Zircon U-Pb Dating of a Tuff Layer from the Miocene Onnagawa Formation in Northern Japan

Geochemical Journal, Vol. 55, pp. 185 to 191, 2021 doi:10.2343/geochemj.2.0622

NOTE

Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Formation in Northern Japan

JUMPEI YOSHIOKA,1,2* JUNICHIRO KURODA,1 NAOTO TAKAHATA,1 YUJI SANO,1,3 KENJI M. MATSUZAKI,1 HIDETOSHI HARA,4 GERALD AUER,5 SHUN CHIYONOBU6 and RYUJI TADA2,7,8

1Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan 2Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 3Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China 4Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 5Institute of Earth Sciences, University of Graz, NAWI Graz Geocenter, Heinrichstrasse 26, 8010 Graz, Austria 6Faculty of International Resource Sciences, Akita University, 1-1 Tegatagakuenmachi, Akita, Akita 010-8502, Japan 7The Research Center of Earth System Science, Yunnan University, Chenggong District, Kunming, Yunnan 650500, China 8Institute for Geo-Cosmology, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan

(Received December 10, 2020; Accepted March 3, 2021)

During the Middle-to-Late Miocene, diatomaceous sediments were deposited in the North Pacific margin and marginal basins. The Onnagawa Formation is one of such deposits, which shows cyclic sedimentary rhythms reflecting oscil- lations of the marine environment in the Japan Sea. However, the age of the Onnagawa Formation is still poorly constrained due to the poor preservation of siliceous microfossils. To better constrain its age, we performed U-Pb dating of zircon grains from a tuff layer from the middle part of the Onnagawa Formation, and obtained an age of 11.18 ± 0.37 Ma. By combining our data with previously reported radiolarian biostratigraphy from the same section, we improved the age model of the diagenetically altered and lithified sediments of the Onnagawa Formation.

Keywords: U-Pb dating, tuffaceous zircon, the Onnagawa Formation, the Middle-to-Late Miocene, NanoSIMS

alternation rhythms have been interpreted as reflecting INTRODUCTION orbital-forced climate changes (Tada, 1991), timescales During the Middle-to-Late Miocene, diatomaceous of the sedimentary cycles have not been precisely con- sediments were deposited in the North Pacific margin and strained. Knowledge of the mechanism controlling the marginal basins including the Japan Sea (e.g., Koizumi sediment cycles of the Onnagawa Formation is critical to and Yamamoto, 2018; Vincent and Berger, 1985). Depo- understand the evolution of the Japan Sea basin and its sition of these diatomaceous sediments are thought to have associated changes in the environment during the Mid- been related to the global cooling prevailing during that dle-to-Late Miocene, when the global climate faced a sig- time, and they occasionally display significant lithological nificant cooling phase (e.g., Holbourn et al., 2013). changes such as a shift from calcareous to siliceous de- Chronostratigraphy of the Onnagawa Formation has posits reflecting the cooling of the surface water (Koizumi been developed by diatom biostratigraphy of and Yamamoto, 2018; Tada, 1994). The Onnagawa For- diatomaceous sediment in the Oga Peninsula (Koizumi mation, which is widely exposed in Akita Prefecture, et al., 2009). However, many outcrops of the Onnagawa Northern Japan, is one of these diatomaceous sediments Formation have been suffered from silica diagenesis, deposited along the Northwestern Pacific margin (Fig. 1a). which dissolved most diatom frustules composed of opal- The Onnagawa Formation is characterized by cyclic A and reprecipitated as opal-CT (e.g., Koizumi et al., changes in alternation rhythms of biosiliceous and detri- 2009; Tada and Iijima, 1983). Consequently, preserva- tus-rich beds (Tada, 1991). Although cyclic changes in tion of diatom frustules became very poor and diatom biostratigraphy could be barely applicable. Although the Onnagawa Formation in the studied area is suffered from *Corresponding author (e-mail: [email protected]) silica diagenesis, it is well exposed, and its continuous Copyright © 2021 by The Geochemical Society of Japan. sequence can be obtained by splicing several sections.

