Origin of Siderite Concretions In Microenvironments of Methanogenesis Developed In A Sulfate Reduction Zone: An Exception Or A Rule?

Abstract

The Mississippian shales from Wardie (Scotland) were deposited in a large normally brackish-water basin. They host siderite concretions with d 13 C values ranging from 23.5 to 12.1% which show that sulfate reduction and methanogenesis contributed to the production of bicarbonate. Fossils are found within each of the concretions, suggesting that the decay of these organic remains provided a catalyst for concretionary growth. The pore water was freshened, which is indicated by the very negative d 18 O values (from 212.8 to 24.7%). Many features indicate that the concretions are very early-diagenetic, precompactional precipitates.The concretions are mostly homogeneous; only a few exhibit complex concentric zoning. The microstructure of the homogeneous concretions indicates a pervasive growth. However, their isotopic composition reveals a clear trend towards lighter d 13 C and d 18 O values from the center to the edge. It is proposed that the concretions grew due to methanogenesis in microenvironments around large organic remains, which acted as nuclei. We suggest that this process may be one of the reasons for the very common occurrence of cements as mineral concretions, not as disseminated precipitates along beds. These microenvironments developed entirely within the sulfate reduction zone, which resulted in the establishment of isotopic gradients in bicarbonate dissolved in the pore water across these microenvironments. These gradients were caused by fast consumption of sulfate by the intensive oxidation of organic remains. This resulted in the isotopic trends preserved in the homogeneous concretions. The formation of the zoned concretions proceeded generally concentrically, but detailed observations suggest that pervasive growth also contributed to their formation. Afterwards, all the concretions were deeply buried, probably within the oil window, where late-diagenetic phases crystallized (mainly kaolinite, quartz, and sulfides).

The dolomite nodules enclosing fossil marine vertebrates in the East Pisco Basin, Peru: Field and petrographic insights into the Lagerstätte formation

“…caused by ammonia production); subsequently these may function as nucleation centers for binding additional bicarbonate scavenged from pore-water (Allison, 1988). Geochemical studies of carbonate concretions in marine environments confirm the role of organic matter decay processes (AOM, SBR, methanogenesis) in their early diagenetic formation (Claypool and Kaplan, 1974;Irwin et al, 1977;Coleman, 1993, Bojanowski andClarkson, 2012). Recently, it has been suggested that nodules on large carcasses of vertebrates could be linked to the activity of sulphate reducing bacteria (Shapiro and Spangler, 2009), and a correlation with the evolution of modern whale-falls communities has been hypothesized (e.g., Danise et al, 2012).…”

Neogene terrestrial sedimentary environments of the Orava-Nowy Targ Basin: a case study of the Oravica River section near Čimhová, Slovakia

“…The range of depositional set tings doc u mented in the Oravica River sec tion, to gether with the abun dance of wood frag ments, points clearly to the non-ma rine or i gin of the ba sin fill in this area. This is fur ther sup ported by the sphe roi dal tex tures of sid er ite con cre tions, be cause such tex tures ap pear nei ther in fully ma rine, nor in brack ish-wa ter en vi ron ments (Bojanowski and Clark son, 2012).…”

Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

“…Therefore, some kind of a concentric mechanism was also involved in the concretionary growth (cf. Bojanowski & Clarkson, ). The proposed interpretation proves that the decreasing content of cement towards the edge of the concretions, which generally is typical, is not necessarily related to conventionally presumed concentric outward growth, but can be the result of a combination of pervasive and inward concentric growth mechanisms.…”

“…Therefore, the elemental and stable C and O isotope compositions of siderite concretions commonly provide information about the sources of pore‐water and the DIC therein, and may help to characterize the depositional environment (Mozley, ; Mozley & Wersin, ). Fresh‐water siderite concretions are reported to have precipitated from meteoric or brackish water, mostly due to Fe reduction or microbial methanogenesis (Curtis & Coleman, ; Curtis et al ., ; Mozley & Carothers, ; Mozley & Wersin, ; Middleton & Nelson, ; Bojanowski & Clarkson, ). Sulphate reduction, although common in marine sediments, does not usually operate in fresh‐water settings due to very low sulphate concentrations (Berner, ).…”