亀尾 浩司

The fluctuating position of the boundary between the Kuroshio (warm) and Oyashio (cold) currents in the mid-latitude western North Pacific affects both heat transport and air–ocean interactions and has significant consequences for the East Asian climate. We reconstruct the paleoceanography of Marine Isotope Stages (MIS) 20–18, MIS 19 being one of the closest astronomical analogues to the present interglacial, through multiple proxies including microfossil assemblage data, planktonic foraminiferal isotopes (δ O and δ C), and foraminiferal Mg/Ca-based temperature records, from the Chiba composite section (CbCS) exposed on the Boso Peninsula, east-central Japan. Principal component analysis (PCA) is used to capture dominant patterns of the temporal variation in these marine records, and shows that the relative abundances of calcareous nannofossil and radiolarian taxa are consistent with the water mass types inferred from geochemical proxies. The leading mode (36.3% of total variance) mirrors variation in the terrestrial East Asian winter monsoon (EAWM), reflecting seasonal trends dominated by the winter monsoon system. In the CbCS, this mode is interpreted as reflecting the interplay between the warm Kuroshio and cold Oyashio waters, which is likely related to the latitudinal shift of the subtropical–subarctic gyre boundary in the North Pacific. The second mode (15.4% of total variance) is closely related to subsurface conditions. The leading mode indicates that MIS 19b and 19a are represented by millennial-scale stadial/interstadial oscillations. Northerly positions for the gyre boundary during late MIS 19c, the interstadials of MIS 19a, and early MIS 18 are inferred from the leading mode, which is consistent with a weak EAWM and consequent mild winter climate in East Asia. Nonetheless, the northerly positions for the gyre boundary during late MIS 19c and early MIS 19a were not associated with subsurface warming presumably due to the suppressed gyre circulation itself caused by the weak Aleutian Low. Intermittent southerly positions for the gyre boundary are inferred for the stadials of MIS 19b and 19a. Regional sea surface temperature (SST) comparisons in the western North Pacific reveal that the moderate SSTs during MIS 19a through early MIS 18 were restricted to the mid- to high latitudes, influenced by the weak EAWM. Comparison between MIS 20–18 and MIS 2–1 suggests that glacial MIS 20 and 18 had significantly milder winters than MIS 2, likely related to the relatively weak EAWM. [Figure not available: see fulltext.]. 18 13

The Executive Committee of the International Union of Geological Sciences on January 17, 2020 ratified the Global Boundary Stratotype Section and Point (GSSP) defining the base of the Chibanian Stage/Age and Middle Pleistocene Subseries/Subepoch at the Chiba section of the Chiba composite section, Japan. The Chiba composite section is a continuous and expanded marine sedimentary succession in the east-central Japanese archipelago facing the Pacific Ocean. It contains well-preserved pollen, marine micro-and macrofossils, a tightly-defined Matuyama-Brunhes (M-B) paleomagnetic polarity boundary, two geomagnetic field paleointensity proxies, and numerous tephra beds, allowing the establishment of a robust and precise chronostratigraphic framework. Its open-ocean continental slope setting has captured both terrestrial and marine environmental signals from upper Marine Isotope Stage (MIS) 20 to lower MIS 18. The M-B reversal serves as the primary guide for the Lower-Middle Pleistocene boundary, yield ing an astronomical age of 772.9 ka. The GSSP is positioned 1.1 m below the directional midpoint of the reversal, at the base of a regional lithostratigraphic marker, the OntakeByakubi-E (Byk-E) tephra bed, in the Chiba section. The GSSP has an astronomical age of 774.1 ka and occurs immediately below the top of Marine Isotope Substage 19c.

