Search

Tectonic plates divide the Earth's surfaces and move across the surface of the planet. Their slow but consistent rate of movement causes them to converge, drift apart and rub together. Whilst the speed behind these interactions is extremely low, the sheer mass of these continental plates means large amounts of energy are associated with the interactions. This energy is the source of the formation of mountain ranges. However, when this energy is suddenly released it is destructive and in the form of earthquakes. There are many different specific causes for earthquakes, but they typically occur most strongly at sites of plate convergence. These areas where two plates meet face to face and one plate falls beneath the other to be absorbed into the Earth's mantle. Professor Akihisa Kitamura of Department of Geo Science, Faculty of Science, Shizuoka University is researching to understand more about these processes, specifically what is happening in Japan.

Four tsunami deposits (deposits I–IV) have been identified on Ishigaki Island, southwest Japan. The youngest tsunami deposit (deposit I) was caused by the Meiwa tsunami, which occurred on 24 April 1771 CE, as described in reliable historical documents. Two well-preserved specimens of articulated marine bivalve were collected from the youngest tsunami deposit (deposit I) and an additional two from the second-youngest tsunami deposit (deposit II; 920–620 cal. yr BP). The shells were tightly closed and empty inside. No encrusting epifauna or evidence of erosion was observed on the inner or outer shell surfaces. In each tsunami deposit, the 14C ages of the shells are nearly identical. The mode of occurrence and coincidence of ages mean that these shells were transported and buried alive by tsunamis. We analyzed the oxygen-isotope (δ18O) profiles of these bivalves to determine the seasons of their death, which provides clues to the seasonal timing of tsunamis. Tsunami deposits I and II were formed during spring and fall, respectively. The former supports the proposal that tsunami deposit I was caused by the 1771 Meiwa tsunami and provides regional radiocarbon reservoir age for the late 1700s; the latter provides a chronological constraint on the identification of tsunami deposit II. Thus, a combination of radiometric dating and δ18O profiles of articulated bivalves derived from tsunami deposits provides important chronologic constraints for examining paleo-tsunami events.

Four paleotsunamis deposits are exposed in a trench on the coastal lowland north of the southern Ryukyu subduction zone trench. Radiocarbon ages on coral and bivalve shells show that the four deposits record tsunamis date from the last 2000 yrs., including a historical tsunami with a maximum run-up of 30 m in 1771, for an average recurrence interval of approximately 600 yrs. Ground fissures in a soil beneath the 1771 tsunami deposit may have been generated by stronger shaking than recorded by historical documents. The repeated occurrence of the paleotsunami deposits supports a tectonic source model on the plate boundary rather than a nontectonic source model, such as submarine landslides. Assuming a thrust model at the subduction zone, the seismic coupling ratio may be as low as 20%.

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.

Within the molluscan phylum, the preserved remains of marine bivalves and gastropods in particular may provide important insights into tsunami occurrence and characteristics. This chapter provides an overview of how the taxonomy, taphonomy, and geochemistry of molluscs are helpful for identifying tsunami deposits in onshore and offshore environments worldwide. A comparison of species compositions of molluscan fauna in reported tsunami deposits suggests assemblages are frequently mixed, with species indicative of a range of different source environments. The degree of mixing of different assemblages may be strongly controlled by local topographic features. Taphonomic characteristics reveal additional insights, with tsunami deposits characterized by a high proportion of fractured and fragmented shells, with fewer whole or articulated shells than in other deposits. The identification of allochthonous articulated bivalves that were transported alive by tsunamis enables further insights. Geochemical approaches based on δ18O analysis of these shells may even reveal the season during which a paleotsunami event occurred.

During the Middle Pleistocene, the nature of glacial–interglacial fluctuations changed from low-amplitude and a periodicity of 41 ky to high-amplitude and quasi-periodic of 100 ky. The origin of the Mid-Pleistocene Climate Transition (MPT) is an unsolved mystery. At present, there is a debate about whether the initiation of the MPT was a gradual or an abrupt process. This study investigated the process of initiation of the MPT from reconstructions of eustatic sea-level changes, as a proxy for global ice volume, based on a reexamination of lithofacies and fossil occurrences from shallow-marine sediments (Omma Formation) exposed on the west coast of Japan. The Omma Formation comprises 19 depositional sequences spanning marine isotope stages (MIS) 56–21.3, reflecting sedimentation under alluvial plain to offshore conditions. The data indicate that (1) sea-level was lowest during MIS 22 (∼0.9 Ma); (2) sea-level during MIS 34 (∼1.13 Ma) and MIS 26 (∼0.96 Ma) was lower than during any other glacial stage, except for MIS 22; and (3) sea-level during MIS 22 was at most 20 m lower than during MIS 34 and 26. Together, these findings suggest that the initiation of the MPT was a gradual, rather than abrupt, process.

