Oki Ridge Research Articles

Paleoceanographic evolution of the Japan Sea during the Pleistocene – A benthic foraminiferal perspective

We reconstructed the paleoceanographic evolution of the Japan Sea over the Pleistocene using benthic foraminiferal and organic geochemical records from sediment cores recovered at IODP Site U1426, which was drilled at a water depth of 900 m on the Oki Ridge in the southern Japan Sea. The dominant benthic foraminiferal biofacies and the assemblage composition, total organic and inorganic carbon, coarse fraction percentages indicate a notable reduction in primary productivity between 2150 and 1700 ka, due to the restricted supply of nutrients from the northern Pacific Ocean through the northern connecting strait. The inflow of the Tsushima Warm Current via the southern strait began after 1700 ka and marked a major change in the productivity regime of the Japan Sea, as shown by the increased abundance of calcareous foraminifera and enhanced CaCO3 (%). A marked shift in productivity occurred during the Middle Pleistocene Transition, as indicated by the dominance of low-oxygen tolerant species, and a higher influx of total organic carbon (%). The onset of more extreme glaciation at ~490–470 ka coincident with the Mid-Brunhes Event was linked to marked changes in foraminiferal assemblage composition, reflecting major changes in sea-level, oxygen availability at the seafloor, and carbonate compensation depth during intense glacial-interglacial cycles. Productivity changes were also modulated by 23 kyr variability, related to precessional insolation forcing at low latitudes, and superimposed on the 41 and 100 kyr variability, linked to high-latitude climate fluctuations.

Pleistocene deep-sea ostracods from the Oki Ridge, Sea of Japan (IODP Site U1426) and condition of the intermediate water

AbstractThe Sea of Japan (also termed the East Sea) has a circulation system isolated from the Pacific Ocean and East China Sea. The East Asian winter monsoon drives the circulation system and cools the Tsushima Warm Current (TWC) to form the Japan Sea Intermediate–Proper Water (JSIPW). The intermediate water conveys oxygen to deep-sea floors, which is available for benthic animals. During the Pliocene (3.5–2.8 Ma), Temperate Intermediate Water (TIW) was formed under the weak winter monsoon, and extinct ostracod TIW taxa were found. Little is known about early Pleistocene intermediate water and the extinction mode of benthic ostracods. We studied radiolarians and ostracods from deep-sea sediments between 2.0 and 1.3 Ma (Marine Oxygen Isotope Stage [MIS] 77 to MIS 41) at Integrated Ocean Drilling Program Site U1426, Sea of Japan. The ostracod faunas contained TIW and JSIPW taxa. The radiolarian subtropical-water taxa and the JSIPW ostracods indicate a small influx of the TWC and the JSIPW. The TIW occasionally expanded to the middle bathyal zone. By analogy with the relationship between the modern JSIPW and winter monsoon, weak winter monsoon possibly caused gentle temperature gradients in the water column and the expansion of the TIW. The JSIPW taxa expanded their ranges into the deep sea during interglacial periods.

Paleoceanographic implications and cyclostratigraphy of variations in well-log data from the western slope of the Ulleung Basin, East Sea

Cyclic variations of density and gamma-ray logs achieved from the two drill sites of UBGH2-1_1 and UBGH2-1_2 in the western slope of the Ulleung Basin were investigated to reveal their paleoceanographic implications and establish cyclostratigraphy of the well logs. Integration of the core, log, and seismic data from the two sites indicates that the sediments of the drilled intervals mostly consist of hemipelagic muds, with a few sporadic mass transport deposits intercalated only in the downslope site of UBGH2-1_1. Comparison with variations in sediment composition suggests that ratios of terrigeneous to biogenic materials which were modulated by orbital-scale climate changes should be responsible to the well-log variations. The established cyclostratigraphy of the well logs based on correlation with marine oxygen isotope records indicates such climatic modulation of the sedimentation in the western slope of the Ulleung Basin has persisted at least since 2.6 Ma, consistent with the variation reported from the Oki Ridge.

Chronostratigraphy and paleoceanography of a piston core (EBP-3) from the South Korea Plateau in the East Sea (Japan Sea)

A piston core (EBP-3) collected from the South Korea Plateau in the East Sea (Japan Sea) preserved typical alternations of thick, dark laminated mud layers and bioturbated hemipelagic mud layers, but included several intercalated volcanic lapilli layers. The age estimate of core EBP-3 was aided by correlation of biostratigraphic radiolarian data, the abundance of microfossils (warm-water radiolarians), and geochemical properties (CaCO3) with the proxies of the well-dated core MD01-2407 obtained from the Oki Ridge. A part from the Marine Isotope Stage (MIS) 11 to MIS 9 in the middle of core EBP-3 was missing, presumably because of erosion following the abrupt deposition of a volcanic lapilli layer. Despite some uncertainty, the age estimate of core EBP-3 revealed that the lower part of core EBP-3 represents MIS 13, as was also evident from a comparison with the oxygen isotope stratigraphy.Downcore biogenic opal and CaCO3 variations in core EBP-3 exhibited less distinct orbital-scale cyclic changes than cores from other areas in the East Sea (Japan Sea). However, differences in features were discernible between MIS 2 and MIS 12 in terms of their lithology and CaCO3 constituent, although both periods were characterized by relatively high levels of CaCO3 (about 30% for MIS 2 and about 50% for MIS 12). MIS 2 preserved a typical thick, dark laminated mud layer, whereas MIS 12 preserved a distinctly light mud layer with thin parallel laminations. This difference of the lithologic facies between MIS 2 and MIS 12 highlighted a discrepancy among the major constituents, indicating that planktonic foraminifera were dominant in MIS 2, whereas coccolithophores were dominant in MIS 12. Our results suggest that the thinly-laminated light mud layer, which is characterized by a high CaCO3 content primarily composed of coccolithophores in MIS 12, can be used as a reliable time marker for the establishment of chronostratigraphy in the East Sea.

Different nature of glacial CaCO3 constituents between MIS 2 and MIS 12 in the East Sea/Japan Sea and its paleoceanographic implication

Two piston cores (PC-05 and PC-08) were collected on the Yamato Rise in the East Sea/Japan Sea during the KR07-12 cruise. Both cores showed typical alternations of light (high L*) and dark (low L*) layers, which are characteristic in the hemi-pelagic sediment deposited in the deep part of the East Sea/Japan Sea. A composite core was achieved with the successful replacement of almost half of the upper part of core PC-05 by the entirety of core PC-08 based on the co-equivalence of L* values and the dark layers, because an interval (170 cm–410 cm) of core PC-05 was considerably disturbed due to fluidization during the core execution. Chronostratigraphy of the composite core was constructed by the direct comparison of L* values to the well-dated core MD01-2407 that was obtained in the Oki Ridge. The lower-bottom of the composite core extended to Marine Isotope Stage (MIS) 14, based on the age estimate by LR04 stacks. Downcore opal variation of the composite core exhibited the distinct orbital-scale cyclic changes; high during the interglacial and low during the glacial periods. However, downcore CaCO3 variation showed no corresponding orbital-scale cyclic change between glacial and interglacial periods. Some intervals of both periods were high in CaCO3 content. Frequent and large fluctuations in CaCO3 content seemed to be more related to the presence of dark layers containing thin lamination (TL) within the glacial and interglacial intervals. It is worthy to note that MIS 2 and MIS 12 are characterized by distinctly high CaCO3 content, showing up to 18% and 73%, respectively, among the glacial periods. Furthermore, in terms of lithology, MIS 2 was characterized by a thick dark layer (low L* values) with TL, whereas MIS 12 preserved the distinctly light layer (high L* values) with parallel laminations. Another remarkable dissimilarity between MIS 2 and MIS 12 was the nature of their CaCO3 constituent; the CaCO3 constituent of MIS 2 consisted of mostly planktonic foraminifera, whereas that of MIS 12 was mostly dump of coccolithophorids, regardless the presence of planktonic foraminifera. The distinctness of the CaCO3 constituents between MIS 2 and MIS 12 indicates that the preservation of CaCO3 contents was different temporarily during the glacial periods in the East Sea/Japan Sea. Enhanced CaCO3 preservation in MIS 2 is attributed primarily to less dissolution during the sinking through the water column or at the seafloor, but increased CaCO3 preservation in MIS 12 is mainly due to the high primary production in the surface water. With respect to the different function of the biological pump which controls CO2 cycles, the East Sea/Japan Sea clearly experienced carbonate-ocean-like state during MIS 12, despite normally silica-ocean-like state.

