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Recent studies have revealed an intensification of the Indian Summer Monsoon (ISM) over the past 700 years, but its influence on the sediments in the northern Andaman Sea is unclear. To examine this influence, we used radiocarbon-dated sediment core StMY6, acquired 100 km offshore in the northern Andaman Sea from the Ayeyarwady River mouth, and obtained a 700-year-long record of the grain-size distribution and geochemistry of the sedimentary sequence. The ISM influences precipitation surrounding the Ayeyarwady River, and thus the river water and sediment discharges to the northern Andaman Sea near the river mouth in the Ayeyarwady River and the weathering intensity in the river catchment. Based on the system, we propose that higher sedimentation rates and larger modal grain size (the most abundant size in the grain-size distribution of sediments) and lower carbonate concentrations after about AD 1600 can be attributed to higher ISM intensity. Profiles of total organic carbon (TOC) content and carbon/nitrogen (C/N) ratios, however, did not show any trends after about AD 1600, which suggests that the ISM has had little influence on these proxies at this site.

A submarine cave is a unique environment that is dark, food limited, semi-isolated from the outside, and sheltered from wave action. However, our knowledge of the long-term change in submarine-cave ecosystems remains limited. We document here the community-scale responses toward long-term change in a submarine cave, Daidokutsu in Okinawa in southern Japan. Using both metazoans (ostracods and bivalves) and protozoans (larger benthic foraminiferans) in two sediment cores obtained from the cave, we reconstruct the faunal and diversity changes of the past 7 Kyr. All taxonomic groups showed long-term, gradual linear change of faunal composition from predominantly open-water taxa to predominantly cave taxa, and ostracods showed short-term variability of species diversity. The long-term faunal trend probably reflects gradual isolation of the cave ecosystem due to coral reef development (i.e., development of the cave ceiling) during periods of the Holocene transgression and subsequent sea-level highstand. The short-term diversity changes show substantial similarity to centennial- to millennial-scale Holocene Asian monsoon variability. Ostracod species diversity peaks tend to correspond with periods of strong East Asian winter monsoons. The results indicate that limestone submarine-cave ecosystems, an important cryptic habitat, developed gradually during the Holocene and may be sensitive to rapid climate changes.

Faunal compositions and 14C ages of emerged sessile assemblages at four sites in the southern part of Izu Peninsula, central Japan, indicate that co-seismic uplift occurred at 1256–950 BC, AD 1000–1270, AD 1430–1660, and AD 1506–1815. The data suggest that the stress field in the southern part of Izu Peninsula changed to its current north–south compression at ca. 3100 BP, and that the recurrence intervals for uplift-inducing earthquakes have become shorter during the last 1000 years. The main faults responsible for this seismicity appear to be located offshore from the south part of Izu Peninsula. This study also examined a 1-cm-thick scoria layer deposited between 6940 and 6810 cal BP in the south part of Izu Peninsula. The layer, which had not been previously examined petrographically, consists of scoria grains derived from Izu-Oshima volcano located 40 km east of the study area, rather than from volcanoes located on Izu Peninsula itself.

To clarify the Holocene uplift history of the Izu Islands, Japan, we analyze the elevations and 14C ages of emerged sessile assemblages measured by accelerator mass spectrometry (AMS) on the islands of Niijima, Jinaijima, Shikinejima, and Kouzushima, on the northern Zenisu Ridge. The results suggest that uplift events took place after AD 1950 (uplift event 1), during AD 786–1891 (uplift event 2), during AD 600–1165 (uplift event 3), and during AD 161–686 (uplift event 4), although uplift events 3 and 4 are identified only at Kouzushima. The minimum amount of uplift was estimated to be 0.4–0.9 m in uplift event 1, 2.4–2.7 m in uplift event 2, 3.6 m in uplift event 3, and 3.3–8.1 m in uplift event 4. These events could have been caused by volcanic activity or strong earthquakes. There also remains the possibility that uplift event 2 was caused by the AD 1498 Meio earthquake; in contrast to the previous interpretation, the ages of uplift events are significantly older than the earthquake, based on conventional (non-AMS) methods.

