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A sediment layer (43 cm thick) and surface sediments (5 cm thick) in a submarine limestone cave (31 m water depth) on the fore-reef slope of Ie Island, off Okinawa mainland, Japan, were examined by visual, mineralogical and geochemical means. Oxygen isotope analysis was performed on the cavernicolous micro-bivalve Carditella iejimensis from both cored sediments and surface sediments, and the water temperature within the cave was recorded for nearly one year. These data show that: (1) water temperature within the cave is equal to that at 30 m deep in the open sea; (2) the biotic and non-biotic environments within the cave have persisted for the past 2000 years; (3) mud-size carbonate detritus is a major constituent of the submarine-cave deposit, and may have come mainly from the suspended carbonate mud produced on the emergent Holocene reef flat over the past two millennia; (4) the δ 18 O-derived temperature (Tδ18O )o fC. iejimensis suggests that the species grows between April and July; (5) the Tδ18O of C. iejimensis from cored sediments implies that there were two warmer intervals, at AD 340±40 and AD 1000±40, which correspond to the Roman Warm Period and Medieval Warm Period, respectively. These suggest that submarine-cave sediments provide unique information for Holocene reef development. In addition, oxygen isotope records of cavernicolous C. iejimensis are a useful tool to reconstruct century-scale climatic variability for the Okinawa Islands during the Holocene. © 2006 Published by Elsevier B.V.

Event-related sedimentary layers, which are deposited occasionally due to volcanic eruptions, earthquakes or heavy rains, are often contained in the rhythmical sequences of lacustrine and marine sediments. We have developed an analytical method for separating the sedimentary rhythms and the event layers identified using the scanning X-ray analytical microscope (SXAM) and obtained sequential profiles of seven elements Al, Si, K, Ca, Ti, Mn, and Fe in the lacustrine sediment from Lake Suigetsu, Japan. Two types of event layers could be detected from the elemental composition of 33 layers of sediment: three known volcanic ash layers and 30 clay layers containing 12 turbidites. The recurrence interval of the latter, which may potentially be initiated and archived by locally important earthquakes, is estimated to be an average of 640 ± 160 years by using Sompi event analysis (SEA) based on an autoregressive (AR) model. After removing those portions that represented event layers from the elemental profiles, we obtained event-removed (ER) temporal profiles based on the tephrochronology of the three volcanic ash layers. The ER temporal profiles of manganese and iron, probably representing the siderite content, showed a millennial-scale variation in the Holocene that corresponded well with ice-rafting events in the North Atlantic.

In Lake Suigetsu, central Japan, greenish/light-brown granules identified as cytoplasmic masses had been preserved in siliceous cell walls of freshwater diatoms in annual layers of lacustrine muds since the early Holocene. The lacustrine muds consisted of alternating dark-colored (rich in diatom valves, clay, and organic matter) and light-colored (mainly diatom valves) laminae. The greenish/light-brown granules were predominately preserved in frustules of the genus Aulacoseira preserved in the dark-colored laminae. The dark-colored laminae were inferred to have formed annually under stratified water caused by surface water warming in summer that caused the formation of an organic-rich anoxic layer on the lake bottom that favored granule preservation. The good preservation of cytoplasmic masses in dark-colored laminae suggested a cause for diatom assemblage periodicity, a phenomenon that was commonly noted in temperate lakes: the cells containing these masses could be potential seed stocks for subsequent spring blooms. Frustules of the most abundant granule-containing species, Aulacoseira nipponica (Skvortzow) Tuji, in the dark-colored laminae of the Early Holocene muds were abundant in the overlying light-colored laminae, suggesting that these species reproduced abundantly in springtime yielding a massive diatom bloom.

In order to construct past biome distributions on Kyushu Island, we compiled fossil pollen data that record vegetation changes during the past 20, 000 years. Age models estimated using 14C dates and/or Tephrochronology allowed the data digitized from published pollen diagrams to be compiled in 1000-year intervals from 20, 000 years ago to the present day.We applied a method for identifying biomes from Japanese modern pollen data (Gotanda et al., 2002) to the fossil pollen data set. We obtained 20 biome maps that show the spatial changes in biome distribution through time. During the Last Glacial Maximum, Kyushu Island was covered with cool-mixed forest, and therefore the climate was estimated to be about 7°C cooler than the present. After 17, 000 yr BP cool-mixed forest retreated and warm-mixed forest migrated northward in western Kyushu. In eastern Kyushu, warm-mixed forest had not yet become established, and temperate deciduous forest started grewing. The appearance of warm-mixed forest in eastern Kyushu was delayed until 7, 000 yr BP This late appearance suggests that the refugia of warm-mixed forest during the Last Glacial Maximum not only existed in Yaku Island and southern Kyushu, but also on the Koshiki Islands in western Kyushu. Warm-mixed forest spread over the Kyushu Island, and its maximum coverage occurred at 4, 000 yr BP, which is much later than the interpreted climatic optimum at 6, 000 yr BP. This time lag may reflect the fact that winter temperature at 6, 000 yr BP was too cold for warm-mixed forest to grow and only became warm enough by 4, 000 yr BP.

金沢大学人間社会研究域人間科学系

A continuous section of Pliocene marine sediments was recovered at Ocean Drilling Program Site 978, located in the Alboran Sea between Spain and Morocco. Three Pliocene subunits have been defined at Site 978: the lowermost (Subunit IC, 129.2 m thick) is characterized by alternating beds of lighter, more calcareous, and darker less calcareous, claystone with bioturbated upper and lower contacts (Type 1 cycles); the middle (Subunit 1B, 67.1 m thick) is composed of relatively homogeneous nannofossil claystone; and the uppermost (Subunit IA, 211.6 m thick) contains abrupt-based darker, terrigenous layers interpreted as turbidites that are interstratified with lighter nannofossil claystone (Type 2 cycles). The rhythmically bedded light and dark layers in Subunit IC correlate with those in the Rosello Composite Section of Sicily, a global reference standard for the Pliocene time scale. These sedimentary cycles are products of variations in precession and resulting continental runoff. Missing cycles occur during eustatic highstands. The shift to more homogeneous sedimentation in Subunit IB is represented in similar-aged sequences throughout the Mediterranean which display evidence of submarine mass wasting. Mediterranean-wide slope degradation was likely a response to rapid sea-level change at approximately 3 Ma. This change in sedimentation style was accompanied by an upsection increase in sediment accumulation rates associated with turbidite influx in Subunit IA. Turbidite frequency throughout the Pliocene section can be linked to eustatic changes in sea level, with turbidite maxima corresponding with mid-sequence downlap surfaces and their associated condensed sections. © 2004, SEPM (Society for Sedimentary Geology).

We made rock magnetic measurements of core sediments from a brackish-water lake on the Japan Sea coast, Lake Tougou-ike in Tottori Prefecture. The core samples mostly consist of non-glacial varved sediments and massive clay and silt, including volcanic ash layers. We measured low-field magnetic susceptibility, anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) of discrete samples at about 10 cm interval. These magnetic-concentration parameters show clear correlation with lithology of the sediments. In particular, sandy parts of the non-glacial varves are suggested to contain greater amounts of coarse-grained magnetite. Several horizons within the non-glacial varves are characterized with significantly high values of coercivity parameters, corresponding with intercalations of thin mud layers. Thermal demagnetization experiments of three component IRMs and measurements of hysteresis parameters indicate presence of hematite at these horizons. Factor analysis of the magnetic parameters combined with mineral magnetic data revealed a characteristic component contributed by the magnetic-concentration parameters and the feldspar content, which is interpreted as representing detrital input of volcaniclastic materials. It is suggested that coercivity parameters would be particularly useful for detecting detrital inputs and estimation of their origin.

