Estimation of nearshore groundwater discharge and its potential effects on a fringing coral reef     

Blanco AC  Watanabe A  Nadaoka K  Motooka S  Herrera EC  Yamamoto T 《Marine pollution bulletin》2011,62(4):770-785

Radon (²²²Rn) measurements were conducted in Shiraho Reef (Okinawa, Japan) to investigate nearshore submarine groundwater discharge (SGD_nearshore) dynamics. Estimated average groundwater flux was 2-3 cm/h (maximum 7-8 cm/h). End-member radon concentration and gas transfer coefficient were identified as major factors influencing flux estimation accuracy. For the 7-km long reef, SGD_nearshore was 0.39-0.58 m³/s, less than 30% of Todoroki River’s baseflow discharge. SGD_nearshore was spatially and temporally variable, reflecting the strong influence of subsurface geology, tidal pumping, groundwater recharge, and hydraulic gradient. SGD_nearshore elevated nearshore nitrate concentrations (0.8-2.2 mg/l) to half of Todoroki River’s baseflow -N (2-4 mg/L). This increased nearshore Chl-a from 0.5-2 μg/l compared to the typically low Chl-a (<0.1-0.4 μg/l) in the moat. Diatoms and cyanobacteria concentrations exhibited an increasing trend. However, the percentage contributions of diatoms and cyanobacteria significantly decreased and increased, respectively. SGD may significantly induce the proliferation of cyanobacteria in nearshore reef areas.  相似文献   

Propagating tsunami wave and subsequent resonant response signals detected by HF radar in the Kii Channel,Japan     

Hirofumi Hinata  Satoshi Fujii  Keita Furukawa  Tomoya Kataoka  Masafumi Miyata  Takashi Kobayashi  Masahiro Mizutani  Takahiro Kokai  Nobuyoshi Kanatsu 《Estuarine, Coastal and Shelf Science》2011

Signals from the tsunami waves induced by the March 11, 2011 moment magnitude (M_w) 9.0 Tohoku-Oki earthquake and from subsequent resonances were detected as radial velocity variability by a high-frequency ocean surface radar (HF radar) installed on the eastern coast of the Kii Channel, at a range of about 1000 km from the epicenter along the eastern to southern coasts of Honshu Island. A time–distance diagram of band-passed (9–200 min) radial velocity along the beam reveals that the tsunami waves propagated from the continental shelf slope to the inner channel as progressive waves for the first three waves, and then natural oscillations were excited by the waves; and that the direction of the tsunami wave propagation and the axis of the natural oscillations differed from that of the radar beam. In addition, spectral analyses of the radial velocities and sea surface heights obtained in the channel and on the continental shelf slope suggest complex natural oscillation modes excited by the tsunami waves.  相似文献   

Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid     

Hiroshi Takeda  Hiroshi Mori  Takahiro Hiroi  Jun Saito 《Meteoritics & planetary science》1994,29(6):830-842

Abstract— We studied five new Antarctic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by a similar process but in a different body.  相似文献   

Importance of space weathering simulation products in compositional modeling of asteroids: 349 Dembowska and 446 Aeternitas as examples     

Takahiro Hiroi  Sho Sasaki 《Meteoritics & planetary science》2001,36(12):1587-1596

Abstract— Based on recent progress in simulating space weathering on asteroids using pulse‐laser irradiation onto olivine and orthopyroxene samples, detailed analyses of two of the A and R type asteroid reflectance spectra have been performed using reflectance spectra of laser‐treated samples. The visible‐near‐infrared spectrum of olivine is more altered than that of pyroxene at the same pulse‐laser energy, suggesting that olivine weathers more rapidly than orthopyroxene in space. The same trend can be detected from reflectance spectra of the asteroids, where the more olivine an asteroid has, the redder its 1 μm band continuum can become. Comparison of the 1 μm band continuum slope and the 2/1 μm band area ratio between the asteroids and olivine and pyroxene samples (including the laser‐treated ones) suggests that asteroids may be limited in the degree of space weathering they can exhibit, possibly due to the short life of their surface regolith. Their pyroxenes may also have a limited chemical composition range. Fitting the visible continuum shape and other parts of the spectra (especially the 2μm part) has been impossible with any combination of common rock‐forming minerals such as silicates and metallic irons. However, this study shows, for the first time, excellent fits of reflectance spectra of an A asteroid (Aeternitas) and an R asteroid (Dembowska), including their visible spectral curves, band depths and shapes, and overall continuum shapes. Our results provide estimates that Aeternitas consists of 2% fresh olivine, 93% space‐weathered olivine, 1% space‐weathered orthopyroxene, and 4% chromite, and that Dembowska consists of 1% fresh olivine, 55% space‐weathered olivine, and 44% space‐weathered orthopyroxene. These results suggest that space weathering effects maybe important to the interpretation of asteroid reflectance spectra, even those with deep silicate absorption bands. Modified Gaussian model deconvolutions of the laser‐irradiated olivine samples show that their identity as olivine remained. The most recent submicroscopic mineralogical analyses have revealed that the laser‐irradiated olivine samples contain nanophase iron particles similar to those in space‐weathered lunar samples.  相似文献   

Maximal size of chondrules in shock wave heating model: Stripping of liquid surface in a hypersonic rarefied gas flow     

Takaaki Kato  Taishi Nakamoto  Hitoshi Miura 《Meteoritics & planetary science》2006,41(1):49-65

Abstract— We examined partially molten dust particles that have a solid core and a surrounding liquid mantle, and estimated the maximal size of chondrules in a framework of the shock wave heating model for chondrule formation. First, we examined the dynamics of the liquid mantle by analytically solving the hydrodynamics equations for a core‐mantle structure via a linear approximation. We obtained the deformation, internal flow, pressure distribution in the liquid mantle, and the force acting on the solid core. Using these results, we estimated conditions in which liquid mantle is stripped off from the solid core. We found that when the particle radius is larger than about 1–2 mm, the stripping is expected to take place before the entire dust particle melts. So chondrules larger than about 1–2 mm are not likely to be formed by the shock wave heating mechanism. Also, we found that the stripping of the liquid mantle is more likely to occur than the fission of totally molten particles. Therefore, the maximal size of chondrules may be determined by the stripping of the liquid mantle from the partially molten dust particles in the shock waves. This maximal size is consistent with the sizes of natural chondrules.  相似文献   

Marine active-source seismic studies in the Japan Trench: a seismogenic zone in an ocean-continent collision zone     

Shuichi Kodair  Yasuyuki Nakamur  Gou Fujie  Seiichi Miura 《《地质学报》英文版》2018,92(Z1):94-95

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Noble gases in individual chondrules of the Allende CV3 chondrite     

Yayoi N. Miura  Keisuke Nagao  Makoto Kimura 《Meteoritics & planetary science》2014,49(6):1037-1056

We analyzed noble gases in nine individual chondrules, an assemblage of small chondrules, and four whole‐rock samples of the Allende CV3 chondrite. Major elements were also determined for five chondrules. The cosmic ray exposure ages are calculated from cosmogenic ³He to be 5.17 ± 0.38 and 5.15 ± 0.25 Myr for the averages of the chondrules and whole rocks, respectively, showing no significant pre‐exposure evidence for the studied chondrules. Large amounts of ³⁶Ar, ^80,82Kr, and ¹²⁸Xe produced by neutron capture are observed in most samples; the abundances of these nuclides are correlated among the samples. The epithermal neutron flux and neutron slowing down density are calculated based on [⁸⁰Kr]_n, from which a sample depth of about 30 cm can be calculated. The measured chondrules contain variable amounts of radiogenic ¹²⁹Xe. The abundance ratios of radiogenic ¹²⁹Xe to neutron capture–produced ¹²⁸Xe are rather constant among the studied chondrules; four chondrules give more precise ratios at the high‐temperature fractions, ranging from 1920 ± 80 to 2280 ± 140, which corresponds to a time difference of 3.9 ± 2.4 Myr. It is noticeable that most chondrules also contain ²⁴⁴Pu‐derived fission Xe. The average ²⁴⁴Pu/²³⁸U ratio for nine chondrules is 0.0069 ± 0.0018, which agrees well with the preferred ratio reported for chondrites.  相似文献   

Solidification depth and crystallization age of the Shiaidani Granodiorite: Constraints to the average denudation rate of the Hida Range,central Japan     

