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, δ 18O and δ 13C. 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 δ 13C, less negative in δ 18O, and have broader compositional ranges. In all four species, δ 13C correlates negatively with δ 18O but the regression line for Simpsonibelus dorsalis is displaced 1.5‰ to more positive values of δ 18O. In all species, Mg/Ca does not correlate with δ 18O 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 δ 18O correlate well among and within 3 species of Toarcian belemnites. ► Mg/Ca does not correlate with δ 18O 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.