The calcareous nannofossil crisis in Northern Spain (Asturias province) linked to the Early Toarcian warming-driven mass extinction

Abstract

Quantitative analysis of Late Pliensbachian–Early Toarcian calcareous nannofossil assemblages from the West Rodiles section (Asturias, Northern Spain) has been performed in order to interpret the paleoenvironmental changes that occurred during this time interval, characterized by a major extinction event, and especially around the Lower Toarcian Tenuicostatum/Serpentinum zonal and extinction boundary. Nannofossil data were statisti- cally treated: the Shannon diversity index was calculated, and results were compared to the stable isotope data and the total organic carbon content. To determine the changes recorded in the entire nannofossil communities, a principal component analysis was applied. During the latest Pliensbachian, the nannofossil assemblages were dominated by Schizosphaerella sp. and Tubirhabdus patulus, followed by the dominance of Calcivascularis jansae, taxa that probably thrived in rather cold waters. The progressive decrease in the relative abundances of both Schizosphaerella sp. and C. jansae coincides with a progressive increase in paleotemperatures during the extinc- tion interval, as indicated by the δ18O values measured on diagenetically screened belemnite calcite. Biscutum spp. dominated the nannofossil assemblages during the Early Toarcian Tenuicostatum Ammonite Zone, when seawaters were warm. In the West Rodiles section, the extinction boundary coincides with the deposition of the laminated shales, where especially high relative abundances of Calyculus spp. were recorded. After the extinction boundary, C. jansae becomes extinct, the relative abundance of Biscutum spp. sharply decreases, and the nannofossil assemblages become dominated by the Crepidolithus and Lotharingius species, which have been interpreted as opportunistic taxa. The Shannon Index fluctuates throughout the studied section, although it is especially high after the extinction boundary. The covariance between the nannofossil crisis and the evolu- tion of δ18Obel-based seawater paleotemperatures, as well as the fact that none of the explanations proposed by other authors seems to explain our observations, suggest a clear relationship between the increase in paleotemperature and the changes recorded in our nannofossil assemblages. Nevertheless, we do not discard possible changes in other paleoenvironmental parameters related or not to warming