EGU22-13210
https://doi.org/10.5194/egusphere-egu22-13210
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

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)

Claudio Garbelli1, Miles D. Lamare2, and Elizabeth M. Harper1
Claudio Garbelli et al.
  • 1Department of Earth sciences, University of Cambridge, CB2 3EQ, Cambridge, UK
  • 2Department of Marine Science, University of Otago, PO Box 56, Dunedin, New Zealand

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 (δ18Oshell) 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 (δ18Oeq) were estimated using salinity and temperature data. Results reveal that maxima and minima of δ18Oshell 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 δ18Oeq 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.

How to cite: Garbelli, C., Lamare, M. D., and Harper, E. M.: 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), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13210, https://doi.org/10.5194/egusphere-egu22-13210, 2022.