Paleotemperature variations of Early Jurassic seawater recorded in geochemical trends of belemnites from the Basque–Cantabrian basin, northern Spain

Abstract

Here we present new O-isotope, Mg/Ca and Sr/Ca data of Early Jurassic belemnites from the Basque–Cantabrian basin of northern Spain, a succession deposited in a periodically anoxic, restricted hemipelagic basin. Nearly 200 Pliensbachian–Lower Toarcian belemnites were collected and examined using optical, cathodoluminescence and chemical techniques to constrain the effects of potential diagenetic alteration. After a precise diagenetic screening of samples, 176 belemnites were selected to provide records of changes in seawater geochemistry and temperature. The records show a fairly good correlation of δ 18O with Mg/Ca ratios ( r=−0.65) and a weak correlation of δ 18O with Sr/Ca ratios ( r=0.34). The oxygen isotope data, if interpreted in terms of paleotemperature, have revealed a period of relatively uniform and warm seawater temperatures during the Early Pliensbachian, followed by a sharp temperature drop during the Late Pliensbachian–Earliest Toarcian. The onset of this cooler period occurred within the Protogrammoceras celebratum Subzone and the coolest seawater temperatures were reached at the end Pliensbachian Pleuroceras spinatum Zone. This was followed by a rapid seawater warming during the Harpoceras serpentinus Zone of the Early Toarcian. In addition, the data set provides new information regarding paleoceanographic conditions for anoxic events in a critical region of connection between the Boreal and Tethys realms. It is the background to relatively high δ 18O values that prevailed during the anoxic episodes. Concerning the impact of the Early Toarcian anoxic event in northern Spain, the detected δ 18O and Mg/Ca excursions reached their minimum and maximum, respectively, during the lower H. serpentinus Zone, while the anoxic black shale interval developed earlier, during the upper Dactylioceras tenuicostatum–lowermost H. serpentinus Zones.