We describe in detail the late Pliocene to Quaternary paleoenvironmental change associated with the newly defined Pliocene/Pleistocene boundary based on calcareous nannofossil biostratigraphy. The period from 4.0 Ma to 2.75 Ma just prior to Datum plane A, is characterized by warm water conditions named mid-Pliocene Golden age. In contrast, the paleoenvironment was drastically changed to increase of the ice sheet in the Arctic Ocean at 2.75 Ma (climate crash) which is caused by closure of the Central American Sea Way. The Quaternary style biogeography of calcareous nannoplankton and planktic foraminifer, and the ocean current system flowing from the Bering straight through the Arctic Ocean to the East Greenland Sea, were established at this time. This indicates that the main “Quaternary style climate” system was completed at 2.75 Ma as a response to the emergence of the Isthmus of Panama.
Orbitally tuned magnetic susceptibility and quartz mean grain size records from Chinese loess-red clay sediments reveal that marine oxygen isotope stratigraphy is in harmony with late Pliocene climate evolution. On the Chinese Loess Plateau, the long-term late Pliocene warm-moist climate since ca. 3.6 Ma, recorded in a red clay layer, was interrupted by a sudden and large cooling-drying event, correlated with the MIS G6—G4 ice-sheet expansion (2.73—2.68 Ma). The climate at the time of the event was still warm. The first loess layer (L33) overlying the red clay is correlated with MIS 104. The L33 glacial episode is characterized by the strongest winter monsoon recorded in the Pliocene—Pleistocene, although the ice volume increase during MIS 104 was relatively small (equivalent to approximately 50 m of sea-level change), being comparable to that in MIS 5.1. The Gauss-Matuyama (GM) transition with multiple rapid polarity swings occurred during the lower to middle part of loess layer L33, which is overlain by paleosol layer S32 and is correlated with MIS 103. The lower boundary of S32 coincides with the beginning of the Quaternary. In the Japanese Islands, the cooling-drying event is correlated with MIS G6 to MIS G4-caused termination of the long-term late Pliocene warm-moist climate, and its accompanying large scale impact on forests. However, the climate during the cooling-drying event was as warm as that during interglacial times.
We studied in detail the calcareous nannofossil biostratigraphy of the upper Pliocene to Quaternary sequences in the Osugozawa route of Gojome area, Aikawa and Komasugawa routes of the Oga peninsula, and two oil exploration wells of NKH-1 and 3 located in Honjo area, Akita Prefecture, Japan. The Pliocene/Pleistocene boundary redefined at the base of Gelasian Stage was identified in the lower part of the Sasaoka Formation in the Osugozawa route. The base of Calabrian stage, formerly considered as the Pliocene/Pleistocene boundary, was also identified in the lower part of the Kitaura Formation in the Oga Peninsula, based on magneto- and nannofossil bio-stratigraphy. Pliocene to Quaternary formations in the Akita area were correlated to those in the Japan Sea side area. We also revealed the tectonic movements along the southern coast of Akita Prefecture on the basis of the nannofossils obtained from drill cuttings and the subsurface geology of the area. The results clarified that the Kita-Yuri thrust faults were active between 3.85 and 1.71 Ma. Paleodepth of western Akita Prefecture were also changed from bathyal to shallow marine environments in 2.75 Ma, a consequence of both fault activity and the climate crush associated with drastic latest Pliocene cooling.
We propose a refined chronostratigraphy for the middle section of the Chikura Group, containing the Plio-Pleistocene boundary, from the southernmost part of the Boso Peninsula, Japan, based on magnetostratigraphy and oxygen isotopic stratigraphic analysis of benthic foraminifers. The stratigraphic interval examined here correlates with the chronostratigraphic interval from the upper Gauss chronozone, including the Kaena sub-chronozone, to the lower Matuyama chronozone. Our chronostratigraphic interpretation strongly suggests that the presently defined stratigraphic interval represents LR04 marine isotope stages (MIS) G16 to 93. The Matuyama—Gauss boundary present within the studied sequence is not within MIS 104, as is described in the LR04 time scale, but is in MIS 103. This interpretation is consistent with that of the boundary's stratigraphic horizon as reported in Mediterranean sapropel stratigraphy.
The Plio—Pleistocene succession in southwest Japan is exposed along the Nagatani River (NGT section) in the northern Miyazaki region, southern Kyushu, and on the Pacific side of southwest Japan. The NGT section is a prominent sequence that includes the upper Takanabe Formation of the uppermost part of the Miyazaki Group. In this study, we clarify the paleoceanographic conditions during deposition of the uppermost Miyazaki Group, based on faunal compositions of calcareous nannofossils and planktic foraminifera. We especially focused on two genera, Discoaster and Reticulofenestra, as well as the calcareous nannofossil species Coccolithus pelagicus and the planktic foraminifera species Globigerinoides ruber, Globigerinoides quadrilobatus, Globigerinoides sacculifer, and Neogloboquadrina incompta, as proxies for paleoceanographic conditions. Changes in the relative abundances of these flora and faunal compositions suggest that paleoceanographic conditions in the Miyazaki area shifted from that of a stratified surface-water mass (Kuroshio current) during the late Pliocene to conditions of upwelling (cold-water mass) during the early Pleistocene, via a transitional phase which occurred just after the biohorizon containing the last occurrence of the genus Discoaster.
The Pleistocene Ryukyu Group, composed of reef-complex deposits, crops out in southern Okinawa-jima, Ryukyu Islands, southwestern Japan. Here, based on new investigations, we revise the previous stratigraphic scheme for the Ryukyu Group and provide a formal stratigraphic description. In the study area, the Ryukyu Group comprises the Itoman, Naha, and Minatogawa formations. The Itoman Formation, composed mainly of coralline algal limestone, is thin (>2 m thick) and sporadically distributed. The Naha Formation rests unconformably on the Itoman Formation, reaches 50 m in thickness, and is exposed at elevations of up to ca. 170 m. The Naha Formation is divisible into four units; each unit is composed of coral limestone overlain by rhodolith, Cycloclypeus—Operculina, and detrital limestones, showing a deepening-upward sequence. The contact between the upper two units (units 3 and 4) is unconformable; this unconformity is present in other areas in Okinawa-jima, such as Yomitan, the Motobu Peninsula, and Ie-jima. Calcareous nannofossil assemblages indicate that deposition of the Naha Formation began at 1.392—1.706 Ma, and continued until after 0.853 Ma. The Itoman Formation and the lowest interval of the Naha Formation are coeval, and they represent, respectively, reef and off-reef facies of reef-complex deposits. The Minatogawa Formation rests unconformably on the Naha Formation, reaches 20 m in thickness, and consists of well-sorted detrital and coral limestones that are thought to have been deposited in a shallow lagoon (moat); its surface exposure is confined to elevations of less than 50 m. The geologic age of the Minatogawa Formation is unknown at this time.