185 Fig. 1. (a) The distribution of the Onnagawa and Funakawa Formations in Akita Prefecture after Ozawa and Suda (1978, 1980) and Ozawa et al. (1987). (b) A geological map of the studied area in Yurihonjo. (c) A photograph showing field occurrence of examined tuff layer. (d) A stereo-microscopic photograph of grains of the tuff layer after sieved with 44 µm mesh. Abbreviations; Fm. = Formation, Pref. = Prefecture.

Through detailed field observation, we found several in- GEOLOGICAL BACKGROUND tercalated tuff layers, some of which contain zircon grains. In this study, we performed U-Pb dating of zircon grains According to Ozawa et al. (1988), the Onnagawa For- separated from one of the tuff layers in the middle part of mation conformably overlies basaltic lava and pyroclastic the section to better constrain the chronostratigraphy of rocks of the Aosawa Formation and pebbly sandstone of the Onnagawa Formation. the Sugota Formation, and is conformably overlain by

186 J. Yoshioka et al. dark gray mudstone of the Funakawa Formation (Fig. 1a). The Sugota and the upper part of the Aosawa Formations were formed contemporaneously with littoral and neritic marine deposits of the Nishikurosawa Formation in the Oga Peninsula, whereas the Onnagawa and Funakawa Formations show wide-spreading deep-sea facies (Ozawa et al., 1988; Koizumi et al., 2009). The Onnagawa For- mation is characterized by the alternation of hard siliceous mudstone and softer mudstone deposited during the Middle-to-Late Miocene (e.g., Tada, 1991; Ozawa et al., 1988). In the Oga peninsula, diatomite that is considered to be equivalent to the Onnagawa Formation deposited between ca. 13.0 and 6.5 Ma according to diatom biostratigraphy of Koizumi et al. (2009). The studied sections are located in the Yurihonjo area in the southern part of Akita Prefecture, and four lithological units (Units 1 to 4) are identified (Fig. 1b). The lowest unit (Unit 1) is characterized by the several- meter-scale cyclic alternation of marl and mudstone, which is conformably covered by the second unit (Unit 2) characterized by the several-meter-scale alternation of bedded siliceous mudstone and mudstone. Unit 2 is conformably overlain by a glauconite-rich sandstone bed at the base of the third unit (Unit 3), whose base was defined as 0 m level in the columnar section of Fig. 3b. The lithology of Unit 3 above the glauconitic sandstone is the several-meter-scale alternation of distinctly bedded muddy porcellanite and bedded siliceous mudstone. Fi- nally, weakly bedded to massive siliceous mudstone of Unit 4 overlies Unit 3 and consists of the uppermost part of the sections. There is no significant fault nor unconformity from Units 1 to 4 in the studied sections. Lithologically, Unit 1 might be correlated to the Nishikurosawa Formation, Units 2 and 3 to the Onnagawa Formation, and Unit 4 to the Funakawa Formation (see Section “Results and Discussion”).

MATERIAL The tuff layer from which we separated zircon grains for dating is intercalated in Unit 2 (the star point in Fig. Fig. 2. (a) CL images of dated zircon grains. Numbers are 1b) at –5.6 m level in the columnar section of Fig. 3b. grain No., and circles are measured points by NanoSIMS. (b) 204 206 238 206 The tuff layer is 7 cm thick and composed of very coarse An isochron diagram between Pb/ Pb and U/ Pb ra- sand-size felsic grains with a bluish-gray fine matrix (Fig. tios (N = 15). Errors are portrayed as the 2σ level. The isochron age was calculated by using Isoplot (Ludwig, 2008). 1c). The tuff layer shows a normally graded bedding structure in the upper part. The tuff layer has a distinct contact with siliceous mudstone above and below, and is later- ally continuous. Grains sieved with 44 µm mesh are composed mainly of quartz and feldspars with a minor amount mostly present as colorless to pale pink primary euhedral of dark gray lithic fragments, pyrite, altered volcanic grains smaller than 300 µm. The lack of detrital (rounded) glasses, and zircon (Fig. 1d). Quartz and feldspars grains grains and flow structures (cross laminations), and the are smaller than 2 mm, and show crystal faces or angular lateral continuity of the tuff layer strongly suggest that fraction surfaces. Pyrite is found as small crystals in clus- the tuff layer is not reworked deposit such as debris flow ters or on the surface of quartz and feldspars. Zircon is deposit and turbidite.

Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Fm. in North. Japan 187 METHODS Table 1. NanoSIMS U-Pb results of zircon grains Zircon grains were separated from the volcanic tuff Grain No. †U content 204Pb/206Pb 238U/206Pb layer by using heavy-liquid techniques and acid treatment [ppm] [×10–4] (operated by Kyoto Fission Track, Co. Ltd.). The sepa- 9-1 868 123 (12) 523 (19) rated zircon grains were mounted in an epoxy resin disc. 13-1 2141 21.4 (9.9) 559 (17) The zircons were polished until the midsections of the 34-1 597 42.0 (10.2) 607 (22) 36-2 650 45.9 (10.0) 578 (25) grains were exposed. Final polishing was done with a 0.3- 36-4 676 11.7 (6.7) 598 (23) µm plastic abrasive sheet. Cathodoluminescence (CL) imaging was performed using SEM-EDS (JEOL JSM- 36-5 595 38.4 (21.5) 579 (24) 6610 LV) equipped with a CL system (Gatan Mini CL) at 47-1 426 29.2 (16.5) 530 (24) the Geological Survey of Japan to observe the internal 47-2 333 44.7 (29.6) 533 (29) 62-1 1044 23.8 (7.9) 574 (20) structure and zonation of the separated zircon grains, and 62-2 482 38.9 (16.0) 565 (25) to select suitable sites for the U-Pb dating. The U-Pb dating was performed by a secondary ion 66-1 397 43.9 (21.6) 572 (26) mass spectrometry (Cameca NanoSIMS) at Atmosphere 86-1 1549 20.6 (6.3) 554 (18) and Ocean Research Institution (AORI), The University 123-1 1903 8.25 (4.66) 572 (18) 123-2 2053 4.13 (4.55) 559 (17) of Tokyo. The detailed analytical protocol is given in 155-2 574 14.0 (14.0) 558 (30) Takahata et al. (2008). A 10 nA mass-filtered O– primary beam was used to sputter a 15- m-diameter crater. We Errors were described as (2σ). µ † + + detected 30Si+, 204Pb+, 206Pb+, 238U16O+, and 238U16O + It was calculated by UO /Si ratio regarding the U content of the 2 95000 standard as 80 ppm. ions simultaneously for 300 seconds under a static magnetic field. Then, the magnet was cyclically peak-stepped to measure 207Pb+ ion using the same collector as 206Pb+ ion at the same spots. The typical 206Pb+ total count was Table 2. NanoSIMS Pb-Pb results of zircon grains 5,000 counts, which is enough to obtain good precision, Grain No. †U content 204Pb/206Pb 207Pb/206Pb 206 + whereas some spots poor in U content showed low Pb [ppm] [×10–4][×10–3] counts. We excluded data with lower U contents than 330 5-1 2138 13.8 (7.6) 52.6 (6.0) 204 206 ppm as those data show abnormally high Pb/ Pb ra- 13-1 2419 5.53 (4.4) 45.2 (5.0) tios due to the high contribution of interfered peaks on 34-1 1008 34.8 (11.4) 54.7 (5.2) 204Pb+ counts. The 91500 standard zircon (Wiedenbeck 36-4 840 19.1 (9.8) 49.9 (6.1) et al., 1995), which is a fragment of a single crystal 38-1 542 231 (108) 98.3 (23.5) achieved in the mineralogical collection of Harvard Uni- 47-1 580 40.4 (13.4) 57.8 (5.8) 238 206 versity, was used for U/ Pb calibration based on the 62-2 532 35.1 (19.2) 58.2 (9.9) empirically derived formula using 206Pb+/238U16O+ and 66-1 472 71.7 (22.2) 61.7 (7.7) 238 16 + 238 16 + 86-1 1999 20.2 (8.4) 49.5 (4.6) U O2 / U O ratios (see Takahata et al., 2008). To use this calibration correctly, we excluded data with a 123-1 2219 19.4 (7.0) 47.5 (3.6) 123-2 1952 19.1 (7.7) 51.3 (4.4) different matrix from pure zircon. Isochron age was de- termined using the Isoplot software (Ludwig, 2008). U Errors were described as (2σ). contents were calculated by comparing the measured †It was calculated by UO+/Si+ ratio regarding the U content of the 238U16O+/30Si+ ratios of the sample against those of the 95000 standard as 80 ppm. 91500 standard. The error of U content is approximately 30% at the 2σ level obtained by repeated analyses. The results of the U contents, the 204Pb/206Pb ratios, and the 238U/206Pb ratios are listed in Table 1. Assuming RESULTS AND DISCUSSION that our analytical data shown in Table 1 are U-Pb con- Zircon U-Pb age cordant, the diagram for 204Pb/206Pb versus 238U/206Pb are The grain size of zircon separated from the tuff layer prepared to consider the precise U-Pb age. The plot be- ranges from 100 to 300 µm. CL images show that most tween the measured 204Pb/206Pb ratios and the calculated zircon grains have euhedral hexagonal edges with clear 238U/206Pb ratios after selecting reliable data using the U zoning (Fig. 2a). Some grains are subhedral, with edges contents and the matrix compositions gained an isochron or crystal faces disturbed by other grains during the crys- age of 11.18 ± 0.37 Ma (2σ, MSWD = 3.6, N = 15), which tal growth or broken after the eruption. For the U-Pb dat- we employed as the U-Pb age of our sample (Fig. 2b). ing with NanoSIMS, we avoided broken areas and inclu- The results of the Pb-Pb measurement are shown in Ta- sions. ble 2. The plot between the 204Pb/206Pb ratios and the

