Diagenetic carbonate minerals in upper Cenozoic sediments recovered from the New Jersey slope are a byproduct of organic matter degradation and the dissolution of precursor biogenic calcite, mostly foraminifers and nannofossils. The diagenetic carbonate minerals include dolomite, siderite, and calcite. Dolomite occurs as disseminated rhombs and cemented beds and nodules. Disseminated dolomite rhombs up to 10 µm in diameter are currently forming in the uppermost 200 m of sediment as indicated by the exponential decrease in pore-water Mg and Ca. The dolomite is both calcian and ferroan with an average composition of
(Ca)1.12(Mg)0.8(Fe)0.08(CO3)2. Carbon isotopic values of the dolomite-cemented nodules that occur at greater stratigraphic depths range from -15.0 to +4.5 parts per thousand and indicate dolomitization in the sulfate-reduction and the upper methanogenesis zones. Organic matter degradation by way of sulfate reduction promotes precipitation of dolomite by removing sulfate ion and increasing the carbonate alkalinity.
Siderite occurs as poorly cemented layers and nodules with an average composition of (Fe)0.72(Mg)0.20(Ca)0.08CO3. Carbon isotopic values for siderite range from -2.0 to +11.3 parts per thousand, indicating formation in the methanogenesis zone. Precipitation of siderite and ferroan dolomite is consistent with the large amount of Fe-rich terrigenous siliciclastics in these sediments and high pore-water Fe concentrations. Diagenetic dolomite and calcite are favored over siderite during early burial because pore-water Fe is removed by the precipitation of glaucony in the oxic to suboxic zones and iron sulfide minerals in the sulfate-reduction zone.
Diagenetic calcite is less common, occurring as cemented nodules with an average composition of (Ca)0.91(Mg)0.07(Fe)0.02CO3. Diagenetic calcites have carbon isotopic compositions that range from -0.8 to -37 parts per thousand. The very low carbon isotopic values of some calcite cements suggest that they formed during methane oxidation near the sediment/seawater interface where the relatively high seawater sulfate concentration and low Mg/Ca ratio favor calcite rather than dolomite precipitation. 87Sr/86Sr-derived ages of the diagenetic carbonates generally do not coincide with the biostratigraphic age of the sediment. The availability of precursor carbonate and variations in sedimentation rate related to sea-level fluctuations appear to be the primary factors that determine the abundance and mineralogy of the diagenetic carbonates.
Date of initial receipt: 27 February 1995
Date of acceptance: 6 December 1995
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