Oxygen and carbon isotopic composition of marine carbonate concretions; an overview; discussion and reply

Abstract

In their review of isotopic data from marine carbonate concretions, Mozley and Burns (1993, p. 8 i) incorrectly associate our work (Dix and Mullins 1987) with their interpretation of the effects of recrystallization on isotopic compositions in concretions. Mozley and Burns (1993, p. 81) state, Late-stage calcite and siderite concretions are characterized by a combination of 'normal marine' 6~3C values and depleted 6~sO values. Such isotopic values have been interpreted by some authors as indicating recrystallization (i.e., that the concretion initially had normal marine carbon and oxygen isotopic compositions and that the oxygen isotopic compositions were reset at higher temperatures; Dix and Mullins 1987 . . . . ). The association of our work with this statement is incorrect because we illustrated that: (1) the bulk of the concretions in the Middle Devonian Hamilton Group appear to have formed during very early stages of burial within the upper 10 m (p. 147); (2) the very negative 5~3C values of most concretion samples and systematic shifts with concretion growth illustrate the role of organic diagenesis within the zone of sulfate reduction, and transitional into the upper zone of methanogenesis (our Fig. II); and (3) the only late-stage concretions (containing ferroan calcite) have 6'3C values ranging between 0 and 2%, and this due to a subsurface shift in porewater 6'3C related to organic diagenesis (our Fig. 11), not recrystallization (p. 150). In their discussion of causes for consistently depleted 5'80 values in shallow-burial calcite concretions, Mozley and Bums (1993, p. 77) appear to dismiss the effect of recrystallization partly through their assumption that the original composition of concretions must be stable low-magnesium calcite. They cite, however, only three studies of concretions that unequivocally remove any possibility for recrystallization. Are these the exceptions or the rule? Combined petrographic and isotopic observations still lead us to the interpretation that recrystallization of Hamilton Group concretions (excluding late-stage concretions) was very important. 6180 data fall within the middle of an isotopic group defined by late-stage septarian calcites, and our samples of septarian calcites also fall within this group with ~laO values that are not too dissimilar to the bulk of concretion calcite (our Fig. 12). We presented petrographic evidence illustrating that two stages of septafian calcite have been replaced by subsequent calcite, including ferroan calcite (our Fig. 7), and that replacement calcite is associated with silica diagenesis in the concretion bodies and migration of petroleum-bearing fluids. If septarian calcite can be replaced, why not the concretion-body calcite too? With so many calcite concretions appearing to form under near-surface burial conditions, could the mineral-water interaction that Mozley and Burns (1993) call upon to account for net shifts in ~aO include very shallow recrystallization ofa metastable magnesium-beating carbonate? Given the diverse mechanisms for causing a shift in 6'80 (Mozley and Burns 1993), recrystallization seems to be a plausible factor (see Desrochers and AI-Aasm 1993) that should not be discarded so readily.