Sr and C Isotope Signals from Berriasian-Hauterivian Arctic Belemnites, Northern Eastern Siberia: Implications for the Boreal-Tethyan Correlation

Despite numerous attempts, biostratigraphic methods have some restrictions in Boreal–Tethyan correlation of the Lower Cretaceous, especially in geographically remote areas, such as Arctic Siberia and the Mediterranean region. The most reliable approach to solve the correlation problem is integrated bio-, chemo- and magnetostratigraphy. Among Boreal sites with published isotope data, only a few are characterized by complex C, O, and Sr isotope data. Here we represent new results regarding the Boreal–Tethyan correlation, which show the most justified position of the bases of the Upper Berriasian, Lower Valanginian, Upper Valanginian and Hauterivian in Arctic Siberia. These results are based on C and Sr chemostratigraphic data, as well as bio- and magnetostratigraphy from the Boyarka River, Nordvik Peninsula, and Anabar River sections in northern Eastern Siberia (Ryazanian–lower Hauterivian). The combination of these results provide to correct the age of a number of Siberian ammonite zones and sub-zones and correlate them with the Lower Cretaceous Standard Mediterranean Ammonite Zonation (SMAZ, Western Tethys). The 87Sr/86Sr values with bio- and magnetostratigraphy indicate that the base of the Upper Berriasian (the base of the Boissieri Chronozone) falls into the middle part of the Ryazanian Hectoroceras kochi ammonite Zone. It is shown that the upper boundary of the Ryazanian in Arctic Siberia (the top of the Tollia tolli ammonite Zone) is located below the level of the upper boundary of the Berriasian (the top of the Alpillensis Chronozone), so that it does not coincide with the Berriasian/Valanginian boundary. This follows from the noticeable deviation of the 87Sr/86Sr ratio obtained for the topmost Ryazanian towards lower 87Sr/86Sr compared to the 87Sr/86Sr values for the topmost Berriasian. Therefore, this implies the necessity of simultaneous re-evaluation of the age of the topmost Ryazanian elsewhere in Boreal regions. In Arctic Siberia, the base of the Valanginian should be placed slightly above the base of the Neotollia klimovskiensis ammonite Zone. A rapid positive δ13С shift near the base of the Upper Valanginian allows to correlate the base of the Polyptychites beani ammonite Subzone with the base of the Verrucosum Chronozone. The base of the Hauterivian can be located in the interval of ammonite beds with Neocraspediteskotschetkovi. The age of the overlying Homolsomites bojarkensis Zone is reliably determined as early Hauterivian based on the high average 87Sr/86Sr ratio corresponding to the Radiatus Chronozone. The obtained δ13C data recognize separate phases of the Weissert event in northern Eastern Siberia for the first time. The calculated amplitude of the δ13С shift at the beginning of this event, corresponding to the first of the largest positive δ13С excursions in the Cretaceous, is about 2.3‰. This is a contribution to the FWZZ-2026-0033 project.