Distribution of abundance of planktic foraminifera and calcareous nannofossils at DSDP Sites 17-167 and 62-463

New paleomagnetic and paleontologic data from Pacific DSDP Sites 463 and 167 define the magnetic reversals that predate the Cretaceous Normal Polarity Superchron (K-N). Data from Mid-Pacific Mountain Site 463 provide the first definition of polarity chron M0 in the Pacific deep-sea sedimentary record. Foraminiferal biostratigraphy suggests that polarity chron M0 is contained entirely within the lower Aptian Hedbergella similis Zone, in agreement with foraminiferal data from the Italian Southern Alps and Atlantic Ocean. Nannofossil assemblages also suggest an early Aptian age for polarity chron M0, contrary to results from the Italian Umbrian Apennines and Southern Alps, which place polarity chron M0 on the Barremian-Aptian boundary. Biostratigraphic dating discrepancies caused by the time-transgressive, preservational, or provincial nature of paleontological species might be reconciled by the use of magnetostratigraphy, specifically polarity chron M0 which lies close to the Barremian-Aptian boundary. At Magellan Rise Site 167, five reversed polarity zones are recorded in Hauterivian to Aptian sediments. Correlation with M-anomalies is complicated by synsedimentary and postsedimentary sliding about 25 m.y. after basement formation, producing gaps in, and duplications of, the stratigraphic sequence. The magnitude and timing of such sliding must be addressed when evaluating the stratigraphy of these oceanic-rise environments.

Quaternary calcareous nannofossils from Periantarctic basins

Currently, coccolithophores have a widespread oceanic distribution and are reported from most latitudes, but not those higher than 65°S. Fifteen piston cores were sampled with the aim of investigating the distribution and abundance variation of Quaternary calcareous nannofossils of the Antarctic region, south of the Antarctic Divergence (>65°S), particularly from Maud Rise, Bausan Bank, and from Weddell, Ross and Bellingshausen Seas. A calcareous nannofossil cold-taxa association is present in most of the cores examined and their discontinuous occurrence is thought to indicate key environmental relationships. The presence of calcareous nannofossils is correlated with interglacial intervals with warmer SSTs and may indicate high productivity and an open-ocean environment. Our results confirm that, during short periods of the late Quaternary, coccolithophorids occurred at southern high latitudes, in the western Antarctic basins, while in the eastern Antarctic basins they are nearly absent, suggesting more variable SSTs near West Antarctic Ice Sheet.

Quaternary and Paleogene calcareous nannofossils of ODP Leg 115

The occurrences of ten datum events for the Quaternary and top Pliocene nannofossils are identified at nine Leg 115 sites. A quantitative investigation of Paleogene nannofossils in 470 samples selected from 11 holes at 9 sites yielded 197 taxa, including one new species and 10 unidentified taxa that are likely to be new species. Regional differences in the timing of some biostratigraphically important events are recognized, and a set of datum events useful for biostratigra- phy in the tropical Indian Ocean is presented. Biogeographical differences are minor for Paleogene cores from the tropical sites (Sites 707-716); however, the Quaternary and late early Oligocene floras observed at the two subtropical sites (Sites 705 and 706) differ significantly from the corresponding floras of the tropical sites. Bathymetrically controlled dissolution is recognized by the reduction of species diversity in the Paleogene flora. Selective dissolution of nannofossils is also evidenced by the percentage reduction of three holococcolith taxa, Lanternithus minutus, Zygrhablithus bijugatus, and Holococcolith type A as well as by the increase of Coccolithus pelagicusand Cribrocentrum reticulatumin the deeper sites.

Calcareous nannofossils and planktic foraminiferal assemblages of ODP Hole 198-1210A

Calcareous nannofossil and planktic foraminiferal assemblages from ODP Hole 1210A in the northwestern Pacific Ocean were used to reconstruct surface-water conditions for the past 500 kyr. Stratigraphic control was provided by calcareous nannofossil events that are thought to be synchronous over a broad range of latitudes. Calcareous nannofossil and planktic foraminiferal assemblages and abundance patterns indicate the unlikelihood of long term (Milankovitch-scale) latitudinal shifts of the Kuroshio Extension over the last 500 kyr and illustrate two successive surface water-mass states, one that prevailed prior to 300 ka and one that existed after 300 ka. The relative abundance of very small placoliths and the absolute abundance of the upper photic zone (UPZ) coccolith species decrease abruptly at approximately 300 ka. The relative abundance of the lower photic zone (LPZ) species Florisphaera profunda greatly increases at the same time, although intervals during which the relative abundance of this taxon is very low or absent also occur prior to 300 ka. The absolute abundance of planktic foraminifera gradually increased after the 300-ka boundary, including peaks of Globoconella inflata. These assemblage and abundance changes suggest significant modifications to the surface water-mass structure. Surface water was weakly stratified prior to 300 ka, but alternated between intensely stratified and vertically mixed after 300 ka. Changes in the surface water-mass structure suggest an intensification of the East Asian summer and winter monsoon after 300 ka.

