The 87Sr/86Sr record of Early Cretaceous (Barremian–Albian) marine invertebrates: First evidence from the Caucasus - palaeoenvironmental implications.

We present the first Sr-isotope data on benthic, semi-pelagic and pelagic invertebrates from 10 biostratigraphical zones of Lower Cretaceous deposits in the Caucasus. We provide a ⁸⁷Sr/⁸⁶Sr record for the Lower Cretaceous of the Caucasus that represents a first step towards refining the sequence of palaeoenvironmental events in the late Barremian–early Albian interval in this region. Integration of data from the Caucasus and Middle Volga regions reveals two local minima events in the Sr-isotope record for the Aptian-lower Albian interval. The first decrease in ⁸⁷Sr/⁸⁶Sr to 0.70733–0.70738 (the early Aptian event) was recognized from limited material from the Volgensis-Schilovkensis Zone in the Ulyanovsk Middle Volga region. A second decrease in ⁸⁷Sr/⁸⁶Sr (the late Aptian event), was more pronounced than the first, and occurred within the Abichi, Nolani and Jacobi zones in the Caucasus, where the ⁸⁷Sr/⁸⁶Sr ratio fell to 0.70720–0.70723, 0.70720–0.70723 and 0.70721, respectively. The combined ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ¹³C and redox records of the Caucasus, Ulyanovsk Middle Volga region and western Europe suggest that the ⁸⁷Sr/⁸⁶Sr decreases in the early and late Aptian times were initiated by sea-floor hydrothermal (basaltic volcanism) activity; associated palaeoenvironmental events include (1) an increase in temperature, (2) the development of anoxia (OAE-1a and OAE-b, respectively), (3) transgression and (4) a decrease in bio-productivity. • The Sr-isotope results represent a first step toward refining the sequence of Cretaceous palaeoenvironmental events. • The first Sr-isotope data from the Caucasus help resolve some discrepancies in regional biostratigraphical reconstruction. • The integrated data suggest the legitimacy of raising the question of the existence of two Aptian falls in seawater ⁸⁷/Sr/⁸⁶Sr ratio. [ABSTRACT FROM AUTHOR]