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The Holocene sea-level history of the east coast of India is relatively unexplored. We analysed a 17.37-m-long sediment core from Kolleru Lake, a fresh waterbody located in a deltaic setting along the east coast of India, to reconstruct the climate, environmental, and sea-level history of the region. Sedimentary facies and pollen assemblages, with nine accelerator mass spectrometry 14C dates (two mollusk shells and seven plant samples) from the core revealed Holocene relative sea-level changes, and provide the first insights into the climate of the region from the Last Glacial Maximum (LGM) to present. A layer of anhydrous calcium sulfate at the core bottom, along with a 14C age of 18.4 cal ka BP, combined with the absence of pollen in mottled yellowish clay, indicated a desiccated lake surface, reflecting a dry LGM climate. Palynomorphs in the overlying calcareous-concretion-bearing light brown silty clay showed a change from arid terrestrial herbaceous plants to freshwater taxa, indicating a change from dry to wet climate after the LGM and before 8.0 cal ka BP. Further upward in the core, black, sticky silty clay with abundant mangrove pollen and mollusk shells indicated a marine environment in Kolleru Lake and aggradational sediment stacking related to the middle Holocene sea-level rise from 8.0 to 4.9 cal ka BP. The uppermost sandy/silty clay, with terrestrial/aquatic pollen and a 14C age of 3.7 cal ka BP indicated a freshwater environment during the late Holocene. Our results show that Kolleru Lake, on the east coast of India, changed from a desiccated state during the dry LGM to a brackish lagoon during the middle Holocene, and then to a freshwater lake by the late Holocene, under the influence of climate and relative sea-level changes.

Studies of the evolution of coastal lowlands since the Last Glacial Maximum (LGM) typically ignore radiocarbon data from sediment samples that have undergone reworking. However, these samples contain information on their sediment sources and their timing of redeposition. We analyzed 738 radiocarbon dates obtained from shell and plant material in samples of post-LGM coastal sediment from north of Tokyo Bay. Of these samples, 245 (33%) had reworked ages. Furthermore, the percentage of reworked samples and their average age offsets increased with the depth of the water environment (terrestrial, 15% and 360 ± 250 years, respectively; intertidal, 26% and 470 ± 620 years; subtidal, 39% and 550 ± 630 years). Taking these radiocarbon samples as a proxy for clastic material, our results imply that channel erosion accounted for relatively little clastic removal in the terrestrial and intertidal environments in short duration, whereas ~40% of clastics were removed by storm winnowing and transported in stepwise fashion to deeper water in long duration and ~60% were transported by floods directly from river mouths in the subtidal environments. These findings imply that radiocarbon ages from reworked samples can be used to quantify clastic recycling processes and their history in coastal areas.

Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was 2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels.

The Arakawa incised-valley fill beneath the Tokyo area, Japan, has formed by delta progradation during Holocene transgression and highstand. This paper presents the integrated stratigraphy of this valley fill as deduced from three continuous sediment cores with rich radiocarbon age control and abundant data from the surrounding, and compares the filling history with the adjacent Nakagawa valley. Six depositional units make up the valley fill, distinguished based on sedimentary facies. In a sequence-stratigraphic framework, these units span from lowstand fluvial deposits (latest Pleistocene) through retrogradational to transgressive marine deposits (early and middle Holocene), to highstand fluvio-deltaic deposits (middle and late Holocene). The isochron and stacking pattern of sedimentary facies indicate that around 8 cal ka BP the shoreline trajectory flipped from landwards to seawards and maximum flooding occurred in the Arakawa valley. This is considerably earlier than in the neighboring Nakagawa valley, where this happened between 7 and 6 cal ka BP. Differences in the sediment supply at the time of transgression explain the diachronicity. The findings identify the Tone River, which was at that time feeding the Arakawa valley, as a system of very large sediment supply, compared to average rivers worldwide.

The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones.

To detect long-term change in the reservoir effect of the eastern coast of Korea, we measured the radiocarbon ages of terrestrial plant and marine shell pairs from the same horizons from approximately 12 m of Holocene sediments under reclaimed land around the Mae-ho Lagoon. The lagoon sediment was examined for lithology, and sedimentary structures, as well as mollusk and diatom assemblages. From bottom to top the sediment comprised three units: transgressive lagoon, restricted lagoon, and delta. The offset in the radiocarbon age of nine pairs were also divided into three stages: 300 ± 90 to 410 ± 120 years (transgressive lagoon sediment in 7350–5800 cal BP), 640 ± 150 to 800 ± 150 years (restricted lagoon sediment in 4320–1600 cal BP), and 120 ± 140 to 330 ± 100 years (deltaic sediment in 1200–390 cal BP). The reservoir ages (R) in the sediments of transgressive lagoon and the restricted lagoon include the variation associated with dietary of shells, herbivorous and deposit-feeding, respectively. The R values in the deltaic lagoon sediment were the most representative of those in the Mae-ho Lagoon because they were obtained from the shells of a filter-feeding taxon, Potamocorbula amurensis, in the most recent sediment. These values from the deltaic lagoon sediment are also consistent with previously reported modern reservoir ages from the Korean Peninsula and Peter the Great Gulf around Vladivostok.

