Physical properties of the shallow sediments in late Pleistocene formations,Ursa Basin,Gulf of Mexico,and their implications for generation and preservation of shallow overpressures

Understanding the evolution of abnormally high fluid pressures within sedimentary formations is critical for analysing hydrogeological processes and assessing drilling risks. We have constructed a two-dimensional basin model and have performed numerical simulations to increase the understanding of the history of fluid flow and shallow overpressures in the Pleistocene and Holocene formations in the Ursa basin, deepwater Gulf of Mexico. We measured physical properties of sediments, such as porosity and permeability, in the laboratory and estimated in situ pore pressures from preconsolidation pressures. We obtained porosity–effective stress relationships from measurements of bulk density, grain density and preconsolidation pressures in the laboratory. Porosity–effective stress relationships were also obtained from downhole density logs and measured pore pressures. The porosity–effective stress and porosity–permeability relationships obtained were applied in two-dimensional basin simulations. Results showed that high pore pressures developed shortly after sediment deposition. Peaks in pore pressure ratios were related to high sedimentation rates of mass transport deposits and the incision of the Ursa channel. Lateral flows from the area where the overburden is thick towards the area where it is thin have occurred at least since 30 ka. Present pore pressure and temperature distributions suggest that lateral flows play a role in re-distributing heat in the basin.     

Accurate ocean tide modeling in southeast Alaska and large tidal dissipation around Glacier Bay

An accurate prediction of ocean tides in southeast Alaska is developed using a regional, barotropic ocean model with a finite difference scheme. The model skill is verified by the observational tidal harmonics in southeast Alaska including Glacier Bay. The result is particularly improved in Glacier Bay compared to the previous model described by Foreman et al. (2000). The model bathymetry dominates the model skill. We re-estimate tidal energy dissipation in the Alaska Panhandle and suggest a value for tidal energy dissipation of 3.4 GW associated with the M₂ constituent which is 1.5 times the estimation of Foreman et al. (2000). A large portion of the M₂ energy budget entering through Chatham Strait is dissipated in the vicinity of Glacier Bay. Moreover, it is shown that the developed model has the potential to correct the ocean tide loading effect in geodetic data more efficiently than the model of Foreman et al. (2000), especially around Glacier Bay.     

Correction to: Street-scale storm surge load impact assessment using fine-resolution numerical modelling: a case study from Nemuro,Japan

Natural Hazards - To: Street-scale storm.     

Early Carboniferous HP metamorphism in the Hida Gaien Belt,Japan: Implications for the Palaeozoic tectonic history of proto‐Japan

We report two new eclogite localities (at Kanayamadani and Shinadani) in the high‐P (HP) metamorphic rocks of the Omi area in the western most region of Niigata Prefecture, Japan, which form part of the Hida Gaien Belt, and determine metamorphic conditions and pressure–temperature (P–T) paths. The metamorphic evolution of the eclogites is characterized by a tight hairpin‐shaped P–T path from prograde epidote–blueschist facies to peak eclogite facies and then retrograde blueschist facies. The prograde metamorphic stage is characterized by various amphibole (winchite, barroisite, glaucophane) inclusions in garnet, whereas the peak eclogite facies assemblage is characterized by omphacite, garnet, phengite and rutile. Peak P–T conditions of the eclogites were estimated to be ~600°C and up to 2.0 GPa by conventional cation‐exchange thermobarometry, Ti‐in‐zircon thermometry and quartz inclusion Raman barometry respectively. However, the Raman spectra of carbonaceous material thermometry of metapelites associated with the eclogites gave lower peak temperatures, possibly due to metamorphism at different conditions before being brought together during exhumation. The blueschist facies overprint following the peak of metamorphism is recognized by the abundance of glaucophane in the matrix. Zircon grains in blueschist facies metasedimentary samples from two localities adjacent to the eclogites have distinct oscillatory‐zoned cores and overgrowth rims. Laser ablation inductively coupled plasma mass spectrometry U–Pb ages of the detrital cores yield a wide range between 3,200 and 400 Ma, with a peak at 600–400 Ma. In the early Palaeozoic, proto‐Japan was located along the continental margin of the South China craton, providing the source of the older population of detrital zircon grains (3,200–600 Ma) deposited in the trench‐fill sediments. In addition, subduction‐related magmatism c. 500–400 Ma is recorded in the crust below proto‐Japan, which might have been the source for the younger detrital zircon grains. The peak metamorphic age was constrained by SHRIMP dating of the overgrowth rims, yielding Tournaisian ages of 347 ± 4 Ma, suggesting subduction in the early Carboniferous. Our results provide clear constraints on the initiation of subduction, accretion and the development of an arc‐trench system along the active continental margin of the South China craton and help to unravel the Palaeozoic tectonic history of proto‐Japan.     