Orbital- and millennial-scale paleoceanographic changes in the north-eastern Japan Basin, East Sea/Japan Sea during the late Quaternary

Two gravity sediment cores (GH99-1239 and GH99-1246) obtained from the north-eastern Japan Basin in the East Sea/Japan Sea were analyzed for the orbital- and millennial-scale paleoceanographic changes. Chronostrati- graphically, core GH99-1239 represents a continuous sedimentary record since 32 ka, based on correlation of distinct lithological markers (i.e. dark layer or TL layer) with those in core GH98-1232 collected nearby. For core GH99-1246, the age model is constructed through correlation of lightness (L � ) values and tephra (Aso-4 and Toya) layers with those in the well-dated Oki Ridge core (MD01-2407), indicating about 134 ka of sedimentation since the latest Marine Isotope Stage (MIS) 6. New geochemical data from both cores corroborate orbital-scale paleoceanographic variation, such that surface-water productivity, represented by biogenic opal and total organic carbon (TOC) contents, increased during MIS 1 and MIS 5; CaCO3 contents do not show such distinct glacial-interglacial cycles, but were influenced by dissolution and preservation rather than foraminiferal production. During the glacial periods when sea ice was prevalent, surface- water productivity was low, and bottom-water conditions became anoxic, as indicated by high total sulfur (TS) contents and high Mo concentrations. The geochemical data further document millennial-scale paleoceanographic variability, corresponding to a series of thin TL layers in response to Dansgaard-Oeschger cycles but irrespective of the glacial or interglacial periods. In particular, thin TL layers formed during MIS 3 are characterized by less TOC (about 1%) and TS (about 0.4%) contents and lower Mo (about 5 p.p.m.) concentration, whereas those during MIS 4 and MIS 5 exhibit more TOC (up to 4%) and TS (up to 5%) contents and higher Mo (up to 120 p.p.m.) concentration. Such a discrepancy is attributed to different degree of surface-water productivity and of bottom-water oxygenation, which is closely related to the sea level position and extent of ventilation. Flux of the East China Sea Coastal Water controlled by millennial-scale paleoclimatic events is the most critical factor in deciding the properties of TL layers in the north-eastern Japan Basin. Our results strongly confirm that TL layers in the Japan Basin also validate the unique feature of basin-wide paleoceano- graphic signals in the East Sea/Japan Sea. Copyright # 2011 John Wiley & Sons, Ltd.

Correlation of TL layers for the synchronous paleoceanographic events in the East Sea (Sea of Japan) during the Late Quaternary

TL layers (i.e., dark mud layers), either thin or thick, as determined by the L* values with lithologic observation, are distinct paleoceanographic signatures in the East Sea (Sea of Japan), which were formed in response to the global eustatic sealevel fluctuations between interglacials and glacials during the Late Quaternary. These lithologic markers were consistently correlated among the three sediment cores (MD01-2407, 05GCRP21, and KR05-09PC1) collected from the Oki Ridge of the southern Yamato Basin, the South Korea Plateau of the northern Ulleung Basin, and the Matsumae Plateau of the northeastern Japan Basin, respectively. Reconstruction of age models for cores 05GCRP and KR05-09PC1 was aided in a good way by the AMS 14C ages of planktonic foraminifera and identification of the tephra layers, and mainly by lithostratigraphic correlation with TL layers of well-dated core MD01-2407, based on the L* values. The correlation of TL layers among the three sediment cores was remarkable, indicating that the East Sea (Sea of Japan) experienced the synchronous and basin-wide scale paleoceanographic events during the Late Quaternary. Our study exemplifies the significant usefulness of these lithologic markers for further Late Quaternary paleoceanographic investigation.

