Mineralogical, elemental, stable and clumped isotope composition of modern bryozoan skeletons.

Bryozoans are one of the most invasive phyla on Earth. Since their appearance in the Upper Ordovician period, a fair proportion of these colonial organisms have developed innovative adaptation strategies, like the ability to form a carbonate skeleton. Despite the fact that these reef builders can represent up to 80% of the carbonate production of some sedimentary formation, bryozoans have been poorly studied compared to other bio-carbonate archives. The diversity of bryozoan morphology is an impediment to their identification and their use for paleoenvironmental reconstruction. The morphology of the carbonate chambers (zoecium) varies not only from one species to another, but also as a function of physiological or environmental parameters. Moreover, depending on the species, bryozoan carbonate skeleton can be polycrystalline. The abundance of each carbonate polymorph can vary spatially within the colony, which has implications for the interpretation of the geochemical record. In order to retrieve useful paleoenvironmental information from this extensive record, we thus need to fill the gaps in our knowledge of bryozoan mineralization mechanisms.

In this contribution, we characterized the mineralogical and isotopic composition of different species of bryozoan living in the same microenvironments and identical species from different locations. Samples were collected from the Western Mediterranean (Marine station of Banyuls sur Mer, France) and North Atlantic (Marine station of Roscoff, France) coasts, where environmental parameters are continuously measured. Mineral characterization by XRD measurements were done on portions of each bryozoan colonies from base to the top and completed by 3D X-Ray diffraction imaging at a nanometric scale on a single zoecium. These mineralogical characterizations were matched with δ¹⁸O, δ¹³C analysis and clumped isotope (Δ₄₇) measurements. Using environmental data (T, pH, S, δ¹⁸O_w and δ¹³C_DIC) collected in situ, the measured isotopic signatures were compared to their respective expected values (assuming pseudo-equilibrium carbonate precipitation). This comparative work yields some unexpected discrepancies from the “equilibrium” line and between different species originated from the same site in both δ¹⁸O and δ¹³C compositions. Δ₄₇measurements, performed on 4 selected species (Pentapora foliacea, Cellaria fistulosa, Sertella beaniana, Tubicellepora avicularis) revealed that the magnitude of apparent isotopic disequilibrium observed in bryozoan is not related to the mineralogical composition of the skeleton nor to the species but rather to the living environment of the organisms. Surprisingly only bryozoan originated from the Mediterranean Sea seems to precipitated their skeleton out of isotopic equilibrium for Δ₄₇. These results permit to discuss the origin of this “isotopic vital effect”, its relation to environmental conditions, and the use of bryozoan as a new paleo-tracer.

Key words: Bryozoan – 3D X-Ray map - Clumped isotopes – Stable isotopes - Isotopic disequilibrium.