Postglacial stratigraphic evolution of a current-influenced sandy shelf: offshore Kujukuri strandplain,central Japan     

Naohisa Nishida  Taku Ajioka  Ken Ikehara  Rei Nakashima  Hajime Katayama  Tomoyuki Sato  Seishiro Furuyama  Toru Tamura 《Sedimentology》2020,67(1):559-575

Sandy shelf sediments are important elements of clastic sedimentary systems because of their wide distribution in the geological record and their significance as hydrocarbon reservoirs. Although many studies have investigated shelf sediments influenced by waves or tidal currents, little is known about shelf sediments influenced by oceanic currents, particularly their lithofacies characteristics and stratigraphic evolution. This study investigated the stratigraphic evolution of shelf sediments off the Kujukuri strandplain facing the Pacific Ocean, which is influenced by the strong Kuroshio Current. Sediment cores were obtained from six locations on the Kujukuri shelf (34 to 124 m water depth) using a vibrocorer. The dominant lithofacies is mud-free sand with low-angle cross-lamination associated with alternating beds of finer and coarser sand with cross-lamination. These display depositional processes influenced by storm waves and the Kuroshio Current, respectively. This finding is consistent with the previously presented modern and historical observations of the Kuroshio Current and estimates of the storm-wave base. Radiocarbon dates show that the sediment succession formed during the last transgressive and highstand stages after 13·1 ka. The depositional processes during the stages represent a transition from storm waves with abundant sediment supply to both storm waves and the Kuroshio Current with sediment starvation mainly due to its trapping in the strandplain. Comparison to other Holocene–Modern shelf systems suggests that the sandy shelf successions are strongly influenced by oceanic currents under conditions of limited riverine input and open coastal geometry. The resultant sand-dominated succession is characterized by reversal of the proximal to distal grain-size trend compared to the fining for most other recognized wave/storm-dominated shelf successions. This is because of seaward increase in the influence of the Kuroshio Current. Thus, shelf deposits are naturally complex, and these may be further complicated by the additional influence of oceanic currents above the usual wave-dominated and tide-dominated end members.  相似文献   

Growth stimulation and inhibition of natural phytoplankton communities by model organic ligands in the western subarctic Pacific     

Yoshiko Kondo  Shigenobu Takeda  Jun Nishioka  Mitsuhide Sato  Hiroaki Saito  Koji Suzuki  Ken Furuya 《Journal of Oceanography》2013,69(1):97-115

The influence of organic ligands on natural phytoplankton growth was investigated in high-nitrate low-chlorophyll (HNLC) waters and during a phytoplankton bloom induced by a mesoscale iron enrichment experiment (SEEDS II) in the western subarctic Pacific. The growth responses of the phytoplankton in the treatments with iron complexed with model ligand were compared with those with inorganic iron or a control. Desferrioxamine B and protoporphyrin IX were used as models for hydroxamate-type siderophore and tetrapyrrole-type cell breakdown ligand, respectively. In the HNLC water, iron associated with protoporphyrin IX especially stimulated smaller phytoplankton (<10 μm) growth, 1.5-fold more than did inorganic iron. Surprisingly, only the addition of protoporphyrin IX stimulated small phytoplankton growth, suggesting that these cell breakdown ligands might be more bioavailable for them. The protoporphyrin IX’s stimulatory effect on small phytoplankton was not observed during bloom decline phase. The growth of phytoplankton was inhibited in the treatment with desferrioxamine B-complexed iron, suggesting its low bioavailability for the natural phytoplankton community. Its inhibitory effects were particularly pronounced in pico-eukaryotic phytoplankton. During the iron-induced bloom, the phytoplankton’s iron-stress response gradually increased with the desferrioxamine B concentration, suggesting that the competition for iron complexation between natural ligands and desferrioxamine B affected phytoplankton growth. However, the pico-eukaryotes did seem better able to utilize the desferrioxamine B-complexed iron during the bloom-developing phase. These results indicate that the iron bioavailability for phytoplankton differs between bloom-developing and bloom-decline phases.  相似文献   

Size fraction and class composition of phytoplankton in the Antarctic marginal ice zone along the 140°E meridian during February–March 2003     

Fuminori Hashihama  Toru Hirawake  Sakae Kudoh  Jota Kanda  Ken Furuya  Yukuya Yamaguchi  Takashi Ishimaru 《Polar Science》2008,2(2):109-120

