Planktic foraminiferal distribution across the Cretaceous–Paleogene (K–Pg) boundary from the Neuquén Basin (Cerro Azul section): biostratigraphy and paleoenvironmental significance

Abstract. The Maastrichtian–Danian time interval corresponds to one of the largest mass extinctions in the Earth's geological history. We investigate changes in planktic foraminiferal assemblages and TEX86H-based sea surface temperatures (SSTs), which reflect surface water conditions, across the Maastrichtian–Danian transition at the Cerro Azul section in the Neuquén Basin, Argentina. Within the Maastrichtian, we identify species that became extinct at the Cretaceous–Paleogene (K–Pg) boundary, such as Planoheterohelix globulosa and Muricohedbergella monmouthensis, even though these taxa are not typically employed as biostratigraphic markers. Dominance of guembelitrids, usually characterised as opportunistic taxa, is recognised in the upper Maastrichtian interval and within the first 35 cm of the lower Danian, suggesting broad instability in the water column across the K–Pg boundary. Within the lower Danian, two planktic foraminiferal biozones, with high-latitude paleogeographic affinities, were recognised. The AP0 zone was recognised for the lowermost 45 cm above the K–Pg boundary, within the partial range of Turborotalina nikolasi. The first occurrence of Globoconusa daubjergensis, 45 cm above the K–Pg boundary, defines the base of subzone AP1a, which extends towards the top of the studied interval (90 cm above the K–Pg boundary). The planktic foraminiferal fauna recovered from the early Danian interval exhibits a strong affinity to high latitudes, as evidenced by the presence of Antarcticella pauciloculata. However, at 45 cm above the K–Pg boundary (biozone AP1a), a slight increase in the abundance and/or appearance of tropical and/or subtropical species (Woodringina claytonensis) is observed, suggesting an increase in surface water temperature (SST). This coincides with the trend recorded by TEX86H, which shows a ∼ 1.5 °C rise in SST above ∼ 45 cm into the Danian interval at the Cerro Azul section. Our assessment of planktic foraminiferal assemblages suggests that the Cerro Azul section comprises a continuous record of the Maastrichtian and early Danian time interval in the Southern Hemisphere. Additionally, a typical distribution pattern across the K–Pg boundary is observed, with the bloom of opportunist taxa. The middle- to high-latitude paleobiogeographic affinity observed at the beginning of the Danian period for planktonic foraminiferal assemblages from the Cerro Azul section is based on the occurrence of Antarcticella pauciloculata, which has its paleobiogeographic range in the South Atlantic Ocean extended to mid-latitudes.