High-resolution pollen analysis of annually laminated sediments from Lake Suigetsu in Japan revealed that climate amelioration at 16,500 varve years before present (vy BP) triggered the expansion of cool temperate deciduous broad-leaved forest. The oldest pottery appearance in Japan coincided with this climate amelioration. Significant vegetation change occurred from 15,000 to 14,500 vy BP. The period during 15,000–14,500 vy BP was a time of ecological transition from a Glacial-type ecosystem, eventually destroyed by global warming, to a new Postglacial-type ecosystem represented by that in Lake Suigetsu. It is believed that the completion of this ecological transition actually took nearly 500–800 years. This drastic change in the ecosystem had forced people to create a new strategy of adaptation. The Japanese Neolithic Jomon culture, mainly sustained by gathering nuts and fishing, was widely established at this time. On the other hand, the cold period which correlates with the Younger Dryas in Europe had no significant ecological influence on the Japanese ecosystem and human culture.

Continuous varve sequences were taken from Lake Fukami, a monomictic lake, southern part of Nagano Prefecture, central Japan in 2000. Total 196 varve units were recognized in the upper 2 m horizon of the sediment core. Sedimentation rates in the core have some high peaks, indicating increments of clastic particles supplied to the lake owing to natural disasters including floods and earthquakes, and the artificial construction around Lake Fukami. The results of the correlation between the meteorological records in Iida meteorological station and the total carbon flux change based on the varve chronology, indicates that the total carbon flux shows an inverse relationship to February mean temperature and precipitation. February mean temperature controlled the vertical mixing of lake water column that affects the productivity of phytoplankton in the epilimnion or decomposition of organic matter in the hypolimnion. February precipitation probably has affected the activity of denitrifying bacteria in the stagnation period. Pyrite and siderite minerals are recognized in the core by observation of thin sections and X-ray powder diffraction analysis. The pyrite was deposited in summer, and the siderite was deposited in summer and winter. These iron minerals indicate the decomposition processes of organic matters in the water column in each season.

Stephanodiscus populations were studied from a sediment core from Lake Suigetsu, central Japan. The cells were assignable to two modern freshwater species, S. suzukii and S. pseudosuzukii, described recently from Lake Biwa, which closely resemble S. carconensis and its varieties from Neogene lacustrine sediments in North America. Wide, continuous morphological variation, such as in the degree of central undulation, raised costae, number of central fultoportulae, and ultrastructure of the rimoportula internal opening, was observed in the populations, which consisted of convex, concave, and hemispherical valves. The diameters of convex and concave valves ranged from 5 to 45 μm, with a predominance of small ones measuring 7-12 μm; hemispherical valves, representing initial cells, were 25-50 μm in diameter and their morphological features were homogeneous. The size distributions indicate that the hemispherical valves are the initial valves of all the convex and concave vegetative valves which show continuous variation in size and morphology. All valves therefore belong to the same species and so we reduce S. pseudosuzukii to being a synonym of S. suzukii. We describe for the first time life-cycle-related changes in the morphology of the rimoportula internal openings: the stalked, fan-shaped rimoportula in initial or large valves changes progressively to smaller and finally scissile forms, indicating the need for caution during taxonomic interpretation of this character in other diatoms.

