Vital effects on lithium isotope fractionation – Insights from Recent and Albian molluscs

Molluscs have great potential as geochemical archive due to their wide spatial and temporal abundance. For lithium isotope compositions (δ7Li), a proxy for reconstructing weathering intensity, they would offer a valuable possibility of obtaining data unobscured by clay contamination which is known to be an obstacle when working with sediment samples.

However, recent studies have shown the potential challenge of accounting for vital effects when interpreting δ7Li data from mollusc shells. Here, we assessed the potential of mostly aragonitic molluscs, mainly cephalopods, as an archive for δ7Li.

We analysed Recent cephalopods such as Argonauta hians, Nautilus pompilius, Spirula spirula and Sepia officinalis. This selection includes species that build both exo- and endoskeletons, and provides us the opportunity to assess whether this feature imposes different vital effects on lithium isotope fractionation. Furthermore, we sampled Albian molluscs including nautiloids (two specimens of Cymatoceras sp.) and ammonites (Cleoniceras sp. and Desmoceras sp.) as well as the bivalves Arca cucullaea and Cardium sp. from the Mahajanga Basin in Madagascar which show exceptional aragonite preservation.

Preliminary data for the modern specimens A. hians (a species that precipitates high-Mg calcite), S. spirula and N. pompilius show a narrow range of offsets from seawater δ7Li that resembles the fractionation between water and inorganically precipitated aragonite for biologically relevant growth rates.

For the Albian cephalopods and bivalves, we observe a narrow range of δ7Li values which hints towards possibly similar biomineralisation strategies among molluscs and thus, offsets caused by vital effects compared to their modern aragonitic relatives.