Strontium-isotope stratigraphy of the Early Cretaceous (Valanginian-Barremian): Implications for Boreal-Tethys correlation and paleoclimate

The interpretation of past oceanographic events on a supra-regional scale requires precisely dated and well correlated biostratigraphic schemes. Only synchronous events can be interpreted in a global context. Events of local or regional character have therefore to be accurately correlated with time-equivalent shifts in other areas in order to be interpreted in a wider context. One of the problems of inter-basin correlation based on biostratigraphy lies in floral and faunal provincialism of the relevant index fossils. In order to overcome such limitations, chemostratigraphy can be used as a stratigraphic tool independent of biostratigraphy. This study presents Sr-87/Sr-86-isotope data for the Lower Cretaceous (Valanginian to Barremian) based on previously published findings from Speeton (northeast England) and the Vocontian Basin (southeast France). These belemnite-based Sr-87/Sr-86-isotope data allow a correlation of the Lower Cretaceous sequences of the Boreal Realm and the Tethys independent of biostratigraphy. This chemostratigraphic approach may help to overcome the biostratigraphic problems which have been discussed for more than 40 years. The two different Sr-87/Sr-86-isotope curves modeled for the Boreal Realm and the Tethys show two offsets, asking for the following adjustments of the current correlation scheme. In the Boreal Realm, the base of the lower Hauterivian, currently defined by the first occurrence of the ammonite Endemoceras amblygonium, should be shifted into the Endemoceras amblygonium zone. The base of the upper Barremian, in the Boreal Realm defined by the first occurrence of the ammonite Paracrioceras elegans, correlates with the uppermost part of the Tethyan Kotetishvilla compressissima zone in the lower Barremian. To adjust for this mismatch a shift of the lower/upper Barremian boundary is necessary either in the Boreal Realm or in the Tethys. The combined Sr-87/Sr-86 and delta O-18(Bel) isotope datasets suggest a synchronous negative delta O-18(Bel) shift for the middle Barremian. This shift, seen both in the Boreal and in the Tethys, indicates a major European wide phase of high humidity and increased run-off. (C) 2014 Elsevier Ltd. All rights reserved.