Eighteen metres of laminated sediments were recovered in a Holocene sediment core taken from Lake Tougou-ike in 1992. The lamination, evident from the alternation of light- and dark-coloured laminae, records cyclic deposition of biochemical sediments. Annual environmental changes in and around the lake caused seasonal variations in sediment flux to the lake floor, explaining these cyclic variations in sedimentary facies. AMS 14 C dating of organic remains from 40 horizons in the succession supports the interpretation of each light-to-dark couplet as an annual increment. Each laminated couplet is identified as a non-glacial varve, formed mainly by seasonal changes in diatom productivity. Varve counting and 14 C dating provide a detailed Holocene chronological framework, with time resolution on the order of individual years, within which to reconstruct paleoenvironmental changes recorded in the sediment core. Holocene relative sea-level changes at Lake Tougou-ike were estimated with ±10 yr precision using the variable authigenic iron mineral content of the sediment as a proxy. The results demonstrate that the relative sea level at Lake Tougou-ike rose abruptly at about 8900, 7700, 5600, 4000, 3600 and 2700 cal yr BP, and decreased gradually in the intervening intervals. These relative rises in sea level are demonstrated to be coeval with ice-sheet discharge events recorded in sediment cores from the North Atlantic Ocean. Abrupt coastal environmental changes caused by sea-level changes likely took place throughout the Japanese islands, and will probably be identified in both the Holocene and the last glacial interval, the latter being a time when Dansgaard-Oeschger cycles were clearly active in the North Atlantic region.

Time-series samples were collected by sediment trap at Nagayo-ura in Omura Bay, western Japan, from June 1998 to June 1999. Annual fluxes of diatom frustules in these trap samples, and in surface sediments collected beneath the sediment trap, were analyzed and compared with each other to investigate the ecology and taphonomy of marine planktonic diatoms. Calculated annual fluxes of marine planktonic diatoms from the sediment surface were much smaller than the annual sinking flux calculated from the trap samples. This investigation suggests that major loss of frustules occurs at the sediment–water interface. The preservation rate (annual flux from the surface sediment sample/annual flux from the trap sample) was estimated at 2.8% for total marine planktonic diatoms, and varied from less than 0.1% to 16.5% among specific taxa. These results indicate that the abundance and taxonomic composition of the fossilized flora are a biased reflection of the living flora from which they were derived. Therefore, taphonomic changes in diatom frustule accumulations should be considered when quantifying past diatom productivity or interpreting palaeoecological evidence from diatom records in sediments. Sediment-trap methods can greatly contribute to the development of palaeoenvironmental studies by addressing the ecology of present-day diatoms in the context of their sedimentary environments.

A sediment core, taken from Lake Fukami in southern Nagano Prefecture in the summer of 1999, contained alternating light- and dark-colored laminae. Diatom microstratigraphy was determined for the succession between 60 and 68 cm core depth. The frequency of Synedra spp. varied from more than 40% in the light-colored laminae to less than 10% in the dark laminae. Aulacoseira spp. or Cyclotella (?) spp. were dominant in the dark laminae, in which the proportion of Synedra spp. was relatively low. The correlation of these diatom assemblages with alternating lamina color indicates a seasonal succession within the diatom biota. Each couplet of light- and dark-colored laminae in the sediment is, therefore, considered to be an annual sedimentation increment.

最近,地球表層環境に強いインパクトを与えて,突然かつ急激なグローバルな気候変動を起こすトリガーについて多くの議論がなされている.とくに重要な発現地として,(1)北部大西洋,(2)南極大陸周辺,および(3)西部太平洋暖水域(West Pacific Warm Water Pool: WWP)があげられ,それらの地域における変動の先行および遅延について,新知見が得られつある.日本列島を含む東アジアの気候変動は,基本的に西部太平洋暖水域の高気圧からチベット高原の低気圧に流れ込む東アジア夏季モンスーンによって強い影響を受けている.水月湖の年縞堆積物における風成塵・炭素フラックスの検討によれば,グリーンランドを含む北部大西洋周辺に比べて,東アジアにおける晩氷期のベーリング期に相当する温暖化時期は1,000年程度先行していることが明らかになった.とくに,水月湖における夏季表層水温の上昇の始まりは18,000～16,000年前であり,それによる基礎生産量の増加も確認された.一方,南シナ海における海洋堆積物コアには,西部太平洋暖水域に起動された夏季モンスーン活動の活発化イベントが17,000年前から10,000年前にかけて9回も記録されていたが,この活発化イベントも諏訪湖湖底堆積物に降水量の増加イベントとして,ほぼ同時に記録されていた.すなわち,東アジアにおける更新世以降の気候変動のトリガーは,年縞が認められたり,高分解能な14C年代測定が行われた湖沼堆積物による解析から,WWPの水温変化の影響を受け,多量の湿潤大気を東アジア内陸部へ運搬した東アジア夏季モンスーンであったと考えられる.