房総半島北東部の銚子地域に分布する鮮新–更新統犬吠層群は微化石を豊富に含む半遠洋性堆積物であり，これまでいくつかの年代層序学的研究が行われてきた（酒井，1990）．この犬吠層群を対象として1998年に陸上ボーリングコアが東京大学海洋研究所によって掘削され，微化石層序，古地磁気，酸素同位体比から年代層序が確立され, 海洋酸素同位体比ステージ（MIS）11～24であることが明らかになっている (Kameo et al., 2006)．このコアには，いわゆるEarly-Middle Pleistocene Transition (EMPT)を含み，氷期・間氷期サイクルが4万年周期から10万年周期へと変動する時代の環境変化を研究するためには非常によい試料である．特にMIS 19については離心率が現在と同じような状況に近いことから将来の気候変動予測に重要な時代であり（Head and Gibbard, 2015），気候変動と関連した海洋循環の消長を考える上で重要な地域といえる．そこで本研究では，浮遊性有孔虫化石群集を用いることによって，数千年スケールでの古海洋環境変動を推定することを試みた．本研究では犬吠層群に相当する地層のうち，横根層上部に相当する深度130～160 mを使用した．この層準に該当する年代は，MIS 17～19である．検討した試料間隔は，約1 mで，時間分解能は約2000～3000年である．乾燥させた各試料を熱湯で処理，ボロン法を用いることで泥化処理を行った. その後125 µmのふるいにかけて浮遊性有孔虫化石が200個体程度になるまで分割して顕微鏡下で全て拾い出し, 種の同定を行った. 検討を行った試料から15属42種の浮遊性有孔虫化石が認められた．全層準を通じて,Neogloboquadrina incomptaが高い産出頻度を占めし，次いで, Globigerinita glutinata, Turbolotalita quinqueloba, Globorotalia inflata などが高い産出頻度を示す．このことから, 銚子地域は当時ほとんど混合水域の環境下にあったと考えられる．しかし，黒潮水域に優勢なGlobigerinoides ruber, Pulleniatina obliquiloculataについては酸素同位体比曲線の温暖なピークと概ね一致することから，温暖時には黒潮の影響を強く受けていたことが考えられる．一方，親潮水域に優勢なNeogloboquadrina pachydermaの左巻も寒冷のピークと概ね一致するが, 産出頻度はそこまで高くないことから，銚子地域では親潮の影響は寒冷な時でもそこまで強い影響はなかったと考えられる. T. quinquelobaについては混合水域だけでなく塩分の低い沿岸水の指標とされている(Takemoto and Oda, 1997). T.quinquelobaが高い産出頻度を示した層準は，氷期への移行期に相当するように見えるので，寒冷化に伴って海水準の低下が起こり，その結果，西方からの沿岸水の影響が強くなったことが考えられる．以上より氷期・間氷期サイクルより短い時間スケールでの黒潮・親潮の南北振動が銚子地域で起こっていたと考えられる. 引用文献 Head and Gibbard,2015,Quat.Int.,389,7-46 五十嵐ほか，2002,地質学会講演要旨 Kameo et al.,2006,Island Arc,15,366-377 Takemoto and Oda,1997, Paleontolog. Res.,1,4,291-310 酒井,1990,宇都宮大教育紀要,23,1-34

Oxygen isotope stratigraphy and calcareous nannofossil biostratigraphy in the upper part of the Kiwada Formation, which is part of the Kazusa Group in the Boso Peninsula in the central part of the Pacific coast of Japan, were examined to establish a high-resolution age model and estimate the age of the lower part of Pleistocene nannofossil biohorizons in the northwestern Pacific region. The new age model indicates that the upper part of the Kiwada Formation corresponds to Marine Isotope Stage (MIS) 41 through MIS 36. Two nannofossil biohorizons, the last occurrences of large forms of Gephyrocapsa (>5.5 mu m) and Helicosphaera sellii, were recorded in the examined section. The LO of large Gephyrocapsa spp. coincides with the MIS 37/38 boundary which is 1241.2 +/- 0.4 ka. The LO of H. sellii is located in late MIS 40 and has a calculated age of 1291.4 +/- 1.4 ka. These biohorizons are traceable even though the LO of H. sellii is a diachronous event. Size variations of Gephyrocapsa from approximately 1250 ka are discussed and fluctuations of small size Gephyrocapsa could represent cyclic changes of the Kuroshio and Oyashio currents.

An amendment to this paper has been published and can be accessed via the original article.

