HR: 1340h
AN: B53C-1010    [Abstracts]
TI: Tracking Changes in Ocean Oxygenation with Molybdenum Isotopes
AU: * Williams, G
EM: Gwyneth.Williams@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287
AU: Anbar, A D
EM: anbar@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Tempe, AZ 85287
AU: Anbar, A D
EM: anbar@asu.edu
AF: Dept. of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287
AU: Arnold, G L
EM: gail@gps.caltech.edu
AF: Div. of Geological & Planetary Sciences, Caltech, Pasadena, CA 91125
AU: Lyons, T W
EM: lyonst@missouri.edu
AF: Dept. of Geological Sciences, University of Missouri, Colombia, MO 65211
AB: The oxygenation of the oceans has varied through geologic time but the timing and extent of these variations are not well understood, nor do we have a good understanding of the connections between changes in atmospheric and ocean oxygenation. The development and refinement of ocean paleoredox proxies is therefore a high priority in unraveling the history of the atmosphere. Because of the connections between oxygen and the carbon cycle, such proxies are also important to understanding climate change on long timescales. The molybdenum (Mo) stable isotope system has emerged as a valuable new tool for the investigation of ocean paleoredox. To a first approximation, Mo enters the oceans via rivers and is removed by adsorption to Mn oxides in oxic sedimentary environments and via scavenging of Mo oxythiomolybdates in sulfidic settings. There is an isotopic contrast between these sinks of ca. 2\permil. Hence, the steady-state Mo isotope composition of the oceans can be considered to reflect the balance between fractionated Mo removal to Mn-oxides vs. near-quantitative extraction in sulfidic settings, with removal to Mn-oxides forcing a steady-state isotopic offset between average crustal Mo (0\permil) and seawater (ca. 1.6\permil) today. The source-seawater offset would have been smaller - i.e., seawater isotopically lighter - during extended periods of expanded ocean anoxia because a smaller fraction of the Mo removed would have been buried as fractionated (isotopically light) Mo associated with Mn-oxide bearing sediments. The Mo isotope system is particularly valuable because it can provide information about regional or global ocean redox, rather than only local redox, as a consequence of the long ocean residence time of this element. We will review the promise and limitations of this new proxy and present an update of our most recent findings. These include: evidence of enhanced ocean anoxia between 1.8 and 1.0 Ga despite rising atmospheric oxygen, consistent with other emerging evidence about the oceans during the "middle age" of Earth history; indications of modestly enhanced ocean anoxia during the mid-to-late Devonian; and data that may suggest short-lived episodes of ocean anoxia during the late Cretaceous preceding and following Ocean Anoxic Event 2.
DE: 0325 Evolution of the atmosphere
DE: 0330 Geochemical cycles
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting