English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Sediments recovered during Leg 57 provide a nearly complete lower Miocene to Holocene composite reference section for diatoms in the northwestern Pacific. Highand middle-latitude diatoms dominate the assemblages; however, because of the influence of the warm-water Kuroshio Current, low-latitude diatoms are consistently present in low abundance. Low-latitude diatom datums tied to paleomagnetic stratigraphy in the equatorial Pacific show little or no displacement in time at the 40°N latitude of the Leg 57 sites. Both highand low-latitude diatom zones are recognizable in the upper Miocene to Quaternary of Sites 438 and 440 and thus allow the first direct detailed correlation of these zonations for the upper Miocene. Eleven new subzones, which offer refinement of the existing high-latitude diatom zonation of the North Pacific, are proposed. In addition, the base of the Denticula lauta Zone is defined, and a new zone, the Actinocyclus ingens Zone, is proposed for the upper lower Miocene. A late Miocene hiatus spanning the interval from about 5.6 to 6.7 m.y.B.P. is identified at Sites 438 and 440. Correlative hiatuses occur throughout the middleand high-latitude North Pacific both on the continental slopes and in the ocean basins. Onshore in California, this interval corresponds with an increase in the rate of sediment accumulation associated with the transition from laminated diatomites to overlying, more massive diatomaceous mudstones. These latest Miocene sedimentologic events apparently are related to a global cooling event. INTRODUCTION Neogene diatom biostratigraphy in the North Pacific has progressed rapidly in recent years (Burckle and Opdyke, 1977; Koizumi, 1977). In low latitudes the diatom zonation of Burckle (1972, 1977) has found wide acceptance, whereas Koizumi's (1973, 1975b) zonation can readily be applied in high latitudes and in Japan. Several diatom datum levels (first and last occurrences) have been correlated directly with paleomagnetic stratigraphy both in low latitudes (Burckle, 1972; 1978) and in high latitudes (Donahue, 1970; Koizumi, 1975d). Burckle and Opdyke (1977) and Koizumi (1977) have provided cross-correlations of the highand low-latitude diatom zonations for the Pliocene and Quaternary; however, the lack of suitable and complete stratigraphic sections has prevented detailed correlations of the zonations for the Miocene. During the Japan Trench Transect of the Deep Sea Drilling Project (Legs 56 and 57, September to December, 1977), a series of excellent Miocene to Quaternary siliceous reference sections were cored along an east to west transect at about 40°N latitude across the Japan Trench (Figure 1). Leg 57 recovered a nearly complete Neogene siliceous record as a composite at two sites (Sites 438 and 440) which contains fairly common, moderately well-preserved to well-preserved diatoms throughout the section. The Leg 57 sites lie near the present-day confluence of the cool-water Oyashio Current and the warm-water Kuroshio Current, and the mixture of tropical and highlatitude diatom assemblages that results allows detailed correlation of lowand high-latitude zonations for the middle Miocene through the Holocene. In addition, the Leg 57 sites are near enough to shore (about 130 to 200 km) that nearshore planktonic diatoms, which are useful in correlations in Japan and in California, are consistently present. ZONATION The diatom zonation of this chapter is modified slightly from the high-latitude North Pacific diatom zonation of Koizumi (1973, 1975b) (Figure 2). Eleven new subzones and one additional zone, the Actinocyclus ingens Zone, are proposed, and the base of the Denticula lauta Zone is defined. This zonation is summarized below, and additional comments appear in the biostratigraphy section. Denticula seminae Zone (Koizumi, 1973) Definition: The interval from the last occurrence of Rhizosolenia curvirostris to the present. Subzones: None. Remarks: The last occurrence of Thalassiosira nidulus var. nidulus coincides with the base of the zone (Koizumi, 1977).

Lower Miocene to Quaternary Diatom Biostratigraphy of Leg 57, off Northeastern Japan, Deep Sea Drilling Project