HR: 1340h
AN: PP43B-0687    [Abstracts]
TI: High-Resolution Isotope Records of the Late Ordovician and Late Carboniferous: A Comparative Perspective on Glacial Carbon and Sulfur Cycles
AU: * Gill, B C
EM: bgill003@ucr.edu
AF: University of California-Riverside, Department of Earth Sciences, Riverside, CA 92521 United States
AU: Lyons, T W
EM: timothyl@ucr.edu
AF: University of California-Riverside, Department of Earth Sciences, Riverside, CA 92521 United States
AU: Saltzman, M R
EM: saltzman.11@osu.edu
AF: The Ohio State University, Department of Geological Sciences, Columbus, OH 43210 United States
AB: Carbonate-associated sulfate (CAS) faithfully tracks the sulfur isotope composition of seawater in both modern and ancient environments. Therefore, analyses of carbonate rocks permit the generation of parallel, high-resolution carbon and sulfur isotope data for seawater spanning geologic history. Our previous work in the early and middle Paleozoic has revealed parallel, short-term (1-4 Myrs) carbon and sulfur isotope excursions. The relationship between the two isotope systems seems to change over time, perhaps tracking longer-term evolution of the marine sulfur reservoir and of the primary loci of carbon burial, including increased burial on land. CAS isotope records therefore have the potential to shed essential mechanistic light on the causes (global versus regional) for carbon isotope excursions observed throughout the geological record. Previous work on the Late Carboniferous and Late Ordovician documented the existence of carbon isotope excursions of varying magnitude during these glacial episodes. The Late Carboniferous glaciations classically show evidence for multiple glacial-interglacial cycles with repeated, low magnitude (1 to 3 per mil) carbon isotope excursions. By contrast, the Late Ordovician was characterized by a short-lived glaciation with a corresponding single 4-6 per mil carbon isotope excursion. The modes and rates of carbon cycling reflected in the differing styles of C isotope behavior are the subject of debate, making these time-slices ideal for the CAS isotope approach. Carbon and sulfur isotope data from Pennsylvanian (Missourian Stage) cyclothems exposed in Kansas City, Missouri, show rapid isotope variability. We have preliminarily attributed these rapid changes to local reservoir effects linked to fluctuating sea level and its relationship to black shale deposition within the midcontinent basin and weathering on the basin margin during lowstands. Other work on the cyclic Carboniferous Bird Spring Formation, Nevada, is also contributing to an improved understanding of the local and global controls on carbon and sulfur cycling during this major glaciation. For comparison, our ongoing work in the Upper Ordovician (Hirnantian Stage) Hanson Creek Formation is exploring carbon-sulfur relationships through the single, large (6 per mil) carbon excursion observed during this glacial episode.
DE: 0330 Geochemical cycles (1030)
DE: 0488 Sulfur cycling
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1615 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 4805, 4912)
DE: 4806 Carbon cycling (0428)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005