Fluctuations in the East Asian monsoon over the last 144 ka in the northwest Pacific based on a high-resolution pollen analysis of IMAGES core MD01-2421
Introduction
The natural environment in Japan is strongly affected by marine currents and the East Asian monsoon due to its location at the eastern edge of the Asian continent, in the middle latitudes of the northwest Pacific. The Kuroshio Warm Current flows northeastward, while the Oyashio Cold Current flows southwestward along the southeastern coast of Hokkaido (Fig. 1, Fig. 2). At present, these two currents meet off the eastern coast of central Japan and produce a mixed water mass between them (Fig. 1). The Tsushima Warm Current flows northeastward along the western coast of Japan in the Sea of Japan and partly flows out to the Pacific Ocean through the Tsugaru Strait between Hokkaido and Honshu, the main island of Japan.
In addition, the East Asian monsoon, which forms as a result of the thermal difference between the Asian continent and the Pacific Ocean, is a principal climatic factor in Japan. The East Asian summer monsoon affects much of Japan, producing heavy rainfall associated with warm, humid southwesterly winds during summer. The winter monsoon is a product of the difference between the Siberian high and Aleutian low in the north Pacific. When an extremely cold, dry air mass forms over Siberia and crosses the Sea of Japan, it receives heat and water vapor from the sea surface where the Tsushima Warm Current flows, creating heavy snowfall on the western coast of Japan during winter.
The present vegetation in Japan is distributed throughout different forest types from south to north: warm-temperate evergreen broad-leaved forest, cool-temperate deciduous broad-leaved forest, pan-mixed forest, and subarctic conifer forest (Fig. 1; Tatewaki, 1955; Yoshioka, 1973; Yamanaka, 1979). The distributions of these forests have shifted in latitude and elevation between glacial and interglacial periods in response to past climatic change (e.g., Tsukada, 1986). Two conifer tree species, Cryptomeria japonica (Linn. fil.) D. Don (Japanese cedar) and Sciadopitys verticillata Sieb. et Zucc. (Japanese umbrella-pine) characteristically grow in Japan, in high-precipitation areas influenced by the summer and winter monsoons (Hayashi, 1960). Cryptomeria is a native tree of the family Taxodiaceae that occurs naturally only in southern China and Japan. The main distribution area in Japan is 30–41°N along the western coast and 30–35.5°N along the eastern coast (Fig. 2). Cryptomeria is broadly distributed along the western coast because of the heavy snowfalls resulting from the East Asian winter monsoon. In contrast, the distribution of Cryptomeria on the Pacific side depends on the heavy rainfall accompanying the East Asian summer monsoon. It is adapted to temperatures between −8.2 °C (mean minimum temperature) and 31.6 °C (mean maximum temperature), which occur from the subalpine to the warm-temperate zone. Annual precipitation in the main distribution area of Cryptomeria exceeds 2000 mm (Tsukada, 1981, Tsukada, 1986).
Sciadopitys, an endemic Japanese evergreen conifer, is distributed mainly in the mountainous area of western Japan between 32°N and 36°N along the Pacific coast (Katoh, 1948). It grows on well-drained soils and is adapted to temperatures between −7.1 °C (mean minimum temperature) and 28 °C (mean maximum temperature). Sciadopitys grows in areas receiving high precipitation during the summer monsoon, where the total annual precipitation is 1500–3000 mm (Fig. 2; Katoh, 1948). Accordingly, fluctuations in the East Asian summer monsoon during the past can be reconstructed by tracing the pollen frequencies of Cryptomeria and Sciadopitys.
Studies of the East Asian paleomonsoon began with investigations of loess-paleosol sequences in central China (An et al., 1990a, An et al., 1991a, An et al., 1991b). Since then, studies have examined monsoon records starting from the late Pleistocene in China, Japan, and the northwest Pacific (e.g., Kukla et al., 1988; Inoue and Naruse, 1991; Porter and An, 1995; Sun et al., 1996; Vandenberghe et al., 1997; Zhang et al., 1997; An and Thompson, 1998). High-resolution analysis of pollen and radiolarians in marine core RC14-99 (Fig. 2) from the northwest Pacific has provided a correlation between the summer monsoon and summer insolation through three glacial/interglacial cycles (Heusser and Morley, 1997). Most interglacial core samples have contained high percentages (>20%) of Cryptomeria pollen, reflecting increasing precipitation in response to intensification of the summer monsoon and the interglacial sea-surface temperature (SST). Maxima in Cryptomeria presence have systematically lagged the maxima in solar insolation at 30°N by several thousand years. This correlation between the summer monsoon and summer insolation corroborates the link between the strength of the Asian monsoon and variation in Northern Hemisphere solar radiation (Heusser, 1989a, Heusser, 1990; Heusser and Morley, 1990, Heusser and Morley, 1997; Heusser et al., 1992; Morley and Heusser, 1997).