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<p>鹿児島県種子島に分布する熊毛層群西之表層の地質，地質年代，そして堆積環境を検討した．砂岩が卓越する西之表層において，生痕化石が多産する泥岩部を大久保泥岩部層として区別した．放散虫化石からは西之表層上部が31.1Maから28.5Maの地層であることが，石灰質ナンノ化石からは西之表層下部が30.00Maから26.84Maに堆積したことが判明した．以上の事実から，西之表層の地質年代が，前期漸新世の後期（30.0-28.5Ma）であることが判明した．</p><p>岩相分布と地質年代の結果から，（1）西之表層は同層準が褶曲と衝上断層によって繰り返すこと，（2）西之表層の層厚は従来の推定よりも薄いこと，（3）西之表層は九州南部に分布する日南層群に対比されること，そして（4）産出する生痕化石から西之表層は水深が2000mよりも深い海底で堆積したこと，が明らかとなった．今後，西之表層の起源を詳しく解明するためには，堆積学的・構造地質学的観点からの検討が不可欠である．</p>

Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the Sao Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn-Fe-10x(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained Os-187/Os-188 data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over similar to 30 Myr with growth rates of 0.5-3 mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the Sao Paulo Ridge over the past similar to 30 Myr. The Pb isotopic compositions show very limited ranges (e.g.,Pb-206/Pb-204=18.80-18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the Sao Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since similar to 30 Ma.

Major climatic reorganizations, including changes in the nature of glacialeinterglacial cycles through the Pleistocene, are a key issue for improving the understanding of Earth's climate system. Highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them. The Kazusa Group, located in the central part of the Japanese island chain, is well-exposed and contains a remarkably continuous and thick, deep-and shallow-water marine sedimentary succession. This group also contains well-preserved marine micro-fossils, pollen, paleomagnetic reversal events, geochemical signatures, and a large number of tephra beds. These features allow us to establish a robust chronological and stratigraphic framework for the Kazusa Group, and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the Lower and lower Middle Pleistocene. In the Boso Peninsula, Chiba Prefecture, the Kazusa Group is deeply incised, yielding spectacular river-cut exposures. The Matuyama-Brunhes paleomagnetic reversal is observed immediately above the widespread Byakubi-E (Byk-E) tephra in the Chiba composite section within the Kokumoto Formation (Kazusa Group). Because the Matuyama-Brunhes boundary customarily serves as the primary guide for the Lower-Middle Pleistocene Subseries boundary, the Chiba composite section is considered an excellent candidate for its global boundary stratotype section and point (GSSP). This study reviews the published data for the Kazusa Group (hitherto almost exclusively in Japanese) in order to place the Chiba composite section within its broader depositional and chronostratigraphic context. (C) 2015 Elsevier Ltd and INQUA. All rights reserved.

We present field and core observations, nannofossil biostratigraphy, and stable oxygen isotope fluctuations in foraminiferal tests to describe the geology and to construct an age model of the Lower Pleistocene Nojima, Ofuna, and Koshiba Formations (in ascending order) of the middle Kazusa Group, a forearc basin-fill succession, exposed on the northern Miura Peninsula on the Pacific side of central Japan. In the study area, the Nojima Formation is composed of sandy mudstone and alternating sandy mudstone and mudstone, the Ofuna Formation of massive mudstone, and the Koshiba Formation of sandy mudstone, muddy sandstone, and sandstone. The Kazusa Group contains many tuff beds that are characteristic of forearc deposits. Thirty-six of those tuff beds have characteristic lithologies and stratigraphic positions that allow them to be traced over considerable distances. Examination of calcareous nannofossils revealed three nannofossil datum planes in the sequences: datum 10 (first appearance of large Gephyrocapsa), datum 11 (first appearance of Gephyrocapsa oceanica), and datum 12 (first appearance of Gephyrocapsa caribbeanica). Stable oxygen isotope data from the tests of the planktonic foraminifer Globorotalia inflata extracted from cores were measured to identify the stratigraphic fluctuations of oxygen isotope ratios that are controlled by glacial-interglacial cycles. The observed fluctuations were assigned to marine isotope stages (MISs) 49-61 on the basis of correlations of the fluctuations with nannofossil datum planes. Using the age model obtained, we estimated the ages of 24 tuff beds. Among these, the SKT-11 and SKT-12 tuff beds have been correlated with the Kd25 and Kd24 tuff beds, respectively, of the Kiwada Formation on the Boso Peninsula. The Kd25 and Kd24 tuff beds are widely recognized in Pleistocene strata in Japan. We used our age model to date SKT-11 at 1573ka and SKT-12 at 1543ka.

