第 四 紀 研 究 (The Quaternary Research) 30 (2) p. 187-196 July 1991 Holocene Sea-Level Changes and Coastal Evolution in Japan1) Masatomo UMITSU2) Recent progress in Holocene sea-level studies and studies on coastal evolution in Japan are reviewed. Several studies recorded either a slight fall or slow rise of sea-level in the early Holocene, and some studies recognized minor regressions after the culmination of rapid postglacial transgression. Coastal landforms have changed remarkably during the Holocene. Many drowned valleys were formed in the middle Holocene, and the coast lines in Japan were very rugged at the time. Various types of coastal evolution have been reported in numerous studies. Some of the studies were carried out as cooperative research using a variety of research techniques. published by OTA et al. (1982, 1990), YONEKURA and I. Introduction OTA (1986), OTA and MACHIDA (1987) and ISEKI The Japanese Islands are located along the (1987). Recent studies on sea-level changes in boundaries of the Eurasian, Pacific Ocean and Japan were compiled in the "Atlas of Holocene Sea Philippine Sea Plates, and the landforms of the Level Records in Japan" (OTAet al., 1981) and the islands have been strongly influenced by the "Atlas of Late Quaternary Sea Level Records in Japan, plates movements. Coastal landforms of Japan vol. I" (OTA et al., 1987a). The coastal during the late Quaternary have also changed environments in the Late Quaternary and the and developed under the influence of both Holocene were illustrated in the "Quaternary tectonic and eustatic movements. Regional Maps of Japan" (JAPAN ASSOCIATION FOR QUATERNARY differences and variations can be found in the RESEARCH ed., 1987) and the "Middle Holocene processes of evolution of the coastal landforms, Shoreline Map of Japan" (OTA et al., 1987b). An "Inventory of Quanternary Shorelines" was also and relative sea-level curves have been clarified in various locations of Japan. complied by YOSHIKAWA (1987). Many studies of sea-level changes, coastal In this article, sea-level changes and coastal landform evolution, and coastal environments evolution in Japan during the Holocene are during the Holocene have been made, and reviewed according to the recent studies and various sea-level curves have been produced. publications. Some of these studies were based on strati- II. Holocene sea-level changes graphic and microfossil analyses and 14C dating of the sediments. In some cases, uncontamina- The sea-level has risen rapidly with some ted soil samples collected with peat corers were oscillations, from the lowest sea-stand in the last used for the studies. glacial maximum towards the present level. Symposia were held on the Late Quaternary This postglacial transgression in Japan is called sea-level changes and special issues were the "Jomon transgression", being named after published by the ASSOCIATION OF JAPANESEthe prehistoric Jomon culture of Japan. The GEOGRAPHERS (1975) and the JAPAN ASSOCIATION postglacialFOR transgression is considered to have QUATERNARY RESEARCH (1982). started c. 20,000-18,000BP, and several oscilla- Recently, several review papers on Holocene tions during the transgression have been re- coastal evolution and sea-level changes were ported. One of the major sea-level drops was in 1) Received 19 February 1991. Accepted 24 May 1991. 2) Dept. of Geography, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01. 188 The Quaternary Research Vol. 30 No. 3 July 1991 the period of 11,000BP to 10,000BP. That sea- in contrast, other stable or subsiding regions had level drops can be seen in a common occurrence, it at or after c.5,000BP. and the coarse sediments of this period called The southern Boso Peninsula in eastern Japan "HBG (Holocene Basal Gravel Bed)" can often be is one of the most notable uplift areas in Japan, seen in the middle horizon of the "Chuseki-so" and the maximum sea-level in the Jomon trans- formation which has been deposited since the gression was 23-26m above sea level (c.6,700- last glacial maximum, in and on the buried 6,200BP) (NAKATA et al., 1979, 1980; FRYDLE, 1982). valleys under the coastal lowlands. This tem- The sea-level at the maximum of the trans- porary sea-level regression is considered to cor- gression, in the coastal areas of Sagami Bay, east respond with the cold climate episode in the of the Izu Peninsula, along the extension of the latest stage of the last glacial epoch. Sagami Trough, was reported as 21-22m The Holocene sea-level rise started after this (6,300-6,200BP) by ENDO et al. (1982) and temporary regression, from a sea-level estimated MATSUSHIMA (1982). The highest shoreline c.