We examined the spatial distributions and 14 C ages of sessile marine organisms in the submarine cave Ginama at Okinawa, Japan. The cave is the northernmost and only known submarine cave with an air chamber in the northwestern Pacific region. The upper limit of living individuals of the coralline sponges Acanthocaetetes wellsi and the bivalve Pycnodonte taniguchii in the cave is located at a depth of approximately 10 m relative to the surface of the pool in the cave. Low temperatures and low salinities prevent these organisms from colonizing water depths shallowerthan 10m. On the other hand, the upper limit of fossil individuals, dated between 5117 and 387 cal yr BP, is at a depth of 2.5 m, implying that at present the influence of fresh water on the cave pool is stronger than it has been at other times in the past 5000 years. This increase in the flux of fresh water may be explained by continuous cavern-forming activities such as dissolution.

Reconstruction of natural hazards from tidal flat sediments is essential for assessment of coastal hazards, and is based on knowledge of recent sediments. This study therefore examined recent sediments deposited on a narrow, muddy tidal flat on Miura Peninsula, metropolitan Tokyo, Japan, by means of sedimentological analysis, C, N, and S elemental analysis, carbonate content, 137Cs dating, and analysis of benthic foraminifera (species composition and degree of preservation). 137Cs profiles of four sediment cores of ∼20 cm in length showed that the sediments were deposited after the release of 137Cs by the accident at the Fukushima No. 1 Nuclear Power caused by the Tohoku-oki tsunami at March 2011. Results of C/S ratios of mud fractions indicated that the intertidal deposits in the central bay are characterized by low C/S ratios (4.9–11.4) and the bayhead deposits of the upper intertidal and supratidal zones by higher ratios (13.9–24.9). A fluid–mud layer identified in the central bay had the same C/S ratios and carbonate contents as marine sediments, indicating that the fluid-mud deposits probably resulted from resuspension of mud caused by storm-wave action after March 2011. Faunal analysis of benthic foraminifera showed that very low density of foraminifera can be regarded as one of the defining characteristics of fluid-mud deposits.

Evidence for abrupt coastal uplifts has been found in emerged sessile assemblages in a sea cave at the southern end of the Izu Peninsula, central Japan. We identified five sessile assemblage zones: Zones I to V, in ascending order. The uppermost zone (Zone I), located at an elevation of 2.7–3.5 m above the present-day mean sea level (amsl), is a hard massive shellcrust consisting mainly of the barnacles Chthamalus challenger and the tube worm Pomatoleios kraussii. Zone II, at 2.35–2.7 m amsl, is dominated by well-preserved individuals of C. challenger. Zone III, at 2.0–2.35 m amsl, is strongly eroded and consists mainly of C. challenger and P. kraussii. Zone IV, at 1.6–2.0 m amsl, is characterized by the co-occurrence of very fresh shells of C. challenger and P. kraussii. Zone V (the lowest zone), at 1.0–1.60 m amsl, is characterized by the co-occurrence of very fresh shells of Saccostrea kegaki and P. kraussii, and by the absence of C. challenger. Radiocarbon dating by accelerator mass spectrometry (AMS) and the presence of modern taxa in the sessile assemblages suggest that three episodes of coastal uplift have occurred in the area, during AD 570–820, AD 1000–1270, and AD 1430–1660, with magnitudes of 0.9–2.0 m, 0.3–0.8 m, and 1.9–2.2 m, respectively.

The aragonitic micro-bivalve Carditella iejimensis, which is less than 3.5 mm in height and length, inhabits the sediment surface in a submarine cave off the Okinawa Islands, Japan. To evaluate the use of this species as a quantitative paleoceanographic proxy, we analyzed the δ18O values of 50 living C. iejimensis specimens collected from the Daidokutsu submarine cave (ca. 30 m water depth). Results show that most individuals (96%) preserve δ18O values corresponding to the mean annual temperature and δ18O of seawater. The mean δ18Oshell of the 50 specimens was -1.10 ± 0.18‰. These results, when applied to fossil shell δ18O records from Daidokutsu cave, indicate that its mean annual surface water temperature is about 1°C higher than temperatures during the Medieval Warm Period and the Middle Holocene Climatic Optimum, and that the recent warming is likely exceptional during the past 7000 years.