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is designed to investigate fault mechanics and seismogenesis along a subduction megathrust, with objectives that include characterizing fault slip, strain accumulation, fault and wall rock composition, fault architecture, and state variables throughout an active plate boundary system. Integrated Ocean Drilling Program (IODP) Expedition 338 was planned to extend and case riser Hole C0002F from 856 to 3600 meters below the seafloor (m b.s.f.). Riser operations extended the hole to 2005.5mb.s.f., collecting logging-while-drilling (LWD) and measurement-while-drilling, mud gas, and cuttings data. Results reveal two lithologic units within the inner wedge of the accretionary prism that are separated by a prominent fault zone at ̃ 1640mb.s.f. Due to damage to the riser during unfavorable winds and strong currents, riser operations were suspended, and Hole C0002F left for re-entry during future riser drilling operations. Contingency riserless operations included coring at the forearc basin site (C0002) and at two slope basin sites (C0021 and C0022), and LWD at one input site (C0012) and at three slope basin sites (C0018, C0021 and C0022). Cores and logs from these sites comprehensively characterize the alteration stage of the oceanic basement input to the subduction zone, the early stage of Kumano Basin evolution, gas hydrates in the forearc basin, and recent activity of the shallow megasplay fault zone system and associated submarine landslides. © Author(s) 2014.

本論では，房総半島西部に分布する安房層群安野層と上総層群最下部層の石灰質ナノ化石を検討し，安野層の年代を明らかにするとともに，安房層群と上総層群の間の不整合，すなわち黒滝不整合の時間間隙を考察した．本研究の結果，安野層には後期鮮新世の石灰質ナノ化石基準面のうち，3つの化石基準面が認められ，安野層の年代は約4.5–3 Maに相当することが明らかになった．また，上総層群と安房層群との時間間隙はおおむね100万年程度であり，房総半島の東部に比べて両層群の年代ギャップは小さいことがわかった．太平洋側の西南日本前弧海盆地域では，後期鮮新世から前期更新世にかけての時代に，同じような不整合や堆積速度の低下層準などが認められていることから，この時期に広い範囲で堆積盆地の構造を大きく変えるようなテクトニックな変動があったと考えられる．

沖縄県西表島に分布する中新統八重山層群西表層最上部の地質年代と堆積環境を検討した．西表層最上部は内側陸棚までの海進と干潟までの海退がセットとなる4つの堆積サイクルで構成される．石灰質ナンノ化石が生痕化石Thalassinoides suevicusの充填物からのみ検出され，西表層最上部は中期中新世初期（CN4帯：14.91 Ma～13.53 Ma）であることが判明した．充填物に石灰質ナンノ化石を含むT. suevicusは，サイクル境界から下位側の堆積サイクルに伸長するものに限られた．この理由は，1）充填物中心部はコンクリ-ション化や風化の影響を受けなかった，2）石灰質ナンノ化石を含む高海面期の海底表層堆積物が生痕内部に流下した，そして3）生痕が堆積物深部に形成されたため，生物攪拌作用や物理的海底浸食による破壊を被らなかった，といった好条件が重なったためと考えられる．生痕化石充填物中の微化石群は，地層年代を考察する際に重要である．

The El Nino/Southern Oscillation (ENSO) system during the Pliocene warm period (PWP; 3-5 million years ago) may have existed in a permanent El Nino state with a sharply reduced zonal sea surface temperature (SST) gradient in the equatorial Pacific Ocean(1). This suggests that during the PWP, when global mean temperatures and atmospheric carbon dioxide concentrations were similar to those projected for near-term climate change(2), ENSO variability-and related global climate teleconnections-could have been radically different from that today. Yet, owing to a lack of observational evidence on seasonal and interannual SST variability from crucial low-latitude sites, this fundamental climate characteristic of the PWP remains controversial(1,3-10). Here we show that permanent El Nino conditions did not exist during the PWP. Our spectral analysis of the delta(18)O SST and salinity proxy, extracted from two 35-year, monthly resolved PWP Porites corals in the Philippines, reveals variability that is similar to present ENSO variation. Although our fossil corals cannot be directly compared with modern ENSO records, two lines of evidence suggest that Philippine corals are appropriate ENSO proxies. First, delta(18)O anomalies from a nearby live Porites coral are correlated with modern records of ENSO variability. Second, negative-delta(18)O events in the fossil corals closely resemble the decreases in delta(18)O seen in the live coral during El Nino events. Prior research advocating a permanent El Nino state may have been limited by the coarse resolution of many SST proxies, whereas our coral-based analysis identifies climate variability at the temporal scale required to resolve ENSO structure firmly.