Spatial and temporal variation of geochemical properties and paleoceanographic implications in the South Korea Plateau (East Sea) during the late Quaternary

Geochemical (opal, total organic carbon (TOC), and CaCO 3) analyses of three sediment cores from the South Korea Plateau in the East Sea (Japan Sea) revealed that the late Quaternary (<190 kyr BP to the present) paleoceanographic conditions are characterized by the orbital variations of geochemical properties that have recorded the changes of surface- and bottom-water conditions. Chronostratigraphy of the sediment cores was accomplished by the direct correlation of L * values to the well-dated core MD01-2407 in Oki Ridge, supplement to the AMS 14C dates, well-known lithostratigraphic and tephrostratigraphic markers. Biogenic opal concentrations are high during Marine Isotope Stage (MIS) 1 and MIS 5.5, and low during the glacial period, suggesting that diatom production was enhanced during the warm period when the surface-water exchange between the East Sea and adjacent marginal seas was active. In contrast, glacial opal paleoproductivity appears to have been depressed, probably because of insufficient nutrient availability, resulting from limited vertical ventilation. Overall TOC contents follow the orbital variations; high during the interglacial and relatively low during the glacial time. However, high-resolution TOC fluctuations are independent of opal concentration, which may be attributed more to the preservation of organic matter rather than to the surface-water production. The high TOC contents in the thin dark mud (i.e., TL) layers may be due to the enhanced surface-water production as well as the intensified bottom-water anoxia. On the other hand, high CaCO 3 contents associated with the TL layers during the glacial and stadial period may be attributed to the reduced dissolution. The temporal variations of these geochemical properties are likely to follow the global orbital cycles, indicating the significant changes of surface- and bottom-water properties due to the eustatic sea-level fluctuations between the interglacial and glacial period. The comparable values of these geochemical properties among the sediment cores suggest that the spatial paleoceanographic conditions in the South Korea Plateau were consistent and uniform during the late Quaternary, in spite of a little local discrepancy in amounts of properties and timing of events.

Japan Sea oxygen isotope stratigraphy and global sea-level changes for the last 50,000 years recorded in sediment cores from the Oki Ridge

We obtained twenty AMS (Accelerator Mass Spectrometry) radiocarbon dates, 2 tephra layer chronologies, and a planktonic foraminiferal δ 18O stratigraphy for the last 50,000 years from Japan Sea cores from the Oki Ridge. In this paper we report AMS 14C results that allow us to reconstruct a high-resolution chronology of the cores and discuss millennial scale climate changes recorded in Japan Sea sediments. The sedimentation rate for the site increased dramatically from the glacial stage to the Holocene as a result of significant hydrological changes associated with deglaciation and global sea-level rise. A lighter δ 18O anomaly for the planktonic foraminifers was found during the LGM (Last Glacial Maximum). This shift corresponds to a freshening of the Japan Sea surface waters, and is mainly due to a global sea-level drop rather than anomalously warm SSTs (sea-surface temperatures) as previously reported. Our record combined with the global sea-level data suggests that the LGM started about 30 kyr ago and that the salinity of the Japan Sea became fresher than at present. The SSTs during MIS (Marine Oxygen Isotope Stage) 3 were relatively stable and global sea-level was not lower than ca. − 80 m. The surface water of the Japan Sea attained “normal” saline levels following the Younger Dryas cold event and reached the present-day δ 18O values 7 kyr ago.

