Kyushu University Academic Staff Educational and Research Activities Database
Search by Keyword How ro search
Keyword
Search Criteria
 
Detailed Search

List of Papers
Yusuke Okazaki Last modified date：2024.04.17

Associate Professor / (Department of Earth and Planetary Sciences, Graduate School of Sciences) / Department of Earth and Planetary Sciences / Faculty of Sciences


Papers
1. Sagawa, T., T. Hasegawa, Y. Narita, M. Yokoyama, Y. Kubota, Y. Okazaki, A.S. Goto, Y. Suzuki, K. Ikehara, T. Nakagawa, Millennial-scale paleotemperature change in the Japan Sea during Marine Isotope Stage 3: Impact of meridional oscillation of the subpolar front, Palaeogeography, Palaeoclimatology, Palaeoecology, https://doi.org/10.1016/j.palaeo.2023.111713, 626, 111713, 2023.09.
2. Horikawa, K., Y. Kozaka, Y. Okazaki, T. Sagawa, J. Onodera, H. Asahi, K.‐C. Shin, Y. Asahara, and K. Takahashi, Neodymium Isotope Records From the Northwestern Pacific: Implication for Deepwater Ventilation at Heinrich Stadial 1, PALEOCEANOGRAPHY AND PALEOCLIMATOLOGY, 10.1029/2021PA004312, 36, 10, 2021.10.
3. Hiroki Nakamura, Yusuke Okazaki, Susumu Konno, and Takeshi Nakatsuka, An assessment of diatom assemblages in the Sea of Okhotsk as a proxy for sea-ice cover, Journal of Micropaleontology, 10.5194/jm-39-77-2020, 39, 77-92, 2020.06.
4. S. Iwasaki, K. Kimoto, Y. Okazaki, M. Ikehara, Micro-CT Scanning of Tests of Three Planktic Foraminiferal Species to Clarify DissolutionProcess and Progress, Geochemistry, Geophysics, Geosystems, 10.1029/2019GC008456, 20, 12, 6051-6065, 2019.12, Evaluation of foraminiferal test dissolution in deep-sea sediments facilitates reconstruction of seawater chemistry. Here we observed test dissolution processes of the planktic foraminifera Trilobatus sacculifer, Globigerinoides ruber, and Neogloboquadrina dutertrei from midlatitudes of the western North Pacific; in these three species, we tested the ability of a new dissolution index using data from X-ray micro-computed tomography scanning. Although the dissolution process of foraminiferal tests differed slightly among species, dissolution of all species was equally assessed by the calcite density distribution (%Low-CT-number calcite volume) calculated from the CT number histogram. In T. sacculifer and G. ruber, the test area density, a conventional proxy for assessing test condition based on weight measurement, is affected by variations in the thickness of the outermost chamber wall; thus, this conventional proxy can be affected by sea surface conditions during test calcification. In contrast, the relationship between the %Low-CT-number calcite volume of tests and the deep seawater calcite saturation state suggests that X-ray micro-computed tomography scanning is applicable for evaluating the intensity of foraminiferal test dissolution at the undersaturated deep seafloor in this area and is an invaluable proxy for detecting deep seawater carbonate ion concentration changes on glacial-interglacial timescales..
5. Shinya Iwasaki, Katsunori Kimoto, Yusuke Okazaki, Minoru Ikehara, X‐ray micro‐CT scanning of tests of three planktic foraminiferal species to clarify dissolution process and progress, Geochemistry, Geophysics, Geosystems, 10.1029/2019GC008456, 20, 6051-6065, 2019.11.
6. Hiroki Matsui, Keiji Horikawa, Shun Chiyonobu, Takuya Itaki, Minoru Ikehara, Shungo Kawagata, Hitomi Wakaki-Uchimura, Yoshihiro Asahara, Osamu Seki, Yusuke Okazaki, Integrated Neogene biochemostratigraphy at DSDP Site 296 on the Kyushu–Palau Ridge in the western North Pacific, Newsletters on Stratigraphy, 10.1127/nos/2019/0549, 2019.09, We revisited the stratigraphy at Deep Sea Drilling Project (DSDP) Site 296, which has promise as a reference sequence for the mid-latitude northwestern Pacific. We constructed the biostratigraphy (calcareous nannofossils, planktic foraminifera, and radiolaria) and chemostratigraphy (strontium isotope ratios, and stable carbon and oxygen isotope ratios) to reveal continuous sedimentation at Site 296 through the past 20 million years (Myr). The biostratigraphy of calcareous nannofossils comprised 27 biohorizons from biozones NN2 to NN21 (early Miocene to Pleistocene), which are consistent with the biohorizons of planktic foraminifera and radiolaria. The uninterrupted sedimentation throughout the past 20 Myr was further supported by strontium isotope stratigraphy aligned tuned to nannofossil datums and by correlation of stable isotope data from benthic foraminifera with an isotopic compilation from the Pacific Ocean. The refined age–depth model showed low sedimentation rates (
7. Takahito Ikenoue, Katsunori Kimoto, Yusuke Okazaki, Miyako Sato, Makio C. Honda, Kozo Takahashi, Naomi Harada, Tetsuichi Fujiki, Phaeodaria An Important Carrier of Particulate Organic Carbon in the Mesopelagic Twilight Zone of the North Pacific Ocean, Global Biogeochemical Cycles, 10.1029/2019GB006258, 33, 8, 1146-1160, 2019.08, Phaeodaria, which comprise one group of large, single-celled eukaryotic zooplankton, have been largely ignored by past marine biological studies because Phaeodaria and their delicate skeletons are liable to collapse. As a result, collection and quantification of specimens are difficult, and seasonal changes of phaeodarian abundance have not been thoroughly studied. The transport of biogenic elements by sinking phaeodarians has been estimated for only a few representative species. Sinking particles >1 mm in size and swimmers have traditionally been excluded when estimating sinking particle fluxes. The focus of this study is the large number of phaeodarians among the >1-mm sinking particles collected in the western North Pacific from June 2014 to July 2015. Careful sorting by microscopic examination and chemical analyses revealed that phaeodarians accounted for up to about 10% of the organic carbon in all sinking particles and accounted for a mean of 33% of the organic carbon in the >1-mm sinking particles. The high-standing stocks of phaeodarians at depths of 150–1,000 m in the mesopelagic twilight zone suggested that particles sinking from the euphotic zone as aggregates and fecal pellets can be efficiently exported to the deep sea by the ballasting effect of large phaeodarian particles rich in organic carbon..
