Formation of fossiliferous concretions in the Cretaceous Santana Formation - Assessing the role of microbial processes
Zusammenfassung der Projektergebnisse
Exceptional fossil preservation (incl. phosphatization of soft-tissues) within organic-rich mudstones is often associated with the formation of a protective carbonate shell surrounding the fossil specimen. Whereas the mechanisms controlling soft-tissue mineralization during the earliest stage of fossilization are considerably well understood, only limited information is currently available on the complex biogeochemical processes which lead to the precipitation of the concretionary carbonate mantle around the fossils. In general, carbonate concretions are thought to form via mineralization of respiratory carbon dioxide derived from organic carbon oxidation by anaerobic bacteria and archaea. In rare cases, early cementation of the surrounding host sediment may allow for a three-dimensional preservation of delicate fossil specimens. Examples for this type of preservation are the fishes, turtles and pterosaurs from the Brazilian Santana Formation. Besides the spectacular macrofossil content, early cementation may also result in the preservation of biogeochemical products of specific microbial processes and reactions, which occur during concretion growth. The inorganic and organic geochemical approach used in this study (incl. high-resolution δ13C and δ18O analysis, micro-XRF scanning, petrography and CL microscopy, carbonate clumped-isotope thermometry, biomarker analysis) provides novel insights into the multistage diagenetic history of the Santana concretions. Characteristic petrographic and geochemical patterns, which can be reproduced within single concretions from different localities, clearly support that the original geochemical signatures were preserved. Secondary processes like surficial weathering can therefore be excluded. Based on petrographic observations, micro-XRF scanning and stable isotope data, a genetic model is proposed to explain the early cementation of the concretions. The observed geochemical variations can be best explained with a scenario linking anaerobic methane oxidation (fueled by methane generation in the decaying carcass in the center of the concretion) with sulfate-reducing conditions in the outer parts. Such a model could well explain (1) the high carbonate carbon and low pyrite content characterizing the inner part of the nodule and (2) the circular pyrite distribution patterns restricted to the outer zone. These results are in line with findings of biomarkers in the concretions and the hosting black shale indicative of archaea. However, the stable isotope signatures, combined with petrographic evidence and data from carbonate clumped-isotope thermometry show clear evidence for diagenetic overprinting of this earliest cementation phase by at least two additional phases. The occurrence of an outermost “cone-in-cone” sparite rim can be associated with comparatively deep burial, probably in the range of 1.5 to 2 km. In summary, the combination of petrographic, inorganic and organic geochemical methods reveals a relatively complex diagenetic history for the Santana concretions with several stages of cementation under varying physical and chemical conditions.
