Oxygen-induced chemocline precipitation between Archean Fe-rich and Fe-poor carbonate seas

Abstract

The removal of dissolved iron by oxygen was a key control on marine Ca-carbonate precipitation during the Archean, but many details remain poorly resolved. We examined thick carbonate platforms, ∼2.9–2.8 Ga, in the western Superior Province of Canada. Interlayered Fe and Ca-carbonate minerals suggest platform margin precipitation, both as seafloor crusts and in the water column, during mixing of anoxic offshore and oxygenated onshore water masses in dynamic chemoclines. Platform interiors were subject to short-lived influxes of Fe-rich anoxic offshore waters that variously precipitated aragonite, herringbone calcite, Fe-dolomite, and rare siderite. Overall, these observations are consistent with chemical equilibrium analysis assuming three co-existing confluent carbonate water masses: (i) offshore anoxic iron-rich seawater, precipitating Fe-carbonate, (ii) a chemocline mixing zone variously precipitating interlayered aragonite, calcite and Fe oxide, (iii) onshore iron-poor seawater precipitating calcite and aragonite in an ‘Oxygen Oasis’. In this system, Fe2+, at ∼ 3 orders of magnitude lower level, supersedes the effect of Mg2+ on Ca-carbonate mineralogy. Only when sufficient Fe2+ was removed by oxygenation did the relative concentrations of Mg2+ and Ca2+ determine CaCO3 mineralogy. Platform margin chemoclines created by oxygen production likely persisted throughout the Archean.