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism. Citation: Lin, W., et al. (2010), Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009, Geophys. Res. Lett., 37, L13303, doi:10.1029/2010GL043158.

本論では，房総半島の中部に分布する安房層群清澄層の地質時代を決定するために，清澄層から産出する石灰質ナンノ化石を検討した．併せて，西部赤道太平洋の深海底コアであるHole 806Bの下部鮮新統のReticulofenestra属の大きさの変化を検討し，その大きさが変化するバイオイベントを明らかにして，標識種に乏しい清澄層の年代決定に役立てた．本研究の結果，清澄層からは多くの石灰質ナンノ化石の産出が確認できたが，確認された化石基準面は石灰質ナンノ化石帯CN10帯とCN11帯の境界（Amaurolithus spp.の産出上限）のみであった．また，清澄層に見られるReticulofenestra属の大きさの変化によるイベントは，西部赤道太平洋の下部鮮新統に見られるそれと同様であり，それに基づく年代推定は古地磁気層序の結果と調和的である．このことはReticulofenestra属の大きさの変化に基づく化石イベントが年代決定に適用可能であることを示している．

房総半島中部の小糸川上流域を中心に安房層群天津層，清澄層の地質を調査し，放散虫化石層序を検討した． 天津層・清澄層からは温暖種と寒冷種を共に含む放散虫化石が比較的豊富に産出した．その結果，低緯度域の放散虫化石帯区分ではRN5帯（D. alata帯）からRN10-RN11帯（P. doliolum-L. audex帯）に，北太平洋中・高緯度域の帯区分ではE. inflatum帯からS. pylomaticus-C. sakaii帯に相当することが明らかになった．放散虫および他の微化石・放射年代データを総合し堆積速度曲線を描いたところ，中新統／鮮新統境界は清澄層下部のKy6層準付近に推定された．また，天津層中上部では堆積速度の著しい低下が認められ，広域的な地質イベントとの関連性について考察を行った．

静岡市,有度丘陵(最高標高307m)の東縁に分布する中部更新統久能山層(村松礫・シルト部層)から産した単体サンゴに,αスペクトル²³⁰Th/²³⁴U法を適用し,176.5^+3.7_-3.6ka(誤差は1σ)の年代値を得た.この年代値から,久能山層堆積期間は海洋酸素同位体ステージ6.5に対応することが明らかとなった.

We introduce a revised method to make microslides with concentration of calcareous nannofossils for the light microscopic observation modified after Hay (1977). This technique for sample preparation enables us to observe a large number of specimens without large sediment particles and organic materials compared with other preparation methods. It should be useful for detailed quantitative analysis of nannofossil abundances after preliminary observations using smear slides and/or microslides by the suspension method. <br>

Selected area electron diffraction (SAED), high-resolution transmission electron microscopic and scanning electron microscopic analyses were performed to determine crystal faces of two nannoliths from Florisphaera profunda and discoasters, the exoskeltons of a certain unicellelar alga, calcareous nannofossils. A specimen of F. profunda is a pentagonal-shaped plate and its [0001] direction (c-axis) is oriented parallel to its surface and in the elongation axis of the plate-like element. Its crystallographic nature mostly resembles a proximal shield element of Emiliania huxleyi, one of the most common coccolithophorids observed in the present ocean. On the other hand, a nannolith of Discoaster specimens is a star-shaped calcite crystal and is composed of several numbers of bent, bar-shaped calcite elements, called 'rays.' Based on results of the SAED observation and calcite decoration on specimens, each ray apparently seems to be a single calcite crystal in all types of discoasters. The [0001] direction is likely perpendicular to the tangential plane of the central area and each ray joins together in the plane that is parallel to [0001]. The difference of crystallographic nature between nannoliths and heterococcoliths suggests that the phylogenetic origin and the evolutionary history of nannolith-producing phytoplankton are different from those of heterococcolith-bearing coccolithophores.

