A detailed reconstruction of paleoclimate over the past 160 ka has been achieved with a 20-40 year interval, based on the total organic carbon (TOC) and total nitrogen (TN) contents of lake sediments from Lake Nojiri and the Takano Formation, central Japan. The validity of the reconstruction is supported by an analysis of pollen grains from the same sediments. The result represents one of the most detailed and continuous reconstructions of paleoclimate for a land area in Japan during the late Quaternary. The reconstruction provides a key to our understanding of late Quaternary climate in Japan, because the studied sediments contain many widespread tephra marker layers, thereby enabling the correlation of the reconstructed paleoclimate with other areas in and around the Japanese Islands.
The TOC content of lake sediment is controlled mainly by the biological productivity of lake water (aquatic organic matter) and the input rate of organic matter from land areas (terrestrial organic matter), as well as the degree of decomposition after deposition. Biological productivity appears to be the main controlling factor in lakes within mid-latitude Japan that contain silt- and clay-rich sediments. Short- and long-term fluctuations in TOC are well correlated with the marine d18O pattern of SPECMAP and with d18O profiles from ice cores in Greenland. The validity of the paleoclimate reconstructed based on TOC is supported by the results of pollen analysis, although the pollen data have relatively low temporal resolution. Therefore, in our reconstruction we can identify marine isotope stages (MIS) 1 to 6 and many of the interstadial events recognized in Greenland ice cores.
A detailed comparison of the obtained TOC curve with the SPECMAP curve reveals a slight lag in the TOC peaks, which are typically recognized in the MIS 5b and 5d stages. This lag may reflect local factors specific to the Japanese Islands.

We reviewed the results of previous studies on paleoclimate and paleoenvironment in and around the Japanese Islands during the Last Interglacial period. Sediment cores recovered from deep-sea settings around the Japanese Islands provide valuable information in understanding the ocean paleoenvironment and age framework since marine isotope stage 6. Analyses of pollen, diatoms, and total organic carbon in lake sediments have contributed to clarifying the land climate in detail, in combination with age constraints provided by tephra marker beds. Great progress has been made in tephra stratigraphy and the identification of tephra marker beds, which provide the key to correlating land and marine data, as shown in the case studies on Lake Biwa and the Takano Formation. Because several widespread tephra marker beds are 90 to 100 ka in age, it is possible to reconstruct the paleoclimate of this period at high resolution, both temporally and spatially, integrating local and regional environmental information for both land and marine settings. Such results may be important in the near future for local and global climate forecasting.

This study presents a new perspective of North Pacific paleoceanography over the last two glacial cycles, based on assessments of U37K’-derived paleotemperature records from IMAGES Core MD01-2421 off central Japan and Ocean Drilling Program (ODP) Sites 1014 and 1016 off California. The temperature differences between ODP Sites 1014 and 1016 (?SSTNEP=SSTODP1014-SSTODP1016) reflect the intensity of the California Current. Comparison of ?SSTNEP and SST derived from Core MD01-2421 reveals anti-phase variations: high ?SSTNEP (weakening of the California Current) corresponds to low SST at the Japan margin (southward displacement of the NW Pacific subarctic boundary and weakening of the Kuroshio Extension), and vice versa. This finding suggests that the intensity of the North Pacific subtropical gyre varies in response to precessional forcing, and that the response is linked to temporal changes in tropical ocean-atmosphere interactions.

(1) In the field of paleoceanography, there exists ongoing discussion regarding how to investigate past changes in ocean systems. Micropaleontology has contributed to the field of geology via investigations of stratigraphy, paleoenvironments, and paleoclimate, and biology-based investigations of microfossils have led to the establishment of the field of evolutionary micropaleontology. (2) Based on recent investigations, we discuss the timing of the beginning of the Quaternary, the timing of the initiation of 100-kyr cyclicity of glacial-interglacial periods and their cause, the timing of rapid warming that occurs when a glacial period changes to an interglacial (i.e., termination), and the timing of Termination II at the beginning of the last interglacial. (3) Cyclic environmental change over the past 150,000 years is inferred from the diatom assemblages in six piston cores from the Tohoku Area and two piston cores from the Japan Sea. The periodic changes recognized in the Tohoku Area are similar to cyclic changes indicated by various proxies in the Northern Hemisphere.

Core MD01-2421, located off Kashima, in the Northwest Pacific, contains a continuous record of deposits over the past 144 ky, as dated by the AMS 14C method, tephrochronology, and the oxygen isotope stratigraphy of benthic foraminifera. Temporal changes in Tp (pollen temperature index) values reveal two warm periods (MIS 5.5 and late MIS 2 to MIS 1), three cool periods (mid-MIS 5.4 to the first half of MIS 5.3, the latter half of MIS 5.1, and early MIS 3), and two cold periods (MIS 6 and all of MIS 2 except for the latter part). Cool temperate deciduous broad-leaved forests (mainly Quercus subg. Lepidobalanus, Fagus, Quercus subg. Cyclobalanopsis, and Juglans/Pterocarya) developed during the warm periods, whereas the cold periods were dominated by subarctic coniferous forests of Picea, Tsuga, and Larix. Following each of the warm and cool periods, Cryptomeria japonica and Sciadopitys verticillata flourished under wet climatic conditions that arose due to strengthening of the East Asian Summer Monsoon. The climate cooled after each of the maximum monsoon periods, resulting in an increase in subarctic conifers. In the southern Kanto district, near the MD01-2421 site, the vegetation history since MIS 5.5 has been divided into four periods (A-D) based on pollen assemblages and plant macrofossils, with age control provided by several tephra layers. The data obtained from the marine and land sites are well correlated, showing similar patterns of vegetation change and comparable age data. Period A is correlated with MIS 5.5 (130-117.3 ka), B with MIS 5.4-5.1 and 4 (117.3-59 ka), C with MIS 3 and 2 (59-12 ka), and D with MIS 1 (12 ka to the present). In comparing pollen data since MIS 5.5 with eight other areas in Japan, data from Lake Biwa, Lake Suwa, and Okute Basin in central Honshu Island correspond well to those from core MD01-2421.

The reconstruction of past ocean environments by the analysis of microfossils from seafloor sediments enables us to forecast the environment in the near future. The species of plankton that occur as microfossils in sediments deposited 20,000 years ago can be found in the present ocean. Consequently, an understanding of the present ocean environment is the key to reconstructing the past oceanic environment.
This study analyzed microfossil communities and calculated the radiolarian index (Sr) by mapping the water mass in the eastern area off Honshu, based on the present ocean environment. Three cores from northern sites, directly affected by the Oyashio, show a minimum seawater temperature at 18 ka. One of the southern sites records the inflow of a water mass, including the Oyashio, at 17.1 ka. At 9.0 ka, the water mass including the Oyashio was located in its present location in the eastern area off Tohoku. During the Jomon transgression (from 8.3 to 6.3 ka), the Kuroshio-Oyashio intermediate area was expanded slightly northward compared with the present location, and from 0.3 to 0.2 ka it expanded southward to the eastern area off Boso peninsula. This article also discusses the interpretation of the structures of plankton species and the current axis of the Kuroshio.