Using Zooid Size Variation and Stable Isotopes in Skeletal Carbonate to Infer Seasonality in Bryozoans - a Multi-Proxy Approach

In this study, two independent proxies are used to infer the annual temperature regime experienced by three Recent colonies of the cheilostome bryozoan Pentapora foliacea (Eliis & Solander) from Skomer, Wales. Intracolonial variation of zooid size has been shown to be directly proportional to the mean annual range of temperature (MART) experienced by the colony throughout its life. Measurements of zooid area, taken from SEM images of the colony, allow an estimate of the MART to be made adapting the method of O'Dea and Okamura (2000). It is also believed that many bryozoans secrete their carbonate skeleton in isotopic equilibrium with seawater, such that in warmer temperatures the carbonate deposited has lower δ¹⁸O than in colder temperatures. Detailed sampling and µg analysis along a transect of the bryozoan colony provides a reconstruction of seawater temperature regime experienced. Good correlations were found between estimates of seasonality (MART) obtained from bryozoan zooid size variation, inferred isotopic temperatures and values acquired in the field by a datalogger. Indications that P. foliacea ceases growth in the winter are consistent with previous studies. This modern study confirms that the MART and stable isotope analysis can be used to investigate palaeoclimatic conditions in fossil material, particularly when used together as a multi-proxy approach.