The problem of isolation and freshening of the Jurassic–Early Cretaceous Middle Russian Sea: A new 87Sr/86Sr, δ18O and δ13C record from the Saratov Volga region

Abstract

We provide the first strontium isotope data on molluscs from the lowermost Bathonian (Besnosovi Zone) and lower Aptian (Tenuicostatus and Volgensis zones) of the Saratov Volga region and some new O- and C-isotope results on early Aptian fossils from this region. Despite minor diagenetic changes in the earliest Bathonian bivalve shells from the Besnosovi Zone, they show 87Sr/86Sr values very close to that (0.70706) of a well-preserved belemnite rostrum from the same zone. New data on the Sr-isotopic composition of the Jurassic and Early Cretaceous (early Aptian) molluscs from the Saratov Volga region, combined with information on the stable isotope content of these molluscs, suggest that the isolation and partial freshening of the Middle Russian Sea occurred both in the late Oxfordian-Kimmeridgian, as has recently been reported, and in the early Aptian (during the Volgensis, to a lesser extent the Tenuicostatus chrons). This information on the lower Aptian is consistent with the available stable isotope, palynological and palaeoecological data. The pronounced early Aptian 87Sr/86Sr value deviation detected in many well-preserved fossils mainly from the Volgensis Zone seems to have contributed to the active input of radiogenic strontium into the basin during a series of repetitive events associated with some episodic isolations and partial freshening of the Middle Russian Sea, starting at the Tenuicostatus Chron and intensifying at the onset of the Volgensis Chron associated with the Oceanic Anoxic Event 1a (OAE 1a). New isotope data, collected from certain molluscs residing in the lower Aptian Tenuicostatus Zone of the Saratov Volga region and living in normal seawater salinity conditions, when combined with literature evidence sourced from other locations such as Ulyanovsk Volga region, Central Pacific and Germany suggest the presence of early Aptian climatic optimum. Further molluscs from the lower Aptian Volgensis Zone of the study area offer insight into the habitat conditions of Mesozoic ammonoids.