Date of this Version
Nannofossil data from the Naturaliste Plateau (DSDP Legs 26 & 28), Falkland Plateau (DSDP Legs 36 & 71), Maud Rise (ODP Leg 113), Northeast Georgia Rise (ODP Leg 114), and Kerguelen Plateau (ODP Leg 120) yield a set of consistent, reliable biohorizons that form the basis of a revised calcareous nannofossil zonation for the Southern Ocean Upper Cretaceous. Analysis of this zonation indicates significant changes in the circum-Antarctic climatic regime during the Campanian and Maastrichtian that foreshadow the more dramatic climatic shifts of the Palaeogene. Calcareous nannofossils from the Turonian through the Lower Campanian of the studied sections are characterized by low diversity assemblages consisting largely of cosmopolitan species. Low-latitude taxa are absent from these assemblages, but relatively few <5) high-latitude endemic species are evident. In general, these can be characterized as somewhat depauperate low latitude assemblages with the addition of a few high latitude species. Biostratigraphical subdivision of this interval is based only on cosmopolitan taxa used in more low latitude zonations. A substantial disconformity separates the Lower and Upper Campanian throughout the Southern Ocean sites drilled to date. A disconformity of similar biostratigraphical placement and hiatus has also been identified on the Western Australian margin. Above this disconformity, there is a significant increase in the number of austral or bipolar species in the Upper Campanian and Maastrichtian. Evolutionary radiations within the genera Biscutum and Monomarginatus, and in the Cribrosphaerella-Psyktosphaera-Nephrolithus plexus in the austral region indicate that the Southern Ocean was ecologically distinct from low latitude areas during the late Campanian and Maastrichtian. The high degree of provinciality evident during this time suggests the development of a (cooler) Southern Ocean surface water mass that was oceanographically distinct from the (warmer) low latitude surface waters. Significant declines in the species richness of the nannofossil assemblages throughout the Maastrichtian suggest progressive climatic cooling. This is most evident in the progressive exclusion of Watznaueria bamesae during the middle to late Maastrichtian. A poleward migration of W. bamesae during the last 500,000 years of the Maastrichtian suggests a brief pulse of warming just prior to the Cretaceous-Tertiary boundary.