U-Pb analyses of a fossil crab cuticle from the Chattian Doberg strata (NW Germany)

A high abundance of arthropod cuticles in the fossil record are well preserved, even at the microstructural level. The age of these fossils is usually constrained chronostratigraphically, but incorporation of radioactive trace elements during fossilization makes cuticles also amenable to radiometric dating. Our study aimed to assess to what extent the current mineralogical, chemical, and isotopic composition of fossil cuticles can provide a record of the long-term diagenetic evolution.

We combined electron microprobe element mapping, Raman spectroscopy, and laser ablation ICP-MS analyses to study the exoskeleton of a fossil raninoid crab from Chattian (Late Oligocene) strata of the Doberg (near Bünde, NW Germany). The investigated exocuticle had been replaced by apatite, while the endocuticle mostly consists of uraniferous calcite (up to 500 ppm U), yielding a LA-ICPMS U-Pb age of 17.5 ± 1.4 Ma (Burdigalian, Late Early Miocene). This age is consistent with the view that the present mineralogy formed millions of years after the death of the individual. A Late Early Miocene age, however, was also obtained from another Oligocene raninoid crab cuticle from Bodenburg (100 km to the ESE), suggesting a possible regional geological significance of this age within the Lower Saxony Tectogene.

In conclusion, fossilized arthropod cuticle can remain a closed U-Pb system over geologic time scales. However, the fossil cuticles were altered by distinct geological events affecting the diagenetic conditions in depositional basins, such as, for instance, an increased fluid flow. The U-Pb system of arthropod fossil cuticles might allow for tracing these long-term diagenetic processes.