During the Last Glacial Maximum (LGM), ice volume reached a relative maximum and global temperature was lower than today. Understanding continental ice volume change requires accurate reconstruction of relative sea level, but tectonic uplift and isostatic adjustment effects complicate many records. Using marine sediment cores from the Bonaparte Gulf in northwestern Australia, a so-called “far field” tectonically stable site, Yokoyama et al. (2000) reported that the LGM terminated abruptly at 19 ka with a rapid sea-level rise (19 ka event). Their sea-level reconstruction determined the age of the LGM termination, but the timing of its inception remained less well constrained, partly because the number of radiocarbon analyses was insufficient to clarify LGM duration. Here we return to the Bonaparte Gulf and present a new relative sea level that better constrains LGM duration with high-resolution radiocarbon dating of a recently recovered marine sediment core (water depth: 120 m, length: 583 cm). Radiocarbon dating of 23 molluscs and 26 bulk organic-matter samples, together with total organic carbon, total nitrogen, and stable carbon isotopes, provide a record of paleoenvironmental variability in response to sea level change. Our results indicate that the LGM sea-level minimum occurred at 20.8 cal ka BP and the LGM duration was shorter than previously reported.

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.

We present a detailed stratigraphy of the lithology, sedimentary facies, magnetic susceptibility, and chemical composition of a 54-m thick core sequence drilled adjacent to the Chiba section, a candidate for the Global Boundary Stratotype Section and Point of the early–middle Pleistocene boundary. Siltstones rich in trace fossils are dominant throughout the core, except for the lowermost sand beds. The Byakubi tephra is divided into three units, of which the uppermost (39.54–39.49 m core depth) is identified as a primary bed showing evidence of fallout. The preliminary Matuyama–Brunhes magnetic polarity boundary lies 1 m above the top of the fallout unit. Maximum and minimum susceptibility axes for anisotropy of magnetic susceptibility have shallow ( 73°) inclinations, respectively, throughout the core. The results suggest that sedimentary structures formed in calm environments remain undisturbed in the sediment. Clustered maximum axis directions indicate the influence of a persistent NW–SE contour current. The homogeneous depositional environment for the siltstone interval is supported by relatively smooth variations in magnetic susceptibility and chemical composition. Stratigraphic correlation with published oxygen isotope data indicates that the depositional age of the core may extend from the end of marine oxygen isotope stage (MIS) 20 to just after mid-MIS 19, from which an average sedimentation rate of 2–2.5 m/ky is estimated. These results suggest that the Chiba section has a high potential to provide an excellent stratotype for the early–middle Pleistocene boundary.

Magnitude 8 megathrust earthquakes that have generated large tsunamis with wave heights of >5 m in coastal areas of southwestern Japan have occurred every 100–150 years along the Nankai–Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. Using sedimentological, geochemical, and paleontological analyses, as well as 14C dating, we examined tsunami deposits in 12 sediment cores collected from the Hamatome lowland, Yaizu Plain, Shizuoka, which faces the Suruga Trough. Four sandy tsunami deposits (Sands 1–4) are identified in sediments deposited during the last 5000 years. Sand 1 was deposited between 805 and 405 BC, suggesting that only one tsunami left a sedimentological imprint in the Hamatome lowland lagoon during the period of ∼4000 years between 3090 and 2913 BC and the AD 1096 Eicho-Tokai earthquake. This very low frequency of tsunami inundation may suggest that this tsunami was larger than those typically associated with megathrust earthquakes. Sands 2–4 are correlated with tsunamis associated with the Eicho-Tokai, AD 1361 Shohei-Koan, and AD 1498 Meio-Tokai earthquakes, respectively. Our data indicate that a sand spit, which had previously protected the lagoon from oceanic waves, was abruptly destroyed following the AD 1096 Eicho-Tokai earthquake and did not re-form at the same location. This disappearance of the sand spit is inferred to have involved an earthquake-induced submarine landslide. The data presented in this paper, together with information obtained from historical documents, indicates that M8 megathrust earthquakes in AD 1096 and AD 1498 caused submarine landslides in the study area, resulting in coastal subsidence and damage to coastal areas additional to that caused by the tsunamis.