-40m. During the early stage of the during the Holocene along Kagoshima Bay in Holocene, between c.10,000 and 8,000BP, the southern Kyushu was reported as 15m sea-level rise seems to have been slow and (6,300-6,000BP) by MORIWAKI et al. (1986), and accompanied by a slight sea-level drop in some that of the Kikai Island was reported as c.10m places. ENDOet al. (1982) recognized a slight sea- (6,500-6,000BP) and 9-13m (6,800- level drop around c.8,300-8,000BP in the 6,000BP) by NAKATAet al. (1978) and OTA Paleo-Oshikiri and Paleo-Isumi Bays in the et al. (1978), respectively. Kanto District, eastern Japan. A slow sea-level Most of the highest sea-levels during the rise during the early Holocene is also reported for Holocene in relatively stable or subsiding regions the Oku-Tokyo Bay and the Tama River delta, were between 0-5m a.s.l. In the Okhotsk around Tokyo Bay (ENDOet al., 1989; MATSUSHIMA coast of Hokkaido, northern Japan, the ed., 1987). MAEDA (1980), UMITSU (1979) and ISEKI maximum sea-level was 3-4m a.s.l. at et al. (1982) also reported temporary regressions 6,000-5,500BP (MAEDA, 1984; SAKAGUCHI et al., or slow sea-level rise in the early stage of the 1985; HIRAI,1987). Similar heights and ages for Holocene in both the Osaka Bay coast area and the maxima of postglacial transgressions were the Nobi Plain, of western and central Japan reported for other regions such as, the Tsugaru respectively. The temporary regressions or Plain in northern Japan (UMITSU, 1976), the Tama slow sea-level rises of the early Holocene, River delta in eastern Japan (MATSUSHIMA, 1987), however, have not been well clarified in other the Toyama Bay region on the Japan Sea coast of areas of Japan, and whether the phenomena took central Japan (FUJI and FUJII, 1982), Osaka Bay place as general or as local occurrences has not (MAEDA,1980), the Harimanada coast of the been proven yet. Inland Sea (NARUSE et al., 1984, 1985) and the From the early to middle Holocene, during San'in coast of western Japan (TOYOSHIMA, 1978). c.8,000-6,500BP, the rate of sea-level rise was There are some places where sea-levels during very rapid, attaining 1.5-2.0cm/yr. The the Holocene did not reach their maxima lower part of coastal lowlands and valley plains between c.6,500-5,500BP. Some of them had in this period were drowned, and many drowned maximum sea-level from the middle to late valleys were formed along the coasts of Japan. Holocene or in the late Holocene, including the Most of the relative sea-level curves in Japan present time. show their maxima during the period of In the southern part of the Izu Peninsula, c.6,500-5,000BP. OTA et al. (1982) pointed out which is situated at the northern tip of the that the age of the maximum level of the Jomon Philippine Sea Plate, the highest sea-level was transgression exhibited regional variations. recorded in the period of 3,000-2,000BP with a Some regions which are characterized by a high height of 2-3m a.s.l (OTAet al., 1986). OTAet al. rate of uplift had a maximum at c.6,500BP, and (1986) mentioned that the study area subsided 1991年7月 第 四 紀 研 究 第30巻 第3号 189 Fig. 1 Various types of relative sea-level curve in Japan from about 6,000BP until uplift began at shown minor oscilations. Minor sea-level drops 3,200-2,000BP, and they suggested that the after that period have been recognized, in emergence of the area may have been due to c.5,000-4,000BP and c.3,000-2,000BP. coseismic uplift. The Holocene sea-level The former is called the "Middle Jomon minor maximum in the Ryukyu Islands, southwestern regression" and the latter is called the "Yayoi Japan, also occurred in the period of regression". The "Middle Jomon minor re- 3,500-1,700BP and was less than 1.0m a.s.l. gression" was discovered in the Boso Peninsula (KOBA et al., 1982). (YOKOTA, 1978), the San'in coast in western Japan Sea-level curves determined for the Shonai (TOYOSHIMA, 1978) and Kikai Island of the Plain by ARIGA (1984), and the Sendai Plain by northern Ryukyus (OTA et al., 1978) and was OMOTO et al. (1978) show maxima in the period of named by OTA et al. (1982). The regression of c.2,000BP to the present. The sea-level curve that period has also been recognized in other for the Sendai Plain presented by OMOTOet al. places such as the Okhotsk coast in northern (1978) had a maximum at c.1,000BP. The Hokkaido (MAEDA, 1984; SAKAGUCHI et al., 1985; curve, however, was based on data from a single HIRAI, 1987), the Chita Peninsula in central Japan out crop in the plain, and MATSUMOTO (1981) has (MAEDA et al., 1983), the Osaka Bay coast (MAEDA, presented a different sea-level curve in the plain.