carbon isotopes (δ13Corg), as well as iron speciation and trace metal chemostratigraphy to reconstruct the marine redox state during the early Silurian (Llandovery to earliest Wenlock). Three globally correlated positive carbon isotope excursions (+ 2 to + 3‰ magnitude shifts) are recorded within the Baltic Basin. The late Aeronian CIE and coincident positive δ34Spy (~ + 22‰ magnitude) shift are recorded in the proximal shelf and distal slope sections within the Lituigraptus convolutus- Sprirograptus guerichi Graptolite Zones. The rising limb of the Valgu CIE and coincident positive δ34Spy (+ 10‰ magnitude) shift are recorded in the distal slope section within the Streprograptus crispus Graptolite Zone. Additionally, the rising limb of the Ireviken CIE and coincident positive δ34Spy (+ 8‰ magnitude) shift are recorded in the proximal shelf section within the Cyrtograptus murchisoni- Monograptus firmus Graptolite Zones. These parallel isotope trends can be explained by changes in marine redox state that resulted in increases in microbially mediated pyrite formation and burial along with enhanced organic matter burial rates. Iron geochemistry independently constrains euxinic deposition (i.e., anoxic and sulfidic bottom waters) during the rise of the late Aeronian CIE with the possibility of similar reducing conditions during the rise of the Valgu and Ireviken CIEs. Coincident enrichments in vanadium (V) and molybdenum (Mo) concentrations during the rising limb of the late Aeronian CIE suggest the expansion from oxygen-deficient but non-sulfidic waters to fully euxinic within the Baltic Basin. Sedimentary iron and trace element enrichment trends are broadly consistent with oxygen minimum zone (OMZ)-type redox conditions in the water column and sediments porewaters with additional contributions from a manganese (Mn)- and Fe- shuttle. Intermittent expansion of an OMZ was likely controlled by local fluctuations in sea-level and provides a reasonable explanation for graptolite and conodont extinction events that occur during the late Aeronian and Valgu CIE intervals. An expansion of reducing conditions would have reduced habitable environments for marine organisms and geochemical evidence for reducing conditions is broadly consistent with extinction events recorded in the Baltic Basin during this period of the early Silurian. === A Thesis submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. === Spring Semester 2019. === April 5, 2019. === biogeochemistry, carbon isotopes, euxinia, extinction, sulfur isotopes, trace metals === Includes bibliographical references. === Seth A. Young, Professor Directing Thesis; Jeremy D. Owens, Committee Member; Yang Wang, Committee Member.