HR: 17:15h
AN: V54B-06    [Abstracts]
TI: Iron Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State
AU: * Rouxel, O J
EM: orouxel@whoi.edu
AF: Geomicrobiology Group, Woods Hole Oceanographic Institution, Marine Chemistry & Geochemistry Dept. MS#8, Woods Hole, MA 02543 United States
AU: Bekker, A
EM: a.bekker@gl.ciw.edu
AF: Geophysical Laboratory; Carnegie Institution of Washington., 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: Edwards, K
EM: katrina@whoi.edu
AF: Geomicrobiology Group, Woods Hole Oceanographic Institution, Marine Chemistry & Geochemistry Dept. MS#8, Woods Hole, MA 02543 United States
AB: The rise of atmospheric oxygen level, which started by 2.3 Ga, was one of the most dramatic environmental changes in the Earth's history. 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. Study of S isotope composition of sedimentary sulfides over geological time has placed important constraints on the S cycle and the evolution of ocean chemistry and here we apply a similar time-record approach to explore potential changes of Precambrian Fe oceanic cycle. We analyzed Fe isotope compositions of ~ 150 pyrites from 20 black shale units, specifically focusing on Late Archean to Paleoproterozoic time. $\delta$$^{56}$Fe values of handpicked sulfides were obtained using a Neptune MC-ICPMS at WHOI and are reported relative to IRMM-14 with an external precision of 0.1$\permil$ at 2$\sigma$ level. The emerged general pattern of Fe isotope record allows dividing the Earth's history into four stages which are strikingly similar to the stages defined by the $\delta$$^{34}$S, $\Delta$$^{33}$S as well as other indicators of the redox state of the atmosphere and ocean (1) Stage 1 from before 2.8 to 2.45 Ga is characterized by highly variable and negative $\delta$$^{56}$Fe values of pyrite. The entire range between 0.22 to -3.5$\permil$ is often observed within single section of black shales but individual pyrite nodules from the same stratigraphic level have similar $\delta$$^{56}$Fe values. These extremely variable and low $\delta$$^{56}$Fe values are interpreted to reflect the reservoir effect during partial oxidation of hydrothermally-derived Fe(II). Because the Fe oxide sink associated with BIFs preferentially remove Fe heavy isotope, an enrichment in Fe light isotope for the residual oceanic Fe(II) pool is expected and is recorded during pyrite formation and burial in Black Shales. (2) Stage 2, representing a transition period, covers the time interval from 2.45 to 2.2 Ga and is characterized by a smaller range of negative $\delta$$^{56}$Fe values (above -1.8$\permil$) and by positive $\delta$$^{56}$Fe values up to 1.0$\permil$. Surprisingly, these isotopic signatures reflect changes in the oceanic Fe cycle on the same stratigraphic level where the most sensitive indicators for the rise of atmospheric oxygen appear. (3) Stage 3 from 2.2 to ~1.6 Ga ago, is characterized by $\delta$$^{56}$Fe values ranging from -0.3 to 1.2$\permil$ . The ubiquitous positive $\delta$$^{56}$Fe values in 2.4 to 1.8 Ga black shales are striking and might be related to the increased effect of sulfide precipitation in a redox-stratified ocean. (4) Stage 4 from 1.6 Ga through most of the phanerozoic is characterized by sedimentary pyrite having a limited range of $\delta$$^{56}$Fe variations (less than 0.5$\permil$ around igneous value at ~0$\permil$). Because pyrite formation in modern organic-rich marine sediments is mediated by sulfate-reducing bacteria and proceed through the reduction of Fe-oxides to Fe(II), this rather limited variability of $\delta$$^{56}$Fe values suggests that bacterial reduction of Fe-oxides is unlikely to explain the extreme light $\delta$$^{56}$Fe values found during the Archean.
DE: 8424 Hydrothermal systems (8135)
DE: 9619 Precambrian
DE: 1045 Low-temperature geochemistry
DE: 0330 Geochemical cycles
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting