An Isotopic analysis of belemnites from the Speeton Clay Formation (Lower Cretaceous) of Filey Bay, North Yorkshire, England.

The Speeton Clay Formation (Lower Cretaceous, Berriasian to Albian) exposed on the northeastern coast of Yorkshire, England, is rich in history, but additional data and interpretation is needed for paleoenvironment and paleoclimate interpretations. The depositional environment originated with a transgressive event, which later transitioned into a regressive event. This is reflected through the marine interbedded calcareous mudstones and clays. These transgressive and regressive sequences correlate to Boreal (cooler temperature) and Tethyan (warmer temperature) realms. The boundaries of the realms fluctuated along Speeton Clay Formation. Dark marine fossiliferous calcareous clays are distinctive for the Speeton Clay. The formation itself is divided into four main beds (A, B, C, and D) and a basal bed. Each of these beds has a distinctive paleoenvironmental and paleontological history. The calcareous clay outcrops are often slumped. Belemnoidea genera are used to define each of the beds: A (Neohibolites), B (Oxyteuthis), C (Hibolites), and D bed (Acroteuthis). The calcitic belemnites can be used as proxy records to see the boundary fluxes of the Boreal and Tethyan Realms of the Early Cretaceous. New major and minor element data (Mg, Mn, Fe and Sr) from the Speeton belemnites were retrieved to determine the degree diagenetic alteration in the specimens. Two element ranges suggest that the calcite in the Speeton belemnites has retained its original isotopic balance with seawater. It is possible that high levels of major ions (Mg vs. Na, Fe vs. Na, and K vs. Na) suggest the Tethyan samples were formed under arid conditions. In addition to elements, δ 18O and δ 13C were used to interpret the paleoclimate. δ 13C suggests the overall marine carbon burial at the time, so it cannot be used to suggest paleoclimate of Boreal and Tethyan Realms. Positive δ 18O records show \u3c7°C at the base to 13°C at the top of the Speeton Clay Formation. Calcitic belemnites retained their isotopic composition from the Speeton Clay Formation, assuming that the specimens have an insignificant degree of diagenetic alteration, and can be used to suggest paleoclimate trends during the Early Cretaceous