Strontium-isotope stratigraphy and oxygen- and carbon-isotope record of Middle Jurassic to Early Cretaceous belemnites in the South Atlantic

The nannofossil, dinoflagellate and molluscan assemblages provide comparable age determinations. The strontium-isotope analysis of the youngest belemnites points to a Hauterivian-Barremian age, whilst age interpretations based upon the fauna provide a wide age range from the Barremian to early Albian. Strontium-isotope stratigraphy of this succession hence offers increased age resolution providing data regarding the timing of episodes of bottom water anoxia which have been recorded throughout the South Atlantic Basin.

Well-preserved belemnite specimens display an oxygen-isotope range between +0.08 and -2.22‰ (PDB, Peedee belemnite international standard) and a carbon-isotope range from +2.35 to -1.33‰ (PDB). Delta13C values become increasingly negative through the Late Jurassic-Early Cretaceous and in concert with the 87Sr/86Sr data reveal a trend that could be accounted for by increasing levels of weathering and erosion. The oxygen-isotope data if interpreted in terms of palaeotemperature are consistent with warm palaeotemperatures in the Kimmeridgian and slightly cooler temperatures for the Tithonian and Early Cretaceous parts of the succession.

The proposed relative Kimmeridgian warmth (based upon strontium-isotope age assignments) is thus in good agreement with other published palaeotemperature records.