Pliensbachian calcareous nannofossil paleoecology in the E Rodiles section (Asturias, N Spain): A key location connecting the Boreal and Tethyan realms

Abstract

Here, we present the results of a quantitative analysis performed on Pliensbachian calcareous nannofossil assemblages from the E Rodiles section (northern Spain), which allowed us to determine their changes in response to paleoenvironmental variations in a key connection between the Boreal and Tethyan realms. Nannofossil data are compared to published carbon and oxygen stable isotope data and organic matter content. During the Early Pliensbachian, when seawater temperatures were around the Pliensbachian average value, calcareous nannofossil assemblages were dominated by the nannolith Schizosphaerella punctulata and the ancestral coccoliths Crucirhabdus primulus and Parhabdolithus liasicus. Mitrolithus elegans, Similiscutum avitum, Similiscutum cruciulus and, to a minor extent, Schiszosphaerella show peaks in their abundances coinciding with the deposition of black-shales and, probably, they thrived in periods of enhanced marine primary productivity and under eventually poorly oxygenated and water stratified conditions. A significant change in calcareous nannofossil assemblages is observed in the Late Pliensbachian, with the dominance of Tubirhabdus patulus, the newly emerged species Lotharingius hauffii and a sharp increase in the abundance of Calcivascularis jansae, coinciding with the rise in the δ18Obel values due to a cooling event that led to the increase of taxa with affinities for rather low temperatures. The species of Crepidolithus, interpreted as opportunistic, show fluctuations in their abundances throughout the studied section, although they are more abundant during the Early Pliensbachian warming, together with the species of Similiscutum, and especially coinciding with the δ13Cbel positive peak in organic-poor carbonates, in opposition to Similiscutum spp., which show their abundance peaks in organic-rich layers.