Diagenetic modification of magnetic properties observed in a piston core (MD01–2407) from the Oki Ridge, Japan Sea

We investigated the magnetic properties of u-channel samples from a giant piston core (IMAGES MD01–2407), in order to obtain high-resolution records of past geomagnetic field and paleoenvironmental changes in the Japan Sea. Measurements of natural remanent magnetization (NRM) show occurrence of anomalous magnetic directions at several horizons, which may represent geomagnetic polarity excursions in the Brunhes Chron. However, most of them occur close to the lithological boundary including tephra layers, suggesting that these features might have been caused by deformation of core sediments. Intensity of NRM normalized by artificially induced remanence shows no significant correlation with a standard paleointensity record. This is probably caused by a variation of lithology including volcanic ash layers and effect of the diagenetic changes of magnetic minerals. Down-core variations of magnetic properties in the topmost part depict in detail the process of dissolution and alteration of magnetic minerals during reductive diagenesis, which is to be expected for hemipelagic sediments characterized by a high sedimentation rate and a high organic matter content. Magnetic concentration parameters including low-field susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization (IRM) indicate that a loss of fine-grained magnetite occurs from 0.6 to 1.2 mbsf. While remanence intensities below the level of the early diagenesis are one or two orders of magnitude lower than those in the topmost part, most of the thinly-laminated (TL) dark layers showed rather enhanced magnetic intensities. Typically, TL2 shows a marked peak at the top with a gradual decrease towards the bottom. Variation of S-ratio, which gives the proportion of a low-coercivity component in saturation IRM, suggests that low-coercivity and fine-grained ferromagnetic minerals are dominant. These data suggest new magnetic minerals formed during deposition of the TL layers, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea.

Age of the SKP-I and SKP-II tephras from the southern East Sea/Japan Sea: Implications for interstadial events recorded in sediment from marine isotope stages 3 and 4

The ages of two newly identified tephras in marine oxygen isotope stages (MIS) 3 and 4 from the southern East Sea/Japan Sea (ES/JS) have been determined. Analysis of chemical composition, grain-size distribution, sediment texture, and pumice micromorphology of material from 24 marine sediment cores was combined with a high-resolution age model established for core MD01–2407. The marine sedimentary record of the ES/JS is characterized by a drastic, millennium-scale variation in paleoceanographic conditions that is closely linked to Dansgaard–Oeschger (D–O) events. The chemical composition of the two tephras, which are stratigraphically located between previously dated AT (29.24 ka) and Aso-4 (88 ka) tephras, varies from phonolitic to trachytic (57.11–62.87 wt% SiO 2 with 5.03–6.81 wt% K 2O and 6.32–9.87 wt% Na 2O). They exhibit a conspicuously bimodal grain-size distribution, and have randomly oriented vesicles with thick walls. The dispersal patterns of these tephras differ markedly from that of the U-Oki tephra, which is originated from a subaerial eruption on Ulleung Island. The two newly identified tephras, named SKP-I and SKP-II, reach their maximum thickness and grain-size at a location near the South Korea Plateau (SKP). An age model for a core from the Oki Ridge (MD01–2407) yields eruptive ages of about 40–41 ka for the SKP-I tephra and about 60–61 ka for the SKP-II tephra. A comparison of sediment lightness ( L ⁎) with GISP2 oxygen isotope data suggests that the SKP-I tephra is stratigraphically between D–O Interstadial (IS) 9 and 10, and the SKP-II tephra is between D–O IS 17 and 18. The SKP-I and SKP-II tephras are equivalent to the U-Ym tephra described by Arai et al. [Arai, F., Oba, T., Kitazato, H., Horibe, Y., Machida, H., 1981. Late Quaternary tephrochronology and paleo-oceanography of the sediments of the Japan Sea. Quat. Res. (Daiyonki-kenkyu) 20, 209–230 (in Japanese, with English Abstr.)] and that described by Ikehara et al. [Ikehara, K., Kikkawa, K., Chun, J.-H., 2004. Origin and correlation of three tephras that erupted during oxygen isotope stage 3 found in cores from the Yamato Basin, central Japan Sea. Quat. Res. (Daiyonki-kenkyu) 43, 201–212 (in Japanese, with English Abstr.)], respectively. They not only provide marker horizons within the stratigraphic interval corresponding to MIS 3 and 4 but also can be useful to identify D–O events for marine cores from the southern part of the ES/JS.