SUMMARY Rock magnetic measurements of Chinese loess–palaeosol samples from the Beiyuan section in Linxia City suggest the presence of two magnetic components: a background component and an enhanced component. The magnetic properties of the enhanced component suggest strong grain-size control, which is in contrast with variable grain-size distribution in loess–palaeosol from the central Loess Plateau. Chemically formed magnetic grains do not fulfil the requirements in the case of the western Loess Plateau because they may show shifts in the grain-size distribution. The difference in climate between the western and central Loess Plateau may lead to different origins and different preservation conditions of the pedogenic magnetite.

Loess samples from the 22-m-thick Shajinping section on the second-lowest terrace of the Yellow River in Lanzhou, Gansu Province, China, were dated by infrared stimulated luminescence (IRSL) and by thermoluminescence (TL).Isothermal preheating tests for IRSL dating were made at two different temperatures, 120 and 160°C, to determine the preheating condition most appropriate for isolating the stable IRSL signal. The irradiated/non-irradiated IRSL ratio reaches a plateau after preheating for 24 to 144 hours at 120°C, or for 1.5 to 5 hours at 160°C. The IRSL signal with preheating at 120°C was enhanced by increasing the artificial gamma irradiation, but the IRSL signal with preheating at 160°C was saturated at a low dose, so it was difficult to use the dose-response curve to calculate an equivalent dose (DE). The 120°C preheating was determined to be most appropriate; therefore, all samples were preheated at 120°C for 96 hours before IRSL measurement.Except for one sample, the IRSL and TL dates for samples from the upper 12m of the section agreed with each other, within the error range, but the IRSL dates for samples from the lower 9m of the section were younger than the TL dates. This discrepancy between IRSL and TL dates might have been caused by underestimation of the IRSL dates, due to the samples' low saturation value of IRSL DEs at about 200Gy. The IRSL and TL dates indicate that the loess deposition rate during the last 60ka has changed several times. A very high loess deposition rate was obtained for the late marine isotope stage (MIS) 3, whereas the deposition rate seems to have been lower during MIS 2.

The 28-m high-resolution Shajinping loess section in Lanzhou on the Chinese western Loess Plateau records a detailed history of millennial Asian summer monsoon change since the earlier last glaciation. Summer monsoon proxies of soil magnetic susceptibility, carbonate content and soil color show that Asian summer monsoon experienced a series of rapid episodic pulse enhancements spanning only ca. 1–2 ka and have sub-Milankovitch cycles of progressive weakening in low frequency domain since ca. 60 kaBP. Soil formation responds to these by deepening colors, accumulating organic matter, increasing biological channels and leaching certain carbonates. The pedogenic degree generally increases with the degree of enhancement of summer monsoon. These may reflect the impact of the last glacial fast climatic change in the North Atlantic region on Asian summer monsoon.

The 28 m high-resolution Shajinping loess section in Lanzhou on the western Chinese Loess Plateau records a 60 ka, millennial summer monsoon variation. The record shows that Asian summer monsoons have rapid episodic pulse enhancements spanning only ca. 1–2 ka in high-frequency domain and having sub-Milankovitch cycles of progressive weakening in low frequency domain in the last glaciation. Soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements, resulting in weakly or moderately developed paleosol sequences. Both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm (Dansgaard–Oeschger) episodes and long-term cooling (Bond) cycles of the North Atlantic climatic records, indicating a possible teleconnection between tropic oceanic air masses and the North Atlantic climatic system. But differences exist for the transition of MIS 2/3 and the Holocene, where extraordinarily heavy dust-input events and fairly variable climatic fluctuations occur for the former and latter, respectively. A westerlies-swing model is proposed to interpret this link.