8. Kozaka, Y., K. Horikawa, Y. Asahara, H. Amakawa and Y Okazaki, Late Miocene-mid-Pliocene tectonically induced formation of the semi-closed Japan Sea, inferred from seawater Nd isotopes, Geology, 10.1130/G45033.1, 46, 10, 903-906, 2018.10.
9. Yukiko Kozaka, Keiji Horikawa, Yoshihiro Asahara, Hiroshi Amakawa, Yusuke Okazaki, Late Miocene-mid-Pliocene tectonically induced formation of the semi-closed Japan Sea, inferred from seawater Nd isotopes, Geology, 10.1130/G45033.1, 46, 10, 903-906, 2018.10, The Japan Sea in the western North Pacific was connected to the deep Pacific via deep seaways before it became a semi-closed marginal sea. However, the timing of the semi-closure and its cause(s) remain debatable. To further constrain the timing, for the first time we analyzed Nd isotopic compositions of fossil fish teeth and fish debris deposited in the Japan Sea (Integrated Ocean Drilling Program [IODP] Site U1425, ~2000 m water depth) over the past 10 m.y. Neodymium isotope values (-4.5 to -3.2 εNd) in the Japan Sea prior to 4.5 Ma illustrate that the deep Japan Sea was composed of an admixture of less-radiogenic Lower Circumpolar Deep Water (avg. -4.3 εNd) and more-radiogenic North Pacific Deep Water (avg. -2.2 εNd), potentially with inflow from the Okhotsk Sea. At 4.5 Ma, εNd values fell to -5.5 εNd within a span of 140 k.y., suggesting that the deep-water communication between the Japan Sea and the North Pacific rapidly declined. We argue that active mountain building in the northern Japan arc, initiated by accelerated Pacific plate motion from the late Miocene to mid-Pliocene, caused the shallowing/narrowing of the Japan Sea- Pacific seaways and rapidly made the Japan Sea semi-closed at 4.5 Ma. As a result, the Japan Sea circulation patterns were realigned; a cyclonic circulation was developed and drew less-radiogenic Amur River-influenced Okhotsk Sea water into the Japan Sea..
10. Takahito Ikenoue, Yusuke Okazaki, Kozo Takahashi, Tatsuhiko Sakamoto, Pliocene to Pleistocene radiolarian biostratigraphy and paleoceanography at IODP Site U1341 in the Bering Sea, Deep-Sea Research II, 10.1016/j.dsr2.2015.03.004, 125-126, 38-55, 2016.03.
11. Kozo Takahashi, Ana Christina Ravelo, Yusuke Okazaki, Introduction to Pliocene-Pleistocene paleoceanography of the Bering Sea, Deep-Sea Research II, https://doi.org/10.1016/j.dsr2.2016.03.001, 125-126, 1-7, 2016.03.
12. Kozo Takahashi, A. Christina Ravelo, Yusuke Okazaki, Introduction to Pliocene-Pleistocene paleoceanography of the Bering Sea, Deep-Sea Research Part II: Topical Studies in Oceanography, 10.1016/j.dsr2.2016.03.001, 125-126, 1-7, 2016.03, High resolution paleoceanography of the Pliocene-Pleistocene is important in understanding climate forcing mechanisms and associated environmental changes during this major transition from global warmth to the Ice Ages. This is particularly true in high latitude marginal seas such as the Bering Sea. The Bering Sea has been very sensitive to changes in global climate during interglacial and glacial, or Milankovitch, time scales. This is due to significant changes in water circulation, land-ocean interaction, and sea-ice formation. With the aim to reveal the climate and oceanographic history of the Bering Sea over the past 5 My, IODP Expedition 323 cored a total of 5741 m of sediment (97.4% recovery) at seven sites in 2009 on D/V JOIDES Resolution covering three regions: the Umnak Plateau, the Bowers Ridge, and the Bering Slope. The water depths of the drill sites range from 818 m to 3174 m, allowing for the characterization of past vertical water mass distribution including changes in the oxygen minimum zone. The four deepest holes range from 600 m to 745 m below the seafloor, and resulted in the recovery of long sediment sequences ranging from 1.9 My to 5 My in age. Following the expedition, two sampling parties at Kochi Core Center (for acquisition of ca. 58,000 subsamples) and two scientific meetings were conducted in order to proceed with the analyses of sediment core samples and discussions. Here, pertinent results, primarily from IODP Expedition 323, are consolidated as a single special volume of Deep-Sea Research Part II Topical Studies in Oceanography..