銚子地域の犬吠層群小浜層に挟在するテフラと, 房総半島の上総層群黄和田層～梅ヶ瀬層に挟在するテフラを, 重鉱物組成, 火山ガラスの形態およびその化学組成に基づいて対比した. その結果, 小浜層のテフラ鍵層Ob1, Ob2, Ob4a, Ob4c, Ob5a, Ob5.1, Ob5.6, Ob5.7, Ob6a, Ob6cおよびOb7bはそれぞれ, 黄和田層のテフラ鍵層Kd39, Kd38, Kd25, Kd24, Kd16およびKd5A, 大田代層上部のテフラ鍵層O7, O3, 梅ヶ瀬層中部のテフラ鍵層U8, U7およびU6Aに対比された. あわせて行った石灰質ナンノ化石の検討によってもこの結果は支持される. したがって, 小浜層は黄和田層下部～梅ヶ瀬層中部に相当することになる. また年代学的なデータに基づくと, 小浜層中部～上部の堆積速度は極めて遅いものの, 従来この層準に指摘されていた約50万年に及ぶ堆積間隙は存在しないことが判明した. この層準の極めて遅い堆積速度と堆積物の粗粒化の原因を明らかにするためには, 古環境学的および古海洋学的な検討が必要である.

横浜地域の大船層中部～小柴層下部に挟在するテフラ鍵層と,房総半島の黄和田層下部～中部に挟在するテフラ鍵層を重鉱物組成,火山ガラスの形態及び化学組成に基づいて対比した.その結果,大船層中部のテフラSg3,小柴層下部のテフラSg2及びSg1はそれぞれ,黄和田層下部～中部のテフラ鍵層Kd38,Kd25及びKd24こ対比された.あわせて行った石灰質ナンノ化石と浮遊性有孔虫化石の検討によってもこの結果は支持される.したがって,大船層中部～小柴層下部は,従来言われていたように黄和田層中部～梅ヶ瀬層に対比されるのではなく,テフラ鍵層Kd19より下位の黄和田層に相当することになる.このことは,横浜地域の第四系の年代論に大幅な見直しが必要であることを意味している.

秋田県北部に分布する天徳寺層,笹岡層の石灰質ナンノ化石層序調査結果は,峰浜地域の笹岡層最上部が更新世最初期に対比されるのを除けば,いずれの地域においても両層の年代は3.85-1.73Ma間の後期鮮新世に対比されることを示す.また,北極地域の急激な氷床拡大と関連する基準面Aは峰浜地域と柾山沢地域で天徳寺層・笹岡層境界に追跡されること,これら対比から,秋田地域の大部分が鮮新世末に陸化したことを明らかにした.この調査結果と,新潟,北陸地域の大桑・万願寺動物群産出層準との対比結果から,秋田県内の大桑・万願寺動物群産出層準が,更新世に対比される新潟・北陸地域とは明らかに異なることを指摘した.一方,日本海側地域石灰質ナンノ化石群集は北太平洋-北極海域で認めた群集と極めて類似し,暖海性種を伴う太平洋側地域と対立すること,その生物地理形態が大桑-万願寺動物群と共通することなどもあわせて明らかにした.

房総半島南部に分布する千倉層群布良層と南朝夷層の石灰質ナンノ化石を検討し,それらの地質時代と後期鮮新世における本邦太平洋側中部海域の表層海洋環境を考察した.おもな化石種の層位分布に基づけば,布良層と南朝夷層はおおむね3.3Maから2.4Maに相当する.一方,環境指標となる化石種の増減から,3.3Maから3.0Maの間,かつての房総海域には非常に温暖で,かつ栄養塩に乏しい外洋水が存在したことが指摘できる.ところが3.0Ma以降,この海域は寒冷化し始め,とくにその傾向は2.7Ma以降強くなる.このことはこの時期以降,房総海域に混合水塊の影響が強く及ぶようになったことを意味している.また,2.7Ma以降,東アジア大陸周辺からの表層水の流入が増加した可能性もあり,東アジア大陸の風化作用の強化に連動して,房総海域の海洋環境が変化したことも示唆される.