Late Cretaceous to Eocene calcareous nannofossil distribution near Gubbio, Italy

Using a modified sample preparation technique, we have been able to establish a detailed lower Campanian to upper Eocene nannofossil stratigraphy in the Bottaccione and Contessa Highway sections near Gubbio. Appearance and extinction levels of virtually all the commonly used calcareous nannofossil zonal markers have been recognized and can now be closely correlated with the planktonic foraminifera zonation and the magnetic reversal stratigraphy previously established in these sections. Comparisons with the nannofossil calibrations of the oceanic magnetic anomaly sequence in Deep Sea Drilling Project (DSDP) sites suggest that magmetic Subchrons C17N and C25N are missing in the Bottaccione section. The observed variability of the relative stratigraphic position of most plankton events is confirmed to less than one magnetic subchron. Absolute abundance, paleobiogeographic restriction, and differential preservation render some of the traditionally used biostratigraphic events less reliable than others.

Neogene calcareous nannofossil abundance and datums of ODP Site 186-1150 and 186-1151 sediments

Calcareous nannofossil assemblages were studied from Ocean Drilling Program Holes 1150A, 1150B, 1151A, 1151C, and 1151D in order to estimate the age of sediments drilled in the Japan Trench of the western Pacific Ocean. The abundance and species diversity of nannofossil flora are generally low but are sufficient to show that the sedimentary sequences range from Quaternary to Miocene in age (nannofossil Zones CN15-CN3). The abundance of Coccolithus pelagicus, a cold-water indicator, was studied from sediments younger than 3.83 Ma from both Holes 1150A and 1151A in order to elucidate past climate conditions. Between 3.83 and 2.82 Ma, the abundance of C. pelagicus was generally low, but abundance increased significantly after 2.82 Ma. In agreement with previous studies, this increase appears to be related to a change in the current system around the western Pacific Ocean and eastern Atlantic Ocean that occurred in response to the final elevation of the Isthmus of Panama.

Calcareous nannofossils in sediments of the Japan Sea

Preliminary results of the biostratigraphic analysis of calcareous nannofossils recovered from Ocean Drilling Program Leg 128, Sites 798 and 799, provide clues to the Quaternary oceanography of the Japan Sea. The distribution of calcareous nannofossils from the Quaternary sediments at Site 798 (903 m water depth) may record the position of an Oceanographic frontal boundary between warm water derived from a branch of the Kuroshio Current as it entered the Japan Sea through the Tsushima Straits to the south, and colder water introduced into the western portion of the Japan Sea derived from the winter chilling of northern Japan Sea surface waters. This Oceanographic front probably oscillated north-south over Site 798 in response to glacial/interglacial cycles, or perhaps to some other climatic event or combination of events unique to the Japan Sea. During the last 1.5 m.y., six major intervals are recognized when the Oceanographic front may have been north of Site 798 separated by five major intervals when the frontal boundary may have been south of the site. These migrations were centered around approximately 0.125, 0.29, 0.56, 0.62, 0.85, 0.91, 0.98, 1.0, 1.11, and 1.5 Ma, which correspond to the boundaries separating nannofossil-rich sediments from barren or nearly barren, low-carbonate intervals. Nannofossil-rich intervals may represent times when the frontal boundary was north of Site 798, and the site was above the CCD. Barren or nearly barren intervals represent times when the frontal boundary may have been south of Site 798 and the CCD was probably higher. The distribution of calcareous nannofossils at Site 799 (2073 m water depth) appears to be controlled more by the depth of the CCD than by any climatic effects. The FOD (first occurrence datum) of Emiliania huxleyi, the LOD (last occurrence datum) of Psuedoemiliania lacunosa, Helicosphaera sellii, Calcidiscus macintyrei (10 ?m), and the FOD and LOD of Reticulofenestra asanoi are recognized from Site 798 cores. The LOD of P. lacunosa is observed in sediments from Site 799. Only in the sediments younger than 1.5 Ma are the nannofossils from Sites 798 and 799 preserved well enough and sufficiently numerous for age dating and paleoceanographic conjecture. In-situ dissolution in older sediments at both sites precludes any dating or paleoenvironmental interpretations.