188 J. Yoshioka et al. Fig. 3. (a) Radiolarian fossils from Samples 1 to 5 in Kurokawa (2015). The occurrence ages of radiolarians are based on Kamikuri et al. (2017). (b) A diagram showing age models for the studied section. *The age models were calculated assuming that each unit had a constant sedimentation rate.

Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Fm. in North. Japan 189 207Pb/206Pb ratios gained a Pb-Pb age of –55 ± 170 Ma boundary between Units 1 and 2, between 10.8 and 11.4 (2σ, MSWD = 0.60, N = 11), which is concordant with Ma (48%) for Units 2 and 3, and between 9.1 and 11.4 the U-Pb age. Ma (83%) for Units 3 and 4, respectively. The U-Pb age of this study much improved the age model based only Comparison of U-Pb age with radiolarian biostratigraphy on the fossil records. Radiolarian biostratigraphy of the Onnagawa Forma- tion has previously been reported in the Yurihonjo area Significance of the sediment age of the Onnagawa For- by Kurokawa (2015). Samples reported by Kurokawa mation (2015) are shown as Samples 1 to 5 in Figs. 3a and 3b. The age of the tuff layer (11.18 ± 0.37 Ma) in the We compared the radiolarian species occurrence of Sam- Onnagawa Formation provides a critical anchor point for ples 1 to 5 with the radiolarian biostratigraphy established constraining the timing of the lithological change from for the drill cores at Site U1430 in the southwestern Ja- carbonate-rich to biogenic silica-rich sediment in North- pan Sea (Fig. 3a) (Kamikuri et al., 2017), which were ern Japan during the Middle Miocene. As described above, drilled during Integrated Ocean Drilling Program (IODP) the lithology of the Miocene sedimentary rocks in the Expedition 346 (Tada et al., 2015). Samples 1 to 3 are Yurihonjo area continuously changes from marl and assigned to the Lychnocanoma magnacornuta biozone mudstone alternation of Unit 1, via siliceous mudstone- based on the occurrence of L. magnacornuta or dominant sediment of Unit 2, to muddy porcellanite of Dendrospyris uruyaensis, whereas Sample 5 is assigned Unit 3, which was followed by siliceous mudstone-domi- to the Eucyrtidium inflatum biozone based on the occur- nant Unit 4 (Fig. 3b). Units 2 and 3 lithologically corre- rence of E. inflatum and Calocyclas motoyamai (Fig. 3b). spond to the Onnagawa Formation, and our improved age The radiolarian biostratigraphy defined by Kamikuri et model constrained its bottom and top ages as 12.4–16.7 al. (2017) was established assuming that the ages of the Ma and 9.1–11.4 Ma, which agree with main cooling steps radiolarian bio-horizons in the Japan Sea are synchronous of 13.9 or 13.1 Ma and 10.6 or 9.9 Ma during the Middle- with those defined in the North Pacific at Ocean Drilling to-Late Miocene (Holbourn et al., 2013). Program (ODP) Sites 887, 884, and 1151 (Kamikuri et Despite the lack of age constraints in Unit 1, we con- al., 2004, 2007; Kamikuri, 2010). In Kamikuri et al. sider that the calcareous sediment of Unit 1 could be cor- (2017), the radiolarian bio-horizons