Distribution of radiolaria in Oligocene to Pleistocene sediments of the North-West Pacific

Neogene stratigraphy of the tropical and subtropical Pacific on radiolaria is studied in the book. A detailed comparison of coeval systems from tropics and subtropics is given. A possibility of use of a uniform zonal scale in these areas is proved. Magnitude of changes of complexes on borders of Neogene zones is studied in detail. Six stages in development of radiolarians are identified in the tropics in Neogene. Stratigraphic levels, where the greatest changes of fauna occurred, are natural boundaries of these stages. 72 species of radiolarians (two of which are new) are described in the book.

Radiolarians from the Southwest Pacific

Radiolarians are very rare in all Leg 90 sites. They are relatively more frequent only in Neogene sediments from Sites 586 and 594, and in Eocene sediments at Site 592. In this chapter radiolarian abundances are recorded as comparative percentages for 92 Neogene morphotypes at Site 586B. Relative abundances only are estimated at Sites 592 and 594, where preservation is poor to moderate. A tentative correlation of radiolarian events at Hole 586B and Site 594 shows that only a few species can be found in both tropical and subantarctic areas. New evolutionary lineages are proposed. 1. Middle Miocene eucyrtids like Eucyrtidium teuscheri group evolved into a widespread species (E. teuscheri teuscheri) ranging from middle Miocene to Holocene and a temperate species (E. teuscheri orthoporus) ranging from middle Miocene to early Pleistocene. 2. Phormostichoartus pitomorphus appears to be a temperate descendant of the cosmopolitan P. fistula and disappears in early Pleistocene time. 3. The discovery of Lamprocyrtis daniellae n.sp. calls into question the lineage L. heteroporos -> L. nigriniae. 4. The evolution of Lamprocyclas maritalis from an ancestor group (L. aff. maritalis) is located in the early part of the Pterocanium prismatium Zone.

Relative abundances of selected radiolarian species in sediments of ODP Leg 199 sites

During Ocean Drilling Program (ODP) Leg 199, sediments were recovered from eight sites in the Central Pacific. Late Oligocene and early Miocene radiolarians are common to abundant and moderately well preserved in Cores 199-1218A-8H through 11H and 199-1219A-5H through 9H. More than 110 radiolarian species were encountered during this study. Of these species, 100 are identifiable forms and the rest are undescribed or unfamiliar forms. This report presents the relative abundances of described forms from the upper Oligocene to lower Miocene sediments.

Radiolarians at DSDP Leg 58 Holes

Radiolarians were observed at all five sites drilled during DSDP Leg 58. Three sites (442, 443, 444) are south of Japan in the Shikoku Basin. The remaining two sites (445, 446) are east of Okinawa, in the Daito Ridge and Basin areas. The observations made on radiolarians during Leg 58 are understood best by considering these two areas separately. The basement ages, preservation, diagenesis, and paleoecology are similar within each area, but different between the two areas. The radiolarian zones of Riedel and Sanfilippo (1978) were used to determine the sediment age. Because of the mixed nature of the fauna, there was an opportunity to test the tropical zonation in middlelatitude sediments. A middle- to high-latitude biostratigraphy for the Pliocene and Pleistocene has been formulated (Hays, 1970; Kling, 1973; Foreman, 1975), but there is no Miocene radiolarian zonation for these latitudes. The tropical elements of the present fauna are sufficient to use the low-latitude zonation, although there is a loss of resolution in the Pleistocene. Because of poor preservation, zone boundaries are indistinct in much of the cored sediment. Determination of abundance in any sample is always subjective and varies among investigators. This work was in its final stages at the publication of Westberg and Riedel (1978), and the guidelines outlined therein are not closely followed. The abundances recorded in Tables 1 through 5 are based on strewn slides which were searched entirely if an individual of a species was found, or for 8 to 10 minutes if the species was not found.

(Table 1) Radiolarians in the Oligocene-Miocene at DSDP Hole 72-515B

The Neogene of the southwestern Atlantic is virtually barren of biogenic silica. Of the four sites drilled on Leg 72, only two contained identifiable radiolarian specimens. In the southwestern Brazil Basin (Site 515), radiolarians are present only from the upper Oligocene (Anomaly 8, about 28 Ma) to the middle Miocene (Zone NN8, about 11.5 Ma). On the Rio Grande Rise (Site 516), radiolarians are present only within a short interval of the lower Miocene (Zones N5-N6, about 18-20 Ma). The abrupt cessation of silica deposition in the upper middle Miocene is characteristic of many drill sites in the tropical and temperate Atlantic and implies that a major oceanographic "threshold" was exceeded at this time, allowing the Atlantic waters to become either less productive or relatively silica deficient. Siliceous microfossils are notably more abundant in Oligocene-Miocene sediments of deep regions where carbonate preservation is poor (Site 515) than in equivalent carbonate-rich strata nearby (Site 516). This discrepancy suggests that the presence of calcareous microfossils may act to enhance post-depositional dissolution of biogenic silica tests by elevating the pH of the surrounding pore waters. Carbonate-free clays, by contrast, may provide a more favorable chemical environment for silica preservation.