13. Takahito Ikenoue, Yusuke Okazaki, Kozo Takahashi, Tatsuhiko Sakamoto, Bering Sea radiolarian biostratigraphy and paleoceanography at IODP Site U1341 during the last four million years, Deep-Sea Research Part II: Topical Studies in Oceanography, 10.1016/j.dsr2.2015.03.004, 125-126, 38-55, 2016.03, Radiolarian assemblages in sediment cores were investigated at the Integrated Ocean Drilling Program Site U1341 on Bowers Ridge in the southern Bering Sea. Radiolarian biozones at Site U1341 spanned the last 4 My from the youngest Amphimelissa setosa Zone (late Quaternary), via the Stylatractus universes Zone, the Eucyrtidium matuyamai Zone and a part of the Cycladophora sakaii Zone (middle to late Pliocene). The A. setosa Zone, newly proposed in this paper, is well correlated with the Botryostrobus aquilonaris Zone in the North Pacific Ocean. The bottom of the S. universus and top of the E. matuyamai Zones are emended in this paper by using the first common occurrence of A. setosa. Seventeen radiolarian datum points were identified at Site U1341 and tied to the geomagnetic and oxygen isotope stratigraphy. Radiolarian assemblages during the last 4 My showed a turnover from subarctic-transitional species (Spongopyle osculosa and Larcopyle buetschlii) to subarctic species (Ceratospyris borealis) between 3.6 and 2.4 Ma, corresponding to the intensification of the Northern Hemisphere Glaciation (NHG). Recent polar species (A. setosa and Actinomma boreale) appeared abundantly after the Mid-Pleistocene Transition (MPT, 1.2-0.8 Ma). Repeated numbers of individual peaks of the abundances of Cycladophora davisiana, dwelling in cold and well-ventilated intermediate water, suggest intermediate to deep water formation in the Bering Sea during the last 1 My..
14. Kozo Takahashi, A. Christina Ravelo, Yusuke Okazaki, Introduction to Pliocene-Pleistocene paleoceanography of the Bering Sea, Deep-Sea Research Part II: Topical Studies in Oceanography, 10.1016/j.dsr2.2016.03.001, 125-126, 1-7, 2016.03, High resolution paleoceanography of the Pliocene-Pleistocene is important in understanding climate forcing mechanisms and associated environmental changes during this major transition from global warmth to the Ice Ages. This is particularly true in high latitude marginal seas such as the Bering Sea. The Bering Sea has been very sensitive to changes in global climate during interglacial and glacial, or Milankovitch, time scales. This is due to significant changes in water circulation, land-ocean interaction, and sea-ice formation. With the aim to reveal the climate and oceanographic history of the Bering Sea over the past 5 My, IODP Expedition 323 cored a total of 5741 m of sediment (97.4% recovery) at seven sites in 2009 on D/V JOIDES Resolution covering three regions: the Umnak Plateau, the Bowers Ridge, and the Bering Slope. The water depths of the drill sites range from 818 m to 3174 m, allowing for the characterization of past vertical water mass distribution including changes in the oxygen minimum zone. The four deepest holes range from 600 m to 745 m below the seafloor, and resulted in the recovery of long sediment sequences ranging from 1.9 My to 5 My in age. Following the expedition, two sampling parties at Kochi Core Center (for acquisition of ca. 58,000 subsamples) and two scientific meetings were conducted in order to proceed with the analyses of sediment core samples and discussions. Here, pertinent results, primarily from IODP Expedition 323, are consolidated as a single special volume of Deep-Sea Research Part II Topical Studies in Oceanography..
15. Hirofumi Asahi, Yusuke Okazaki, Minoru Ikehara, Boo-Keum Kim, Seung-Il Nam, Kozo Takahashi, Seasonal variability of O-18 and C-13 of planktic foraminifera in the Bering Sea and central subarctic Pacific during 1990-2000, PALEOCEANOGRAPHY, 10.1002/2015PA002801, 30, 10, 1328-1346, 2015.10.
