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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.

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.

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.

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...

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.

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.

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．

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.

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.