Mangroves can deliver important socio-economic benefits and store significant volumes of carbon along tropical to temperate coastlines, but the distribution is controlled by complex geomorphologic, hydrodynamic and salinity conditions, as well as human-related pressures. In the Mekong delta, the interaction of tide, wave and riverine hydrodynamic processes clearly impacts sedimentation and mangrove distribution. Extensive mangroves presently occur along the northeastern delta margin (Can Gio, located 40 km southeast of Ho Chi Minh City), in an embayed, back-barrier environment subject to the highest tidal range (c. 4 m) and away from major fluvial distributary channels. However, the development of the Can Gio mangrove system during the Holocene evolution of the Mekong delta is not fully understood. This study aims to constrain the sedimentary evolution of the back-barrier mangrove in Can Gio based on stratigraphic, palynological and geochronological analyses of sediment cores. Two deep borehole cores (15 and 30 m deep) and four short auger cores (up to 4 m deep) were taken at the modern mangrove to characterize a 20-km-long shore-normal cross section dated with radiocarbon and optically-stimulated luminescence (OSL) ages. The cores consist of a thick (> 20 m) subtidal mud from the base of the Holocene unit to the level of 3–4 m deep below the ground level, overlain by carbonaceous, intertidal mud. Chronological data reveal major episode of subtidal mud aggradation from 6 to 3.5 ka, forming the topography for mangrove development. Radiocarbon ages in the overlying intertidal mud range from 4 ka to modern and show some inconsistencies with the stratigraphy, suggesting mangrove root penetration and sediment mixing. The beach-ridge structure and OSL ages of the Can Gio barrier indicate that barrier formation initiated just before 3.2 ka. The transition from subtidal to intertidal mud aggradation and initiation of barrier formation, are broadly correlated with the onset of the subaqueous delta progradation and shift to a wave-dominated regime in the central delta. However, after 3.5 ka, the Can Gio shoreline had negligible sediment accretion while the central delta prograded up to 50 km seawards. Reduced sediment supply to Can Gio is consistent with more asymmetric, southwestward-dominated longshore drift after 3.5 ka. The negligible progradation at Can Gio also indicates minor sediment supply from the nearby Dong Nai River, which prevented upper intertidal accretion and translation of mangroves into a flood plain.

Studies on the sediment distribution along the fluvial to marine transition zone are rare and have focused on large-scale systems. Here, we report a case study from the Dong Nai River System (DNRS), a small-scale, tide-dominated river delta from southern Vietnam. The study is based on 80 channel bed sediment samples, together with channel depth, temperature, and salinity measurements, identification of mangroves and mollusks, and grain-size analysis. Year-round meter-scale water-level fluctuations due to tides are present across the entire study area, whereas saline-water intrusion affects the entire studied part of the Vam Co Dong–Vam Co River and the majority of the studied part of the Dong Nai–Soai Rap River. The sediment distribution of both channels highlights the presence of a downstream area characterized by soft mud indicating active deposition, and upstream areas characterized by sand and gravel sediments with abundant bedrock clasts and other evidence of the extremely limited sedimentation (e.g., encrusting biostructures, borings, coating, and agglutinated mats). The coarser-grained sand and gravel are likely reworked from local bedrock and transported for short distances as also suggested by their angularity and the similarities with Pleistocene sedimentary and igneous rocks in the study area. The sediment of the DNRS is overall coarser and muddier than the sediment of the Mekong, likely reflecting the smaller scale of this system, the lower discharge, and the local sediment supply from the bedrock.

To measure the spatial and chronological changes of the reservoir effect around the Korean Peninsula, the radiocarbon ages of 38 marine shell and terrestrial plant pairs from the same horizons of six cores of Holocene sediments collected from the southern coast and western coast sites of the peninsula were measured. These reservoir ages ( R ) were distributed in the range of 430 ± 190 yrs within 60 ± 60 to 1000 ± 60 yrs starting in 9000 cal BP. The average R values of the cores obtained from large rivers, such as the S13 and YAR-4 cores (340 and 190 yrs), were clearly smaller than the R values of the sites far from a large river, such as the S15 and W09 cores (470 and 650 yrs). This is thought to be associated with the mixing process of old brine and young freshwater. On the other hand, the R values of the S13, W17, and YAR-4 cores gradually increased during the time span from 6700 to 8200 cal BP. The R values for the S15 core also increased in the period from 2800 to 3800 cal BP. Such tendencies result from the mixing ratio increase of brine due to the rising sea level.

Spatial and quantitative analysis of infilling processes of the tide-dominated incised valleys beneath the Tokyo Lowland during the last 14 kyr was undertaken by using data from 18 sediment cores, 467 radiocarbon dates and 6100 borehole logs. The post-Last Glacial Maximum valley fills consist of braided river, meandering river, estuary, spit and delta systems in ascending order. The boundary between the estuary and delta systems is regarded as the maximum flooding surface. The maximum flooding surface beneath the Tokyo Lowland is dated at 8 ka in the Arakawa Valley and 7 ka in the Nakagawa Valley. This age difference is due to the migration of the Tone River from the Arakawa Valley to the Nakagawa Valley at 5 ka, and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea-level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers. The meandering river system is dominated by sheet-like sands that were deposited during lateral migration of channels during the Younger Dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the Younger Dryas. The transition between these channel geometries is controlled by a threshold sea-level rise of 4 to 7 mm yr−1. Before migration of the Tone River at 5 ka, the tide-dominated bay in the Nakagawa Valley was filled by upward-fining laterally accreting muds. The muds accreted from the margin to the axis of the bay. Such lateral accretion of suspended particles derived from outside the bay has been documented in other tide-dominated coastal environments and is probably common in other similar settings. After the migration of the Tone River, the bay was filled by upward-coarsening deltaic sediments.

A sparsely documented tsunami in 1454 may subdivide the recurrence interval between the 869 and 2011 tsunamis near Sendai, as judged from geomorphic, stratigraphic, and archival evidence. Pond-filled breaches cut across beach ridges on century-old topographic maps. The basal pond deposit in one of these breaches postdates 1454. Stratigraphy on Sendai Plain includes a sand sheet that contains marine and brackish diatoms. Radiocarbon ages suggest that the sheet dates to 1406–1615 (2σ), and written records for this interval in Tohoku mention a tsunami in 1454. The inferred inundation extended 1.0–2.5 km inland from an approximate medieval shoreline. Simulated tsunamis that best account for the sand sheet require a thrust earthquake of moment magnitude 8.4 or larger. If the sand sheet represents the 1454 tsunami, the two most recent intervals between great thrust earthquakes in Sendai region spanned 585 and 557 years.

Little is known about offshore tsunami deposits, although they have the potential to improve palaeotsunami reconstructions, especially in areas with limited preservation of onshore tsunami deposits. We analysed 44 sediment cores collected in 2012 at water depths of 14–30 m in Sendai Bay, north-eastern Japan, to identify the deposits of the 2011 Tohoku-oki tsunami with the aid of 134Cs released from the Fukushima-Daiichi Nuclear Plant. The preserved tsunami deposits in the inner shelf are interbedded with very fine sand and mud, showing similar features to storm deposits. In the shoreface, a distinct layer of coarse sand overlies fine sand, appearing to have originated as beach sand transported offshore by the tsunami backwash. The coarse sand fades offshore around the boundary between the shoreface and inner shelf. Many of the core sites show evidence that surface sediment was reworked, probably by post-tsunami storm waves, suggesting that further reworking would make the ultimate preservation of the tsunami deposits unlikely.

We investigated carbon sources and cycling in a canal connected to Lake Kasumigaura (Japan) using stable carbon isotope values (δ13C) in dissolved inorganic carbon (DIC) of ambient water and in shell carbonate of the commercially cultured freshwater pearl mussel (Hyriopsis sp., Unionidae). The δ13CDIC showed correlations with DIC concentrations, pH, monthly precipitation and pCO2. Thus, the carbon isotopic composition can be used as a proxy for environmental variables. The pCO2 ranged from 3,046 to 9,535 μatm, with a mean value of 6,035 μatm, and remained well above the atmospheric value throughout the years of study. Changes in CO2 solubility, outgassing and biological processes in the river had only minor influence on the carbon isotope composition. DIC concentration and δ13CDIC were controlled primarily by external inputs of soil and weathering-derived DIC associated with precipitation. The δ13C values in different growth zones of the Hyriopsis shells were similar, suggesting that given a constant δ13CDIC source throughout shell formation, the influence of size-related metabolic variations on shell δ13C was negligible. Comparison of δ13C values in DIC and Hyriopsis shell indicated that mussel ontogeny did not influence carbon isotope fractionation, but inter-specimen differences suggested that physiological factors exert systematic control on δ13C values in Hyriopsis. Results also suggest that if the annual mean δ13CDIC value is to be inferred for a fluvial system from shell δ13C values, multiple specimens should be sampled, and the annual mean value yields a temporally integrated measure of the stable isotopic composition of the river system. Given the carbon isotope fractionation between ambient DIC and shell is +2.0 ± 0.7 ‰ (2 se), we estimated that pCO2 averaged ~6,850 μatm during shell growth periods from 2002 to 2007. The pCO2 concentration was an order of magnitude greater than the atmospheric equilibrium value in all years, suggesting that CO2 was lost from surface water to the atmosphere or downstream to the lake. These losses must be accounted for when considering the regional redistribution of CO2. Our results extend the utility of δ13C measures in freshwater bivalves for inferring the δ13CDIC history of ambient waters. This tool provides information that is complementary to that gained from direct δ13CDIC measurement and should enable inference of long-term changes in critical variables such as pCO2.

The Changjiang (Yangtze) delta in eastern China is one of the largest tide-dominated deltas in the world. To obtain a more detailed information on its initiation, we drilled three sediment cores, PK01, HG01 and XJ03, from the uppermost region of the delta plain. On the basis of sedimentary facies and AMS 14C dating, we identified three depositional systems: (1) a fluvial system, (2) an estuarine system, and (3) a deltaic system including an initial delta (estuary–delta transition) and a typical delta. The maximum flooding surface separates the estuarine and deltaic systems and occurred at ca. 8.0 cal kyr BP, when brackish water intruded landward and reached the Nanjing area. The Changjiang delta was initiated at the area between Nanjing and Yizheng when the sea-level rise decelerated after the rapid rise in sea level during 9.0–8.2 cal kyr BP. The subsequent evolution of the Changjiang delta has been shown as a seaward development model of river-mouth bars with shoreline migration. The oldest river-mouth bar called Hongqiao sand body, which is located most landward, was dated at 6.0–5.5 cal kyr BP in this study, nearly one thousand years younger than the previous estimates. A tide-dominated, funnel-shaped estuary was formed at 8.0 cal kyr BP, and lasted until the Hongqiao sand bar became exposed. This stage is regarded as an aggradation-dominated initial stage of the Changjiang delta, followed by a progradation-dominated stage that is characterized by the formation of a series of seaward migrating river-mouth bars. A relative sea level highstand in the mid-Holocene may be associated with glacio-hydro-isostatic tilting.

We used micro X-ray fluorescence (μ-XRF) to investigate element profiles of sulfur in the inner layer of an aragonitic clam shell (Hippopus hippopus), and we used sulfur K-edge X-ray absorption near-edge structure (XANES) to evaluate the local environment of sulfur in aragonitic (H. hippopus, freshwater pearl mussel Hyriopsis sp.) and calcitic (Japanese scallop Mizuhopecten yessoensis) bivalve shells. The spectra of S K-edge XANES collected from bivalve shells and S-bearing organic and inorganic reference materials indicated that inorganic sulfate was present in marine aragonitic and calcitic bivalve shells. However, XANES results did not permit us to discriminate between organic and inorganic sulfate in freshwater aragonitic shells. The first-order fluctuations of sulfur in the inner shell layer showed clear annual fluctuations, with sulfur concentrations being lower during periods of faster growth. Our results suggest that trace sulfur profiles in aragonitic shells could be the result of cyclic changes of shell growth related to insolation or some influencing factors of shell growth.

Shell microstructures of five solemyid species from Japan were characterized by scanning electron microscopy. All five species examined had outer and inner shell layers, and were categorized into four groups according to the shell microstructure composition. Group 1 consist of Solemya (Petrasma) pervernicosa and Solemya (Solemya) tagiri. The outer layer of their shells is characterized by radially elongate simple prismatic structure (RESP) type A, and the inner layer by irregular prismatic structure. Group 2 is represented by Solemya (Solemya) pusilla. The outer layer of its shell has RESP type B structure, and the inner layer structure is homogeneous. Acharax japonica is in Group 3. Its outer shell layer has RESP type C, and the inner layer is characterized by laminar, homogeneous, and irregular complex crossed lamellar structures. Group 4 is represented by Acharax johnsoni. Its outer shell layer has a reticulate structure and the inner layer has a cone complex crossed lamellar structure. Retic...

The Godavari delta in India is a major wave dominated delta of a tropical monsoon-fed river with one of the largest sediment deliveries in the world. While several earlier studies revealed the nature of landforms and progradation style of the delta plain during the Holocene, the present study attempts to reconstruct the depositional environment of the Godavari delta through the analysis of core sediment and accelerator mass spectrometry (AMS) 14C dating from three locations. The sediment core obtained from a 27.06 m deep borehole at Vilasavilli (VV) supported by 13 14C dates revealed the complete succession of the Holocene deposits unconformably overlying Pleistocene sediments. Textural analysis indicates the lower upward-fining and upper upward-coarsening units in the Holocene succession. Total organic carbon (TOC)/total nitrogen (TN) ratios of > 20; high content of TOC around 1.5–2.5%; and black to very dark colored sediment throughout the muddy part of the Holocene succession indicated a predominance of terrigenous material. The VV core and two other cores (DRP and SDG) with 14 14C dates indicated the thickness of the Holocene sediments in the Godavari delta plain is in the order of ~ 20–50 m, unconformably resting on a seaward sloping Pleistocene basement. Sediment facies and sediment accumulation of the three cores show the evolution of the Godavari delta. A transgressive phase is recognized as an upward-fining succession in the VV core, 8.4–8.0 cal ky BP, followed by a low accumulation period, 8.0–6.3 cal ky BP including the Holocene maximum transgression in the Godavari delta. After 6.3 cal ky BP, areas of high accumulation rates have changed laterally between the central part (VV site) and southwestern part (DRP and SDG sites), may be controlled by the location of river-mouths. Further analysis coupled with 11 more 14C dates compiled from earlier works indicated that the strandplain of the Godavari delta prograded seaward in three stages, and that the rate of progradation accelerated during the past ~ 3 ky, particularly in the last millennium. However, pronounced shoreline erosion led to a net negative growth of the delta during the recent decades due to sediment retention by upstream dams. Although this study provided insights into the sedimentation patterns and rate of seaward progradation of the Godavari delta, further studies on three-dimensional volume analysis of the deltaic sediments are necessary to estimate the overall rates of past sediment discharge and the Holocene growth of the Godavari delta.