16. Hirofumi Asahi, Yusuke Okazaki, Minoru Ikehara, Boo Keum Khim, Seung Il Nam, Kozo Takahashi, Seasonal variability of δ18O and δ13C of planktic foraminifera in the Bering Sea and central subarctic Pacific during 1990-2000, Paleoceanography, 10.1002/2015PA002801, 30, 10, 1328-1346, 2015.10, We evaluated a 10 year time series of δ18O and δ13C records from three planktic foraminifers (Neogloboquadrina pachyderma, Globigerina umbilicata, and Globigerinita glutinata) in the Bering Sea and central subarctic Pacific with a focus on their responses to environmental changes. Foraminiferal δ18O followed the equilibrium equation for inorganic calcite, with species-specific equilibrium offsets ranging from nearly zero (-0.02‰ for N. pachyderma and -0.01‰ for G. umbilicata) to -0.16‰ (G. glutinata). Equilibrium offsets in our sediment trap samples were smaller than those from plankton tow studies, implying that foraminiferal δ18O was modified by encrustation during settling. Habitat/calcification depths varied from 35-55 m (N. pachyderma and G. umbilicata) or 25-45 m (G. glutinata) during warm, stratified seasons to around 100 m during winter, when the mixed layer depth increases. Unlike δ18O, foraminiferal δ13C showed species-specific responses to environmental changes. We found a dependency of δ13C in G. umbilicata on CO32- concentrations in ambient seawater that agreed reasonably well with published laboratory results, suggesting that δ13C of G. umbilicata is subject to vital effects. In contrast, δ13C of N. pachyderma and G. glutinata are likely affected by other species-specific biological activities. Seasonal flux patterns reveal that fossil records of N. pachyderma and G. glutinata represent annual mean conditions, whereas that of G. umbilicata most likely indicates those of a specific season. Because none of these three taxa was abundant from December to February, their fossil records likely do not reflect isotope signals from cold seasons..
17. Shinya Iwasaki, Katsunori Kimoto, Osamu Sasaki, Harumasa Kano, Makio Honda, Yusuke Okazaki, Observation of the dissolution process of Globigerina bulloides shells (planktic foraminifera) by X-ray micro-computed tomography, Paleoceanography, 10.1002/2014PA002639, 2015.04.
18. Shinya Iwasaki, Katsunori Kimoto, Osamu Sasaki, Harumasa Kano, Makio C. Honda, Yusuke Okazaki, Observation of the dissolution process of Globigerina bulloides tests (planktic foraminifera) by X-ray microcomputed tomography, Paleoceanography, 10.1002/2014PA002639, 30, 4, 317-331, 2015.04, We performed a 9 day dissolution experiment with tests of the planktic foraminifer Globigerina bulloides at pH 6.7 ± 0.1 in water undersaturated with respect to calcite. The initial stage of the dissolution process, which is not recognizable from the surface structure of the tests, was quantitatively evaluated by X-ray microcomputed tomography (XMCT). XMCT revealed three distinct test structures: early-developed calcite formed during the juvenile stage of G. bulloides, an inner calcite layer, and an outer calcite layer. The test ultrastructure was observed by scanning electron microscopy, and CT number evaluated the density distribution in the test. The early-developed calcite and inner calcite layer had low CT numbers (500-1300; low density, porous) and were sensitive to dissolution, whereas the outer calcite layer had high CT numbers (2 = 0.87) between % Low-CT-number calcite and % Test weight loss was found. This relationship indicates that the amount of test dissolution can be estimated from the distribution of CT numbers. We propose that XMCT measurements will be useful for quantitatively estimating the amount of carbonate loss from foraminiferal tests by dissolution..
19. Akira Ijiri, Masako Yamane, Minoru Ikehara, Yusuke Yokoyama, Yusuke Okazaki, Online oxygen isotope analysis of sub-milligram quantities of biogenic opal using the inductive high-temperature carbon reduction method coupled with continuous-flow isotope ratio mass spectrometry, Journal of Quaternary Science, 10.1002/jqs.2716, 29, 455-462, 2014.07.
20. Akira Ijiri, Masako Yamane, Minoru Ikehara, Yusuke Yokoyama, Yusuke Okazaki, Online oxygen isotope analysis of sub-milligram quantities of biogenic opal using the inductive high-temperature carbon reduction method coupled with continuous-flow isotope ratio mass spectrometry, Journal of Quaternary Science, 10.1002/jqs.2716, 29, 5, 455-462, 2014.07, We present a method for determining sub-milligram quantities of biogenic opal. The method employs the inductive high-temperature carbon reduction method for dehydration of opal and reduction of silica, and a continuous-flow isotope ratio mass spectrometry system for direct analysis of the oxygen isotope ratio in the evolved carbon monoxide. The accuracy and precision of the online analytical system were evaluated by isotopic analysis of various quantities of standard SiO2 in the range 40-538μg. The time required to analyse a single sample was relatively short (50min); hence, our method is suitable for routine analysis for paleoenvironmental studies that require large amounts of time-series data. The method was validated for samples in the sub-milligram range and can be applied to oxygen isotope analysis of various types of biogenic opal that have not been analysed because of their small amounts in natural samples. We successfully applied our method to (i) the first oxygen isotope analysis of monospecific radiolarian skeletons and (ii) high-resolution oxygen isotope analysis of Holocene diatom frustules from the Southern Ocean..