Calcareous nannofossil assemblages from land sections of the Japanese Islands and DSDP Holes in the equatorial to high latitude regions of the Pacific Ocean were analyzed in an effort to reconstruct their late Pliocene paleobiogeography. While the late Pliocene assemblages in the equatorial to middle latitude regions are comprised primarily of discoasters, both the high latitude and Japan Sea samples record an abrupt change from a Reticulofenestra-Dictyococcite assemblage to a Coccolithus pelagicus assemblage at 2.75 Ma. The northernmost boundary of the Discoaster assemblage moved southward in the East China Sea (western Pacific Ocean) at that time. These changes in the calcareous nannofossil paleoblogeography indicate the strong influence of heavy glaciation in high latitude to arctic regions of the Pacific Ocean. This also correlates with the final closure of the Central American seaway at 2.75 Ma.

房総半島中部に分布する安房層群天津層の石灰質ナンノ化石を検討し,その地質時代を考察した.天津層からは指標種を含むさまざまな石灰質ナンノ化石が比較的豊富に産出する.おもな石灰質ナンノ化石種の層位分布に基づけば,天津層に計14の化石基準面とバイオイベントが認定できる.その結果,天津層は石灰質ナンノ化石帯CN5帯(NN6帯)からCN10帯(NN12帯)に相当し,中部中新統から下部鮮新統に対比される.相当する年代は概ね13Maから5Maである.今後,天津層でその他の微化石層序や古地磁気層序が詳しく検討されることによって,中緯度海域における中新統複合年代尺度を設定できると考えられる.

Principal components analysis of nannofossil assemblages in five ODP sites in the Caribbean and the eastern equatorial Pacific was used to monitor Neogene surface water circulation changes. In 15.83-10.71 Ma, floral distributions clearly show the existence of the Circum-tropical Current between the Caribbean and the eastern equatorial Pacific and no surface water communication between the northern and southern Caribbean. In 10.71-9.36 Ma, the Circum-tropical Current weakened and the northward intra-Caribbean current had been initiated. Northern and southern Caribbean surface waters again became separated in 8.35-3.65 Ma and the Circum-tropical Current was regenerated. After 2.76 Ma, the northward intra-Caribbean current has completely been established and the Circum-tropical Current disappeared. This suggests that a barrier to the Circum-tropical Current had formed, indicating the final closure of the Isthmus of Panama. (C) 2000 Elsevier Science B.V. All rights reserved.

琉球列島久米島に分布する島尻層群真謝層に, 不整合関係にある2つの堆積ユニットを認め, それぞれ下位より, 真謝層, 阿嘉層として再定義した.石灰質ナンノ化石の群集構成に基づくと, 真謝層上部はOkada and Bukry(1980)のCN9帯, 上部中新統に対比され, 阿嘉層上部はCN 12 b亜帯, 上部鮮新統に対比される.また, 軟体動物化石を用いたSr同位体層序の検討より, 真謝層上部から約7.8～7.2 Ma(後期中新世), 一方, 阿嘉層からは約3.2～3.1 Ma(後期鮮新世初期)にそれぞれ収束するSr同位体年代値を得た.さらに, ここで明らかにした層序と地質年代に照らして, 真謝層および阿嘉層を琉球列島に分布するほかの島尻層群相当層と対比し, 両累層の層序的意義を考察した.

We demonstrate size fluctuations of the calcareous nannofossil genus Reticulofenestra in Upper Pliocene sediments from the North Atlantic Ocean and clarify a characteristic evolutionary trend of this genus. Four bioevents, which are based on abrupt decreases in maximum size and on changes of morphologic features of Reticulofenestra specimens, are detected in the sediments. They are the disappearance of R. minutula var. A, the termination of Acme Zone II of R. minutula var. C, the disappearance of R. minutula var. B, and the termination of Acme Zone I of R. minutula var. C, in ascending order. These are nearly synchronous and traceable events. (C) 1999 Elsevier Science B.V. All rights reserved.