The response of calcareous nannoplankton to the latest Pliensbachian-early Toarcian environmental changes in the Camino Section (Basque Cantabrian Basin, northern Spain)

Quantitative analysis performed on latest Pliensbachian-early Toarcian calcareous nannofossil assemblages from the Camino section (Basque Cantabrian Basin) allowed their response to the environmental changes recorded during this time interval to be deciphered, characterized by an extinction event. The results were introduced within a principal component analysis and compared with the stable isotope and total organic carbon curves. During the latest Pliensbachian, the Mirabile and the lowermost part of the Semicelatum Ammonite Subzones, Schizosphaerella, Bussonius prinsii, Biscutum finchii, Calcivascularis jansae and Similiscutum avitum, taxa that probably thrived in rather cold waters, dominated the calcareous nannofossil assemblages. Coinciding with warmer and wetter conditions, which probably led to an increase in surface water fertility, recorded slightly below the extinction boundary, the mesotrophic taxa B. novum, L. hauffii and Calyculus spp. were dominant. Nevertheless, T. patulus and C. jansae, which became extinct just below the extinction boundary, show preferences for oligotrophic conditions. Salinities similar to those of modern oceans have been inferred around the extinction boundary, considering the coupling between the abundances of Calyculus spp. and the species richness together with the absence of black shales. After the extinction boundary, nannofossil assemblages were dominated by the deep-dwelling C. crassus and the shallow-dwelling Lotharingius species, interpreted as opportunistic taxa. This work confirms that calcareous nannofossils are a really useful tool for palaeoceanographic and palaeoenvironmental reconstructions, especially in terms of climatic changes.