21. Yusuke Okazaki, Katsunori Kimoto, Hirofumi Asahi, Miyako Sato, Yuriko Nakamura, Naomi Harada, Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea, Palaeogeography, Palaeoclimatology, Palaeoecology, 10.1016/j.palaeo.2013.12.013, 395, 53-66, 2014.02, As a source region of North Pacific Intermediate Water, the Okhotsk Sea plays an important role in the ventilation of the North Pacific. To understand the detailed oceanographic changes in this marginal sea since the last glaciation, we studied decadal to centennial scale proxy records from new sediment cores from the southwestern Okhotsk Sea. Glacial to Holocene δ14C records of benthic foraminiferal shells suggested enhanced ventilation in the Okhotsk Sea during the early deglacial period between 18 and 15ka, corresponding to Heinrich Event 1. Although the δ14C reconstruction has considerable uncertainties, the Okhotsk Sea may have acted as a source for vigorous ventilation of the subarctic Pacific during this period. CaCO3 preservation events appear to be better explained by the ventilation history of the Okhotsk Sea than by coccolithophores and foraminifera production. CaCO3 preservation started to improve during 18 to 15ka, and pronounced peaks in the CaCO3 content corresponded to the Bølling-Allerød (15 to 13ka) and Preboreal (11.5 to 10ka) warm periods. Diatom and coccolithophore productivity remained low in the Okhotsk Sea throughout the glacial to deglacial periods, different from the situation in the open subarctic Pacific, where high productivity was observed during the Bølling-Allerød period. After the Preboreal period, biogenic opal gradually increased and δ15N decreased in the southern Okhotsk Sea, suggesting that productivity was enhanced by a relaxation of the nitrate limitation..
22. Yusuke Okazaki, Katsunori Kimoto, Hirofumi Asahi, Miyako Sato, Yuriko Nakamura, Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea, Palaeogeography, Palaeoclimatology, Palaeoecology, 10.1016/j.palaeo.2013.12.013, 395, 53-66, 2014.01.
23. N. Harada, M. Sato, K. Oguri, K. Hagino, Okazaki Yusuke, K. Katsuki, Y. Tsuji, K.-H. Shin, O. Tadai, S.-I. Saitoh, H. Narita, Konno Susumu, R.W. Jordan, Y. Shiraiwa, J. Grebmeier, Enhancement of coccolithophorid blooms in the Bering Sea by recent environmental changes, GLOBAL BIOGEOCHEMICAL CYCLES, 10.1029/2011GB004177, 26, 2012.06.
24. Naomi Harada, Miyako Sato, Kazumasa Oguri, Kyoko Hagino, Yusuke Okazaki, Kota Katsuki, Yoshinori Tsuji, Kyung Hoon Shin, Osamu Tadai, Sei Ichi Saitoh, Hisashi Narita, Susumu Konno, Richard W. Jordan, Yoshihiro Shiraiwa, Jacqueline Grebmeier, Enhancement of coccolithophorid blooms in the Bering Sea by recent environmental changes, Global Biogeochemical Cycles, 10.1029/2011GB004177, 26, 2, 2012.06, Since 1997, ocean color satellite images have revealed large-scale blooms of the coccolithophorid Emiliania huxleyi in the eastern Bering Sea. The blooms are often sustained over several months and have caused ecosystem changes in the Arctic Ocean, as well as in the Bering Sea. We examined continental shelf sediment profiles of alkenone, a biomarker for E. huxleyi, covering the past ∼70 years. The alkenone records suggest that large E. huxleyi blooms are a novel feature in the Bering Sea as they have occurred only since the late 1970s. Recent changes in alkenone content were closely related to the 1976-77 climatic regime shift in the North Pacific, implying that warming and freshening of Bering Sea waters promoted E. huxleyi blooms. The production rate of diatoms (total valves in sediment samples), the dominant primary producers in the Bering Sea, also increased during the past several decades. However, the ratio of alkenone content to total diatom valves in the sediments increased as E. huxleyi production increased, suggesting that the increase in the E. huxleyi production rate frequently exceeded the increase in the diatom production rate. Overall, our results indicate a possible subarctic region ecosystem shift driven by climate change..
25. S. Sugisaki, J. P. Buylaert, A. S. Murray, N. Harada, K. Kimoto, Okazaki Yusuke, T. Sakamoto, K. Iijima, S. Tsukamoto, H. Miura, Y. Nogi, High resolution optically stimulated luminescence dating of a sediment core from the southwestern Sea of Okhotsk, Geochemistry Geophysics Geosystems, https://doi.org/10.1029/2011GC004029, 13, 2012.05.
