Microbial processes: Controls on the shape and composition of carbonate concretions

Abstract

Shape, size and mineralogical/chemical compositions of carbonate concretions vary considerably. The questions to be answered are—Why do they form at all? What processes are involved? Why do they so often form as regular spheroids? Why do they sometimes form laterally-extensive bands? What controls whether the mineral composition is calcite, dolomite, rhodochrosite or siderite (or mixtures of them)? The questions are addressed and the answers illustrated by selected examples. Carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents (organic matter). Microbes take advantage of the energy source and are believed to be involved in most of the various processes. Pore waters in steadily-accumulated sediments show a depth-related succession of chemical and carbon stable-isotope compositions. This results from degradation of organic matter proceeding via an orderly succession of processes during burial, each mediated by a specific microbial group. Aerobic oxidation, using dissolved oxygen occurs first, followed by Mn(IV), reduced to Mn(II), as the oxidant. Fe(III) reduction and sulphate reduction are probably more significant. Methanogenesis is the final, deepest microbial process. The common product of all these reactions is CO 2 or dissolved HCO 3 − which can result in carbonate supersaturation. The specific process controls the chemical and isotopic compositions of pore-water and leads to a characteristic mineralogy of any carbonate precipitated. However, the processes may not proceed successively and localised micro-environments occur. Recent work on siderite concretions in very young salt-marsh sediments (Norfolk, U.K.) has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth. Concretions in ancient rocks can be described in terms of the same processes observed in modern sediments. Laterally-extensive siderite bands in Bullhouse Quarry (Westphalian, U.K.) resulted from interaction of two sets of complementary processes: shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation. The model is supported by a mass and charge balance calculation. Similar, complementary processes formed spheroidal concretions (Gammon Shale, USA) but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction. The spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes. Calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model. Kimmeridge Bay (UK) dolomite ledges are marine analogues to the Bullhouse examples. For the pyrite-rimmed calcite Jet Rock concretions (Early Jurassic, Yorkshire, U.K.) the rate of production of sulphide (from sulphate reduction) locally exceeded availability of Fe(II), to precipitate pyrite. Outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion. Because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore-water evolution. However, all represent an expression in ancient rocks of microbiological processes similar to those operating today.