were updated accord- related to the Nishikurosawa Formation in the Oga Pe- ing to the revised ages of the magnetic chrons in the Geo- ninsula (Koizumi et al., 2009). The Nishikurosawa For- logic Time Scale 2012 (Ogg, 2012). Although the mation is characterized by the sediment deposited under radiolarian biostratigraphy may potentially have some the influence of “the warm” surface water preferred by regional disparities in the species occurrences, our new calcareous phytoplankton. The Nishikurosawa Formation U-Pb data on zircons from the tuff layer are consistent changes upward into the Onnagawa Formation, which was with the radiolarian age constraints of the Onnagawa For- formed under “the cold” surface water preferred by sili- mation in the Yurihonjo area by Kurokawa (2015). ceous phytoplankton (Koizumi et al., 2009). The transi- Furthermore, the age model of the Onnagawa Forma- tion occurred around 13 Ma in the Oga Peninsula tion was strongly improved by the numerical age of zir- (Koizumi et al., 2009), which is consistent with 12.4– cons with a smaller error range (Fig. 3b). Assuming that 16.7 Ma in our studied section. The decrease in biogenic each lithological unit (Units 1 to 4) has a constant sedi- silica or the increase in detritus from Unit 3 to Unit 4 mentation rate, we can construct two simple age models may represent the lithological change from the Onnagawa with and without our U-Pb age (shadow and dot areas of Formation to the Funakawa Formation in the northeast- Fig. 3b). Both age models satisfy all age constraints by ern coastal basin along the Japan Sea (e.g., Tada, 1991). radiolarians; 9.1–10.2 Ma (Sample 1), 10.8–11.9 Ma Tada (1994) proposed a hypothesis that the reduction of (Sample 2), 9.1–11.9 Ma (Sample 3), 9.9– Ma (Sample surface productivity resulted from the decrease in the in- 4), and 11.7–13.1 Ma (Sample 5). For example, in the flow of the nutrient-rich intermediate water from the Pa- age model without the U-Pb age, the age of the boundary cific due to the eustatic sea-level drop. The sea-level drop between Units 3 and 4 is estimated between 9.1 Ma (up- might be related to the Antarctic ice sheet expansion at per limit of Sample 1) and 11.9 Ma (lower limits of Sam- the global cooling steps (Holbourn et al., 2013), although ples 2 and 3) by considering the maximum sedimentation other factors would have also affected the change in rate of Unit 3 or 4 (Fig. 3b). The age model without the depositional facies. Since the discussion about U-Pb age also brings the ages of the boundaries, between paleoenvironment requires well-constrained sediment age, 12.4 and 18.9 Ma for Units 1 and 2, and between 10.8 the improved age model should play a critical role in in- and 12.1 Ma for Units 2 and 3, respectively. The U-Pb vestigating the significance of the Onnagawa Formation age makes these ages between 12.4 and 16.7 Ma (67% of and its distinct orbital-scale sedimentary cycles during the age range estimated without the U-Pb age) for the the period of global cooling.

