Archean and Paleoproterozoic marine oxygen estimated from I/(Ca+Mg) in shallow water Ca-carbonate sediments

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.

太古宙（Archean）与古元古代（Paleoproterozoic）的海洋氧化作用目前约束程度极低。碳酸盐岩中的碘元素可作为潜在的氧化还原指示剂。我们对加拿大与美国境内苏必利尔构造省南部的浅海相钙碳酸盐（灰岩、白云岩）台地沉积物开展分析，获取了2.93–2.31 Ga样品的碘与钙镁摩尔比（I/(Ca+Mg)）。所获I/(Ca+Mg)最大值分别为0.13、0.26、0.05与0.22 μmol/mol，对应2.93 Ga、2.86 Ga、2.80 Ga及2.31 Ga的沉积物样品。本次研究还首次报道了太古宙沉积物中I/(Ca+Mg)为正值的结果。基于上述数值的保守估算显示，2.93 Ga、2.86 Ga、2.80 Ga及2.31 Ga时，海水最大溶解氧浓度分别约为23、40、11与23 μM。本次数据集将当前海洋溶解氧的估算记录向前推进了约9亿年。该结果支持如下假说：太古宙时期，钙碳酸盐沉积于紧邻缺氧富铁海域的局域性浅海「氧化绿洲」之中。将平衡模型应用于已发表的中元古代（mid-Proterozoic，~1.57 Ga）I/(Ca+Mg)数据，在pH值为7.9的假设条件下，可预测浅海水体中曾出现短时溶解氧浓度高达174 μM的时段，该浓度足以支撑动物生存。