Japanese historical documents reveal that Mw 8 class earthquakes have occurred every 100–150 years along the Suruga and Nankai troughs since the 684 Hakuho earthquake. These earthquakes have commonly caused large tsunamis with wave heights of up to 10 m in the Japanese coastal area along the Suruga and Nankai troughs. From the perspective of tsunami disaster management, these tsunamis are designated as Level 1 tsunamis and are the basis for the design of coastal protection facilities. A Mw 9.0 earthquake (the 2011 Tohoku-oki earthquake) and a mega-tsunami with wave heights of 10–40 m struck the Pacific coast of the northeastern Japanese mainland on 11 March 2011, and far exceeded pre-disaster predictions of wave height. Based on the lessons learned from the 2011 Tohoku-oki earthquake, the Japanese Government predicted the tsunami heights of the largest-possible tsunami (termed a Level 2 tsunami) that could be generated in the Suruga and Nankai troughs. The difference in wave heights between Level 1 and Level 2 tsunamis exceeds 20 m in some areas, including the southern Izu Peninsula. This study reviews the distribution of prehistorical tsunami deposits and tsunami boulders during the past 4000 years, based on previous studies in the coastal area of Shizuoka Prefecture, Japan. The results show that a tsunami deposit dated at 3400–3300 cal BP can be traced between the Shimizu, Shizuoka and Rokken-gawa lowlands, whereas no geologic evidence related to the corresponding tsunami (the Rokken-gawa–Oya tsunami) was found on the southern Izu Peninsula. Thus, the Rokken-gawa–Oya tsunami is not classified as a Level 2 tsunami.

Coral reef cavities, crevasses, and caves are inhabited by cryptic communities. The sediments within the submarine Daidokutsu Cave (29 m water depth) on the fore-reef slope of Ie Island, Okinawa, Japan, preserve a fossil record of cryptic bivalve species over the past 7000 years. The record suggests that infilling of cavities caused a progressive decrease in the exchange of water between the interior and exterior of the cave, resulting in a decline in food supply to the cave. To test this hypothesis, the light conditions within the cave were reconstructed from the spatial and temporal distributions of algal symbiont-bearing large benthic foraminifers, based on the assumption that infilling of reef cavities would have resulted in reduced light intensity within the cave. The results show that progressive darkening of the cave occurred by about 5130 yr BP, synchronous with a weakening in water flow within the cave. This synchronicity indicates that infilling of cavities and narrowing of the cave entrance might cause declines in the exchange of water between the interior and exterior of the cave, as well as in light intensity within the cave. These resulted in a deficiency in food in the cave, thereby affecting the species composition of cavernicolous bivalves.

The micro-bivalve Carditella iejimensis inhabits the sediment surface within submarine caves at Ie Island, Okinawa, Japan. A comparison of the δ 18 O values ( δ 18 O aragonite ) of empty and living shells indicates that the shell is formed over several seasons, and that the main cause of mortality is low water temperature. According to this hypothesis, it is likely that the shells with heaviest δ 18 O aragonite values (− 0.4‰) among the recent dataset formed under or close to the lower limit of growth temperature for the species. Assuming an unchanging temperature tolerance of the species during the Holocene, samples with δ 18 O aragonite values heavier than − 0.4‰ indicate unusually low temperatures and enrichment of δ 18 O of sea water ( δ 18 O seawater ). The δ 18 O aragonite record of C. iejimensis from sediment cores recovered from Daidokutsu cave shows no clear long-term trend in sea surface temperature or δ 18 O seawater in the East China Sea during the past 7000 years, and indicates anomalously cool and dry events (enrichment in δ 18 O seawater ) at around 6300 and 5550 cal. years BP. These events may have been related to changes in the activity of the East Asian monsoon, related in turn to weakening solar activity. In contrast, these anomalies appear to be obscured during the last 1000 years, including a weak Asian summer monsoon event during the Little Ice Age, thereby indicating that the mode of the East China Sea climatic and hydrologic response to decadal- to centennial-scale variability in the intensity of East Asian monsoon has varied over the past 7000 years.