Suborbital-scale climate variations, possibly caused by solar activity, are observed in the Holocene and last-glacial climates. Recently published bicentennial-resolution paleoceanic environmental records reveal millennial-scale high-amplitude oscillations postdating the last geomagnetic reversal in the Marine Isotope Stage (MIS) 19 interglacial. These oscillations, together with decoupling of post-reversal warming from maximum sea-level highstand in mid-latitudes, are key features for understanding the climate system of MIS 19 and the following Middle Pleistocene. It is unclear whether the oscillations are synchronous, or have the same driver as Holocene cycles. Here we present a high resolution record of western North Pacific submarine anoxia and sea surface bioproductivity from the Chiba Section, central Japan. The record reveals many oxic events in MIS 19, coincident with cold intervals, or with combined cold and sea-level fall events. This allows detailed correlations with paleoceanic records from the mid-latitude North Atlantic and Osaka Bay, southwest Japan. We find that the millennial-scale oscillations are synchronous between East and West hemispheres. In addition, during the two warmest intervals, bioproductivity follows the same pattern of change modulated by bicentennial cycles that are possibly related to solar activity.

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.

This study examines geological evidence for earthquakes and tsunamis based on data obtained from 14 sediments cores and one trench in the coastal area of the Shimizu Plain, Japan. The Shimizu Plain is located onshore of the Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. On the basis of lithologies, fossil contents, and radiocarbon dating, we identified geological and paleontological evidence for abrupt changes in depositional environments related to coseismic uplift associated with the ad 1854 Ansei-Tokai earthquake. We estimated a maximum coseismic uplift of 1.2 m and post-earthquake gradual subsidence of c. 0.6 m. We also identified four prehistoric tsunami deposits (T-I, T-II, T-III, and T-IV) in shallow-water (from low tide to 40 m depth) muddy bay deposits. The tsunami deposits are 20–70 cm thick, have sharp erosional bases, typically a graded structure, and contain rip-up clasts. 14C ages date the deposits to 6180–6010 to 5700–5580 cal. yr BP for T-I, 5700–5580 to 5520–5320 cal. yr BP for T-II, 4335–4125 to 4250–4067 cal. yr BP for T-III, and 3670–3540 to 3500–3360 cal. yr BP for T-IV. The interval between the tsunamis is in the order of 500–1000 years, which is significantly longer than the intervals between the four historical tsunamis that have inundated the Shimizu Plain since ad 1096. We also identified three event deposits that may have formed during the ad 1498 Meio or ad 1707 Hoei earthquakes.

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.

Three washover sand beds, ranging from 15 to 34 cm in thickness, were recorded in a lagoonal mud sequence on the Shizuoka coast of central Japan, which faces the Suruga Trough. The sand beds were c...

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.

No previous studies have investigated changes in the offshore molluscan assemblages of the Japan Sea during the last deglacial period. We examined the sedimentary facies and the stratigraphic distribution of molluscs in cored sediments collected at 132 m water depth off Sado Island, Japan. The ages of 13 specimens of molluscan shells were determined by 14C dating. Moreover, we analyzed the relative abundance of the planktonic foraminifer Globigerinoides ruber, which is a proxy of the Tsushima Current. The results show that the cored sediments were deposited from 13,574 ± 121 cal yr BP at a water depth that increased over time from about 50 m to 132 m. Three sedimentary facies are recognized in the cored sediment, in ascending stratigraphic order; coquina sediments, massive silt, and massive fine sand yielding many molluscan shells. The change in substrate was caused mainly by variability in the input of terrigenous deposits at 10,640–9,300 cal yr BP (the transition from coquina to silt) and at 8...

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 Early Pleistocene fossil record of the Japan Sea reveals that the planktonic foraminifer Globorotalia inflata succeeded in expanding its range from the East China Sea to the Japan Sea during interglacial stages of the Early Pleistocene; however, the species is only found in surface sediment samples from the southwestern-most part of the sea. Based on the ecology of G. inflata and the water mass structure of the Japan Sea, it appears that its geographical distribution is mainly controlled by the low temperature of the intermediate water, which is caused by the cool-water mass of Japan Sea Proper Water. The temporal and spatial patterns of G. inflata occurrence indicate that the intermediate water at 200 m depth during interglacial stages between 1.46 and 1.3 Ma was warmest during the Quaternary. This warm intermediate water probably resulted from the influence of the low-salinity nature of the Tsushima Current due to the large inflow of East China Sea Coastal Water. After 1.3 Ma, the intermediate water at 200 m depth became cooler in a step-wise manner. This cooling can be explained by an intensification of thermohaline circulation in the Japan Sea associated with strengthening of the East Asian winter monsoon and an increase in salinity due to enlargement of the southern strait. Copyright © 2009 John Wiley & Sons, Ltd.

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.

