228 Earth’s Early Atmosphere and Surface Environment

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 101A-C

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

Paper number: 228-6
Program: Technical Sessions
Time: 10:50 AM – 11:40 AM

Authors

Abstract

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 FeIII 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.