We analyzed sediments and cavernicolous bivalve assemblages within cored sediments recovered from the Daidokutsu submarine limestone cave, which occurs in Pleistocene limestone on the fore-reef slope of Ie Island, off Okinawa, Japan. We found sand-sized brown grains within sediments aged 7,000–6,000 cal BP, and identified the grains as debris derived from a red soil layer developed in coastal zones around the Island. Temporal changes in the abundance of the debris suggest that water flow in the cave weakened at about 6,500 cal BP. Temporal changes in the cavernicolous bivalve assemblages indicate that the environmental conditions of the innermost cave area have gradually spread to the cave entrance over the past 5,000 years. These findings indicate that the filling of cavities within the reefal foundations of the cave has continued over time, resulting in a progressive decrease in the flux of water between the interior and exterior parts of the cave during at least the past 6,500 years. This indicates, in turn, a long-term decline in the functioning of the cryptic community within the reef as a source of the nutrients that support coral reef growth. Our findings regarding the distribution of debris from the red soil layer strongly support the occurrence of a mid-Holocene high-energy window, which represents a period of high wave energy prior to the formation of a reefal barrier.

The micro-bivalve Carditella iejimensis is less than 3.5 mm in shell height and lives in the surface sediment within a submarine cave at Ie Island, Okinawa Islands, Japan. A previous study interpreted that δ 18O-derived temperature obtained from C. iejimensis shells indicates the water temperature between May and July of each year. This interpretation is based mainly on a statistically significant correlation between shell size and δ 18O values obtained from empty shells. To assess the validity of this interpretation, we performed high-resolution isotopic analyses on 19 empty shells with the aim of examining ontogenetic variation in δ 18O values. The results reveal no systematic variation in δ 18O values, inconsistent with the previous interpretation. Our data indicate little or no influence of growth duration (shell size) on the δ 18O values of shells greater than 1 mm in height.

A 5000-year fossil series of minute submarine cave bivalves was studied using morphometric and evolutionary analyses. The obtained results indicate that the shapes of larval shells of studied species were labile, whereas the size of the larval shell was stable in each species studied. This result is different than that previously reported in most other studies in which size change is more common than shape change. This unique evolutionary pattern of these bivalves might be attributed to their refugial lifestyle.

We measured the δ18O values of the whole shells of the cavernicolous micro-bivalvia Carditella iejimensis obtained from sediments within a submarine cave (31 m water depth) at Ie Island (Okinawa Island, Japan) in the subtropical Northwest Pacific. Our results show no significant millennial-scale trend in the δ18O record, implying that both springtime temperature and the δ18O of sea water at 30 m depth around the Okinawa Islands have been stable for the past 3000 years at values similar to those of today. Moreover, we found one exceptionally light δ18O value from specimens spanning the past 250 years. The δ18O-derived temperature represents a departure of 2.1 °C from the average value for the past 250 years, being equal to the departure recorded during unusually high temperatures in the spring of 1998. This finding may imply that such high springtime sea surface temperature has been a rare event over the past 3000 years.

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.

There is a continuous record of planktonic foraminifers for oxygen isotope stages 50 to 26 (ca. 1.5–1.0 Ma) in the early Pleistocene Omma Formation near Kanazawa City, Central Japan, on the Sea of Japan coast. The warm-water species Globigerinoides ruber entered the Sea of Japan with the Tsushima Current during all interglacial periods and went locally extinct in the succeeding glacial periods. This implies that the marine climate of the Sea of Japan varied predominantly with the 41,000-year period of Earth's orbital obliquity. However, the relative abundances of G. ruber in marine isotope stages 47, 43 and 31 are significantly higher than those in other interglacial stages. These stages correspond to periods when eccentricity-modulated precession extremes were aligned with obliquity maxima. The Tsushima Current is a branch of the warm Kuroshio Current which is the strong northwestern component of the subtropical North Pacific Ocean gyre. Our data imply that the early Pleistocene climate in the northwestern Pacific was influenced not only by obliquity cycles but also by eccentricity cycles. This study also supports the climate model regarding eccentricity's role in the origin of low-frequency climate changes before the late Pleistocene ice ages.

