ORBITAL CALIBRATION OF PALEOENVIRONMENTAL EVENTS FOR THE LAST 1.2 MYR OF THE CRETACEOUS: INFERENCES FOR OCEAN ACIDIFICATION BEFORE THE MASS EXTINCTION

An integrated study of magnetic mass susceptibility (MS), bulk stable isotopes and calcareous nannofossil paleoecological changes has been performed on the late Maastrichtian of the Elles section, Tunisia, spanning the last 1200 kyr of the Cretaceous. The cyclostratigraphic analysis reveals Milankovitch frequencies and orbital tuning is used to provide ages of important stratigraphic horizons and paleoenvironmental events, relative to the age of the Cretaceous-Paleogene boundary (K-PgB). Principal Component Analysis (PCA) performed on the nannofossil assemblage reveal two main factors, PCA1 and PCA2, which match changes in bulk δ¹³C and δ¹⁸O, respectively, and allow building a nannofossil fertility (NFI) and a nannofossil temperature index (NTI). The NTI, the variations in abundances of high-latitude taxa and the warm-water species Micula murus, and variations in bulk and foraminifer δ¹⁸O point to the following paleoclimatic interpretation: (1) 67.2–66.7 Ma, mild and variable sea-surface paleotemperatures, (2) 66.7–66.52 Ma, enhanced cooling event, (3) 66.52–66.21 Ma, the end-Maastrichtian greenhouse warming, (4) 66.21–66.05 Ma, resumed cooling, (5) an additional warm pulse in the last 50 kyr of the Maastrichtian. The latter warm pulse, which has never been documented so far, and the very early age for the onset of the end-Maastrichtian greenhouse warming in Tunisia indicate a ca. 150 kyr unconformity at the base of the boundary clay in most other K-PgB sections worldwide. Correlation of the climatic trends to that of Bidart, SW France, suggests that an interval of Deccan-induced ocean acidification, identified immediately below the boundary clay and within a short-term cooling at Bidart, correlates with the 66.21–66.05 Ma cooling episode at Elles. Ocean acidification is thus a plausible cause for the global 150 kyr unconformity. In addition, Dwarfing of five calcareous nannofossil species is recorded in the latest Maastrichtian of the Bidart section and correlates with the episode of ocean acidification. These findings confirm drastic environmental changes in the ocean linked to phase 2 of Deccan volcanism.