Constraints on the Archean Surface Environment from Mass-Independent Sulfur Isotopes (Invited)

Details Constraints on the Archean Surface Environment from Mass-Independent Sulfur Isotopes (Invited) Constraints on the Archean Surface Environment from Mass-Independent Sulfur Isotopes (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p14a..02h&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #P14A-02

Mathematics

Logic

[0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [0330] Atmospheric Composition And Structure / Geochemical Cycles, [1030] Geochemistry / Geochemical Cycles

Scientific paper

Anomalous sulfur isotope fractionation (“mass-independent” fractionation, or MIF) in Archean sedimentary sulfates and sulfides is widely interpreted to record low levels of atmospheric oxygen. But in addition to the presence or absence of MIF, the record appears to possess large-scale structure, including an apparent asymmetry, with large positive but only modest negative MIF anomalies preserved. This structure has been previously attributed to unspecified changes in atmospheric composition and chemistry or to a temporary rise in atmospheric O2 concentrations, but to date no clear quantitative explanation exists. Whereas the atmospheric production of MIF has been examined in a number of photochemical modeling studies, the basin- to global-scale processes that affect the anomalous signal since its deposition from the atmosphere onto the Earth’s surface and until its preservation in the rock record have not been quantitatively addressed. We account for these processes in a model of the Archean biogeochemical sulfur cycle and find that in addition to the atmospheric oxidation state, the preservation of the MIF signal is sensitive mostly to three properties of the surface system (ocean and atmosphere): i) the partial pressure of CO2 (pCO2), ii) the rates of biological cycling of sulfur between its different oxidation states, and iii) the ratio of SO2 to H2S in volcanic outgassing (SO2:H2S). Changes in the former two properties (pCO2 and biological cycling) cause variability in the magnitude of preserved MIF, but do so in a nearly symmetric manner around a bulk Earth composition (0‰), inconsistent with the (sometimes extreme) asymmetry of the MIF record. In addition, the required swings in atmospheric pCO2, if it were the dominant control over the magnitude of preserved MIF, would leave a climate signal not observed in the Archean geologic record and an increase in the importance of biological cycling to explain low mid-Archean MIF results in an inability to account for large MIF in the late Archean. Modest variability in volcanic SO2:H2S can explain both the large-scale variability in magnitude and the asymmetry of the MIF record. The increase in volcanic SO2:H2S required to explain the spike in MIF magnitude and asymmetry in the latest Archean may be related to the growing frequency of subaerial versus submarine volcanism and to episodes of continental crust formation.

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