Dust influx reconstruction during the last 26,000 years inferred from a sedimentary leaf wax record from the Japan Sea

We obtained the high-resolution record of terrestrial biomarkers (C 29 and C 31 n-alkanes) for the last 26,000 years from Oki Ridge in the south Japan Sea that enabled us to discuss millennial scale climate changes. Our sampling resolution for the biomarker during the major deglaciation period (10–19.5 cal ka BP) is 300 years and for the elemental analyses (total organic carbon and total nitrogen) is as good as ca 200 years. The estimated mass accumulation rate of these molecules during the last glacial period is substantially higher than during the Holocene. They also exhibited two distinct peaks at 17.6 cal ka BP and 11.4 cal ka BP, which are coincident with Heinrich Event 1 and the latest stage of the Younger Dryas, respectively. The unique oceanographic setting of the Japan Sea tends to preferentially preserve organic material of aeolian origin. The nature of our biomarker record in fact suggests a strong aeolian signal, and hence their flux to the Japan Sea potentially reflects the climate conditions of the dust source regions and transport intensity. Our results are consistent with previously reported monsoon variations based on other proxies that is indicative of a strong linkage between North Atlantic climate and Asian monsoon intensity.

Last stage of the Japan Sea back‐arc opening deduced from the seismic velocity structure using wide‐angle data

The Japan Sea is one of the most well studied back‐arc basins in the northwestern Pacific. The seismic crustal model, however, has been inadequate to elucidate the detailed opening model of the Japan Sea. In 2002, to clarify the late stage of the formation style of the Japan Sea opening, a seismic experiment using 35 ocean bottom seismographs (OBSs), an air gun array, and a multichannel hydrophone streamer was undertaken in the areas from the southwestern Yamato Basin, the Oki Ridge, and the southwestern Oki Trough to the coast of the southwestern Japan Island Arc. The crusts beneath the southwestern Yamato Basin and the Oki Ridge are estimated as having approximately 13 km and 19.5 km, respectively. The upper and lower crusts of the southwestern Yamato Basin are approximately 3.2 km and 8 km thick, respectively. Those of the Oki Ridge are approximately 8.2 km and 10.5 km thick, respectively. The upper crust of the Oki Ridge thickens more steeply than that of the southwestern Yamato Basin; however, the lower crust thickens more gently. The crustal structure of the southwestern Yamato Basin shows the extended continental crust accompanied with the opening of the Japan Sea. A remarkable structural characteristic, the upper crust being thinner than the lower crust, caused by listric or complicated normal faults developed in the upper crust of the southwestern Yamato Basin. This deformed upper crust is a common structural characteristic in the southern Japan Sea, which includes the Yamato Basin. The southern Yamato Basin, including the southwestern Yamato Basin, has the thinnest upper and lower crusts in the Japan Sea. For that reason, it is suggested that the southern Yamato Basin had the strongest deformation by a back‐arc opening and that the period of the opening in the southern Yamato Basin had been longest in the southern Japan Sea. The formation process of the southern Yamato Basin is inferred to have two stages: rifting and extension of continental crust separating the northeastern and southwestern Japan Island Arcs from the Asian continent and, further, the extension affected by the rotation of the southwestern Japan Island Arc.

