Astrobiology: Life in the Universe

NASA Astrobiology Institute (NAI)


  1. Seminar Detail

    Iron Isotope Constraints on the Early Redox History of the Oceans

    Iron Isotope Constraints on the Early Redox History of the Oceans

    Presenter: Olivier Rouxel ()

    February 28, 2005 11:00 AM Pacific

    The response of the ocean redox state to the rise of atmospheric oxygen by
    ca. 2.3 Ga ago is poorly constrained although deposition of BIFs until ca.
    1.8 Ga suggests at least episodical deep ocean anoxia during the
    Paleoproterozoic. Since Fe, along with C and S, are coupled with and
    maintain the redox state of the surface environment, Fe seawater
    concentration and isotopic composition were likely affected by the change in
    the redox state of the atmosphere. Here we will present a study of Fe
    isotope composition of sedimentary sulfides over geological time and provide
    evidence for a change in the ocean Fe cycle at the same time as atmospheric
    redox state changed.



    We analyzed Fe isotope compositions of ~ 150 pyrites from 20 black shale
    units, specifically focusing on Late Archean to Paleoproterozoic time. δ56Fe
    values of handpicked sulfides were obtained using a Neptune MC-ICPMS at WHOI
    and are reported relative to IRMM-14 with an external precision of 0.1‰. The
    emerged general pattern of Fe isotope record allows dividing the Earth's
    history into three stages which are strikingly similar to the stages defined
    by multiple S isotopes as well as other indicators of the redox state of the
    atmosphere and ocean. (1) Stage 1 (>2.8 to 2.3 Ga) is characterized by
    highly variable and negative δ56Fe values of pyrite (down to –3.5‰) that are
    interpreted to reflect the reservoir effects during partial oxidation of
    hydrothermally-derived Fe(II) and precipitation of Fe-oxides. (2) Stage 2
    (2.3 to ~1.6 Ga) is characterized by δ56Fe values ranging from -0.3 to 1.2‰
    that might be related to the increased effect of sulfide precipitation in a
    redox-stratified ocean. (3) Stage 3, from 1.6 Ga through the Phanerozoic, is
    characterized by sedimentary pyrite having a limited range of δ56Fe
    variations (less than 0.5‰ around igneous value at ~0‰) reflecting the
    establishment of an Fe-poor oxygenated ocean.

    Participation Instructions

    Sites who would like to connect via Polycom must notify Diane Hawks at
    dhawks@mail.arc.nasa.gov.

    The slides from the seminar can be viewed real-time using WebEx at
    https://naimeetings.webex.com, click on "Director's Seminar", the password
    is 1KFalcon. If you've never joined a WebEx meeting before, please allow an
    extra 5-10 minutes to install the plug-in. Explorer is the recommended
    browser.

    Sites without a Polycom system can listen to the seminar on the telephone
    while viewing the slides in WebEx. The NASA Meet-me number for this is
    650-604-3393.

    Alternatively, participants without a Polycom system can view the webcast
    at: http://vanseg-1.arc.nasa.gov/2005/AB050228-01.ram There is a 30 second
    delay for the webcast, so viewers will need to advance the slides manually
    in WebEx. Questions can be posted in the WebEx chat area to be
    answered by Olivier at the end of his talk.

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