NEW EVIDENCE FROM THE PALEOPROTEROZOIC GUNFLINT IRON FORMATION FOR MICROBIALLY-DRIVEN, EARLY DIAGENETIC PRECIPITATION OF SIDERITE

Petrographic, geochemical, C-, O-, and clumped isotope measurements of drill core samples from the Paleoproterozoic (1.88 Ga) Gunflint Iron Formation containing siderite, ankerite, Fe-silicate clays (dominantly greenalite) and silica, reveal variable but light carbonate δ¹³C values (-21.3 to -7.87‰, VPDB), with heavy and less variable carbonate δ¹⁸O values (20.6 to 24.9‰, VSMOW). We will demonstrate that these measurements are consistent with a model in which siderite is formed during early diagenesis as a by-product of the metabolism of dissimilatory iron reducing (DIR) microbes that consume Fe(III)-oxide and organic carbon delivered from the water column to the sediment-water interface. This model indicates that the oxygen isotope values of reactant Fe-oxide, organic matter and dissolved inorganic carbon were set in equilibrium with seawater at T = 5-35^oC and δ¹⁸O = -1.0 to -6.5‰, prior to being metabolized to form siderite. Clumped isotope measurements on Fe-bearing carbonates, targeted using a timed online digestion method developed in our laboratory, yield temperatures (T(D⁴⁷)) between 40^oC-132^oC, which constrain δ¹⁸Ofluid to values between -6.22 to 7.32‰. These measurements record the onset temperature of DIR followed by low water-to-rock ratio diagenetic recrystallization at elevated temperature. Combined petrographic, chemical, and isotopic measurements reveal that the major phases delivered to the shallow seafloor were a disequilibrium assemblage of Fe-oxide, greenalite, silica, and organic matter that underwent microbially mediated modification to form an assemblage of siderite, greenalite, silica, and later diagenetic ankerite. We contend that observed differences between carbonate derived from Gunflint core and outcrop may be reconciled by removal of siderite during exposure and weathering, leaving outcrop enriched in late diagenetic ankerite. The comparison of outcrop and core samples provides potentially useful lessons for the interpretation of the marine carbonate isotopic record across iron formation deposits of all ages.