11 Oxygen and Evolution on Early Earth I

Tuesday, 9 August 2005
9:00 AM - 12:00 PM

GEOLOGIC EVIDENCE FOR THE EARLY DEVELOPMENTS OF AN OXYGENATED ATMOSPHERE, SULFATE-RICH OCEANS, AND DIVERSE MARINE- AND TERRESTRIAL BIOSPHERES

Paper number: 11-2
Program: THEME SESSIONS
Time: 9:40 AM – 10:00 AM

Authors

Abstract

The current popular model for the evolution of atmosphere, oceans, and biosphere (i.e., the CWHK model) postulates that: prior to ~2.3 Ga, the atmosphere was poor in oxygen (pO2 < 1 ppm) but methane-rich (CH4 ~ 2000 ppm); the oceans were rich in Fe2+ (~10 ppm), but poor in sulfate (<200 µM) and H2S (<1 mM); the geochemical cycles of redox elements (C, S, Fe, Mo, U, etc) were different from today; non-oxygenic photoautotrophs and methanogens were dominant organisms in the oceans, but cyanobacteria, sulfate reducing bacteria (SRB), and eukarya were not; and the land surface was sterile.

However, the CWHK model cannot provide satisfactory explanations for the similarities between pre- and post-2.3 Ga rocks in: (1) the geochemistry of paleosols (Fe3+/Fe2+ ratios; total Fe; organic C contents; δ13C of organics and carbonates; REEs); (2) the mineralogy and geochemistry of sandstones and shales (detrital minerals; Fe3+/Fe2+ ratios; sulfide S-, Mo-, N-, and organic C contents; δ13C of kerogen; and δ15N of kerogen and bulk rock); (3) the geochemistry of carbonates (Fe/Mn/Mg/Ca; δ13C); (4) the biomarkers and microfossils (cyanobacteria, eukarya); (5) the absence of MIF-S in normal sedimentary formations; and (6) the mineralogy (hematite/magneite/siderite) and geochemistry (REEs) of deep-sea hydrothermal deposits (including many BIFs) and pillow basalts. However, these data are consistent with the Dimroth-Ohmoto model, which postulates the development of a full-oxygenated (and methane-poor) atmosphere, the modern-style oceanic chemistry and geochemical cycles of redox elements, and the diverse biospheres on land and in the oceans by ~3.8 Ga. Calculations of the O2 transfers between the atmosphere and the ocean surface zone suggest that a popular concept of “oxygen oases” to explain Archean sedimentary rocks with oxygenated characteristics (e.g., Kasting, 2001) is not a valid model.