Silicoflagellate Biostratigraphy of Middle Eocene to Holocene Subantarctic Sediments Recovered by Deep Sea Drilling Project Leg 71

Abstract

Subantarctic sediments recovered in the Southwest Atlantic during Leg 71 provide well-preserved and often diverse silicoflagellate assemblages of middle Eocene, late Eocene-early Miocene, and late middle Miocene through Holocene age. Two holes, 511 and 513A, comprise a complete 370-meter upper Eocene to lower Miocene composite reference section which is divided into six new or revised silicoflagellate zones, in ascending order: the Mesocena occidentalis Zone, Naviculopsis trispinosa Oppel Zone, N. constricta/Dictyocha deflandrei Concurrent Range Zone, N. constricta-Corbisema archangelskiana Interval Zone, C. archangelskiana Range Zone, and Naviculopsis biapiculata Partial Range Zone. New species described from this upper Eocene to lower Miocene section include D. alta, Mesocena bispicata, and Distephanus crux paulii. Middle Eocene sediments hydraulically piston cored at Site 512 are assigned to the newly constructed Dictyocha grandis Range Zone and are subdivided into three subzones; the D. stelliformis Subzone, D. stelliformis-M. apiculata Interval Subzone, and M. apiculata Subzone. Paleomagnetic measurements and correlations of this middle Eocene section indicate deposition of the D. grandis Zone between-43.7 to 40.9 Ma. New species from this interval include D. anguinea, D. grandis, and D. stelliformis. Miocene to Holocene sediments present in hydraulically piston cored Sites 512 and 514 and the rotary cored Site 513 contain the M. circulus Zone, M. circulus/M. diodon Zone, M. diodon Zone, and Distephanus boliviensis Zone. The upper boundaries of these zones are correlated directly with magnetostratigraphy and lie in upper Chronozone 9, upper Chronozone 6, the lower Gilbert Chronozone, and the lower Gauss Chronozone, respectively. A detailed analysis was made of all silicoflagellate skeletal morphotypes present in the upper Eocene-lower Oligocene to determine the limits of species variability. Numerous sporadically occurring species were determined to be intraspecific variations of more abundant species and are placed in synonymy with these species.

Neogene Paleoceanography of the Norwegian Sea Based Upon Silicoflagellate Assemblage Changes in ODP Leg 104 Sedimentary Sequences

“…Corbisema was also a minor contributor, less than 5% of the assemblage, except at the early/middle Miocene bound ary where it comprises 9.5% and corresponds to a peak abun dance of Dictyocha. The low percentages of these warm-water genera are similar to those recorded in the lower and middle Mi ocene of the Southern Ocean (Ciesielski, 1975;Busen and Wise, 1976;Shaw and Ciesielski, 1983;and others). Although more temperate conditions are indicated by the high abundance of quadrate Distephanus during this period, much lower abun dances of Corbisema and Dictyocha than at lower latitudes in dicate a significant latitudinal thermal contrast.…”

“…These assemblages are indicative of subpolar conditions with surface-water temperatures between 1° and 4°C. Bachmannocena is absent; however, the genus has not been noted in high latitudes after the earliest Pliocene (Shaw and Ciesielski, 1983), and its absence cannot be interpreted as due to a lack of surface-water fertility.…”

“…The biogeographic distribution of extinct and extant spe cies of each group reveals a distinct temperature control on their distribution during the Pliocene-Quaternary, with Dictyocha most abundant in temperate to tropical regions (Bukry 1985b;Mandra, 1969;Martini 1971;and others) and hexagonal Distephanus dominant over Dictyocha in subpolar to polar regions (Ciesielski, , 1975Shaw and Ciesielski, 1983;and others). Because both groups account for more than 95% of the Plio cene-Quaternary silicoflagellate assemblages, these temperature preferences allow for an unambiguous interpretation of relative paleotemperatures based on their abundances, assuming little change in their paleotemperature affinity.…”

The Stratigraphy of Neogene Silicoflagellates from the Norwegian Sea, ODP Leg 104

“…Recent detailed quantitative analyses of silicoflagellates with multiple windowed apical structures by Shaw and Ciesielski (1983) and herein, reveal justification for emendation of this species. This taxon is emended to include forms with a six-to eight-sided basal ring similar to or slightly larger in size than Ds.…”

Site 642: Norwegian Sea

“…Correlations: The zone is correlative with the lower to mid dle Denticulopsis hustedtii Zone (NSD8) to upper D. hustedtii/ D. lauta Zone (NSD7) of the Southern Ocean Shaw and Ciesielski, 1983;. Correlations with other diatom zones of the North Pacific, Rockall Region, and Norwegian Sea are given in the diatom section, this volume.…”