We described the shell microstructure of the cold seep-associated bivalve Conchocele bisecta using the acetate peel method and scanning electron microscopy, and revealed complex microstructural changes with ontogeny. The shell of the bivalve has outer, middle, and inner layers that consist of aragonite. The outer layer consists of spherulites and spindles of various sizes and shapes; these microstructures are identified as spherulitic, planar spherulitic, spherulitic prismatic, and spindle-like structures. The middle layer is characterized by cone complex crossed lamellar structure in the outer part and crossed lamellar structure in the inner part. The inner layer is composed of cone complex crossed lamellar, fine complex crossed lamellar, and irregular prismatic structures. On the basis of the observations from the umbo to the ventral margin of each shell layer, we recognized two growth stages that are divided by microstructural distributions in the outer and inner layers and the positions of d...

We investigated oxygen isotope and Sr/Ca, Ba/Ca and Mg/Ca ratios in shells of a commercially cultured freshwater pearl mussel ( Hyriopsis sp., Unionidae) and ambient water. The first-order fluctuations of oxygen isotope ratios of the outer shell layer along the maximum growth axis recorded the mussels' annual growth from 2002 to 2007. Clear annual Sr/Ca cycles suggest that Sr was incorporated into the shells at a high rate during warm seasons. All three examined shells showed a clear decrease in annual average Sr/Ca with age. Inter- and intra-annual trends suggest that Sr incorporation was higher during periods of faster growth. The Ba concentration and the Ba/Ca ratio of ambient water showed neither an annual periodicity nor a seasonal trend. In contrast to the findings of an inorganic aragonite precipitation experiment, Ba/Ca was positively correlated with ambient temperature. In addition, Ba/Ca and Sr/Ca ratios were positively correlated with each other. These results suggest that not thermodynamic equilibrium partitioning but the shell growth rate controls Ba incorporation into Hyriopsis shells. The Mg/Ca profiles of the three shells suggest that complex factors control the incorporation of Mg into shell aragonite. Water temperature and annual growth rates did not significantly influence Mg incorporation, and substantial interspecimen differences suggested that physicochemical factors do not exert any systematic control on Mg/Ca ratios in Hyriopsis . Instead, the Mg content of shell aragonite might reflect biological contamination with organic material or adsorption phenomena during crystal precipitation rather than regulated substitution into the aragonite crystal lattice. Our results suggest that trace element profiles in aragonitic bivalve shells could be interpreted as the result of strong biological controls over-riding environmental influences on trace element incorporation.

Approximately 4100 km of high-resolution seismic profiles recently obtained from the western South Yellow Sea were subdivided into seven seismic units by six major seismic surfaces. Three sediment cores between 60 and 71 m in length, obtained from localities on the seismic profile lines, were analyzed for sedimentary characteristics, clay mineral components, and benthic foraminiferal and ostracod assemblages and were also dated by AMS 14 C and optically stimulated luminescence to document the environmental history of the region. Stratigraphic units in the cores were confidently correlated with seismic units in the profiles. Stratigraphic correlation among these three cores and the previous well-studied core QC 2 was facilitated by the cold-water deposits in marine isotope stage (MIS) 5. The three cores document the sedimentary sequence during the period from MIS 6 up to MIS 1. Strata formation and sedimentary environment changes since MIS 5 are strongly controlled by sea-level fluctuations. Most of the successions were formed in MIS 5, MIS 3, and MIS 1 with major erosions in MIS 4 and MIS 2. Particularly our data revealed the presence of a large delta active during MIS 3 (likely from the early stage of MIS 3 to about 40 cal kyr BP) that was subsequently affected by channel incision. The delta succession and incised-channel fills have a high concentration of smectite in their clay mineral components, suggesting that the paleo-Yellow River was the main contributor to the delta development and channel incision. The delta sequence has a coarsening-upward trend and consists of two seismic facies. The upper facies shows lateral alternations of chaotic and oblique or very low-angle reflectors, which correspond to sandy (proximal) delta-front and delta-plain facies, and the lower facies is characterized by seaward-dipping clinoforms, which correspond to muddy (distal) delta-front and prodelta facies. The top surface of the delta inclines very gently seaward, consistent with a deltaic development associated with the sea-level fall in MIS 3. The incised-channel system consists of two main channels with multiple tributaries in a dendritic pattern. The incised-channel fills begin with fluvial and then estuarine sediments, which are truncated by a transgressive ravinement surface and capped by transgressive deposits. The main incision channels have a terraced cross-sectional morphology, which is interpreted to reflect multiple episodes of sea-level fall during incision, presumably from the late MIS 3 to the Last Glacial Maximum.

