EVIDENCE FOR THE FULLY-OXYGENATED OCEANS AND ATMOSPHERE DURING THE ARCHEAN EON

The current paradigm for the early Earth is that the “Great Oxidation Event (G.O.E.)” occurred at ~2.45 Gyr; prior to the G.O.E., the atmospheric pO₂ was <1 ppm and the oceans were O₂-free and U-poor, but Fe-rich. Evidence supporting this paradigm includes: (a) loss of Fe from paleosols, (b) abundance of banded iron formations (BIFs), (c) presence of “detrital” grains of uraninite and pyrite, (d) lack of U- and Mo enrichments in black shales, and (e) presence of anomalous isotope fractionations of sulphur (AIF-S), in Archaean rocks. However, such evidence is equivocal, because characteristics (a)-(e) have also been found in rocks of Proterozoic and Phanerozoic age, and because AIF-S signatures can be produced by thermochemical sulphate reduction in sediments, as well as by atmospheric photochemical reactions.

We present the following lines of evidence to suggest the atmospheric pO₂ level and the ocean chemistry have been basically the same since at least ~3.5 Ga ago: (1) abundances and d¹³C values of carbonates and kerogen in sedimentary rocks, indicating the oxidative recycling of biogenic methane and kerogen to CO₂; (2) d¹⁵N record of kerogen, indicating the importance of NO₃^-; (3) abundance and d³⁴S values of semimentary pyrite, indicating SO₄-rich oceans; (3) abundance and d³⁴S values of barite in submarine hydrothermal deposits, indicating SO₄-rich oceans; (4) enrichments of Fe^III in paleosols and submarine volcanic rocks, indicating reactions with O₂-rich groundwater and seawater; (5) abundance of sedimentary pyrite, requiring the abundance of Fe(OH)₃ formation during soil formation; (6) Fe-, Mo-, and Cr isotope variations due to redox variations; (7) Ce anomalies in BIFs, cherts and submarine volcanic rocks, indicating Ce anomalies in ocean waters; (8) dissolution/reprecipitation of U in paleosols; (9) enrichments of U in submarine volcanic rocks and shales due to reactions with U-rich seawater; (10) enrichments of radiogenic Pb in sedimentary rocks and submarine volcanics, illustrating reactions with U-rich seawater. Since ~3.8 Ga, the atmospheric pO₂ level has been regulated at near the present level by the coupling of two negative feedback mechanisms: the responses of the burial and weathering fluxes of kerogen to a change in the atmospheric pO₂.