HR: 13:55h
AN: PP33D-02    [Abstracts]
TI: What can Fe Isotopes in Anoxic Environments Tell us About the Rise of Oxygen in the Coupled Land-Ocean-Atmosphere System? - Clues From the Black Sea
AU: * Severmann, S
EM: Silke.Severmann@ucr.edu
AF: Dept of Earth Sciences, University of California, Riverside, CA 9521 United States
AU: Lyons, T W
EM:
AF: Dept of Earth Sciences, University of California, Riverside, CA 9521 United States
AU: McManus, J
EM: mcmanus@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: Anbar, A D
EM: anbar@asu.edu
AF: Dept of Geological Sciences and Dept of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287 United States
AU: Gordon, G
EM: gawillia@asu.edu
AF: Dept of Geological Sciences and Dept of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287 United States
AB: Pyrite burial in marine sediments and its oxidation during weathering are two of the dominant processes that regulate atmospheric oxygen levels on geological time scales, thereby linking the geochemical cycles of O, C, S and Fe. Models of these burial and weathering fluxes, and by extension the history of atmospheric oxygen through time, are traditionally built upon the isotope records of marine authigenic minerals. For example, the increasing S isotope variability from the Archean to the present appears to track increasing sulfate availability in the ocean, thus reflecting an increased oxic weathering flux (Canfield, 1998). Fe isotope data for authigenic pyrite from the same time interval broadly follow - although in the opposite direction - the first-order divisions observed for S isotope variations (Rouxel et al., 2005). A detailed understanding of the inverse secular pattern for Fe isotope variability is currently lacking but almost certainly reflects an ocean that shifted from S to Fe limitation over time scales of 10⁸ to 10⁹ years. Insights into the evolution of the Fe cycle and the redox balance of the atmosphere-ocean system in the early Earth hinges on a better understanding of the isotopic expressions in modern sedimentary systems. An important challenge to make Fe isotopes widely applicable as a paleoproxy is to demonstrate that the cumulative isotope fractionations in natural systems can be used to gain straightforward information about diagenetic processes and depositional environments. In this study we present Fe isotope data from the Black Sea, a commonly used modern analogue for ancient marine environments. Our simplified interpretation of isotopic fractionations assumes that variations in Fe isotope compositions represent, in the broadest sense, a transformation of Fe from an oxidized to a reduced reservoir. Further, we calibrate the utility of Fe isotopes as a paleo-chemical tool against some general concepts that have previously been derived from S isotopes and paleoproxies based on Fe reactivity (degree of pyritization, reactive Fe enrichment). Mass-balance considerations in a closed system demand that the near-quantitative conversion of reactive Fe to pyrite, which is diagnostic of euxinic settings, eliminates the necessity to correct for fractionations associated with pyrite formation and diagenetic overprinting. Consequently, low δ⁵⁶Fe values for pyrite in the deep basin sediments of the Black Sea are interpreted to reflect intra-basinal shuttling of ferrous Fe from the suboxic shelf into the deep euxinic basin. This unique isotopic signature provides us with a fingerprint to distinguish different Fe shuttling mechanism in the past and present sedimentary record, on both basin and ocean scales. Canfield, D. E. A new model for Proterozoic ocean chemistry. Nature 396, 450-453 (1998) Rouxel, O., Bekker, A. & Edwards, K. J. Iron isotope constraints on the Archean and Paleoproterozoic ocean redox state. Science 307, 1088-1091 (2005)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
DE: 1065 Major and trace element geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005