190 J. Yoshioka et al. CONCLUSION 272, 85–98. Kurokawa, S. (2015) Orbital and suborbital-scale sedimentary We obtained a U-Pb age of 11.18 ± 0.37 Ma for zircon rhythms in bedded siliceous rocks of the middle Miocene grains separated from a tuff layer in the middle part of Onnagawa Formation, Northeastern Japan. Master’s The- the Onnagawa Formation in the Yurihonjo area, Northern sis, The Univ. of Tokyo, 50 pp. (in Japanese with English Japan. The age data are consistent with the previous stud- abstract). ies on biostratigraphy based on radiolarian fossils in the Ludwig, K. R. (2008) Isoplot 3.70: A Geochronological Toolkit same section, but improve the previous interpretations on for Microsoft Excel. Berkeley Geochronology Center Spe- chronostratigraphy of the formation. It will help us to cial Publication, 4, 77 pp. construct a precise age model of Middle-to-Late Miocene Ogg, J. G. (2012) Geomagnetic polarity time scale. The Geo- logic Time Scale 2012 (Gradstein, F. M, Ogg, J. G., Schmitz, sediments along the Japan Sea margin and to understand M. D. and Ogg, G. M., eds.), 85–113, Elsevier. the environmental change in the Japan Sea after the ma- Ozawa, A. and Suda, Y. (1978) Geological map of Japan jor global cooling, and its relationship with the open 1:200,000, Hirosaki and Fukaura. Geol. Surv. Japan (in Japa- oceans. nese). Ozawa, A. and Suda, Y. (1980) Geological map of Japan Acknowledgments—We thank Dr. A. Yamaguchi and Mrs. H. 1:200,000, Akita and Oga. Geol. Surv. Japan (in Japanese). Matsumoto, T. Kochi, and T. Tada (The Univ. of Tokyo) for Ozawa, A., Hiroshima, T., Komazawa, M. and Suda, Y. (1987) their assistance in the fieldwork and sample collection, Dr. T. Geological map of Japan 1:200,000, Shinjo and Sakata. Kagoshima (The Univ. of Tokyo) for his assistance in the sam- Geol. Surv. Japan (in Japanese). ple preparation for SIMS analysis, and Dr. T. Miki (The Univ. Ozawa, A., Katahira, T., Nakano, S. and Tsuchiya, N. (1988) of Tokyo) for his assistance in the SIMS measurement. Dr. N. Geology of the Yashima district, 1:50,000. Geol. Surv. Ja- Akizawa (The Univ. of Tokyo) helped us for the lithological pan, 87 pp. (in Japanese with English abstract). description of the tuff layer. We also thank two anonymous re- Tada, R. (1991) Origin of rhythmical bedding in middle Miocene viewers for their critical but helpful and constructive comments siliceous rocks of the Onnagawa Formation, northern Ja- on the previous manuscript and the revised manuscript. This pan. J. Sediment. Petrol. 61, 1123–1145. research has been financially supported by JSPS Kakenhi Tada, R. (1994) Paleoceanographic evolution of the Japan Sea. (19K04054). Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 487–508. Tada, R. and Iijima, A. (1983) Petrology and diagenetic changes of Neogene siliceous rocks in Northern Japan. J. Sediment. REFERENCES Petrol. 