Geochemical paleogradient in pore waters controlled by AOM recorded in an Oligocene laminated limestone from the Outer Carpathians

Abstract

A 20-cm thick laminated limestone of Oligocene age from the Outer Carpathians is composed mainly of coccoliths and authigenic calcite. δ 13C values unequivocally indicate that the authigenic calcite precipitated as a result of methane oxidation. The δ 13C and δ 18O values change gradually from the base to the top of the bed from − 38.0‰ to − 17.6‰ and from − 1.1‰ to − 5.7‰, respectively. This trend reflects a geochemical gradient which developed in the zone of anaerobic oxidation of methane (AOM) due to the consumption of sulfate by methane. This gradient was related to an inverse relationship between the concentrations of both compounds in the pore waters of the AOM zone. Methane was the source of isotopically light carbon. It was accompanied by heavy oxygen-containing water liberated from decomposition of gas hydrates. The methane-bearing fluids migrated upwards to the base of the sulfate reduction (SR) zone, where methane was oxidized (CH 4 + SO 4 2 − → HCO 3 − + HS − + H 2O) and the AOM zone developed. The bicarbonate produced in this reaction was significantly depleted in 13C and 16O. In the SR zone organic matter was oxidized by sulfate: 2CH 2O + SO 4 2 − → H 2S + 2HCO 3 −. The bicarbonate released in that reaction was the source of isotopically heavier C and lighter O. The proportion of bicarbonate produced by oxidation of methane to bicarbonate produced by oxidation of organic matter decreased upwards. The resultant isotopic composition of bicarbonate dissolved in the pore waters changed upwards towards higher δ 13C and lower δ 18O values and has been recorded in the limestone's isotopic composition.