26. S. Sugisaki, J. P. Buylaert, A. S. Murray, N. Harada, K. Kimoto, Yusuke Okazaki, T. Sakamoto, K. Iijima, S. Tsukamoto, H. Miura, Y. Nogi, High resolution optically stimulated luminescence dating of a sediment core from the southwestern Sea of Okhotsk, Geochemistry, Geophysics, Geosystems, 10.1029/2011GC004029, 13, 1, 2012.05, Optically stimulated luminescence (OSL) dating is now widely accepted as a chronometer for terrestrial sediment. More recently, it has been suggested that OSL may also be useful in the dating of deep-sea marine sediments. In this paper, we test the usefulness of high resolution quartz OSL dating in application to a 19 m marine sediment core (MR0604-PC04A) taken from the southwestern Sea of Okhotsk, immediately to the north of Hokkaido, Japan. Fine-grained quartz (4 to 11 μm) was chosen as the dosimeter, and a single-aliquot regenerative-dose protocol was used for the determination of equivalent dose (De), with stimulation by both infrared and blue light. The suitability of the measurement procedure was confirmed using dose recovery tests. A high resolution record (∼2 OSL ages/m) identified clear sedimentation rate changes down the core. The OSL ages are significantly dependent on the water content model chosen; two alternative interpretations are discussed, and the geologically preferred model identified. However, ages resulting from the observed (non-modeled) water content lie closest to the available radiocarbon ages (in the range back to 20 ka). Our OSL ages confirm the known high sedimentation rates in this locality, and for the first time demonstrate clear differences in sedimentation rate before, during and after deglaciation. Although the apparent accuracy of single sample ages is not always consistent with expectations, average ages are accurate, and our data show that OSL dating can be a powerful method for establishing high resolution marine chronologies..
27. Harada, N., M. Sato, O. Seki, A. Timmermann, H. Moossen, J. Bendle, Y. Nakamura, K. Kimoto, Y. Okazaki, K. Nagashima, S.A. Gorbarenko, A. Ijiri, T. Nakatsuka, L. Menviel, M.O. Chikamoto, A. Abe-Ouchi, and S. Schouten, Sea surface temperature changes in the Okhotsk Sea and adjacent North Pacific during the last glacial maximum and deglaciation, Deep-Sea Research II, 10.1016/j.dsr2.2011.12.007, 61-64, 93-105, 2012.03.
28. Chikamoto, M.O., L. Menviel, A. Abe-Ouchi, R. Ohgaito, A. Timmermann, Y. Okazaki, N. Harada, A. Oka, and A. Mouchet, Variability in North Pacific intermediate and deep water ventilation during Heinrich events in two coupled climate models, Deep-Sea Research II, 10.1016/j.dsr2.2011.12.002, 61-64, 114-126, 2012.03.
29. Menviel, L., A. Timmermann, O. Timm, A. Mouchet, A. Abe-Ouchi, M.O., Chikamoto, N. Harada, R. Ohgaito, and Y. Okazaki, Removing the North Pacific halocline: effects on global climate, ocean circulation and the carbon cycle, Deep-Sea Research II, 10.1016/j.dsr2.2011.03.005, 61-64, 106-113, 2012.03.
30. Hu, A., G. A. Meehl, W. Han, A. Abe-Ouchi, C. Morrill, Y. Okazaki, and M.O. Chikamoto, The Pacific-Atlantic seesaw and the Bering Strait, Geophysical Research Letters, 10.1029/2011GL050567, 39, L03702, 2012.02.
31. Okazaki, Y., T. Sagawa, H. Asahi, K. Horikawa, and J. Onodera, Ventilation changes in the western North Pacific since the last glacial period, Climate of the Past, https://doi.org/10.5194/cp-8-17-2012, 8, 17-24, 2012.01, [URL].
32. Y. Okazaki, T. Sagawa, H. Asahi, K. Horikawa, J. Onodera, Ventilation changes in the western North Pacific since the last glacial period, Climate of the Past, 10.5194/cp-8-17-2012, 8, 1, 17-24, 2012.01, We reconstructed the ventilation record of deep water at 2100 m depth in the mid-latitude western North Pacific over the past 25 kyr from radiocarbon measurements of coexisting planktic and benthic foraminiferal shells in sediment with a high sedimentation rate. The 14C data on fragile and robust planktic foraminiferal shells were concordant with each other, ensuring high quality of the reconstructed ventilation record. The radiocarbon activity changes were consistent with the atmospheric record, suggesting that no massive mixing of old carbon from the abyssal reservoir occurred throughout the glacial to deglacial periods..
33. Bernhard Chapligin, Melanie J. Leng, Elizabeth Webb, Anne Alexandre, Justin P. Dodd, Akira Ijiri, Andreas Lücke, Aldo Shemesh, Andrea Abelmann, Ulrike Herzschuh, Fred J. Longstaffe, Hanno Meyer, Robert Moschen, Yusuke Okazaki, Nicholas H. Rees, Zachary D. Sharp, Hilary J. Sloane, Corinne Sonzogni, George E.A. Swann, Florence Sylvestre, Jonathan J. Tyler, Ruth Yam, Inter-laboratory comparison of oxygen isotope compositions from biogenic silica, Geochimica et Cosmochimica Acta, 10.1016/j.gca.2011.08.011, 75, 22, 7242-7256, 2011.11, Several techniques have been introduced in the last decades for the dehydration and release of O2 from biogenic silica (opal-A) for oxygen-isotope analysis. However, only one silica standard is universally available: a quartz standard (NBS28) distributed by the IAEA, Vienna. Hence, there is a need for biogenic silica working standards. This paper compares the existing methods of oxygen-isotope analyses of opal-A and aims to characterize additional possible working standards to calibrate the δ18O values of biogenic silica. For this purpose, an inter-laboratory comparison was organized. Six potential working standard materials were analysed repeatedly against NBS28 by eight participating laboratories using their specific analytical methods. The materials cover a wide range of δ18O values (+23 to +43‰) and include diatoms (marine, lacustrine), phytoliths and synthetically-produced hydrous silica. To characterize the proposed standards, chemical analyses and imaging by scanning electron microscopy (SEM) were also performed. Despite procedural differences at each laboratory, all methods are in reasonable agreement with a standard deviation (SD) for δ18O values between 0.3‰ and 0.9‰ (1σ). Based on the results, we propose four additional biogenic silica working standards (PS1772-8: 42.8‰; BFC: 29.0‰; MSG60: 37.0‰; G95-25-CL leaves: 36.6‰) for δ18O analyses, available on request through the relevant laboratories..