We investigated the size fraction and pigment-derived class compositions of phytoplankton within the euphotic zone of the Antarctic marginal ice zone between 63.3°S and 66.5°S along the 140°E meridian on two consecutive cruises in the late austral summer and early austral autumn of 2003. We observed significant temporal and spatial variations in phytoplankton size and taxonomic composition, although chlorophyll a concentrations were generally below 1 μg l⁻¹ during both periods. Microphytoplankton (>20 μm), mainly diatoms, were prominent in the euphotic zone in the southernmost area around 66.5°S during late summer. In the rest of the study area during both cruises, the phytoplankton community was dominated by pico- and nano-sized populations (<20 μm) throughout the euphotic zone. The small-size populations mostly consisted of diatoms and haptophytes, although chlorophytes were dominant in extremely cold water (−1.5°C) below the overlying warm water around 65.5°S during late summer. From late summer to early autumn, chlorophytes declined in abundance, probably due to increasing temperature within the euphotic zone (−1 to 0°C). These pico- and nano-phytoplankton-dominated populations were often accompanied by relatively high concentrations of ammonium, suggesting the active regeneration of nutrients within the small-size plankton community.  相似文献   

Evolution processes of Ordovician–Devonian arc system in the South‐Kitakami Massif and its relevance to the Ordovician ophiolite pulse          下载免费PDF全文

Kazuhito Ozawa  Hirokazu Maekawa  Ken Shibata  Yoshihiro Asahara  Masako Yoshikawa 《Island Arc》2015,24(2):73-118

The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.  相似文献   

Characterization of the crust of the Coast Mountains Batholith,British Columbia,from P to S converted seismic waves and petrologic modeling     

Josh A. Calkins  George Zandt  James Girardi  Ken Dueker  George E. Gehrels  Mihai N. Ducea 《Earth and Planetary Science Letters》2010,289(1-2):145-155

The late Triassic to early Tertiary Coast Mountains Batholith (CMB) of British Columbia provides an ideal locale to study the processes whereby accreted terranes and subduction-related melts interact to form stable continental crust of intermediate to felsic composition and complementary ultramafic residuals. Seismic measurements, combined with calculated elastic properties of various CMB rock compositions, provide a window into the deep-crustal lithologies that are key to understanding the processes of continental growth and evolution. We use a combination of seismic observations and petrologic modeling to construct hypothetical crustal sections at representative locations across the CMB, then test the viability of these sections via forward modeling with synthetic seismic data. The compositions that make up our petrologic forward models are based on calculations using the free energy minimization program Perple_X to predict mineral assemblages at depth for the bulk compositions of exposed plutonic rocks collected in the study area. Seismic data were collected along two transects in west-central British Columbia: a southern line that crossed the CMB near the town of Bella Coola (near 52° N), and a northern line centered on the towns of Terrace and Kitimat (near 54° N). Along both transects, seismic receiver functions reveal high V_p/V_s ratios near the Insular/Intermontane terrane boundary and crustal thickness increasing from 26 ± 3 km to 34 ± 3 km (at the 1 sigma certainty level) from west to east across the Coast Shear Zone (CSZ). On the southern line, we observe an anomalous region of complex receiver functions and diminished Moho signals beneath the central portion of the CMB. Our petrologic and seismic profiles show that observed seismic data from much of the CMB can be well-matched in terms of crustal thickness and structure, average V_p/V_s, and amplitude of the Moho converted phase, without including ultramafic residual material in the lower crust.  相似文献   

Concentrations of 137Cs, 90Sr, 108m Ag, 239+240 Pu and atom ratio of 240Pu/239Pu in tanner crabs, Chionoecetes japonicus and Chionoecetes opilio collected around Japan     

Morita T  Ohtsuka Y  Fujimoto K  Minamisako Y  Iida R  Nakamura M  Kayama T 《Marine pollution bulletin》2010,60(12):2311-2322

The anthropogenic radionuclides, ¹³⁷Cs, ⁹⁰Sr, ^108mAg, ^239+240Pu, were measured in two Chionoecetes species, red queen crab (Chionoecetes japonicus) and snow crab (Chionoecetes opilio) collected around Japan during 1996–2007. There was no increase in the concentrations of these radionuclides and no large variation of the atom ratio of ²⁴⁰Pu/²³⁹Pu during this research period. These results indicated that the source of the radionuclides was not the radioactive wastes dumped by the former USSR and Russia and originated from past nuclear weapon tests. The higher atom ratio in the crab species than that from global fallout would be contributed by the Pacific Proving Grounds close-in fallout. The variability of the concentration of radionuclides in the crab species would result from the variability of the composition and quantity in the diet. However, the decrease in the concentration of radionuclides with sampling depth would depend on the concentration in the seawater and diet.  相似文献   

‘Thailand was a desert' during the mid‐Cretaceous: Equatorward shift of the subtropical high‐pressure belt indicated by eolian deposits (Phu Thok Formation) in the Khorat Basin,northeastern Thailand     

Hitoshi Hasegawa  Suvapak Imsamut  Punya Charusiri  Ryuji Tada  Yu Horiuchi  Ken‐Ichiro Hisada 《Island Arc》2010,19(4):605-621

