Campanian‐Maastrichtian intermediate‐ to deep‐water changes in the high latitudes: Benthic foraminiferal evidence

During the latest Cretaceous cooling phase, a positive shift in benthic foraminiferal δ18O values lasting about 1.5 Myr (71.5–70 Ma) can be observed at a global scale (Campanian‐Maastrichtian Boundary Event, CMBE). This δ18O excursion is interpreted as being influenced by a change in intermediate‐ to deep‐water circulation or by temporal build‐up of Antarctic ice sheets. Here we test whether benthic foraminiferal assemblages from a southern high‐latitudinal site near Antarctica (ODP Site 690) are influenced by the CMBE. If the δ18O transition reflects a change in intermediate‐ to deep‐water circulation from low‐latitude to high‐latitude water masses, then this change would result in cooler temperatures, higher oxygen concentration, and possibly lower organic‐matter flux at the seafloor, resulting in a major benthic foraminiferal assemblage change. If, however, the δ18O transition was mainly triggered by ice formation, no considerable compositional difference in benthic foraminiferal assemblages would be expected. Our data show a separation of the studied succession into two parts with distinctly different benthic foraminiferal assemblages. Species dominating the older part (73.0–70.5 Ma) tolerate less bottom water oxygenation and are typical components of low‐latitude assemblages. In contrast, the younger part (70.0–68.0 Ma) is characterized by species that indicate well‐oxygenated bottom waters and species common in high‐latitude assemblages. We interpret the observed change in benthic foraminiferal assemblages toward a well‐oxygenated environment to reflect the onset of a shift from low‐latitude toward high‐latitude dominated intermediate‐ to deep‐water sources. This implies that a change in oceanic circulation was at least a major component of the CMBE.