34. Chapligin, B., M.J. Leng, E. Webb, A. Alexandre, J.P. Dodd, A. Ijiri, A. Lücke, A. Shemesh, A. Abelmann, U. Herzschuh, F.J. Longstaffe, H. Meyer, R. Moschen, Y. Okazaki, N.H. Rees, Z.D. Sharp, H.J. Sloan, C. Sonzogni, G.E.A. Swann, F. Sylvestre, J.J. Tyler and R. Yam, Inter-laboratory comparison of oxygen isotope compositions from biogenic silica, Geochimica et Cosmochimica Acta, https://doi.org/10.1016/j.gca.2011.08.011, 75, 7242-7256, 2011.01.
35. Okazaki, Y., A. Timmermann, L. Menviel, N. Harada, A. Abe-Ouchi, M.O. Chikamoto, A. Mouchet, and H. Asahi, Deepwater formation in the North Pacific during the last glacial termination, Science, 10.1126/science.1190612, 329, 5988, 200-204, 2010.07.
36. Katsuki, K., B.K. Khim, T. Itaki, Y. Okazaki, K. Ikehara, Y. Shin, H.I. Yoon, and C.Y. Kang, Sea-ice distribution and atmospheric pressure patterns in southwestern Okhotsk Sea since the Last Glacial Maximum, Global and Planetary Change, 10.1016/j.gloplacha.2009.12.005, 72, 99-107, 2010.01.
37. Horikawa, K., Y. Asahara, K. Yamamoto, and Y. Okazaki, Intermediate water formation in the Bering Sea during glacial periods: Evidence from neodymium isotope ratios, Geology, 10.1130/G30225.1, 38, 435-438, 2010.01.
38. Katsuki, K., B.K. Khim BK, T. Itaki, N. Harada, H. Sakai, T. Ikeda, K. Takahashi, Y. Okazaki, and H. Asahi, Land-sea linkage of Holocene paleoclimate on the Southern Bering Continental Shelf, Holocene , 10.1177/0959683609105298, 19, 747-756, 2009.01.
39. Yusuke Okazaki, Kozo Takahashi, Hirofumi Asahi, Temporal fluxes of radiolarians along the W-E transect in the central and western equatorial Pacific, 1999-2002, Micropaleontology, 54, 1, 71-86, 2008.07, Coarse size (>63μm) radiolarian fluxes along the Equator in the central and western Pacific between 145°E and 160°W (Stations MT3, MT4, MT5, MT6 and MT7) were examined during January 1999 to January 2003 in order to evaluate how radiolarian assemblages respond to the El Niño Southern Oscillations (ENSO). Radiolarian fluxes were constrained by the water masses: the Western Pacific Warm Pool (WPWP) located in the west and the Equatorial Upwelling Region (EUR) east of the WPWP in the central equatorial Pacific. In general, radiolarian fluxes gradually increased from the western sites to the eastern sites, responding to the increases in nutrient supply and primary production in the euphotic layer. Notable annual cycles of radiolarian fluxes observed at the two Stations with nearly complete time series (MT3 and MT5) were likely corresponding to the rainy seasons of Papua New Guinea (MT3) and the extent of upwelling intensity in the equatorial Pacific (MT3 and MT5). With transition from La Niña to El Niño during 2001 to 2002, high radiolarian fluxes were coincident with cooler surface water. Among the radiolarian taxa, Lithomelissa sp. group and Pseudocubus obeliscus Haeckel 1887, which were mainly dwelling in the surface water in the EUR, responded distinctively to the WPWP excursion and decreased their fluxes as surface water temperature rose..
40. Yoshiyuki Ishitani, Kozo Takahashi, Yusuke Okazaki, Seiji Tanaka, Vertical and geographic distribution of selected radiolarian species in the North Pacific, Micropaleontology, 54, 1, 27-39, 2008.07, In this paper, we compile the plankton tow and sediment trap data previously obtained in the North Pacific and discuss the relevant results for ten representative radiolarian taxa. Didymocyrtis tetrathalamus and Tetrapyle octacantha group are controlled by warm water currents. Tetrapyle octacantha group is a Tropical-Subtropical indicator whose living zone is broader than that of D. tetrathalamus. Pseudocubus obeliscus is a productivity indicator. Spongotrochus glacialis appears to be advected from the Subarctic Current through the California Current into the central Equatorial Pacific. Pseudodictyophimus gracilipes, Stylochlamydium venustum, Larcopyle butschlii, and Litharachnium tentorium show distributions conformable with the "tropical submergence" hypothesis. Botryostrobus aquilonaris and Stylodictya aculeata seem to be associated with upwelling and we define them as "upwelling" taxa..