Detailed mapping and radiocarbon dating of emergent marine sessile assemblages show that coseismic uplift occurred at 1256–950 BC, AD 1000–1270, AD 1430–1660, and AD 1506–1815 in the southern Izu Peninsula, central Japan. Employing a characteristic earthquake model, this study reconstructed the source fault for the uplift events from the spatial distribution of coseismic vertical displacements and historical documents. The source is inferred to be a reversal fault located about 3 km off the southern Izu Peninsula that is 25 km long and 13 km wide (strike = 250°, dip = 52° to the north) and slip of 2.7 m and has generated a Mw 7 class earthquake.

The Song Hong (Red River) delta, located on the western coast of the Gulf of Bac Bo (Tonkin) in the South China Sea, formed as a result of the Song Hong sediment discharge throughout the Holocene. The river's sedimentary basin upstream from the delta plain is not large. The delta plain comprises emerged tidal/mangrove flats formed during the sea-level highstand at + 2–3 m (6–4 cal. kyr BP) and a beach-ridge strandplain, with straight-to-lobate beach ridges, on the landward and seaward sides of the delta plain, respectively. The delta affords us the opportunity to examine river-mouth morphodynamics comprehensively in relation to sediment discharge and sea-level changes. In this paper, we describe the Holocene evolution of the Song Hong delta system and the river-mouth morphodynamics on the basis of seven sediment cores, each 30–70 m long, taken from the delta plain during 1999–2001 and 101 radiocarbon dates obtained from the core sediments. Sediments from the seven cores, consisting of incised-valley fills since the last glacial maximum, can be divided, in ascending order, into fluvial sediments composed of gravelly sand and mottled clay, tide-influenced estuarine sediments containing shell and wood fragments, and deltaic sediments composed of tide-influenced sand and mud deposits, in which the contents of sand and wood fragments increase upward. By applying sequence stratigraphic concepts, a transgressive surface can be identified at the gravelly sand-mottled clay boundary, and the maximum flooding surface can be identified at the estuarine–deltaic sediments boundary. The lowstand systems tract, transgressive systems tract, and highstand systems tract record ∼15, 15–9, and 9–0 cal. kyr BP, respectively. During the past 9 kyr, the majority of the sediment discharged by the Song Hong accumulated in the incised valley and enhanced progradation at the river mouth. The river mouth prograded from the bay head toward the gulf, and its morphology changed from funnel-shaped (9–6 cal. kyr BP) to straight (6–2 cal. kyr BP) and finally to lobate (2–0 cal. kyr BP). The morphological change from a funnel-shaped to straight coast is interpreted to have been closely related to a hydrologic regime shift from a tide-dominated bay-head setting to a wave-influenced open-coast setting. The cessation of the sea-level rise at 6 cal. kyr BP may have played only a small role in the change of river-mouth morphology because the progradation rate decelerated from 22 to 4 m/yr, instead of accelerating at that time. The river-mouth morphodynamic change from straight to lobate was largely caused by an increase in the Song Hong sediment discharge from 17–27 (9–2 cal. kyr BP) to 49 million t/yr (2–0 cal. kyr BP) as a result of anthropogenic deforestation along the upper reaches of the Song Hong.

This study examines the deposition of highly laminated muddy sediment in vibrocores recovered from a depth of 40–55 m of water and located 120 km east of the Yangtze River mouth. X-radiographs show numerous sharp-based sedimentary rhythmics with nearly parallel and undulated laminations, interbedded with silty lenses and interbeds. The laminated sediment varies from clayey silt to silty clay. AMS radiocarbon dates on well-preserved bivalves are primarily younger than 5500 y BP, indicating the formation of the laminated sediment under the present sea-level conditions. Macro- and microfossils in the sediments are consistent with the modern offshore sedimentary setting. The laminated sediment originated from the Yangtze estuary, but the sedimentation processes were not deltaic. The highly laminated sediment that lies below the normal wave base of 10 m was deposited on the seaward margin of the Yangtze subaqueous delta, closely associated with submarine tidal and storm-generated currents.

