OXYGEN AND THE ARCHEAN INCEPTION OF ARAGONITE SEAS

The earliest known well-preserved marine Ca-carbonate sediments were precipitated on shallow oxygenated shelves (‘Oxygen Oases’) beside iron-rich anoxic seas ~3.5 Ga. Platform thickness increased from <20 m ~3.5 Ga, to >200 m ~2.8 Ga. Dissolved ferrous ion (Fe²⁺) concentrations, ~0.1 – 1 mM, widely inferred for Archean seawater favor siderite (FeCO₃) over aragonite or calcite (CaCO₃) precipitation. Oxidation lowered Fe²⁺ to below siderite solubility, at a concentration (<<100 µM) that permits aragonite precipitation but inhibits calcite. Thus, Fe²⁺ can supersede magnesium (Mg²⁺) effects on Ca-carbonate mineralogy at ~3 orders of magnitude lower levels. Three co-existing confluent carbonate water masses were associated with these deposits during the Archean: (i) anoxic iron-rich ‘Siderite Seas’, (ii) oxygenated iron-poor ‘Ca-carbonate Seas’ variously precipitating aragonite, calcite and dolomite, separated by (iii) a relatively narrow ‘Fe-influenced Aragonite Sea’ located at an oxycline in which dissolved Fe²⁺ levels were often initially sufficiently high to inhibit calcite, but allowed aragonite to precipitate. Under these conditions Mg/Ca molar ratio could only exert a controlling influence over Aragonite-Calcite mineralogy in dissolved Fe-free platform interior seas; these likely were the precursors of Mg-influenced Aragonite-Calcite Seas that persist at the present-day. The variety of Archean ‘Carbonate Seas’ therefore reflects the localized influence of oxygenic removal of Fe²⁺. Only when this was accomplished did the relative concentrations of Mg²⁺ and Ca²⁺ influence CaCO₃ mineralogy. We infer oxygenic photosynthesis, which must have been extant by 3.5 Ga, as the primary oxygen source. The apparent scarcity of Ca-carbonate minerals in offshore Iron Formation during the Archean suggests that the oxyclines delimiting early carbonate platforms may have been steeply angled and did not extend far offshore. Over time, as oxygenation progressed, the oxycline rotated outward, extending and then descending into deeper water. Relatively deep oxyclines are associated with platform foreslope carbonates in the Paleoproterozoic, but oxycline position continued to fluctuate considerably throughout the Proterozoic.