Tetsuo Kawakami  Shigeru Sueoka  Tatsunori Yokoyama  Saya Kagami  Georgina E. King  Frédéric Herman  Sumiko Tsukamoto  Takahiro Tagami 《Island Arc》2021,30(1):e12414

Solidification pressure and crystallization age of the ~5 Ma Shiaidani Granodiorite (Hida Mountain Range, central Japan) are determined based on Al-in-hornblende geobarometry and U–Pb zircon dating. Al-poor patchy replacements developed in amphiboles are common in this granite and petrographic study revealed that the replacements include chloritized biotite and albitic plagioclase. These are probably the hydrothermally recrystallized domains, and should not be used for solidification pressure estimates. Magmatic rim of amphibole is characterized by Si < 7.3 a.p.f.u. (Al_IV > 0.7 a.p.f.u.), and utilized in solidification pressure estimate that yielded 0.17–0.29 GPa. The solidification age of the granite is estimated as ~5.6–5.2 Ma using U–Pb zircon dating. From these data, the lower limit of an average denudation rate after ~5.6–5.2 Ma for the area where Shiaidani Granodiorite is exposed is estimated as 0.93–2.5 mm/year.  相似文献   

Geochemistry of caldera-forming and resurgent magmas in the Pliocene Hiwada caldera,northeast Japan: An example of a monotonous intermediate system     

Takahiro Yamamoto 《Island Arc》2021,30(1):e12415

The 2.9-Ma Hotokezawa Ignimbrite, which was ejected from the Aizu caldera cluster in the northeast Japan arc, is a typical monotonous intermediate ignimbrite, with 40–50 vol% crystals and an eruptive volume of >140 km³ dense-rock equivalent. This ignimbrite filled Hiwada caldera and was deformed by post-caldera plutonic intrusions that formed a resurgent dome. The Hotokezawa Ignimbrite is a calc-alkaline, medium-K dacite to rhyolite with SiO₂ contents of 67.9–71.3 wt%, and has homogeneous trace-element abundances and Sr–Nd isotopic ratios. These geochemical features suggest that the Hotokezawa magma was formed by partial melting of amphibolitic crustal rocks. This crystal-rich magma did not appear during the post-caldera stage. Therefore, it is plausible that the chamber of eruptible magma was emptied by the caldera-forming eruption. In contrast, post-caldera plutonic rocks exhibit a variety of compositions and have a clear SiO₂ gap corresponding to the caldera-forming magma: the early pluton (tonalite) and later ones (quartz porphyry, granite porphyries, and granite) contain 62.0–66.6 and 71.2–76.5 wt% SiO₂, respectively. The tonalite and the Hotokezawa Ignimbrite form a continuous trend in their major-element variations. The Sr–Nd isotopic ratios of the ignimbrite and tonalite overlap, but those of the porphyries and granite are more enriched. The early tonalite represents the more basic part of the Hiwada caldera system that was held in small pockets separate from the main magma chamber, because its trace-element abundances are varied and distinct from those of the Hotokezawa Ignimbrite. The distinct compositional change from the Hotokezawa Ignimbrite to the late porphyries and granite indicates that the partial melting crust generating felsic magma was renewed by the subsequent intrusion of the mantle melts. The new felsic magma ascended through subsidence-related faults into the shallow caldera system and emplaced as laccoliths forming the resurgent dome.  相似文献   

Fractional-order derivative and time-dependent viscoelastic behaviour of rocks and minerals     

Yusuke Kawada  Takahiro Yajima  Hiroyuki Nagahama 《Acta Geophysica》2013,61(6):1690-1702

A general constitutive equation for viscoelastic behaviour of rocks and minerals with fractional-order derivative is investigated. This constitutive law is derived based on differential geometry and thermodynamics of rheology, and the fractional order of derivative represents the degree of time delay. Analyzing some laboratory experimental data of high temperature deformation of rocks and minerals such as halite, marble and orthopyroxene, we propose how to determine the orders of fractional derivative for viscoelastic behaviours of rocks and minerals. The order is related to the exponents for the temporal scaling in the relaxation modulus and the stress power-law of strain rate, i.e., the non-Newtonian flow law, and considered as an indicator representing the macroscopic behaviour and microscopic dynamics of rocks.  相似文献