Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions

Abstract

Coal measures often contain concretions; segregations of diagenetic minerals originally formed within unconsolidated sediments. Three different types (calcite/pyrite, dolomite/pyrite and siderite) occurring spatially quite close together in the Central Pennine Region of England vary widely in carbon isotope composition (+10.35%. > δ 13 C > −21.49%.) and in major cation chemistry (Ca, Mg, Fe, Mn). Within some siderite concretions, very high Mn Fe ratios were found in central subsamples; these were also most enriched in 13C. The Fe Mg ratio decreases systematically from centre to edge (early, shallow to deeper, later precipitation). The calcite/pyrite and dolomite/pyrite concretions developed completely prior to significant burial. Both have high Mn Fe ratios but negative δ 13C values (calcite −21.49%., dolomite −8.67 to −10.48%.). All of these patterns can be equated precisely with theories of pore water evolution developed on the basis of geochemical investigations of modem sediments. Microbial processes (sulphate reduction, methanogenesis) contributed significantly, as did thermal decarboxylation (to siderite precipitated at considerable burial depth). Mn(IV) and Fe(III) acted differentially as oxidants; producing CO 2 and increasing alkalinity. The interplay of fresh and marine depositional waters is seen most obviously in the presence or absence of sulphate reduction. This controlled mineral type (iron sulphide or carbonate) as well as isotopic and mineral chemistry.