Benefits and drawbacks of employing the carbonate shell of brachiopods as an archive of δ18O seasonality: facts and clues from the living Calloria inconspicua (Sowerby, 1846)

The carbonate shell of rhynchonelliformean brachiopods is a relevant archive of seawater temperature / δ¹⁸O for the Phanerozoic. However, its application to investigate seasonality is under-exploited despite its wide employment to reconstruct the chemical conditions of past oceans. Indeed studies of brachiopod sclerogeochemistry are still rare compared to some other biogenic archives. In order to better understand if ontogenetic time-series faithfully record the seasonal variability of the surrounding environment, we investigated the incorporation of δ¹⁸O in the shell (δ¹⁸O_shell) of a modern temperate brachiopod species, Calloria inconspicua, living in the intertidal zone of Otago Harbor (New Zealand).  After reconstructing the life history and ontogenetic age using morphological features of the valves, carbonate samples were collected from the inner fibrous layer along the maximum growth axis with a sub-millimetric resolution. Databases provided environmental parameters of seawater for comparison, and time-series of δ¹⁸Ofor equilibrium calcite (δ¹⁸O_eq) were estimated using salinity and temperature data. Results reveal that maxima and minima of δ¹⁸O_shell are well related to morphological markers highlighting yearly growth steps. The count of yearly cycles allowed an estimate of the timing of shell secretion and comparison with the estimated δ¹⁸O_eq of the surrounding environment.

We found that C. inconspicua can be a robust archive for seasonal variation since it records with high fidelity the local condition of seawater in certain phases of its life. Although the application of the brachiopod shells in sclerogeochemistry and studies of past seasonality is promising, future researches should consider a few potential drawbacks which need to be taken into consideration. These include the fact that growth rate changes over the life-span and that shell is not deposited throughout the entire year (leading to under-estimation of the full seasonal range). Additionally, it is important to ensure that sampling does not sample different thickness within the shell, this returning time-averaged values. Knowledge of likely general biology, population composition, life cycle and shell structure are important prerequisites for the interetation of δ¹⁸O time-series measured on fossil brachiopod shells.