The Tibetan Plateau is a key factor in controlling the present‐day climate and atmospheric circulation pattern in Asia. The pattern of atmospheric circulation after the uplift of the plateau is well known, whereas direct evidence is lacking regarding the nature of the circulation pattern prior to the uplift. The distribution of desert directly reflects the position of the subtropical high‐pressure belt, and the prevailing surface‐wind pattern recorded in desert deposits reveals the position of its divergence axis. Cretaceous eolian sandstone of the Phu Thok Formation is extensively exposed in the northern Khorat Basin, northeastern Thailand. We conducted a sedimentological study on this formation to reconstruct temporal changes in the latitude of the subtropical high‐pressure belt in low‐latitude Asia during the Cretaceous. Spatio‐temporal changes in the paleo‐wind directions recorded in the Phu Thok Formation reveal that the Khorat Basin mainly belonged to the northeast trade wind belt and subtropical high‐pressure belt was situated to the north of the Khorat Basin during the initial stages of deposition, shifted southward to immediately above the basin during the main phase of deposition, and then shifted northward again to the north of the basin during the final stages of deposition. The paleomagnetic polarity sequence obtained for the Phu Thok Formation comprises three zones of normal polarity and two of reversed polarity, correlating to chrons M1n to C34n of the geomagnetic polarity time scale. This result suggests that the Phu Thok Formation is mid‐Cretaceous in age (from c. 126 Ma to c. 99–93 Ma), similar to the age of eolian sandstone in the Sichuan Basin, southern China (the Jiaguan Formation). These results, in combination with paleo‐wind direction data, suggest the development of low‐latitude desert and an equatorward shift of the subtropical high‐pressure belt (relative to the present‐day) in Asia during the mid‐Cretaceous.  相似文献   

Influence of shear angle on hangingwall deformation during tectonic inversion     

Yasuhiro Yamada  Ken McClay 《Island Arc》2010,19(3):546-559

Tectonic inversion is a common phenomenon in island arc settings, especially in back‐arc basins. The reactivation of normal faults as thrusts, triggered by tectonic inversion, produces typical inversion fault‐related folds and thrusts in the hangingwall. These hangingwall inversion geometries are affected by two factors: the geometry of the underlying master fault and the angle of inclined simple shear relative to the regional dip of strata, in the case that the deformation is approximated by simple shear. This study employed numerical simulations to analyse the influence of the antithetic shear angle on the geometry of the hangingwall and displacement along the master fault. The simulation results reveal that a steeply inclined shear vector during extension produces a narrow, steep‐sided half‐graben, whereas a gently inclined shear produces a wide, open basin. After tectonic inversion, a tight anticline is formed under steeply inclined shear, whereas an open anticline is formed under gently inclined shear. Antithetic shear results in reduced total displacement along the master fault, and the greater the angle between the shear direction and the regional dip, the greater the displacement along the master fault. Because the deformation geometry of syn‐extension layers is affected by extension followed by contraction, a change in the shear angle during tectonic inversion produces a wide variety of deformation geometries. Comparison of the simulation results with the results of analogue modelling suggests that the shear angle decreases by 5° during the transition from extension to tectonic inversion and that such a change may be commonly observed in natural geological structures. These results highlight the benefits of numerical simulations, which can be used to readily examine a variety of constraining parameters and thereby lead to a better understanding of the mechanism of hangingwall deformation, avoiding erroneous estimates of the amount of fault displacement.  相似文献   

Mixed Land Use and Obesity: An Empirical Comparison of Alternative Land Use Measures and Geographic Scales     

Ikuho Yamada  Barbara B. Brown  Ken R. Smith  Cathleen D. Zick  Lori Kowaleski-Jones  Jessie X. Fan 《The Professional geographer》2013,65(2):157-177

Obesity is a growing epidemic in the United States. Walkable neighborhoods, characterized as having the three Ds of walkability (population Density, land use Diversity, and pedestrian-friendly Design), have been identified as a potentially promising factor to prevent obesity for residents. Past studies examining the relationship between obesity and walkability vary in geographic scales of neighborhood definitions and methods of measuring the three Ds. To better understand potential influences of these sometimes arbitrary choices, we test how four types of alternative measures of land use diversity measured at three geographic scales relate to body mass index for 4,960 Salt Lake County adults. Generalized estimation equation models demonstrate that optimal diversity measures differed by gender and geographic scale and that integrating walkability measures at different scales improved the overall performance of models.  相似文献   

Mechanisms of tropical precipitation biases in climate models     

Kim  Hanjun  Kang  Sarah M.  Takahashi  Ken  Donohoe  Aaron  Pendergrass  Angeline G. 《Climate Dynamics》2021,56(1-2):17-27

Climate Dynamics - We investigate the possible causes for inter-model spread in tropical zonal-mean precipitation pattern, which is divided into hemispherically symmetric and anti-symmetric modes...  相似文献