有孔虫の殻の酸素・炭素同位体比に基づく過去１５万年間の日本海と鹿島沖の環境変化

Five distinct paleoenvironmental changes in the Japan Sea over the last 95 ka are revealedfrom the oxygen and carbon isotopes of foraminiferal tests in a piston core recovered from the Oki Ridge. Between 95 and 27 ka, the warm Tsushima Current did not flow into the Japan Sea.The environment at the seafloor fluctuated between anoxic to weakly oxic conditions. Between 27and 17 ka, freshwater input to the Japan Sea, probably from the surrounding land, stratified the water column, and the resulting severe anoxic conditions eliminated most benthic fauna. Between 17 and 10 ka, the cold Oyashio Current flowed into the Japan Sea through the Tsugaru Strait, reestablishing deepwater ventilation. Between 10 and 8 ka, bottom conditions changed from anoxic to oxic. At 10 ka, the warm Tsushima Current started to flow into the Japan Seathrough the Tsushima Strait, establishing the modern oceanographic regime at 8 ka.From high-resolution oxygen and carbon isotope analysis of both benthic and planktonic foraminifera from an IMAGES core collected from off Kashima, central Japan, very large (20°C) sea surface temperature (SST) fluctuations can be reconstructed for the last 150 kyr, with minimum SSTs of 3-4°C during Marine Isotope Stage (MIS) 2 and MIS 6/5e transition, and with peak SSTs of 22-23°C during early MIS 1 and MIS 5a/4, 5c/5b, and 5e/5d transitions. The SSTs varied in parallel with changing carbon isotope differences between Globorotalia inflata and Globigerina bulloides, which suggests that the SST changes were primarily caused by latitudinal displacements of the Kuroshio-Oyashio Currents. A strong correlation between the SST shifts and orbital forcing indicates that latitudinal displacements of the Kuroshio-Oyashio Currents were influenced by summer insolation at 65°N.

日本近海の長尺ピストンコアに見いだされたスープ状擾乱層

Soupy disturbances appeared in the long piston cores taken from the Oki Ridge, off Shiretoko, off Kashima and off Kochi during IMAGES Vil-WEPAMA Leg 2 (May-June 2001). The soupy disturbance layers were cold when the core was recovered, and they expelled the mixture of gas, water and mud through drilled holes of the core liner tube. This suggests the dissociation of gas hydrates in the core onboard. Because the presence of soupy disturbance does not prove the existence of gas hydrate by itself, we need to obtain additional evidences in a future long piston coring study.

Carbon isotopic compositions of long-chain n-alkanes in the Japan Sea sediments: implications for paleoenvironmental changes over the past 85 kyr

Carbon isotopic compositions of long-chain n-alkanes in cores KH-79-3 (C-3 and L-3) taken from the Oki Ridge (37°04′N, 134°42′E, water depth 935 m) in the Japan Sea were measured by gas chromatography/isotope ratio mass spectrometry. Fluctuations in the carbon isotopic composition of terrestrially derived material, as determined from δ 13 C n-alkane (weighted average of δ 13 C values for higher plant-derived n-C 29 and n-C 31 alkanes), range from −29.1 to −32.1‰ for the past 85 kyr in core KH-79-3, C-3. δ 13 C n-alkane values in colder climates ( 18 O stages 2 and 4) are depleted in 13 C by approximately 1‰ relative to those in warmer climates (stages 1, 3 and 5. 1). This suggests that the temporal variations of the δ 13 C values for higher plant-derived n-alkanes exhibit a close relationship with glacial–interglacial cycles. The results over the past 30 kyr ( 14 C age) reveal that the δ 13 C n-alkane values shifted abruptly from −31.1±0.5‰ to −29.9±0.4‰ at around 10 kyr B.P., with the isotopic values remaining relatively constant thereafter. This positive (1.2‰) shift in δ 13 C n-alkane values can be attributed to one or both of the following factors: (i) enhancement of primary productivity on land; and (ii) change in vegetational types (e.g. C4/C3 plants ratios and/or changes in the species of C3 plants).

Organic molecular and carbon isotopic records of the Japan Sea over the past 30 kyr

The organic and isotopic geochemical study of two sediment cores (KH‐79‐3, L‐3, and KH‐79‐3, C‐3) from the Oki Ridge in the Japan Sea has revealed that total organic carbon (TOC) mass accumulation rates are extremely high in the 12–11 ka (calendar age) interval and TOC in the sections in the 24–17 ka interval is depleted in 13C by 3.5‰ relative to Holocene sediments. Alkenone sea surface temperature (SST) shows a decrease from 18° to 14°C from 17.5 to 11.6 ka and a sharp increase from 14° to 19°C from 11.6 to 11.1 ka. The SST changes are associated with the inflow of cold seawater with the vertical water mixing and the inflowof warm Tsushima Current into the Japan Sea. The δ13C values for both 24‐methylcholesta‐5,22‐dien‐3β‐ol (diatom marker) and dinosterol (dinoflagellate marker), are at their minimum from 24 to 17 ka, while those for long‐chain alkenones are not. The theoretical considerations on δ13C for biomarkers suggest low photosynthetic carbon demand of diatoms and dinoflagellates from 24 to 17 ka.

