A sulfate control on carbonate concretion formation throughout geologic time

Abstract:

Early diagenesis of marine organic matter dramatically impacts Earth’s surface chemistry, alters the burial potential of carbon and promotes the formation of authigenic mineral phases including carbonate concretions. Carbonate concretions tend to form as a result of anaerobic diagenetic reactions that degrade organic matter, some of which require an external oxidant. When considered in the context of concretion-producing factors, concretions tend to occur in sediments underlying sulfate-rich marine waters, implicating sulfate reduction pathways (dissimilatory sulfate reduction and the anaerobic oxidation of methane) as important reactions through time. A compilation of concretion abundance and carbon isotope data indicate significant temporal variability since the Late Archean. The Paleoproterozoic is interrupted by an anomalous concretion-containing interval between ~2.15 to 1.75 Ga, the beginning of which coincides with the enigmatic Lomagundi event. The Phanerozoic exhibits an increase in the range of concretion carbon isotope compositions, manifested as an increase in maximum (and positive) and decrease in minimum isotope values, perhaps reflecting enhanced signal inheritance potential of methanogenesis-produced carbonate in an otherwise sulfate reduction dominated concretion-forming diagenetic time period. The first appearance of conclusively anaerobic oxidation of methane-derived carbonate concretions occurs at ~375 Ma and broadly coincides with a Paleozoic rise in marine sulfate and the first appearance of methane seep authigenic (non-concretionary) carbonates with similar ¹³C depletions. Our findings indicate the overall importance of sulfate in driving carbonate concretion formation through time. Other potential second order controls include organic matter nature and availability, although the degree to which these impact concretion precipitation remains unresolved.