Annually laminated (varved) lacustrine sediments provide a potentially continuous, high-resolution records of the last glacial and Holocene paleoenvironments. In 1991-1993, long sequences of laminated lacustrine sediments were successfully taken from two lakes in southwestern Japan, Lake Suigetsu and Lake Tougouike. Based on lamina counting and AMS 14C dating, we clarified that these laminated sediments were varves. Clay mineral composition in these varved sediments would be reflected by eolian dust concentrations, transported from Chinese Loess Plateau, and by precipitations around both lakes. Also, formation of iron sulfides and carbonates in varves would be caused by cyclic changes of sea-water invasion, originated from sea-level changes. Annual to decadal oscillations of dust and iron mineral concentrations since the last glacial were detected in varved sequences of both lake sediments. These detailed sedimentological analysis of these sediments revealed varve chronology, process of varve formation and annual to decadal changes of sea-level and climate. Some of the climatic changes may correlate with abrupt changes (Younger Dryas and Heinrich events) observed in Greenland ice cores and marine sediments.

Recent atmospheric circulations over the Asian Continent have been influenced by the Himalayan Range and the Tibetan Plateau as a wind barrier. Based on recent meteorological data, the existence of the Himalaya-Tibet within the troposphere has caused meandering of the Westerlies, intense activity of the Asian Monsoon and appearance of the Easterlies. Meandering of the Westerlies has led to increase precipitation in the East Asia region including northeastern China and Japan, and has formed the soils with poorly crystalized illite minerals in the Chinese Loess Plateau and the Japanese Islands. In nothern India and Nepal including the frontal basin of the Himalayan Range such as the Kathmandu Basin, intense activity of the Asian Monsoon has led abruptly to increase precipitation named “onset of monsoon”. Also, appearance of the Easterlies from the Himalaya-Tibet to northern Africa within the upper part of the troposphere has led to dry and spread the arid area in the northern Africa.There are many sedimentary records indicating Late Quaternary climatic changes around the Himalayan-Tibetan Plateau. We expected to determine the formation age of recent atmospheric circulation. This age indicates when the Himalayan-Tibetan Plateau was abruptly uplifted and elevated. In other words, formation ages of more humid climate in northeastern China, the Japanese Islands and the Kathmandu Basin, and the driest climate in the northern Africa are useful to determine age of uplift and elevation of the Himalayan-Tibetan Plateau.On the base of above-mentioned evidences and hypothesis, we attempted to investigate magnetic susceptibility of the loess-paleosol sequence in the Luochuan area of th e northeastern China, and illite crystallinity of loess-paleosol sequence in the Kathmandu Basi n and deep-sea sediments of the Japan Sea and the Mediterrannean Sea. Because characteristic changes of magnetic susceptibility and illite crystallinity indicate dry -wet oscillations as useful detectors.As results, we clarified three evidences as follows:1) Large amplitudes of magnetic susceptibility oscillations by large pricipitation in interglacial periods appeared in the Luochuan loess -paleosol sequence during last 600ka.2) Large amplitudes of illite crystallinity oscillations by large precipitations caused decomposition of illite minerals appeared in the deep-sea sediments of the Sea of Japan during the last 800ka and in the loess-paleosol sequence of the Kathmandu Basi n during the last 1.1Ma. 3) Amplitudes of illite crystallinity oscillation during the last 800k a were smaller than those amplitudes since 800ka ago.Investigation mentioned above indicates that critical uplift and elevations of the Himalayan-Tibetan Plateau achieved to induce activity of westerlies, easterlies and the Asian Monsoon, and suggests that determined ages of critical elevation caused atmospheric circulation changes range from 0.6 million years to 1.1 million years ago.