A 35.60-m-long core (ECS-0702) recovered from a water depth of 22 m in the muddy area off the Yangtze River estuary was analyzed for sedimentary characteristics, clay and detrital mineral components, and element geochemistry as well as by AMS 14 C dating to document sediment provenance changes and environmental evolution during the postglacial period in the study area. On the basis of the lithology, the benthic foraminiferal and ostracod assemblages, and the AMS 14 C ages, we divided the core into four depositional units (DU1–DU4 in descending order) representing the postglacial sedimentary sequence during the last ∼13,000 years. DU 4 was deposited in a littoral to tidal-flat environment during ∼13,000–11,500 cal yr BP; DU 3 in a nearshore, subtidal environment upwards to the nearshore shelf with tidal influence in response to the postglacial sea-level rise during ∼11 500–7400 cal yr BP; DU 2 in a tide-affected, nearshore shallow-sea environment during ∼7400–540 cal yr BP, on the delta-front of the Yangtze River when the delta was actively prograding; and DU 1 in a nearshore shallow-sea environment during the last ∼540 years, on the delta-front under modern marine conditions. In DU 1, the depositional rate was much higher than in the underlying units, and the sediment composition reflects more influence from the Yellow River; this unit can therefore be logically linked to the period when the Yellow River last discharged into the South Yellow Sea, from AD 1128 to 1855. According to historical records, this period can be divided into an early stage (AD 1128–1494), when the shoreline near the Yellow River mouth was advancing slowly eastward, and a late stage (1495–1855), when the shoreline was advancing rapidly. The initiation of DU 1 deposition corresponds to the latter part of the early stage, suggesting that since that time, a relatively large amount of sediment from the Old Yellow River mouth area has been transported to the offshore area of the Yangtze River mouth. Our study has identified a high-resolution sedimentary signature associated with the last course shift of the Yellow River in the Yangtze River delta-front, thus highlighting the intricate relationship between sediment sources and sinks in coastal areas. A significant contribution of the Old Yellow River sediments to the Yangtze subaqueous delta during the last ∼600 years must be considered when the evolutional history of the Yangtze delta is examined and especially when the influence of the Three Gorge Dam construction on the future changes of the Yangtze Estuary and ecological environments in the East China Sea is estimated.

中川開析谷の沖積層には泥質な海成層が厚く分布する．この海成層は完新世中期の海水準高頂期以降の奥東京湾奥から湾口にかけたデルタの前進によって形成されたと考えられてきた．開析谷を横断する3本のボーリングコア堆積物の堆積相と放射性炭素年代値を検討したところ，中川開析谷における沖積層の埋積過程は，ステージ I（～10 cal kyr BP）：蛇行河川性堆積物の累重（aggradation），ステージ II（6～10 cal kyr BP）：エスチュアリー性堆積物の開析谷中軸において厚く，開析谷縁辺において薄い累重，ステージ III（0～6 cal kyr BP）：デルタ性堆積物の湾奥から湾口，開析谷の西縁から東縁にかけた前進（progradation），に分けられることが明らかになった．ステージIIからIIIにかけて開析谷中軸では潮流が強く影響し，デルタ性堆積物の西縁から東縁にかけた堆積もこれに起因すると考えられる．

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley-fills in the Shibakawa Lowland are revealed based on sedimentary facies, physical properties and chemical element contents in the GS-KZK-1 core, Kawaguchi City, Saitama Prefecture. A transverse section based on several existing core data is also examined.The sedimentary succession is composed of Pleistocene Shimosa Group and Post-glacial incised-valley fills. The latter is subdivided into six units in ascending order; river channel fills, floodplain deposits, inner-bay deposits, tidal channel deposits, inner-bay deposits, salt marsh to back marsh deposits.The transverse section indicates that the basal gravels in the valley fill may have been derived from the gravelly layers in the Pleistocene Shimosa Group and sandy layers in the middle and upper parts do not cover the entire valley, which is mostly filled with muddy sediments.

We present oxygen and carbon isotope ratios and the morphological structure of the cultured freshwater pearl mussel (Hyriopsis sp., Unionidae) shell and pearl. The number of first-order fluctuations of δ18O of the outer shell layer along the maximum growth axis was consistent with the number of cultured years. The dominant factor controlling annual δ18O fluctuations was water temperature with a minor contribution from the variation in δ18O of ambient water, especially during the rainy season. The δ13C values were approximately constant throughout the life of the mussel, suggesting that the contributions of body size to δ13C of the shell were minor. We observed nine distinct disturbance rings on the outer surface of the shell. Five rings coincided with the five winter peaks of the δ18O profile, indicating winter growth cessation below approximately 10°C, probably because of either inactive growth at low water temperatures or reproduction. Summer disturbance rings were not observed in all years. Moreover, some summer rings showed discontinuity in the inner structure. These findings suggest that summer growth cessation may be caused by occasional events such as heavy rains, as the decrease of dissolved oxygen concentration. The δ18O profile and shell structures indicated that shell aragonite was precipitated at close to equilibrium conditions with respect to the oxygen isotope composition of the ambient water. Hyriopsis sp. shells can potentially be used for reconstruction of past hydrologic conditions. The δ18O of a pearl indicated that calcification occurred over a temperature range of at least 13–23°C. The optimal temperature for pearl calcification in this species is lower than that for marine pearl calcification.

The stratigraphy and transitions of sedimentary environments in the latest Pleistocene to Holocene incised valley fills in the Arakawa Lowland are revealed based on sedimentary facies, fossil shell assemblages, physical properties, chemical element contents and radiocarbon dating in the GS-TKT-1 core, Toda City, Saitama Prefecture.The sedimentary successions are composed of Pleistocene Shimosa Group (Unit 1), gravelly fluvial deposits (Unit 2), sandy fluvial to salt marsh deposits (Unit 3), inner bay to delta front deposits (Unit 4) and sandy fluvial deposits (Unit 5) in ascending order. Unit 2 and upper units are the latest Pleistocene to Holocene incised valley fills, indicating a transgression (Unit 2-3) followed by a regression (Unit 4-5). Depositional ages are 10000 cal yBP or before for Unit 2, 10000-9000calyBP for Unit 3, 9000-5000calyBP for Unit 4 and 5000calyBP to present for Unit 5.In a transverse section, the gravelly fluvial deposits and the sandy fluvial deposits fill the narrow axis of the incised valley under the Arakawa Lowland, the salt marsh deposits and lower part of the bay deposits extend over a buried terrace south of the valley axis, and upper part of the bay deposits, the delta front deposits and the fluvial deposits cover the entire lowland.

