Can we improve the accuracy of climate reconstructions from fossil shells by measuring internal water in their carbonate?

Bivalve mollusc shells have proven to be promising recorders of environmental variability on short time-scales: incremental growth over their lifetimes (~ 1 – 100 years) allows for high resolution temporal sampling in their carbonate shells¹. Seasonal and even daily environmental variability have successfully been reconstructed using fossil shells, e.g. ^2–4.

However, these shells are not made up of pure carbonates but also contain organic matter and internal fluids⁵. Understanding the formation pathways and associated isotopic and trace elemental fractionation of these components of the shell carbonate system is important to deconvolute the bulk carbonate chemical signal. Furthermore, measurements of oxygen isotopes (δ¹⁸O) of internal fluids and carbonate coupled with clumped isotope (Δ₄₇, Δ₄₈) measurements of the carbonate can constrain disequilibrium precipitation and diagenetic alteration processes^6,7. Accounting for these processes allows for improved δ¹⁸O-based temperature reconstructions. As it is yet not well-constrained where internal fluids are present in biogenic carbonates, their significance for shell formation and as an environmental indicator is currently largely unknown⁸.  

Utilizing bivalve molluscs cultivated under closely monitored environmental conditions, we develop a method to quantify the different components of the shell carbonate system, analyse their respective isotopic and elemental signatures and correlate these with conditions experienced during growth. Modern bivalve shells collected from a wide range of present-day climate zones allow us to assess the performance of mollusc shells as archives for environmental conditions. This approach aims to provide a robust framework for improved future mollusc-based climate reconstructions and more accurate interpretation of chemical and isotope proxies in carbonate archives from past climates and environments.

 

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2. de Winter, N. J. et al. Amplified seasonality in western Europe in a warmer world. Sci. Adv. 10, eadl6717 (2024).

3. Kniest, J. F. et al. Dual clumped isotopes from Mid-Eocene bivalve shell reveal a hot and summer wet climate of the Paris Basin. Commun. Earth Environ. 5, 1–10 (2024).

4. Arndt, I. et al. 20,000 days in the life of a giant clam reveal late Miocene tropical climate variability. Palaeogeogr. Palaeoclimatol. Palaeoecol. 112711 (2025) doi:10.1016/j.palaeo.2024.112711.

5. Lécuyer, C. & O’Neil, J. R. Stable isotope compositions of fluid inclusions in biogenic carbonates. Geochim. Cosmochim. Acta 58, 353–363 (1994).

6. Nooitgedacht, C. W., van der Lubbe, H. J. L., Ziegler, M. & Staudigel, P. T. Internal water facilitates thermal resetting of clumped isotopes in biogenic aragonite. Geochem. Geophys. Geosystems 22, e2021GC009730 (2021).

7. Staudigel, P. et al. Fingerprinting kinetic isotope effects and diagenetic exchange reactions using fluid inclusion and dual-clumped isotope analysis. Geochem. Geophys. Geosystems 24, e2022GC010766 (2023).

8. de Graaf, S. et al. Analytical artefacts preclude reliable isotope ratio measurement of internal water in coral skeletons. Geostand. Geoanalytical Res. 46, 563–577 (2022).