Climatic fluctuations and sea-surface water circulation patterns at the end of the Cretaceous era: Calcareous nannofossil evidence

Abstract

New paleoecological data are presented for late Maastrichtian calcareous nannofossil assemblages of the Indian Ocean and the Boreal epicontinental Chalk Sea. These data are compiled with recent results in the tropical Atlantic, Pacific, and Tethys oceans in order to characterize environmental changes by the end of the Cretaceous era. The paleobiogeographic distribution of the warm-water species Micula murus is updated and indicates the existence of major sea-surface currents in the late Maastrichtian Atlantic Ocean similar to the present day. The end-Maastrichtian greenhouse warming is characterized at tropical and subtropical latitudes by an increase in abundance of M. murus and the temporary disappearance of the high-fertility marker Biscutum constans. In the Boreal realm, the greenhouse episode is marked by a contemporaneous acme of Watznaueria barnesiae coincident with very rare occurrences of M. murus and other tropical nannofossil species which have never been reported before at boreal latitudes. A review of cyclostratigraphic and calcareous nannofossil data in the Atlantic, Pacific, Indian, and Tethys oceans points to the following evolution of sea-surface paleotemperatures for the last ca. 350–380kyr of the Cretaceous: the end-Maastrichtian greenhouse warming lasted on average a little more than 200kyr and was followed by a ca. 100–120kyr cooling. In the Tethys, a 30–40kyr additional pulse of warming is highlighted immediately below the Cretaceous–Paleogene boundary. These findings indicate an important instability of the climate system at the end of the Maastrichtian, most likely caused by Deccan volcanism. The calcareous nannofossil species richness dropped during the end-Maastrichtian greenhouse warming, which may indicate environmental stress and/or ocean acidification. However, nannoplankton diversity returned rapidly to higher values after this climatic episode and remained high up to the Cretaceous–Paleogene boundary. No significant extinction is recorded in this biotic group prior to the boundary clay.