Late Neogene Radiolarian Biostratigraphy in the Subarctic Northwest Pacific

The radiolarian assemblages recovered from Upper Miocene and Pliocene sequences at DSDP Sites 192, 302 and 438A and from an onshore section in northern Japan lack many of the species used as zonal markers in low and middle latitudes. A new high-latitude radiolarian zonation based on nine easily recognized events is proposed. Forty-five radiolarian events are identified and the sediment accumulation rate curve of Site 438A allows most of these events to be correlated with the geochronologic time scale. Five new species, Lithelius barbatus, Cycladophora nakasekoi, Cycladophora sakaii, Dictyophimus robustus and Lychnocanoma parallelipes, are described. INTRODUCTION Low-latitude Cenozoic zonations for calcareous and siliceous microfossils including foraminifera, radiolaria, calcareous nannofossils and diatoms have been greatly improved by the study of deep-sea sediments. The combination of these multiple zonations offers high stratigraphic resolution (Berggren et al. 1985). High-latitude biostratigraphy has remained poorly resolved because of the low diversity of calcareous plankton and little knowledge of siliceous microfossils except for diatoms whose zonations have been improved in middle and high latitudes (e.g. Koizumi 1973; Barron 1985). Radiolarians are diverse in high latitudes and are abundant and well-preserved in deep-sea sedimentary sequences which are poor in calcareous microfossils. Early biostratigraphic studies of radiolarians in midto highlatitude Northwest Pacific were based on piston cores that recovered mostly Pliocene and Pleistocene sediment. With the advent of the Deep Sea Drilling Project, much thicker Neogene sedimentary sequences became available for micropaleontological investigations. Radiolarians have contributed to biostratigraphic subdivision and correlation of Neogene sections drilled by the DSDP Legs 19 (Aleutian region), 31 (Japan Sea), 32 (Shatsky Rise, Hess Rise), 55 (Emperor Sea Mounts), 56, 57 (offshore Japan), 86 (Shatsky Rise) 127 and 128 (Japan Sea). However, radiolarian studies showed that low-latitude zonations could not be easily applied to mid to high latitudes. Alternative radiolarian zonations were developed for mid latitudes but not for high latitudes or the Japan Sea. Neogene radiolarian studies of Japanese land-based sections generally focused on contributing to oil-research, i.e. correlation of Neogene strata in the oilbearing field. Hays (1970) first developed a zonation from piston core samples in the northern North Pacific. He established four zones for the late Pliocene to Quaternary and correlated them with the paleomagnetic time scale. Ling (1973) reported occurrences of radiolarians from the Upper Miocene to Quaternary sequence drilled by DSDP Leg 19 but proposed no zonation. Foreman (1975) established a radiolarian zonation for the Upper Miocene to Quaternary from DSDP Leg 32. She distinguished eight zones including four upper zones similar to those of Hays (1970), a new Pliocene zone and three Upper Miocene zones of the lowlatitude zonation of Riedel and Sanfilippo (1970). Subsequently, Foreman's zonation has been applied to the Upper Miocene to Quaternary sedimentary sequences of Legs 55, 56, 57 and 86 in the mid-latitude Northwest Pacific (Ling 1980; Sakai 1980; Reynolds 1980; Morley 1985). In the Japan Sea, Ling (1975, 1992) and Alexandrovich (1992) studied radiolarians but established no zonation for Upper Miocene to Quaternary sediments of Legs 31, 27 and 128. The most important radiolarian biostratigraphic work from onshore Japan is that of Nakaseko and Sugano (1973). They established four assemblage zones in the upper Lower Miocene to Pliocene of onshore Japan based on radiolarian assemblages reported previously by Nakaseko and co-workers (e.g. Nakaseko 1954, 1955, 1959, 1960, 1969; Nakaseko et al. 1965, 1972; Sugano and Nakaseko 1968, 1971). Funayama (1988) defined four new zones and two new sub-zones from the upper Lower Miocene to lower Upper Miocene of the Suzu area in the Noto Peninsula, central Japan. In spite of these efforts, a Neogene radiolarian zonation for the high-latitude Northwest Pacific and Japan Sea was not established. Few thorough stratigraphic studies of radiolarians from the Upper Miocene to Pliocene of onshore Japan were conducted after Nakaseko and Sugano (1973). The objectives of this paper are to record the stratigraphic distribution of radiolarians in the DSDP Sites 192, 302 and 438A and a section in the Tsugaru Peninsula, northern Japan, in order to establish a radiolarian zonation for the upper Neogene of the subarctic Northwest Pacific region, including northern Japan and Japan Sea. It is also demonstrated that the mid-latitude radiolarian zonations proposed by Hays (1970) and Foreman (1975) cannot be applied in this region. Site 192 lies in the present-day source area of the cool-water Oyashio Current. It was chosen for this study to elucidate subpolar Neogene faunas. Site 438A provides one of the most continuous Neogene sedimentary sequences near northern Japan. micropaleontology, vol. 42, no. 3, pp. 221-262, text-figures 1-11, plates 1-7, tables 1-5, 1996 221 This content downloaded from 207.46.13.57 on Mon, 08 Aug 2016 05:53:13 UTC All use subject to http://about.jstor.org/terms Isao Motoyama: Late Neogene radiolarian biostratigraphy in the subarctic Northwest Pacific TEXT-FIGURE 2 Location of the six outcrops in the Tsugaru section: JQ, JR, JS, JT JX and USI. TEXT-FIGURE 1 Location of DSDP Sites 192, 302 and 438 and Tsugaru section. Site 302 and the Tsugaru section provide a composite sedimentary record from upper Miocene to Quaternary of the Japan Sea. These three sedimentary records approximately cover the same chronological range from Late Miocene to Quaternary. This makes it possible to test the stratigraphic usefulness (synchroneity) of various radiolarian events based on parallelism of sequential order of evolutionary events in these sections across a broad latitudinal distance (40?-53?N) or between the marginal sea and the open ocean. MATERIALS AND METHODS