| This dissertation is a collection of three projects utilizing calcareous nannoplankton as biostratigraphic and paleoceanographic indicators. After an introductory chapter, Chapter 2 details the results from Site 1258, drilled during ODP Leg 207 on Demerara Rise off the northern coast of South America. This cruise recovered organic-rich Albian sediments that contain abundant, moderately to well preserved calcareous nannofossils. Biostratigraphic analysis shows the section primarily spans Roth's (1978) middle to late Albian Zone NC9. An unconformity separates these sediments from overlying uppermost Albian laminated shales from Zone NC10. The presence of Seribiscutum primitivum within the Albian section represents the first known occurrences of this species at such low latitudes, as Demerara Rise was located within 15° of the equator during the mid-Cretaceous. Its presence on Demerara Rise indicates cooler water incursions either through changes in surface circulation or upwelling conditions during the opening of the Equatorial Atlantic.Chapter 3 details the results of a study of calcareous nannofossils in clasts obtained during the SHALDRIL II NBP0602A cruise to the Antarctic Peninsula. Site NBP0602A-9 includes two holes located in the northern James Ross Basin in the western Weddell Sea. Sediment from both holes consists of very dark grey, pebbly, sandy mud, grading to very dark greenish grey, pebbly, silty mud in the lower 2.5 m of the second hole. In addition to abundant pebbles found throughout the cores, both holes contain numerous sedimentary clasts. Biostratigraphic analysis of diatom assemblages from the glaciomarine muds yielded rare to few, poorly preserved diatoms that suggest the sediment is late Pleistocene in age. The sedimentary clasts, on the other hand, are nearly barren of diatoms, but contain rare, moderately to well preserved calcareous nannofossils. The clasts contain three distinct assemblages. Two clasts are assigned an early Maastrichtian age based on the presence of Biscutum magnum and Nephrolithus corystus, whereas one clast is of late Maastrichtian age based on the presence of Nephrolithus frequens. These samples also contain other characteristic Late Cretaceous species, including Biscutum notaculum, Cribrosphaerella daniae, Eiffellithus gorkae, Kamptnerius magnificus, and Prediscosphaera bukryi . Two samples contain an early Paleocene assemblage dominated by Hornibrookina teuriensis. The Maastrichtian assemblages are similar to those found in the Lopez de Bertodano Formation on Seymour and Snow Hill Islands, making it the likely source area for the Cretaceous clast material. Although no calcareous nannofossils have been reported from Paleocene formations on these islands, the occurrence of calcareous foraminifers suggests other calcareous plankton may be present thus, the Paleocene clasts likely also originated from the Seymour Island area.The fourth chapter presents results from a Pleistocene study of calcareous nannoplankton assemblages spanning 480--355 ka at IODP Site U1313. Calcareous nannoplankton assemblages from Marine Isotope Stage (MIS) 12-10 record changes in surface-water conditions over this interval. The assemblage is dominated by family Noelaerhabdaceae, and spans a single biostratigraphic event, the last occurrence of Pseudoemiliania, dated to 427 ka at this site. Most species indicate paleoecological preferences similar to those found in the literature, although Gephyrocapsa oceanica is more abundant during MIS 12, even though it is thought to prefer warmer waters. Similarly, Helicosphaera, another warm-water taxon, is also more abundant during MIS 12. Both prefer higher nutrient conditions that occur during the glacial stage. The first factor of a CABFAC factor analysis explained nearly 92% of the variability in the assemblage. This factor is dominated by G. oceanica, and the varimax factor scores correlate well with the alkenone-based temperature record, suggesting that the distribution of G. oceanica at Site U1313 is controlled by temperature. The N ratio, based on the ratio of lower photic zone dweller Florisphaera profunda to upwelling indicators, shows deep stratification during much of MIS 12, usually associated with an increase in IRD and freshwater proxies indicating the presence of icebergs in the area. Finally, most productivity indicators suggest higher productivity during MIS 12, in contrast to the nannofossil accumulation rate (NAR), which was lower during that time. Other phytoplankton groups may have increased productivity during MIS 12, although further work is needed to explain why the abundance of alkenones indicates higher productivity in the calcareous nannoplankton when the NAR does not. (Abstract shortened by UMI.)... |