The history of Cryptomeria since the last interglacial age has been revealed by several palynological studies of terrestrial sediments in Japan (e.g., Tsuji, 1980; Tsuji and Minaki, 1982; Tsuji et al., 1984; Miyoshi, 1989; Hibino et al., 1991; Miyoshi et al., 1991; Takeuti and Manabe, 1993; Oshima et al., 1997; Takahara and Kitagawa, 2000). However, those terrestrial sediment records were not continuous, and their chronologies before 44 ka, based on fission track ages of a few widespread tephras and paleomagnetic stratigraphy, were not always accurate.
Marine core MD01-2421 obtained from the northwest Pacific off central Japan, for which a detailed chronology has been determined, consists of a continuous sediment record covering the last 144 ka (Oba et al., in press). At present, the land near the coring site is not within the main distribution area of Cryptomeria, and Sciadopitys is absent, due to the low amount of summer precipitation. In this study, we reconstructed a detailed history of East Asian monsoon fluctuations over central Japan during the past 144 ka using the pollen frequencies of Cryptomeria and Sciadopitys and the vegetation history of the area.
Section snippets
IMAGES core MD01-2421
During the IMAGES (International Marine Global Change Study) VII-WEPAMA (Western Pacific Margin) Leg 2 cruise in 2001, piston core MD01-2421 (45.82 m long) was recovered by the French R/V Marion Dufresne (Fig. 1). The site is characterized by a relatively flat seafloor (36°01.4′N, 141°46.8′E; 2224 m water depth; ca 100 km offshore of central Japan) on a slightly convex submarine plateau on the continental slope. The core consisted of homogenous olive-gray silty clay containing many calcareous and
Pollen transportation and sedimentation
How do pollen and spores become sediments deposited on the sea bottom? Sea currents can transport pollen 3000–5000 km from land (Kawahata and Oshima, 2002). Heusser and Morley (1985) demonstrated that pollen assemblages from marine surface sediments off Japan generally correspond to vegetation patterns in adjacent coastal regions, although marine and air currents do carry minor amounts of pollen from other regions. In addition, an investigation of pollen from surface sediments of the continental
Oscillation of Tp
The distribution of plants depends on an annual accumulated temperature of more than 5 °C/month during the growing season of plants (Warmth Index; Kira, 1949). Warm-temperate evergreen broad-leaved forests and cool-temperate broad-leaved forests are distributed in the regions where the Warmth Indexes are between 85 and 180 °C/month, and 45 and 85 °C/month, respectively. To reconstruct thermal fluctuations, the pollen temperature index (Tp) was calculated using the formula []
Dynamics of Cryptomeria and Sciadopitys
The northern Kanto region (Fig. 2) is situated near the northern distribution limit of warm-temperate evergreen broad-leaved forests (Fig. 1; Miyawaki, 1977). In the region north of 36°N along the Pacific coast, Cryptomeria is not abundant and Sciadopitys does not grow at present (Fig. 2) because of the small amount of summer precipitation (ca 1500 mm; Wadachi, 1958). The frequency of Cryptomeria pollen in recent terrestrial sediments from 36°N along the Pacific coast is around 10% (Tsuji and
Vegetation and climate
The vegetation, inferred from pollen assemblages and climate based on Tp values and the frequencies of Cryptomeria and Sciadopitys during the past 144 ka was reconstructed as follows (Table 2 and Fig. 5).
A cool-temperate broad-leaved forest developed during MIS 55 and between late-MIS 2 and mid-MIS 1 under warm climate conditions. A mixed forest composed of cool- and warm-temperate trees coexisting with Cryptomeria was distributed during late-MIS 1 under warm/humid climatic conditions. During
Correlation with terrestrial pollen data and data from marine core RC14-99
We compared the Cryptomeria pollen data obtained from terrestrial sediments in central Japan for the period since MIS 6 with the pollen data from MD01-2421. The chronostratigraphy of terrestrial sediments (including sediment cores from lake bottoms) older than 44 ka was determined using fission track ages of one–three layers of widespread tephras. Accordingly, there were differences in the accuracy of the chronology among the terrestrial sites.
In Lake Suwa (Loc. 1 in Fig. 2; 36°02′N, 138°06′E),
Relationships among pollen data, insolation, and SST in the northwest pacific
Fig. 5 shows the down-core Tp values, summer insolation curves at 36°N and 65°N, the SST curve calculated from the oxygen isotope difference between benthic and planktonic foraminifera (Oba and Murayama, 2004; Oba et al., in press), and the relative abundances of Cryptomeria and Sciadopitys in core MD01-2421. Except after 8 ka, the Tp increased during the periods of high summer insolation, which had maxima around 127, 105, 83, and 11 ka and decreased during the low-insolation peaks around 138,
Conclusions
Periods of high Tp occurred during 129–119, 115–100, 82–76, 53–49 ka, and from 15 ka to the present. During these periods, cool-temperate deciduous broad-leaved forests developed, whereas subalpine coniferous forests grew during periods of low Tp. The fluctuations in Tp and the summer insolation strength at 36°N were roughly in phase, except after 8 ka. High-insolation periods around 127, 105, 83, and 11 ka were roughly correlated with high Tp periods, and low-insolation periods around 138, 117,
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