This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2AE 3t o 2AE6 and 1AE 8t o 2AE0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.

A subaqueous clinoform wraps around the end of the Shandong Peninsula in the Yellow Sea. The clinoform, previously called the Shandong mud wedge, is up to 40 m thick nearshore and thins seaward. Its origin has been controversial since it was first noted in the 1980s. For our recent investigation offshore of the northeastern Shandong Peninsula, high-resolution shallow seismic profiles covering about 1700 km were obtained, and two drilling cores penetrating the clinoform were recovered. We described the lithofacies characteristics, identified benthic foraminifera and ostracoda, measured grain size, analyzed the mineral components of the cores, and obtained AMS 14 C dates on 28 samples, to decipher the sedimentary evolution of the clinoform in response to postglacial sea-level changes. The shallow seismic profiles show that the clinoform comprises three units: a lowermost seismic unit (SU 3, less than 2–3 m thick) showing a retrogradational/aggradational stacking pattern with subhorizontal internal reflectors, a middle seismic unit (SU 2, up to 35 m thick) showing a prograding reflection pattern with mostly seaward-stepping reflectors separated by several erosive surfaces, and an uppermost seismic unit (SU 1, maximum thickness 18 m) exhibiting an aggradational reflection pattern with subparallel reflectors. The three seismic units are bounded on the bottom by distinct erosion surfaces. Analyses of the two cores indicate that the clinoform comprises three depositional units, DU 1, DU 2, and DU 3 in descending order, corresponding respectively to SU 1, SU 2, and SU 3. The depositional units are distinguished by lithofacies characteristics and the downcore distributions of benthic foraminiferal and ostracod assemblages. Mineral components of the cores suggest that the clinoform's provenance is predominantly Yellow River-derived sediments. AMS 14 C dates suggest that DU 3 was formed from about 11.6 to 9.6 cal kyr BP, DU 2 from 9.6 to 6.5 cal kyr BP, and DU 1 after ca. 6.5 cal kyr BP. Some major meltwater pulse (MWP) events during the deglacial transgression were registered in the clinoform stratigraphy, with the basal surfaces/sediments of DU 3 and DU 2 corresponding respectively to MWP-1B (ca. 11.6–11.3 cal kyr BP) and MWP-1C (ca. 9.6–9.2 cal kyr BP) events, and with DU 2 being largely formed after the 8.4–8.2 cal kyr meltwater event. Rapid centennial- to multidecadal-scale flooding events were presumably responsible for the erosive surfaces separating the downlapping internal reflectors in SU 2. The change of depositional pattern from progradational in SU 2 to aggradational in SU 1 is attributed to a change from strong, dominantly to-and-fro tidal currents to the modern circulation in the Bohai Sea as well as to the weakening of the tidal-current field during the highstand after about 6500 cal yr BP. As a product of complex interactions among sediment supply, sea-level changes, and hydrodynamic regimes, the clinoform can be regarded as the distal part of the tidal depositional system in the eastern Bohai Sea, which was largely derived from the Yellow River.

大宮台地の地下に分布する更新統下総層群木下層の形成過程について，コアの層相および産出する化石群集を基にシーケンス層序学的な検討を行った．大宮地域の木下層は，開析谷システムにより形成された下部とバリアー島システムによって形成された上部に分けられ，下部および上部のそれぞれに上方細粒化と上方粗粒化のセットからなる堆積相累重様式が認められる．花粉化石とテフロクロノロジーおよびMISカーブの対比に基づくと，下部はMIS6～5e前期，上部はMIS5e後期に対比され，下部の開析谷システムは低海面期堆積体および海進期堆積体，上部のバリアー島システムは高海面期堆積体と解釈される．

川口町田麦山地区は2004年新潟県中越地震によって最も甚大な被害を受けた地区の一つである．当地区の家屋の大部分は，姶良Tnテフラ降下前に形成された魚野川の河成段丘面と，段丘面を開析した谷を埋めて分布する扇状地面上に立地する．扇状地堆積物は地すべり地形が発達する鮮新統～更新統の堆積岩に由来し，軟弱な腐植まじりの砂泥互層を主体とする．地震動による家屋の被害は，扇状地堆積物の厚さが約20 mに達する扇状地中央部で最も激しく，礫層を主体とする段丘と扇状地の縁辺部では比較的軽微であった．地震被害と細粒な扇状地堆積物の厚さの密接な関係は，この堆積物が地震動を増幅し，局地的に激しい被害を引き起こしたことを示す．このことは地震ハザードマップ作成の上で注意すべき事実である．

