Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah, USA)

Abstract

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ13C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ13C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ13C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ34S) and silica (δ18O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.

Authigenic carbonate burial in the Late Devonian–Early Mississippian Bakken Formation (Williston Basin, USA)

“…However, the δ 13 C carb data indicate that the majority of carbonate was sourced from dissolved inorganic carbon rather than oxidized organic matter, in contrast with low‐ δ 13 C authigenic cements from similar geological deposits (Stakes et al ., ). These findings are similar to other surveys of intermediate‐ δ 13 C siliciclastic‐hosted cements in the Cambrian (Gaines & Vorhies, ) and Late Permian (Saitoh et al ., ). On the basis of these commonalities, this study hypothesizes that conditions in the Palaeozoic which supported episodes of widespread carbonate authigenesis were fundamentally different from the modern ocean in that poorly‐oxygenated basins facilitated anaerobic oxidation of organic carbon at or above the sediment–water interface, allowing authigenic precipitation adjacent to an augmented DIC reservoir and significant mixing with this isotopically enriched pool (Saitoh et al ., ).…”

Growth mechanisms and environmental implications of carbonate concretions from the ~ 1.4 Ga Xiamaling Formation, North China

“…Early diagenetic carbonate concretions are authigenic mineral aggregates formed in sediments at the expense of microbial respiration of organic matter (e.g. Sellés-Martínez 1996; Raiswell and Fisher 2000;Gaines and Vorhies 2016). Various microbial processes in suboxic to anoxic zones of marine sediments have the ability to induce the formation of carbonate concretions by increasing the environmental alkalinity and pH (Curtis et al 1986;Loyd et al 2012).…”

Formation of large carbonate concretions in black cherts in the Gufeng Formation (Guadalupian) at Enshi, South China

“…Therefore, the upward decrease in the proportion of framboidal pyrite relative to euhedral pyrite (Figure ) reflects a process of concretionary growth associated with burial. More enriched values are interpreted to have precipitated during a later stage of growth (Gaines & Vorhies, ). Thus, the upward increase in , combined with an increasing proportion of euhedral pyrite (Coleman & Raiswell, ), suggesting that the lower part of the studied concretion formed at an earlier stage than the upper part (Loyd et al, ).…”

“…Therefore, the upward decrease in the proportion of framboidal pyrite relative to euhedral pyrite (Figure 7) reflects a process of concretionary growth associated with burial. More enriched 34 S py values are interpreted to have precipitated during a later stage of growth (Gaines & Vorhies, 2016). Thus, the upward increase in 34 S py , combined with an increasing proportion of euhedral pyrite (Coleman & Raiswell, 1981), suggesting that the lower part of the studied F I G U R E 7 Chemostratigraphic profiles of 13 C carb , 18 O carb , Pyrite content, 34 S py , CAS content, 34 S cas , and framboidal pyrite proportion (percentage) in the carbonate concretion from the Tianfengping section, South China.…”