53, 911–930. Holbourn, A., Kuhnt, W., Clemens, S., Prell, W. and Andersen, Tada, R., Murray, R. W., Alvarez Zarikian, C. A., Anderson, W. N. (2013) Middle to late Miocene stepwise climate cool- T., Jr., Bassetti, M.-A., Brace, B. J., Clemens, S. C., da Costa ing: Evidence from a high-resolution deep water isotope Gurgel, M. H., Dickens, G. R., Dunlea, A. G., Gallagher, S. curve spanning 8 million years. Paleoceanography 28, 688– J., Giosan, L., Henderson, A. C. G., Holbourn, A. E., 699. Ikehara, K., Irino, T., Itaki, T., Karasuda, A., Kinsley, C. Kamikuri, S. (2010) New late radiolarian species from the mid- W., Kubota, Y., Lee, G. S., Lee, K. E., Lofi, J., Lopes, C. I. dle to high latitudes of the North Pacific. Rev. C. D., Peterson, L. C., Saavedra-Pellitero, M., Sagawa, T., Micropaleontol. 53, 85–106. Singh, R. K., Sugisaki, S., Toucanne, S., Wan, S., Xuan, C., Kamikuri, S., Nishi, H., Motoyama, I. and Saito, S. (2004) Zheng, H. and Ziegler, M. (2015) Site U1430. Proc. IODP, Middle Miocene to Pleistocene radiolarian biostratigraphy 346 (Tada, R., Murray, R. W., Alvarez Zarikian, C. A. and in the Northwest Pacific Ocean, ODP Leg 186. Isl. Arc 13, the Expedition 346 Scientists, eds.), 113 pp., College Sta- 191–226. tion, TX. Kamikuri, S., Nishi, H. and Motoyama, I. (2007) Effect of Takahata, N., Tsutsumi, Y. and Sano, Y. (2008) Ion microprobe Neogene climatic cooling on North Pacific radiolarian as- U-Pb dating of zircon with a 15 micrometer spatial resolu- semblages and oceanographic conditions. Palaeogeogr. tion using NanoSIMS. Gondwana Res. 14, 587–596. Palaeoclimatol. Palaeoecol. 249, 370–392. Vincent, E. and Berger, W. H. (1985) Carbon dioxide and polar Kamikuri, S., Itaki, T., Motoyama, I. and Matsuzaki, K. M. cooling in the Miocene: the Monterey Hypothesis. The Car- (2017) Radiolarian biostratigraphy from middle Miocene bon Cycle and Atmospheric CO2: Natural Variations to late Pleistocene in the Japan Sea. Paleontol. Res. 21, 397– Archean to Present (Sundquist, E. T. and Broecker, W. S., 421. eds.), 455–468, Am. Geophys. Union, Geophys. Monogr. Koizumi, I. and Yamamoto, H. (2018) Diatom ooze and Ser. 32. diatomite-diatomaceous sediments in and around the North Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W. L., Meier, M., Pacific Ocean. JAMSTEC Rep. Res. Dev. 27, 26–46. Oberli, F., von Quadt, A., Roddick, J. C. and Spiegel, W. Koizumi, I., Sato, M. and Matoba, Y. (2009) Age and signifi- (1995) Three natural zircon standards for U-Th-Pb, Lu-Hf, cance of Miocene diatoms and diatomaceous sediments from trace element and REE analyses. Geostand. Newsl. 19, 1– northeast Japan. Palaeogeogr. Palaeoclimatol. Palaeoecol. 23.

Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Fm. in North. Japan 191