41. Ishitani, Y., K. Takahashi, K., Y. Okazaki, and S. Tanaka, Vertical and geographic distribution of selected radiolarian species in the North Pacific, Micropaleontology, 54, 27-39, 2008.01.
42. Okazaki, Y., K. Takahashi, and H. Asahi, Temporal fluxes of radiolarians along the W-E transect in the central and western equatorial Pacific, 1999-2002, Micropaleontology, 54, 1, 71-85, 2008.01.
43. Yusuke Okazaki, Osamu Seki, Takeshi Nakatsuka, Tatsuhiko Sakamoto, Minoru Ikehara, Kozo Takahashi, Cycladophora davisiana (Radiolaria) in the Okhotsk Sea
A key for reconstructing glacial ocean conditions, Journal of Oceanography, 10.1007/s10872-006-0082-2, 62, 5, 639-648, 2006.10, Cycladophora davisiana, a radiolarian species dwelling at mesopelagic depths, is known as a representative glacial fauna due to its unique distribution during glacial periods. In the present ocean, abundant production of C. davisiana is only observed in the Okhotsk Sea, indicating an adaptation of C. davisiana for seasonal sea-ice covered conditions. We found pronounced abundant production of C. davisiana during the early to middle Holocene in the Okhotsk Sea, suggesting more favorable conditions for C. davisiana than the present Okhotsk Sea. In order to clarify the reason, oceanographic conditions during the Holocene were reconstructed based on biomarkers, lithogenic grains including ice-rafted debris (IRD), biogenic opal, and total organic carbon (TOC) in two sediment cores from the Okhotsk Sea. These indicators suggest that the pronounced C. davisiana production may be attributed to: 1) a supply to mesopelagic depths under intensified stratification of fine organic particles derived from coccolithophorids, bacteria, and detrital materials; and 2) cold, well-ventilated intermediate water formation..
44. Okazaki, Y., O. Seki, T. Nakatsuka, T. Sakamoto, M. Ikehara, and K. Takahashi, Cycladophora davisiana (Radiolaria) in the Okhotsk Sea: a key for reconstructing glacial ocean conditions, Journal of Oceanography, https://doi.org/10.1007/s10872-006-0082-2, 62, 5, 639-648, 2006.01.
45. Ono, A., K. Takahashi, K. Katsuki, Y. Okazaki, and T. Sakamoto, The Dansgaard-Oeschger cycles discovered in the up stream source region of the North Pacific Intermediate Water formation, Geophysical Research Letters, 10.1029/2004GL022260, 32, 11, L11607, 2005.01.
46. Okazaki, Y., K. Takahashi, H. Asahi, K. Katsuki, J. Hori, H. Yasuda, Y. Sagawa, and H. Tokuyama, Productivity changes in the Bering Sea during the late Quaternary, Deep-Sea Research II , 10.1016/j.dsr2.2005.07.003, 52, 16-18, 2150-2162, 2005.01.
47. Okazaki, Y., K. Takahashi, J. Onodera, and M.C. Honda, Temporal and spatial flux changes of radiolarians in the northwestern North Pacific during 1997-2000, Deep-Sea Research II , 10.1016/j.dsr2.2005.07.006, 52, 16-18, 2240-2274, 2005.01.
48. Okazaki, Y., K. Takahashi, K. Katsuki, A. Ono, J. Hori, T. Sakamoto, M. Uchida, Y. Shibata, M. Ikehara, and K. Aoki, Late Quaternary paleoceanographic changes in the southwestern part of the Okhotsk Sea: Based on analyses of geochemical, radiolarian, and diatom records, Deep-Sea Research II , 10.1016/j.dsr2.2005.07.007, 52, 16-18, 2332-2350, 2005.01.
49. Okazaki, Y., K. Takahashi, T. Itaki, and Y. Kawasaki, Comparison of radiolarian vertical distribution in the Okhotsk Sea near Kuril Islands and the northwestern North Pacific off Hokkaido Island, Marine Micropaleontology , 10.1016/j.marmicro.2004.01.003, 51, 3-4, 257-284, 2004.01.
50. Okazaki, Y., K. Takahashi, T. Nakatsuka, and M.C., Honda, The production scheme of Cycladophora davisiana (Radiolaria) in the Okhotsk Sea and the northwestern North Pacific: Implication for the paleoceanographic conditions during the glacials in the high latitude oceans, Geophysical Research Letters , 10.1029/2003GL018070, 30, 18, 1939, 2003.01.
51. Okazaki, Y., K. Takahashi, H. Yoshitani, T. Nakatsuka, M. Ikehara, and M. Wakatsuchi, Radiolarians under the seasonally sea-ice covered conditions in the Okhotsk Sea: Flux and their implications for paleoceanography, Marine Micropaleontology , 10.1016/S0377-8398(03)00037-9, 49, 3, 195-230, 2003.01.


Unauthorized reprint of the contents of this database is prohibited.

Copyright © 2006, Kyushu University. All rights reserved.