Activities of Asian monsoon and westerlies during the last 2.4Ma were determined through the investigation of sedimentary features in the Chinese loess-paleosol sequences and the deep-sea sediments of the Sea of Japan. It was already known that sequential changes of magnetic susceptibility in the loess-paleosol sequences of the Chinese Loess Plateau were closely concerned with the glacial-interglacial cycles. Also, sequential changes of illite crystallinity, expressing as full width of half maximum of 10 A peaks by X-ray diffraction patterns in late Quaternary fine-grained sediments at Site 795A of Ocean Drilling Program in northeastern part of the Sea of Japan, were well correlated to standard records of oxygen isotope changes by SPECMAP. The samples of this research were derived from the Luochuan of northwestern China and from deep-sea sediments in ODP site 795A. This research is dealing with the changes of magnetic susceptibility and/or clay mineralogy of the loess-paleosol sequences during the last 2.4 Ma.The results are summarized as follows:1) Illite is one of the main detrital minerals in the loess-paleosol sequences, and illite concentrations in loess are higher than those concentrations found in paleosol. Therefore, illite crystallinity and concentrations are useful indicators to detect concentrations of eolian dust deriving from the Chinese Loess Plateau.2) During interglacial periods, wet atmospheres drifted from the Eastern China Sea into the inland area of the Asian Continent. This increase of moisture has promoted the hydration and decomposition of illite crystals in paleosols.3) According to the investigation of sedimentary features, it is obvious that there was little supply of eolian dust origin minerals transported from the arid area of western China to the Chinese Loess Plateau. This fact cannot always support the formation process of the loess-paleosol sequences, as it was interpreted by Kukla et al.(1988).4) The large amplitudes of Magnetic susceptibility in the loess-paleosol sequences during the last 0.6 Ma and there was a high degree of illite crystallinity in the deep-sea sediments during the last 0.8 Ma appeared. These facts suggest that changes of atmospheric circulations were caused by the uplift of the Himalayan/Tibetan Plateau about 0.6-0.8 Ma ago.

Bulk density and grain density profiles of the uppermost 90 cm portion of lacustrine sediment from Lake Suigetsu, one of the five major lakes of Lake Mikatagoko, located in central Japan, indicate the occurrence of at least 10 depositional events characterized by anomalously high density spikes. Two events are accompanied by turbidites. The dates of the sedimentary events estimated here are correlated with the occurrence of destructive earthquakes since 1100 A.D. Close examination of the sedimentological signatures recorded in the lake sediment and historical literature describing destructive earthquake phenomena provides a new opportunity to detect paleo-earthquake events in a high seismic area having a long history.

年縞(non-glacial varve)を用いた高精度年代測定法は,第四紀の年代決定法の中で最も有力な方法の一つである.本論文では,第1にスウェーデン年縞編年学(Swedish Varve Chronology)を例として取りあげ,年縞研究の現状と問題点をレビューした.その結果,1.最新年縞年代と現在との関係,2.年縞の計数方法および,3.年縞でない葉理の付加および年縞の消失が,編年に大きな影響を与えることが指摘できた.第2に,福井県三方五湖の水月湖における数本の堆積物コアから得られた,過去16,000年間にわたる年縞堆積物を用いた編年学について詳しく述べた.この年縞堆積物では歴史記録から年代をさかのぼる計数,および広域テフラや泥流堆積層による対比や計数の検証が行われつつあり,Younger Dryas期や最終氷期極相期の編年が数年単位で明らかにできる可能性を指摘した.第3に,集水域が狭く,周辺からの粗粒砕屑物が流入しない水月湖の細粒堆積物中の鉄鉱物や粘土鉱物組成の変動が,大陸起源の風成塵変動,日本海の海水準変動および若狭湾周辺の降水量変動を反映しており,年縞計数の結果から過去8,830年間の変動を1年～数年単位で明らかにした.

