Elemental and Isotopic Composition of Siderite in the Kuparuk Formation, Alaska: Effect of Microbial Activity and Water/Sediment Interaction on Early Pore-Water Chemistry

Abstract

ABSTRACT The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin. Authigenic siderite is common throughout the formation, either concentrated in distinct layers or as isolated rhombs. Examination of siderite-rich samples with back-scattered electron imaging reveals complex compositional zonation in the siderite. The samples are principally composed of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich siderite, with mean compositions of (Fe77.5Mg13.8Ca7.9Mn0.7 )CO3 and (Fe55.8Mg32.8Ca10.8Mn0.6 )CO3, respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderit s are most abundant in non-glauconitic sandstones and mudrocks. The whole-rock isotopic composition of the siderite is highly variable, with 13C values ranging from -20.3 to 7.9 PDB, and 18O values from 21.4 to 31.1 SMOW. This variation in isotopic composition correlates with the relative proportions of the two siderite types in a given sample. The Fe-rich siderites have low 13C and high 18O values, whereas the Mg-rich siderites have high 13C and low 18O values. Both the elemental and carbon isotopic compositions of the siderite result from modification of the original marine pore waters during successive stages of microbial decomposition of organic matter. The oxygen isotopic compositions of the siderite indicate that early pore waters were depleted in 18O, perhaps as a result of water/sediment interaction. Authigenic ankerite associated with the siderite-bearing samples is unzoned and has a mean composition of Ca1.11 Fe0.35Mg0.53(CO3)2. The ankerites have higher 13C values and lower 18O values than the Fe-rich siderite, but have 13C values less than and 18O values greater than (with one exception) that of the Mg-rich siderite. The isotopic data suggest that ankerite precipitation began prior to precipitation of Mg-rich siderite and ended subsequent to or during Mg-rich side ite precipitation.