Abundant fossil and modern specimens of the bivalve Panomya collected from Hokkaido, northern Japan, belong to five extant species, including P. norvegica and P. trapezoidis (both no longer present in Japan), and P. beringiana, P. nipponica, and P. ampla for modern specimens. I examine the taxonomy for these species around Japan and also for the extinct P. simotomensis and P. izumo from Japan and discuss the geographic history of Panomya. Panomya originated during the latest Oligocene to middle Miocene in the northeastern Gulf of Alaska. Subsequently, P. simotomensis and P. beringiana appeared in the early to late Miocene of northeastern Asia, while P. izumo and P. trapezoidis appeared in the middle to late Miocene of northeastern Asia and Alaska. Panomya nipponica, P. ampla, and P. norvegica appeared during the Plio–Pleistocene in northern Japan and Bering regions. All extant species evolved in the North Pacific; Panomya trapezoidis and P. norvegica extended their ranges to the North Atlantic in...

Oxygen and carbon isotope profiles from the outer calcite layer of two well-preserved specimens of the Pliocene scallop Fortipecten takahashii exhibit distinct attenuation in their annual ranges throughout the complete growth history of approximately 10 years. The segments of oxygen isotope profiles corresponding to winter or summer extremes were compressed, suggesting growth retardation during extreme water temperatures. These profile compressions correspond well with the positions of growth rings on the outer shell surface. One scallop shell, which evidently grew in warmer environments, based on mean oxygen isotope values, developed growth rings in summer while a second shell with colder mean-temperature values exhibits winter growth rings. A drastic reduction in isotope fluctuation occurred at a shell age of approximately 3 years, and possibly is related to the onset of the reproductive phase, which may have suppressed shell growth in favor of building up reproductive tissue. Sexual maturity brought a change in life habit, from a swimming to a reclining mode, which is clearly shown by the change in shell architecture. Although scallop shells during the juvenile stage may record a complete seasonal cycle, isotopic profiles in the adult stage contain repeated growth breaks. As a result, using isotope signals as paleoenvironmental proxies must be done carefully, at least with scallop shells.

The distribution and migration history of the genus Fortipecten, a characteristic late Cenozoic bivalve, is evaluated on the basis of specimens from Hokkaido, northern Japan. Fossil occurrences indicate that Fortipecten species lived in Hokkaido from about 7.0 to 1.2 Ma. The geological occurrences and age of Fortipecten in the northwestern Pacific imply that the biogeographic history of the species was strongly influenced by climatic fluctuations. The latest Miocene migration of its geographic range from central Hokkaido southward to northern Honshu was caused by global cooling. Its early Pliocene expansion northward to Kamchatka resulted from warm, high-stand conditions, and its subsequent range contraction resulted from stepwise cooling in the late Pliocene to early Pleistocene. The species became extinct at about 1.2 Ma as the result of extreme climatic cooling. Four species of Astartidae co-occurred with Fortipecten takahashii from three horizons of Hokkaido. The lowermost horizon (about 6–5 Ma) in the Atsuga Formation is correlated with the Astartidae-bearing horizon (5.5–4.8 Ma) in the Bear Lake Formation in southwestern Alaska, and indicates the timing of the initial opening of Bering Strait.

ロシアのサハリン島には新生界が広く分布し, サハリンと日本の地質の対比は北西太平洋における古環境変遷や北方系生物の系統解析などに重要な意味を持つ. しかし, サハリンへのアクセスの問題や両国の研究者の分類学的見解の食い違いなどから, 両地域の対比は十分とはいえない. ペレストロイカ以降地質調査でサハリン行きが可能となり, 今後の活発な研究交流が期待される.調査は Makarov 地域に分布する中部中新統 Ausi 層, Kurasi層, 下部鮮新統 Maruyama 層中部 とTaranay 地域の下部鮮新統 Maruyama 層中部で行った. 各層から貝類化石が多産し, 特に Makarov 地域の Maruyama 層中部は本邦新生代の代表化石である Fortipecten taka-hashii(Yokoyama)の模式地として知られる. ロシアではこの種を前期鮮新世の示準化石として扱っているが, この種は中新世末から前期更新世まで生息していた可能性が高く, Maruyama層の地質年代の再検討が必要である.

Landform and damage classification surveys have been conducted on six colonies worst-hit by the 2004 mid-Niigata prefecture earthquake. The landform was classified using aerial photographs taken by the US Air Force in 1947. Damage degree of each structure has been classified into four classes including foundation damage class, and location of each structure has been mapped with damage class. Around 1,000 houses and stores have been classified and mapped. In five of six colonies, buildings on the fans composed of colluvial deposits have been more likely suffered severe damage than those on terraces or dissected valleys. In the other colony located on a flood plain, no clear difference in degree of damage among landforms has been observed.

茨城県つくば市東部を流れる花室川の中流域から,Palaeoloxodon naumanni (Makiyama)の臼歯が発見された.産出層準は,最上部更新統である桜川段丘堆積物に相当する緩斜面堆積物で,約2.7万年前より新しい年代を示す.歯種は左上顎第3大臼歯であり,歯冠長は331mm,咬板数は1/2・22・1/2と,これまでに報告された臼歯の中でも大型であり,特に咬板数は最大であることがわかった.この標本の産出は,P.naumanniの時代的な形態変異を明らかにする上で重要である.