A molecular fossil approach to the origin of ancient carbonate concretions
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
Diagenetic carbonate concretions commonly form during shallow burial via localized lithification of soft sediment and have long been recognized as a source of exceptionally preserved fossils. While concretions are common in many ancient sediments, modern equivalents are very rare and our understanding of the complex biogeochemical processes underlying their formation is incomplete. In this project the organic matter enclosed in ancient concretions was studied with organic geochemical techniques to provide further clues to the formation of these precipitates and test their significance as containers of exceptionally preserved molecular fossils. It was hypothesized that lipid biomarkers and kerogen locked in concretions may represent a ‘snapshot’ of the original sedimentary OM, plus the early microbial processes as they once occurred in the freshly deposited sediment. A particular focus was placed on testing the role of carboxylic acids (particularly fatty acyl-derived salts and other compounds), and their anaerobic degradation products as raw materials for the formation of carbonate concretions. A number of Jurassic concretions rich in siderite, calcite, and phosphate, was studied to test these hypotheses. It was observed that hydrocarbon biomarkers in these concretions do not show individual distributions, but mostly represent host rock compounds that have been trapped in the carbonate matrix. In contrast, carboxylic acids including n-fatty acids, methylbranched fatty acids, and α,ω-diacids were clearly enriched in concretions as compared to their host rocks, and partly differed in compound distributions. This is most likely due to (i) an increased initial abundance of fatty acyl-derived compounds at the place of concretion formation, (ii) additional inputs from microoganisms degrading these substances, and (iii) preferential binding of carboxyl groups to the carbonate lattice. Contributions of organic matter by recent soil microorganisms and other external contamination were also studied, but their influence on the fossil biomarker distributions was found to be negligible in our samples. Summarizing, concretions rather preserve individual organic features related to their microbially mediated, early diagenetic formation, than being a ‘treasure trove’ for the general marine environment. In a microcosm experiment, we further tested the potential of fatty acylrich substrates to fuel microbially mediated carbonate precipitation. Our results clearly testified the involvement of distinctive sedimentary bacteria (namely sulfate reducers shifting to Fe-reduction) on the formation of authigenic siderites as a plausible starting point for the formation of massive Fe-rich carbonate concretions.
