Abstract
Abstract. The Bancs Jumeaux Formation appears as a succession of limestone beds and marly interbeds, of Late Jurassic age (Tithonian), cropping out in the cliffs of the Boulonnais (French shore of the English Channel). This formation is enriched in pyrite present in abundance in the forms of framboids, polyframboids and other concretions of a relatively large size for sedimentary pyrite (a few tens to hundreds of micrometers). Previous works have shown that (poly)framboids appeared during early authigenesis in reducing microenvironments within dysoxic sediments. The question is whether, like what is observed for bacteria causing carbonate mineralization of their outer wall, sulfate-reducing bacteria could have been trapped (locked up) in pyrite “sarcophagi”, induced by their own metabolism. In an attempt to provide answers, the large pyrites were extracted from the Bancs Jumeaux marls by physical separations and acid digests. Their analysis reveals the presence of organic matter in significant quantities. The typical morphology of certain polyframboids present in the studied samples suggests the early fossilization of bacterial colonies by pyrite. If pyrite appears to be a relatively significant trap for organic matter, then pyrite could represent an underevaluated component of the carbon cycle.
Highlights
Heterotrophic bacteria are known to sometimes induce the precipitation of authigenic minerals, as a consequence of their metabolism
Could the sulfate-reducing bacteria, responsible for the precipitation of sedimentary pyrite, be mineralized, that is to say, pyritized, like other bacteria can be calcified? In an attempt to provide some answers to this question, this study focused on relatively largeframboids, easy to extract from marls rich in organic matter, belonging to the Bancs Jumeaux Formation (Fm.) of Tithonian age (Late Jurassic) and present in the cliffs of Boulonnais
Our observations support the hypothesis that bacteria can become covered with a pyrite coating during their lifetime
Summary
Heterotrophic bacteria are known to sometimes induce the precipitation of authigenic minerals, as a consequence of their metabolism. Using organic substrates as electron suppliers of redox reactions, bacterial activity conditions various parameters of the interstitial waters within sediments, such as the pH, Eh and alkalinity. The prominent two bio-induced products are calcite (CaCO3) and pyrite (FeS2). The works of Castanier et al (1999a, b) have shown, during controlled growth of bacteria in vivo, that calcifying bacteria can become calcified very quickly. This calcification can kill the bacteria by isolating them from the external milieu, but, in some cases, it has been observed that the bacteria remained alive inside their calcite “sarcophagus” or “cocoon”
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