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 stratigraphic distribution of warm-water molluscs, planktonic foraminifers and diatoms in Plio-Pleistocene strata (3.5–0.8 Ma) of the Sea of Japan coast of Japan is the basis for reconstructing the history of the Tsushima Current. The results indicate: (1) the Tsushima Current flowed at 3.5, 3.2, 2.9, 2.4 and 1.9 Ma, in the 3.5–1.7 Ma interval; (2) the current flowed at every interglacial highstand, except for MIS 25, 23 and 21.3, in the 1.71–0.8 Ma interval; (3) the salinity and volume of the Tsushima Current during the earlier interval may have been significantly lower and smaller, respectively, than in the latter interval; (4) the Late Pliocene altitude of the southern part of the Sea of Japan was up to 50 m above present-day sea level.

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.

The middle Pleistocene Kunosan Formation is distributed in the Udo Hills (Shizuoka, central Japan) that was tectonically uplifted about 300m during the late Quaternary. The formation consists of about 200-m-thick of coarse-grained fan-delta deposits. The thorium-230 age of a species of solitary coral from the Kunosan Formation in the eastern Udo Hills (Muramatsu Gravel-Silt Member) is 176.5+3.7-3.6ka (error is 1σ). This radiometric age means that the Kunosan Formation was deposited during marine isotope stage 6.5. This interpretation is consistent with previously published chronologic information for the middle and upper Pleistocene in the Udo Hills.

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.

函館湾で採取した堆積物コア試料から,小球状ガラス物質(直径0.5～2.0mm)と石炭粒子を含む級化層理の発達した細礫～砂層を見つけた.函館湾周辺には石炭層が露出していないことと,形態的特徴から小球状ガラス物質は熔融した物質が急冷固結したものと推定されることから,我々は小球状ガラス物質と石炭粒子を1945年の函館空襲による大型船舶の被弾・大破炎上時に放出されたものと考えた.そして級化層は,それらが1954年の洞爺丸台風時の暴浪によって再移動と淘汰を受け,類似した水理特性を持つほかの砂粒子とともに再堆積したものと解釈した.このストーム堆積物は,函館湾の堆積物から過去数百年間の気候・環境変動を解読するために重要な年代マーカーとなりうる.

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.

During oxygen isotope stages 50 to 26, an offshore environment that was unsuitable for both cold- and warm-water molluscs may have temporarily prevailed in the southern Sea of Japan. This inner- to outer-shelf environment existed with a lateral scale of a few kilometers and a vertical scale of a few tens of meters during at least three transitions from oxygen isotope stages 48 to 47, 44 to 43 and 32 to 31. These deglaciation periods coincided with the three highest peaks of July solar insolation at 65°N (495, 493 and 500 W/m2) between oxygen isotope stages 50 and 26. This implies that anomalously high seasonality induced by orbital-insolation cycles is likely to have played an important role in establishing non-analog benthic communities with a very low density and diversity of molluscs in the early Pleistocene Sea of Japan.

本州の日本海沿いに分布する上部鮮新統-下部更新統の暖流系貝類化石および浮遊性有孔虫化石(対馬海流の指標)の層位分布から,日本海の南方海峡の変遷を検討した.既存の化石記録からは,3.9Maから3.1～3.0Maの期間において南方海峡の存在を示唆する確実な証拠は見出せなかった.3.1～3.0Maから酸素同位体ステージ60(1.72Ma)までの期間は,海水準高位期に南方海峡は一時的に出現したが,それ以外の期間は離水していたと考えられる.その後,同位体ステージ26(0.98Ma)までの期間の全間氷期に南方海峡が存在したことは確実であり,また同位体ステージ47(1.45Ma)から41(1.32Ma)の期間には氷期にも南方海峡が存在した可能性が高い.なお,同位体ステージ57(1.66Ma)における南方海峡の急速な形成は,2Ma頃に始まったフィリピン海プレートの沈み込み方向の変化に伴う沖縄トラフ北縁部のリフティングに関連したものと推定される.

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.

Changes in the fossil assemblage of offshore molluscs within each sixth-order (41-kyr) depositional sequence of the early Pleistocene Omma Formation document repeated recolonizations of warm-water species into the Sea of Japan. Each recolonization started as soon as the warm Tsushima Current began to flow in the Sea of Japan during a deglaciation and subsequent interglacial period. Here we examined in detail the stratigraphic distribution of six selected significant warm-water species and calculated confidence intervals for their first (local) appearances in a total of 10 deglaciations using continuously sampled data. They were subdivided into two groups, based on the chronological order of their first appearance during warming phases. Comparison of geological with biogeographic data reveals that these migration patterns are not always consistent with the modern geographic distributions of the species. These results show that fossil records may anticipate how the assemblage of local species would change in response to climate warming.

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.