Delta initiation and subsequent delta growth were investigated by using radiocarbon-dated borehole cores from the Song Hong (Red River) delta, Vietnam. The sediments underlying the present delta plain record three depositional systems since the last glacial maximum: fluvial, estuarine, and deltaic, in ascending order. The landward part of the initial delta was characterized by an active tide-influenced accumulation of sandy sediments. The seaward part of the initial delta is composed of muddy prodelta and seafloor sediments that accumulated slowly. Radiocarbon ages of molluscan shells and organic materials suggest that the transition from estuary to delta occurred at approximately 8500 cal years BP. The transition was induced by deceleration of the postglacial sea-level rise, which was coeval with the delta initiation. The stacking pattern of the delta system is very different from that of the estuary system. The sediments of the estuary that developed during the rapid rise in sea level between 11000 and 8500 cal years BP display a retrogradational stacking pattern, forming a thick transgressive systems tract. On the other hand, aggradational as well as progradational stacking with clinoform architecture characterized the delta system between 8500 and 6500 cal years BP as the sea level continued its slow rise. Progradation has been dominant during the last 6500 years because the cessation of sea-level rise resulted in little accommodation being added. The results of this study are similar to those for other major delta systems, such as the Changjiang and the Ganges–Brahmaputra deltas, despite large differences in sediment discharge, tectonics, basin size, and coastal environment. This implies that the postglacial sea-level rise and the change in the rise rate markedly influenced the evolution of depositional systems at river mouths.

The evolution of the outer lake of Hwajinpo Lagoon in Korea has been reconstructed using environmental proxies (lithologic, geochemical, and fossil data) with a chronology established using 7 accelerator mass spectrometry (AMS) radiocarbon dates. Grain size, water content, and X-ray analyses from the core of outer coastal lakes (HJ99) were used to reconstruct sedimentary environments by using total organic carbon, C/N, S, and C/S chemical proxies. Assemblages of mollusc remains also provided paleoenvironmental information. The environmental changes of the outer lake of Hwajinpo Lagoon can be divided into 6 depositional phases. The basin of the Hwajinpo was exposed and underwent a weathering process before the Holocene period. The muddy sand layer on the weathered bedrock indicated an estuarine system about 6000 BP. The laminated layer implies that the lagoonal system was anoxic between about 5500–2800 BP. The marl layer implies a relatively oxic lagoonal condition with mollusc presence about 2500 B P. The layer of very low sulfur content indicates a freshwater lake system isolated by a sand barrier about 1700 BP. Beginning about 1000 B P, the river system deposits progress progradation on the marl layer. Two erosional landforms could be related with a high standing sea level span during Holocene. These high-stands are dated at 5700 BP and 2200 BP and are supposed to have formed erosional landforms of about 1.6 amsl and 0.8 amsl, respectively. Environmental changes of the outer lake of Hwajinpo Lagoon are considered due mainly to the lake- and sea-level fluctuation during Holocene.

The Song Hong (Red River) delta occurs on the northwest coast of the South China Sea. Its evolution in response to Holocene sea-level changes was clarified on the basis of sedimentary facies and 14 radiocarbon dates from the 40 m long Duy Tien core from the delta plain, and using previously reported geological, geomorphological, and archaeological data. The delta prograded into the drowned valley as a result of early Holocene inundation from 9 to 6 cal. kyr BP, as sea-level rise decelerated. The sea-level highstand at +2–3 m from 6 to 4 cal. kyr BP allowed widespread mangrove development on the delta plain and the formation of marine notches in the Ha Long Bay and Ninh Binh areas. During sea-level lowering after 4 cal. kyr BP, the former delta plain emerged as a marine terrace, and the delta changed into the present tide- and wave-influenced delta with accompanying beach ridges. Delta morphology, depositional pattern, and sedimentary facies are closely related to Holocene sea-level changes. In particular, falling sea level at 4 cal. kyr BP had a major impact on the evolution of the Song Hong delta, and is considered to be linked to climate changes.

