Biogeography of Campanian-Maastrichtian Calcareous Plankton in the Region of the Southern Ocean: Paleogeographic and Paleoclimatic Implications

摘要

Analysis of biogeographic distribution patterns among Campanian-Maastrichtian calcareous nannoplankton and planktonic foraminifera from the southern high latitudes provides insight to changes in circum-Antarctic climate and surface circulation surface routes. Both microfossil groups are similarly characterized in the early Campanian by low-diversity, cosmopolitan species with few or no austral provincial taxa. This changes by late Campanian-early Maastrichtian time as austral species diversified and began to dominate the high-latitude assemblages. Maximum diversity of austral provincial taxa occurs during the late Campanian among the planktonic foraminifera and in the early Maastrichtian among the calcareous nannoplankton. Climatic cooling is considered the cause for the decline from 53 nannofossil species during the early Maastrichtian to 20 species toward the end of the Maastrichtian as well as the equatorward shifts of the nannofossil Nephrolithus frequens and the planktonic foraminifer Abathomphalus mayaroensis during the late Maastrichtian. On the other hand, the poleward migrations of the planktonic foraminifer Pseudotextularia elegans and the nannofossil Watznaueria barnesae less than 500,000 years before the Cretaceous/Tertiary extinction event correspond with a negative δ18O excursion observed at Maud Rise Site 690, suggesting that these species shifts were caused by a brief high-latitude warming event. The high degree of provinciality among the late Campanian-early Maastrichtian calcareous plankton reflects segregation of a cool, high-latitude water mass from warmer, subtropical surface waters. A long-term climatic cooling and paleogeographic changes related to the breakup of the southern Gondwana continents are considered the major factors that caused the paleocirculation and biogeographic changes. Seafloor spreading and subsidence between Antarctica, Australia, and New Zealand, northward drift of South America from the Antarctic Peninsula, and a global rise in sea level during the middle Campanian provided new routes for shallow marine communication between the Indian, Pacific, and South Atlantic ocean basins. Opening of these gateways may have also caused a widespread disconformity that separates lower Campanian from upper Campanian sediments in the Atlantic and Indian ocean sectors of the Southern Ocean. Reemergence of a South American-Antarctic Peninsula isthmus in the middle and late Maastrichtian is postulated to account for poleward migration of several keeled and nonkeeled planktonic foraminifera during a time of gradual climatic cooling of the polar oceans. Closure of this gateway could have been caused by a fall in sea level and renewed volcanism along the Antarctic Peninsula magmatic arc. This could have led to a diminished intensity of surface current flow between the southern South Atlantic and southern Indian ocean basins and enhanced vertical stratification and niche partitioning in the austral surface waters, thus enabling habitation by a greater diversity of depth-stratified planktonic foraminifera. A renewed terrestrial land bridge at this time would explain the selective dispersal of marsupials and terrestrial plants across the southern Gondwana continents that has been postulated in several paleobiogeographic studies.