Many spike-like anomalies of grain densities and clay mineral concentrations in silty sediments have been recognized in the uppermost part (60cm in length) of a piston core, named SG2, in Lake Suigetsu of Mikata area, Fukui Prefecture, central Japan. Grain density spikes indicate large precipitations of iron sulfides and carbonates from interstitial waters in the upper part of muddy turbidites, which were suddenly caused by earthquakes. Spikes of Fe- and Mgrich chlorite/illite ratio indicate large accumulations of detrital clay particles which flowed into lake by sudden flood.After the Urami Canal was constructed between Lake Suigetsu and Lake Kukushi, which connects both lakes directly to the Sea of Japan, in 1664 A. D., sea water began to enter Lake Suigetsu and the lake changed its water conditions from fresh-to brackish-water. Iron mineral concentrations changed from siderite (FeCO3) to pyrite (FeS2) which was formed under sulfate reduction condition, in 35cm depth of a core column. Core column from 29cm to 35cm in depth shows a silty layer. Macroscopic observations and historical informations presumed a silty layer to be a muddy turbidite which was probably formed by Kanbun Earthquake (June 6, 1662 A. D.). This interpretation suggests that 29cm depth of core column represents deposition surface in 1662 A. D.Based on relative spacing between 0cm (=1991 A. D.) and 29cm (=1662 A. D.) depths, formation ages of grain density and chlorite/illite ratio spikes can be dated. Grain density spikes are interpreted to represent historical earthquake events of 1449, 1532, 1586, 1605 (Keicho Earthquake), 1662, 1683, 1707 (Houei Earthquake), 1819, 1854 (Ansei Earthquake), 1891 (Nou-bi Earthquake), 1909 (Gonou Earthquake) and 1963 (Echizenmisakioki Earthquake) of the Wakasa region. Chlorite/Mite ratio spikes are interpreted to represent historical flood events of 1633, 1691, 1701, 1729, 1735, 1786, 1791, 1825, 1842, 1853, 1866, 1895, 1953 (Typhoon No.13), 1959 (Isewan Typhoon), 1965 (Typhoon No.23 and No.24) in the Wakasa region. Five spikes cannot be correlated to historical earthquakes and floods. These events that represent depositions in 1646, 1799, 1848, 1931 and 1936 are probably correlated with historical engineering works of 1642, 1801, 1848, 1932 and 1934-35 around Lake Suigetsu.All events suggest that non-bioturbated fine-graind sediments of meromictic lake near human communities recorded any earthquakes, floods and human activities over a long periods of which no historical literature exists.

A periodic variation on a centennial to millennial scale was observed in the manganese and iron contents of Lake Suigetsu sediments in Japan, dating from 19.0 to 5.7 cal. kyr B.P. (calibrated thousand years before present). The increased manganese and iron contents, which mainly indicate the relative abundance of siderite, result from a greater vertical circulation in the lake during a cold period, with a higher mass of manganese and iron oxide being precipitated. The cold periods inferred from the increased manganese and iron contents coincide with the timing of ice-rafting debris (IRD) events in the North Atlantic, which span from the last termination to the Holocene, suggesting that the climate shift was hemispheric or probably global in nature. The manganese variation resembles the residual atmospheric 14C production (Δ14C) data of the Holocene that reflect variation in solar activity, and its periodicities correspond with those of solar-related climate changes from paleoproxy records in the North Atlantic and East Asia from the last cold stage to the Holocene. In particular, the Mn variation was larger in amplitude and shorter in timescale during the last cold stage when compared with those during the last termination and the Holocene, thereby possibly showing a significant difference in climate response to solar irradiance. The transition of the climate system from a glacial to an interglacial state corresponds to the beginning of the last termination.

Lake sediments represent high-resolution records of important informatin on the local, regional and global environmental changes. Studies of lake sediments are important for improving many models of global changes and for making realistic predictions on the future environmental system.