Experimental studies on the genesis of apatized fish fossils in calcareous concretions

• Applicant: Professor Dr. Thorsten Geisler-Wierwille, since 12/2023
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 348043586

Apatized fish fossils in calcareous concretions are among the most significant and spectacular fossils worldwide. They are especially famous for their soft tissue preservation. In these fossils there are indications of extremely fast mineralization which allowed for the preservation of very delicate structures. Although these concretions have been investigated for a long time, there is no clear model for their origin. This is the purpose of this proposal. Using the fish fossils from the Santana Formation in Brazil and the Gogo Formation in Australia as examples, the formation of these fossils and the concretions around them shall be experimentally reconstructed. The focus of these experiments will be the influence of microbial fauna on the chemistry of the surrounding seawater. The combination of biotic and abiotic factors is essential to achieve this outstanding preservation. In this project the precipitation of calcium carbonate through the influence of ammonia on seawater will be simulated. Ammonia increases the pH-value of seawater, which is normally ~8.1 in modern seawater and therefore not alkaline enough to stabilize CaCO3, to over 9. This causes inorganic CaCO3 precipitation, which could then lead to the formation of a calcareous concretion. Microbial activity seems to be the obvious source for this NH3. When organic matter, such as a dead fish, is decomposed, some bacteria release ammonia; a dead fish would therefore, itself, be the source of the ammonia through which it can later be preserved as a fossil. Besides CaCO3 for concretion formation, the precipitation of apatite (Ca5(PO4)3(OH)), which then causes the actual mineralization of the former organic soft tissue, is also important. Sea water contains almost no phosphate to precipitate apatite. Therefore, a strong accumulation of phosphate must take place to stabilize apatite. Bacterial activity also offers a solution for this as well. In low phosphate environments, such as seawater, some bacteria produce an enzyme that releases phosphate from (dead) organic matter: alkaline phosphatase (AP). Overactivity of this enzyme can lead to a high accumulation of phosphate in seawater. The origin of PO42-, the substrate for the enzyme, is again a dead fish and the bacteria that inhabit it. The alkaline pH value necessary for apatite formation is already explained by the influence of NH3 and calcium is present in excess in seawater. This enables an inorganic precipitation of apatite. The aim of this application is to show experimentally that a fish carcass can trigger its own fossilization and that PO42 for fossilization in concretions comes from the carcass itself and does not have to be provided externally.

• DFG Programme: Research Units
• Subproject of: FOR 2685: The Limits of the Fossil Record: Analytical and Experimental Approaches to Fossilization
• Ehemaliger Antragsteller: Dr. Fabian Gäb, until 11/2023