Mineralogical association and physical properties of sediments with palaeoclimatic implications (ODP Site 798B, Japan Sea): a comparative study from core and downhole measurements

The aim of our approach is to provide a reliable and continuous mineralogical description derived from downhole logs and constrained by core measurements. This study focuses on a series of 742,000 year drilled during Ocean Drilling Program (ODP) Leg 128 at Site 798 on Oki Ridge (Japan Sea). A linear matrix algebraic procedure has been applied to eight logs (bulk density, neutron porosity, natural gamma-ray, uranium, thorium and potassium concentrations, photoelectric effect and electrical resistivity) to compute the proportions of opal, quartz, clay minerals (illite, chlorite, kaolinite, smectite) and water in the sediment. Similarly, a continuous cation exchange capacity (CEC) log is derived from the natural gamma-ray (CGR). A continuous intrinsic formation factor (FF), which depends on the structure of the pore space, is then computed from the CEC. The log-derived mineralogy and CEC are compared to laboratory analysis on samples. High-resolution mineralogy derived from the logs allows recognition of well-known episodes of climate evolution (development of glaciation and of aeolian processes at 2.5–2.3 Ma), as well as new insights into climate dynamics (occurrences of short, warm excursions and of unstable periods). Periodic oscillations in relation to orbital parameters are also deduced from log-derived mineralogy.

Early Holocene climatic instability in Japan: organic geochemical evidence in sediment cores from Lake Biwa, Lake Kizaki and the Japan Sea

Sediment cores from Lake Biwa (Shiga Prefecture) and Lake Kizaki (Nagano Prefecture) provide evidence of variations in the climate of Japan subsequent to the end of the last glacial period. Concentrations of organic carbon increase in sediments deposited after the Pleistocene–Holocene boundary, as wetter climates washed in more soil nutrients and aquatic productivity increased. Organic matter C/N ratios simultaneously decrease, indicating smaller proportions of land-derived plant matter. Lake Biwa C/N ratios and organic carbon isotope ratios fluctuate markedly between 10 and 4 Kya, suggesting that both delivery of land-derived organic matter and production of lake-derived organic matter were variable during this period of time. Similar variability in biotic paleoceanographic indicators has been reported in sediments from the Oki Ridge in the Japan Sea and has been interpreted to record early Holocene fluctuations in the intensity of the Tsushima Current. Such fluctuations in this warm-water current are likely to have caused concordant fluctuations in climate on Honshu, a relationship that explains the variability in rainfall evident in the organic matter of the lake sediments.

.DELTA.13 records of diploptene in the Japan Sea sediments over the past 25 kyr.

Stable carbon isotopic compositions were determined for diploptene in the sediment core KH-79-3, L-3 (core length: 235 cm) taken from the Oki Ridge in the southern part of the Japan Sea. Two major groups of diploptene with different δ13C values were recognized. The first group appears in the 0-130 cm section (present to 10 kyr B.P.) of the core, the δ13C values being -25.0 ± 0.7‰ on average. Diploptene in this section should have originated from cyanobacteria. The second group which is strongly depleted in 13C (-53.1 ± 5.3‰ on average), is observed in the 145-204 cm section (12-21 kyr B.P.). Diploptene in this section might have originated from methanotrophic bacteria. This indicates that an anoxic bottom water condition was developed during 12-21 kyr B.P. (the last glacial period), which was perhaps caused by the development of seawater stratification in the Japan Sea.