Belemnites as an archive for the seawater lithium isotope composition: Evidence in the belemnite rostrum for large vital effects

The lithium isotopic composition of the seawater (δ7Li) can be used as a proxy for silicate weathering. So far, different archives like planktonic foraminifera, brachiopods, mollusks and bulk carbonates have been tested. To date only one study has investigated the δ7Li in belemnite rostra.

A Late Cretaceous belemnite (Belemnitella mucronata) from the Campanian succession of Lägerdorf, Germany was investigated. The lithium isotopic composition of the chalk matrix surrounding the specimen has a δ7Li value of +21.9 ±0.06‰ (2σ). The section across the belemnite rostrum displays signs of diagenetic alteration in its center and at the outer margin. The bulk belemnite was drilled from the intermediate pristine part of the rostrum – the domain that is considered least altered. The data analyzed by solution MC ICP-MS (Multi Collector Inductively Coupled Plasma Mass Spectrometry) revealed a δ7Li value of +36.8 ±1.4‰ (2σ, n=2). We also conducted spatially resolved Li-isotope analyses across the rostrum by laser ablation MC ICP-MS. δ7Li varies between +20.5 and +59.8‰ with the highest values in the intermediate pristine part of the rostrum. In contrast, low values were detected in the milky domains, which are considered as altered by diagenesis.

The distinct positive isotopic offset between B. mucronata and surrounding chalk (15–21‰ for bulk solution chemistry data) exceeds the magnitude of modern mollusks, which are also enriched by 2–8‰ relative to the lithium isotopic composition of modern seawater. This positive offset indicates a high influence of vital effects during the biomineralization process of this species.