Well preserved calcitic belemnite samples from the Basque-Cantabrian basin of northern Spain have been analyzed to improve our current understanding of the chemistry of seawater as an index of paleoceanographic changes during the Early Jurassic, a period punctuated by times of oceanic anoxia and global mass extinctions. Because the oxygen-isotope composition of calcite depends not only on temperature but also on salinity, we present here a combination of Sr/Ca, Mg/Ca, and δ18O analyses that helped us to evaluate both effects separately. Comparison between the temporal records and cross plots of these geochemical parameters shows a good correlation of Mg/Ca with δ18O and a weak correlation of Sr/Ca with δ18O.

Comparison of our belemnite δ18O record with coeval O-isotope profiles from other paleogeographic domains suggests the existence, despite differences in the absolute isotopic values, of a major negative shift during the serpentinus Zone that is regionally reproducible. This event is linked to the Early Toarcian ocean anoxic event (OAE), and suggests the existence of global changes in paleoclimatic and paleoceanographic conditions that are reflected largely in the geochemical composition of seawater. Compared to our results, however, other Early Toarcian δ18O data reported from northern European basins are generally depleted ∼1.3-1.6‰ In contrast, Mg/Ca ratios display similar values. These differences in δ18O data suggest a salinity component for the northern European data and provide evidence for a north-to-south salinity gradient within the north European interior (Boreal) seaway during the Early Jurassic. This study suggests that the secular variation of Mg/Ca is a useful proxy for discriminating between temperature and salinity effects on the oxygen isotope composition of fossil shells, and highlights the prospect of using the Mg/Ca ratios of belemnite calcite to calculate paleotemperatures independently of salinity. The findings of our study may help to calibrate δ18O curves in other basins by removing the salinity component of their δ18O records, which may help to calculate water density gradients between the Jurassic seas of northern and southern Europe.

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