The purpose of the present study is to examine the topographic evolution and sedimentary characteristics of a mud-ridge system in the Yangtze offshore region. As revealed by seismic profiling, 2–3-km-wide, 5–8-km-long, up-to-15-m-thick mud ridges, situated 5–10 km apart, extend here in a southeasterly direction, approximately parallel to the direction of the ebb and flood flow of the modern tide. Directly resting upon the latest Pleistocene terrestrial sediments, some mud ridges are exposed on the present seafloor, while others are buried by landward-thickening Holocene deltaic sediments. An 11-m-long vibrocore recovered from one mud ridge reveals homogenous and mottled silty clay and clayey silt, rich in foraminifera, suggesting an estuarine environment of deposition. Chronostratigraphic correlation in association with acoustic facies indicate that the mud ridge was formed primarily during ∼10–7 ka, corresponding to a period of global decelerating rise in sea level. This mud ridge is underlain by thick, late Pleistocene fluvial to coastal fine to medium sands with an unconformity surface on top, and it is overlain by modern deltaic sediments with an erosional surface in between. We propose that the mud ridges were deposited under tidal-controlled estuarine conditions during the early Holocene deceleration of the rise in sea level. The Holocene transgression in the study area was nearly coeval with the mud ridge deposition, and the deltaic sedimentation occurred after the mud ridges formed. The ridges were subsequently reworked, largely by strong tidal currents.

生きている化石の宝庫として海底洞窟は1960年代より世界的に知られていたが,そこの堆積物に関する研究は行われていなかった.サンゴ礁における古環境解析のための新たなる地質記録としての有効性を検討するため,我々は沖縄県伊江島の海底洞窟(通称“大洞窟”)から堆積物のコア試料(長さ43cm)を採取した.堆積物はシルトからなり,葉理や生痕などの堆積構造は見られない.堆積物粒子は石灰質岩片を主とし,コッコリス,底生有孔虫や浮遊性有孔虫などを含む.貝化石の産状と14C年代測定の結果,コア試料は1,500年間以上の記録を有していることが分かった.また,平均堆積速度は2～4cm/100年と算出された.以上のことから,海底洞窟堆積物はサンゴ礁の後期完新世の環境・気候変動を解明できる記録媒体として有効であるといえる.

The initiation of flow of the warm Tsushima Current created significant changes in Quaternary paleoceanography and ecosystems in the continental shelf of the Japanese islands in the Sea of Japan. In order to understand the behavior of the current at the initiation of its flow, we reconstruct the change in the relative thickness of the current during the initiation of flow based on combining planktonic and shallow-water molluscan fossil analyses of the Early Pleistocene Omma Formation in central Japan. The result indicates that warm-water planktonic foraminifera and cold-water molluscs coexisted during about 2300–2400 years just after the initiation of the intrusion of the Tsushima Current. Migration of a warm-water molluscan fauna to the shelf occurred shortly after the local extinction of cold-water molluscs. This means that the rate of increase in thickness of the Tsushima Current during the initiation of flow was faster than that of the sea-level rise. Using deep-sea core records and modern biogeographic data, the rates of the former and latter are calculated to be 17.2±8.9 m per 1000 years and 1.9±1.1 m per 1000 years, respectively. Because bottom water change might lag behind surface water change during a rapid climatic change in all shallow water, combining planktonic and benthic fossil analyses is necessary to reconstruct shallow paleoceanography.

加賀平野下のボーリングコアから発見された火山灰層は,その年代が約8,800～8,270年前であると推定される.火山ガラスの屈折率・鉱物組み合わせ・年代に基づくと,この火山灰層に一致する広域テフラは報告されていない.一方,加賀平野の南約50kmに位置する白山火山には,ほぼ同時代に噴出した弥陀ヶ原火山灰層が存在し,その重鉱物組成は発見された火山灰層とよく一致する.よって,この火山灰層は弥陀ヶ原火山灰層に対比される可能性が高い.

Major changes occurred in the benthic molluscan fauna of the Sea of Japan continental shelf during the transition from Pleistocene glacial to interglacial stages, owing to rapid warming associated with the inflow of the warm Tsushima Current. Molluscan associations representing this transition occur in the lower Pleistocene Omma Formation in central Japan and suggest that there were two patterns of faunal change. The first was when warm-water species migrated into the Sea of Japan and lived along with cold-water species, accompanied by a northward shift in species ranges. The second pattern involved the migration of warm-water mollusks shortly after the local extinction of cold-water species. In the latter, it is possible that benthic molluscan communities with very low diversity and density existed temporarily and locally at inner shelf depths (

The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.