Archean and Paleoproterozoic marine oxygen estimated from I/(Ca+Mg) in shallow water Ca-carbonate sediments
Description
Marine oxygenation during the Archean and Paleoproterozoic is poorly constrained. Iodine in carbonate is a possible oxygen indicator. We obtained molar ratios of iodine to calcium-magnesium (I/(Ca+Mg)) for 2.93–2.31 Ga from shallow marine Ca-carbonate (limestone, dolostone) platform sediments of the southern Superior Province in Canada and the USA. Maximum I/(Ca+Mg) values obtained are 0.13, 0.26, 0.05 and 0.22 μmol/mol, for 2.93, 2.86, 2.80 and 2.31 Ga samples, respectively. These include the first reports of positive I/(Ca+Mg) values from Archean sediments. Conservative estimates based on these values suggest maximum dissolved O2 levels of ~23, 40, 11 and 23 µM at 2.93, 2.86, 2.80 and 2.31 Ga, respectively. These data extend current estimates of marine O2 back in time by ~900 million years. They support the idea that during the Archean Ca-carbonate was precipitated in localized shallow marine ‘oxygen oases’ adjacent to anoxic iron-rich seas. Application of an equilibrium approach to published I/(Ca+Mg) data for the mid-Proterozoic (~1.57 Ga) would predict transient shallow water dissolved O2 levels up to 174 µM assuming pH 7.9, sufficient to support animal life.