Early Jurassic (Lias) seawater paleotemperatures were determined through analysis of the oxygen isotopic composition of unaltered low-Mg calcitic belemnite rostras from the Basque Cantabrian basin, an area critically located in middle pajeolatitudes- between the Boreal and Tethyan realms. Using optical, cathodoluminescence, and chemical analysis; it has been possible to select very well preserved samples of Pliensbachian-Lower Toarcian belemnite rostra that retain the. original marine isotope signal. Nearly 200 samples have been analyzed for their S'sO and minor and trace-elements Composition (Ca, Mg, Mn, Fe, Sr) to provide records of geochemical changes on seawater composition.. The detected variations In the oxygen isotopic composition of the belemnite skeletal calcite could be' interpreted td reflect seawater palaeotemperature changes and/or palaeosalinity fluctuations related to freshwater input. However, independent evidences provided by variations .in their Sr/Ca and Mg/Ca ratios do not support significant paleosalinity changes but reinforce paleotemperature changes throughout the Pliensbachian-Early Toarcian. Our study shows that Early Pliensbachian paleotemperatures were relatively uniform and warm (mean 17.9°C), while the Late Pliensbachian-Earliest Toarcian period is marked by a strong drop of the mean temperature (14°C) with fluctuations up to 7 0°C. A rapid warming ocurred during the serpentinus Zone of the Early Toarcian, where average seawater temperature reached 2 VC
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