New Constraints on Seawater Chemistry from Mesoarchean Carbonates, Canada

New field observations show seaward margins of 2.8–2.95 Ga carbonate platforms in Red Lake Greenstone Belt (northwestern Superior province, Canada) contain iron oxide and calcium carbonate precipitates interlayered in proportions of ~1:5–1:6 thickness. Precipitation is inferred to have occurred as offshore anoxic seawater mixed with oxygenated water at the ‘Oxygen Oasis’ platform margin [1]. Iron formation deposited in the offshore basin is mainly chert, magnetite, pyritiferous carbonaceous slate and rare siderite [2]. Although likely altered during early diagenesis, these sediments suggest that offshore seawater chemistry favoured Fe-carbonates (e.g., siderite), and inhibited Ca-carbonate precipitation. Oxidation at the platform margin removed Fe2+ by precipitating iron (oxy)hydroxide. This in turn favoured first aragonite and then calcite precipitation to create the carbonate platforms. Previous work suggests that Archean chemical sediments may be used to infer seawater composition and pCO2 if these minerals precipitated in the water column [3, 4, 5]. We used a thermodynamic speciation model to quantitatively evaluate the amounts of precipitates that would form at chemical equilibrium due to Fe2+ removal, by varring temperature, pCO2, pH, Ca2+ and Fe2+ to set initial and equilibrium conditions. We then used the new field observations of the relative abundances of interlayered CaCO3 and iron oxide to constrain Fe and Ca concentrations in solution. This model in turn provides new estimates of pH and pCO2 values during the Mesoarchean, a critical time when large carbonate platforms began depositing for the first time in Earth history.