古海面を記録する地形・堆積物の高度・年代資料から,完新世における海面の推移を考察するために,琉球列島の喜界島で調査を行なった.本島の完新世段丘はI～IVの4段丘面に細分され,4回の相対的な海面停滞期があったことを示す.I面は,その一部のみが,またII面以下はすべてがサンゴ礁からなる.とくにII面は幅が広く,典型的な裾礁として形成された.各段丘面を構成するサンゴの14C年代と旧汀線高度は,I面で6,000～6,800 y. B. P., 9～13m,II面で3,500～5,200 y. B. P., 5～7m,III面で3,000～3,500 y. B. P., 2.5～5m,IV面で1,500～2,500 y. B. P., 1.5～2mである.これらは,新しいものほど低位置の海岸側にあり,サンゴ礁が水平方向に拡張してきたことを示す.1.5m/1,000年という等速な隆起を仮定して相対的海面変化曲線からユースタティックな海面変化曲線をえがくと, 6,500 y. B. P. と4,000 y. B. P.を中心とする2回の高海面停滞期と, 5,500 y. B. P.と2,000 y. B. P.を中心とする2回の海面低下期が現われる.両期の間の海面変化の量はせいぜい±2～3m程度と思われる.2回目の高海面停滞期は長期にわたり,II面の広いサンゴ礁の形成をうながした.サンゴの成長が,いわゆるクライマティックオプティマム期よりおくれて5,000 y. B. P.ごろからさかんになったことは,酸素同位体比の分析結果による融氷状況とも一致している.
Dent numerous Holocene sea-level curves have hitherto been published for various parts of the world. In this paper, about twenty such curves were selected and discussed. It is considered that the following two opinions discussed by Shepard (1964), Guilcher (1969), Fairbridge (1971) and others are the subject of the greatest controversy, although there are several clear disagreements among these curves as shown in Figs. 1_??_4. 1. When had the eustatic sea-level reached at the highest level? 2. Has the sea-level risen with minor fluctuations or without? Based on these two different standpoints, the curves can be classified into the following three types: (a) that in which the eustatic sea-level had risen above the present level with fluctuation (provisional name: high sea-level with fluctuation gro up, e.g. Fairbridge, 1961; Mörner, 1969; etc.); (b) that in which the present is the highest level and the sea-level has fluctuated (low sea-level with fluctuation group, e.g. Ters, 1973; Greensmith & Tucker, 1973; etc.); and (c) that which recognizes neither a high sea-level nor fluctuations (smooth group, e.g. Shepard, 1964; Scholl et al., 1969; etc.) (a) and (c) correspond to the curves known as the “Fairbridge Curve” and “Shepard Curve” respectively. Concerning about the standpoint 1, since Daly (1920) recognized previous shorelines a few feet above the present sea-level for various parts of the world and claimed that such shorelines were the result of ecstasy, many scientists have recognized this so-called “Daly's level”. However, various scientists, mainly those in the United States and Europe, have opposed this idea using the data obtained from North America and Europe, and have claimed that the present sea-level represents the highest level in the Holocene. The controversy on high sea-level during the Holocene period thus constitutes the most interesting problem. Compiling the data on Holocene sea-level changes from areas which were considered locally stable, the author mapped areas where the present sea-level was reported as the highest level and those where the sea level was considered to have risen above its present level in the Holocene respectively (Fig. 5). As a result, regionality with continental scale were recognized: in North America and part of Europe, the present sea-level is the highest in the Holocene, whereas in South America, Africa, Asia, and Australia, a high sea-level is generally recognized. Such differences have been also recognized by Gill (1971), etc., who claimed that this problem should be discussed on a continental scale. However, no concrete explanation has yet been put forward. It is interesting to note that such differences over wide areas have been well explained on the basis of broad crustal movements effected by glacio- and hydro-isostasy as recently stated by Walcott (1972). As shown in Fig. 5, most part of North America and Europe, where the present sea-level is the highest, represents the peripheral zone of the former ice sheets and have submerged by glacio-isostasy, whereas the regions where the sea-level has risen above its present level in the Holocene out have been uplifted by hydro-isostasy. If this explanation of the problem relating to high sea-level by glacio-and hydro-isostasy is justified, it is thus not significant to discuss the problem further from the eustatic point of view. As for the standpoint 2, judging from the data in Table 1, it appears that the difference between a fluctuating curve and a smooth curve does not result from local conditions in the area (e.g. landform, local crustal movements, tidal range, etc.) or the methods employed for determination of the altitude and age of the past sea-level. It might be resulted from the fact that the indivisual researcher's appraisal of errors contained in the altitude and age of past sea-levels has played an important role.
In recent years, there have developed some studies on sea-level changes during the Late Holocene in Japan. When reviewed in detail, however, the results of those studies are found consisting of two cases; one case for a curve-drawing in favor of the Fairbridge curve, and the other for that in favor of the Shepard curve by making different assumptions about the process of crustal movements. This is also true even when the same area is chosen as an objective of those studies. This shows that in Japan where crustal movements have been great, studies about crustal movements themselves must be important in order to investigate ecstatic sea-level changes. Nevertheless, there are in them some common points concerning the height of the past sea-level changes. One is that the sea-level changs at the last glaciation maximum (about 18, 000 B. P.) are thought to have ben about 140 meters below the present sea-level, judging from the depth of the bottom in some buried valleys which can be found throughout Japan. Another is that, judging from the depth of th bottom in marine silt and clay beds that were accumulated in time of the Flandrian transgression, the sea level at the earliest period of the Holocene is supposed to have been about 40 meters below the present sea level. As studis about the recent alluvial formation proceed, there have lately emerged some researchers who insist on sea-level changes in favor of the Shepard curve, while most of the researchers in Japan have reported that the sea level was higher around 6, 000 B. P. than at the present. Although some studies have reported that the sea level fell temporarily and rose again later in some areas, there would be little possibility that the fluctuation was caused by glacial eustasy. It is because it would be almost impossible to assume that the expansion of an ice sheet made the sea level lower at a rate of over 0.1 meter per century. Apart from such fluctuations, traces can be found throughout Japan which indicate that the sea level was two to three meters lower around 2, 000 B. P. than now. It would seem to me that future studies in Japan about the sea level, first of all, should consist of finding out regional characteristics of curves for sea-level changes on the continental scale, by eliminating as much influence as possible caused by crustal movements or orogenic movements limited to a comparatively narrow area. It would not be possible, I assume, to investigate eustatic and absolute sea-level changes until we are through with those problems discussed above.