Origin of authigenic carbonates in sediment from the deep Bering Sea

Abstract

ABSTRACTForty beds of authigenic carbonate were identified from the deep Bering Sea in cores taken on Leg 19 of the Deep Sea Drilling Project. Carbonate minerals were mainly high‐magnesium calcite and protodolomite, less commonly siderite, rhodo‐chrosite, low‐magnesium calcite, and manganosiderite. Authigenic carbonates cement and replace diatom ooze, ash and bentonite beds, and, less commonly, clastic beds. Replacement zones are as much as 60 cm thick. Eighty‐five per cent of carbonate beds occurred below 400 m sub‐bottom depth and 70% in sediment older than 4 m.y. δ13C values averaged ‐17.200/00 PDB and δ18O ranged from 18.59 to 34–110/00SMOW. The carbon was derived from oxidation of organic matter under anaerobic conditions during bacterial reduction of sulphate, or from CO2 produced in concert with CH4 during degradation of organic matter. The cations (Ca, Mg, Fe, Mn) were derived from alteration of ash beds. In Bering Sea deposits, ash beds altered to smectite within about 3–5 m.y. Carbonate precipitated simultaneously at different stratigraphic levels within the 627–1057 m sections at temperatures of 7–85°C. No apparent calcite precursor of biogenic origin was found for these authigenic carbonates.