Lower Jurassic belemnites as indicators of palaeo-temperature

In order to test the validity of palaeo-proxy records in belemnite calcite, four species of belemnite have been compared for Mg/Ca, Sr/Ca, Na/Ca, δ18O and δ13C. They are Acrocoelites (Acrocoelites) subtenuis, Acrocoelites vulgaris, Simpsonibelus dorsalis and Youngibelus simpsoni. The specimens derive from 25 m of Lower Toarcian strata in Yorkshire, UK, that represent a period of ≈ 400 kyrs.Compared to Acrocoelites vulgaris, the species Acrocoelites subtenuis and Youngibelus simpsoni are less positive in δ13C, less negative in δ18O, and have broader compositional ranges. In all four species, δ13C correlates negatively with δ18O but the regression line for Simpsonibelus dorsalis is displaced 1.5‰ to more positive values of δ18O. In all species, Mg/Ca does not correlate with δ18O whilst, excepting in S. dorsalis, Sr/Ca correlates well, suggesting that Sr/Ca in Early Toarcian belemnites, but not Mg/Ca, may be a useful recorder of palaeo-temperature. Compositional differences between species reflect biofractionation, habitat differences, or both, and confirm that element/Ca values in belemnite calcite, if usable as palaeo-environmental proxies, must be interpreted at species level.For each of the species Acrocoelites subtenuis and Acrocoelites vulgaris, indistinguishable compositional ranges and inter-element relations exist in two populations, one from specimens collected from 25 m of strata that encompass ≈ 400 kyrs; the other for specimens collected from a single level in the section that is interpreted to represent a period of no more than a few weeks of time. The similarity suggests that at least some belemnite species inhabited environmental niches that remained unchanged in character, if not necessarily location, over substantial periods of time.

► Sr/Ca and δ18O correlate well among and within 3 species of Toarcian belemnites.
► Mg/Ca does not correlate with δ18O at species level or among all species.
► Sr/Ca in Toarcian belemnites may reflect calcification palaeo-temperature.
► Mg/Ca in Toarcian belemnites is not a useful palaeo-temperature indicator.
► The study of belemnite palaeo-proxies should be done at species level.