Palaeoenvironmental changes during the Danian-Selandian Boundary interval: the ichnological record at the Sopelana section (Basque Basin, W Pyrenees)

Ichnological analysis was conducted in the Danian–Selandian (D–S) boundary interval from the Sopelana section (Basque Basin, northern Spain) to improve characterization of the recently defined Global Stratotype Section and Point of the base of the Selandian Stage (Middle Paleocene) in the nearby Zumaia section, and to interpret the Danian–Selandian boundary event with its associated palaeoenvironmental changes. The trace fossil assemblage of the boundary interval is relatively scarce and shows low diversity, consisting of Chondrites, Planolites, Thalassinoides, Trichichnus and Zoophycos, which cross-cut a diffuse, burrow-mottled background, typical of a normal burrowing tiered community. Distribution of trace fossils shows local drops in abundance and diversity just above the D–S boundary and about half a metre upwards into the succeeding Selandian. Generally, the Selandian part of the section has slightly lower trace fossil diversity and abundance. This is interpreted as due to a higher detrital food supply, corresponding to a sea-level fall, in contrast to a decreased food supply during the Selandian sea-level rise. Smaller-scale fluctuations of trace fossil diversity and abundance are also interpreted as due more to food content fluctuations in the sediment than to oxygenation of pore waters. Results reveal the minor influence of an extreme warming event (hyperthermal conditions) at the D–S boundary which affected the whole benthic habitat. Contrarily, a probable major effect of sea-level fluctuations can be envisaged, which determined variations in siliciclastic input and food content