HR: 1340h
AN: PP13A-1028    [Abstracts]
TI: The Geochemical Figure Print of an Early Paleozoic OAE
AU: * Gill, B
EM: bgill003@ucr.edu
AF: University of California-Riverside, Geology Building 900 University Avenue, Riverside, CA 92521, United States
AU: Young, S
EM: desethtacon@yahoo.com
AF: The Ohio State University, School of Earth Sciences, 275 Mendenhall Lab, Columbus, OH 43210, United States
AU: Kump, L
EM: kump@geosc.psu.edu
AF: Penn State University, Geosciences, 0535 Deike Building, University Park, PA 16802, United States
AU: Saltzman, M
EM: saltzman.11@osu.edu
AF: The Ohio State University, School of Earth Sciences, 275 Mendenhall Lab, Columbus, OH 43210, United States
AU: Lyons, T
EM: timothyl@ucr.edu
AF: University of California-Riverside, Geology Building 900 University Avenue, Riverside, CA 92521, United States
AB: The Paleozoic Era contains many large, commonly globally expressed positive carbon isotope excursions recorded in carbonate rocks. In younger Mesozoic rocks, similar excursions are often easily linked to organic-rich deposits formed from enhanced carbon burial under ocean-scale anoxia –i.e., oceanic anoxic events (OAEs). These events are important since voluminous organic carbon and pyrite burial in anoxic settings can be a central player in modulating the amount of oxygen and carbon dioxide in the atmosphere, and many of Earth's major extinctions are coeval with ocean-scale anoxia. In contrast, physical records of organic carbon burial tied to the carbon isotope record are scarce in the Paleozoic; leading to ambiguity in the interpretation of the isotope data. These data become less cryptic when viewed in light of coeval seawater sulfur isotope trends. For the globally expressed, Late Cambrian (SPICE) carbon isotope excursion, carbonate-C and sulfate-S records reveal parallel, positive isotope shifts suggesting enhanced organic C and pyrite S burial. Additionally, both organic carbon and pyrite sulfur isotope data from the Alum Shale of Sweden record the SPICE Event, putting to rest questions of the primary nature of the carbonate records. Comparison of the SPICE to similar isotope data from the Toarcian OAE and results from geochemical box modeling of both events lead us to conclude that the SPICE Event is a prime candidate for an early Paleozoic OAE. Additional evidence for increased ocean anoxia coincident with the SPICE also comes from the Alum Shale. Molybdenum concentrations show muted enrichment during the extent of the SPICE, despite data that show the basin was persistently euxinic before, during and after the event. Significant increases in molybdenum concentration occur only immediately after the event; suggesting a depleted seawater Mo inventory associated with a greatly expanded global anoxic Mo sink during the SPICE. An interesting result from geochemical box modeling of the SPICE and Toarcian records is the suggestion of large-scale oxidation of ³⁴S-depleted sulfur at the end of both events. A likely source of this sulfur is the oxidation of destabilized euxinic water masses. The driver of this oxidation was most likely increasing atmospheric pO₂ in conjunction with lowered pCO₂-the result of the enhanced organic carbon and pyrite burial that marked the events. Lowered pCO₂ allowed for lower global temperatures, invigorated ocean circulation and thus the delivery of O₂ to the deep ocean, leading to the demise of each anoxic event.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0488 Sulfur cycling
DE: 0489 Trace element cycling (4875)
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 9621 Cambrian
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting