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 Sr-87/Sr-86 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 Sr-87/Sr-86 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 Sr-87/Sr-86 (the late Aptian event), was more pronounced than the first, and occurred within the Abichi, Nolani and Jacobi zones in the Caucasus, where the Sr-87/Sr-86 ratio fell to 0.70720-0.70723, 0.70720-0.70723 and 0.70721, respectively. The combined Sr-87/Sr-86, delta O-18, delta C-13 and redox records of the Caucasus, Ulyanovsk Middle Volga region and western Europe suggest that the Sr-87/Sr-86 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-1 a and OAE-b, respectively), (3) transgression and (4) a decrease in bio-productivity. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study presents Sr-isotope data on invertebrates from different zones in the Lower Cretaceous deposits in the Caucasus, contributing to a refined understanding of palaeoenvironmental events in the 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 decreases in Sr-87/Sr-86 during the early and late Aptian times were possibly initiated by sea-floor hydrothermal activity, leading to changes in temperature, oxygen levels, transgression, and bio-productivity.