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt,southwest Japan: Constraints from inclusions study in garnet porphyroblasts

The Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (?ω₁) of >8.5 cm^?1 and <8.5 cm^?1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ?ω₁ values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.     

An extreme flood caused by a heavy snowfall over the Indigirka River basin in Northeastern Siberia

Flooding is one of the greatest disasters that produces strong effects on the ecosystem and livelihoods of the local population. Flood frequency is expected to increase globally making its risk assessment an urgent issue. In spring-summer 2017, an extreme flooding occurred in the Indigirka River lowland of Northeastern Siberia that inundated a large area. In this study, the extent and climatic drivers of the flooding were determined using the results of field observations, satellite images, and climate reanalysis dataset, and its possible effects on the ecosystem were discussed. In 2017, a significant lowland area of around 16,016 km² was covered with water even in July, which was 5,217 km² (around 4% of the total area) greater than the water-covered area in 2015 when usual hydrological condition in the area was observed. The hydrographic signature obtained for the Indigirka River water level in 2017 was unusual. Although the water level rose sharply at the end of May (which was typical for the Arctic region), it did not fall afterwards and even increased again to an annual daily maximum value in the middle of July. The climate reanalysis dataset obtained for the temporal–spatial variations of snow water equivalent, snowmelt, and runoff over the lowland revealed that a large amount of snowmelt runoff in June and July 2017 produced a large water-covered area and unusually high river water levels that lasted until summer. Snow depth from winter to spring was largest in 2017 over the period from 2009 to 2017, and the surface of the lower reach of the lowland was partially covered with snow even in the end of June due to the extreme snowfall that occurred in October 2016. Such unusual hydrological conditions waterlogged most trees over the lowland, which caused serious ecosystem devastation and changes in the material cycle.     

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of ?1% when the signal level was <3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 μm) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flat-field) variation was 3% or less. The uncertainty in background zero level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 ± 0.03 mm, yielding a field-of-view (FOV) of 5.83° × 5.69°. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.     

The shape distribution of boulders on Asteroid 25143 Itokawa: Comparison with fragments from impact experiments

Laboratory impact experiments have found that the shape of fragments over a broad size range is distributed around the mean value of the axial ratio 2:√2:1, which is independent of a wide range of experimental conditions. We report the shape statistics of boulders with size of 0.1-30 m on the surface of Asteroid 25143 Itokawa based on high-resolution images obtained by the Hayabusa spacecraft in order to investigate whether their shape distribution is similar to the distribution obtained for fragments (smaller than 0.1 m) in laboratory impact experiments. We also investigated the shapes of boulders with size of 0.1-150 m on Asteroid 433 Eros using a few arbitrary selected images by the NEAR spacecraft, in order to compare those with the shapes on Asteroid Itokawa. In addition, the shapes of small- and fast-rotating asteroids (diameter <200 m and rotation period <1 h), which are natural fragments from past impact events among asteroids, were inferred from archived light curve data taken by ground-based telescopes. The results show that the shape distributions of laboratory fragments are similar to those of the boulders on Eros and of the small- and fast-rotating asteroids, but are different from those on Itokawa. However, we propose that the apparent difference between the boulders of Itokawa and the laboratory fragments is due to the migration of boulders. Therefore, we suggest that the shape distributions of the boulders ranging from 0.1 to 150 m in size and the small- and fast-rotating asteroids are similar to those obtained for the fragments generated in laboratory impact experiments.     

Equivalent forcing depth in tropical oceans

The forcing efficiency for the first and the second baroclinic modes by the wind stress in tropical oceans has been discussed by calculating equivalent forcing depth from annual mean, seasonal, and pentadal density profiles of the observational data. In the annual mean field, the first mode is forced preferentially in the western Pacific and the Indian Ocean, whereas the second mode is more strongly excited in the Atlantic and the eastern Pacific. This difference is mostly due to the pycnocline depth; the second mode is more dominantly forced where the pycnocline depth is shallower. We also revealed large seasonal variations of the second mode's equivalent forcing depth in the western Indian Ocean. The first mode is more dominantly forced during boreal spring and fall in the western Indian Ocean, while the second mode becomes more dominantly forced during boreal summer and winter. Those are due to seasonal variations of both the zonal wind and the pycnocline depth. Moreover, we show that the excitation of the second mode in the western Pacific increases after the late 1970s, which is associated with the decreasing trend of the zonal pycnocline gradient. Revealing the variation of the equivalent forcing depth will be useful for understanding the oceanic response to winds in tropical oceans and the improvement in the predictability of air-sea coupled climate variability in the tropics.     

Climate variability in the southern Indian Ocean as revealed by self-organizing maps

Using a non-linear statistical analysis called “self-organizing maps”, the interannual sea surface temperature (SST) variations in the southern Indian Ocean are investigated. The SST anomalies during austral summer from 1951 to 2006 are classified into nine types with differences in the position of positive and negative SST anomaly poles. To investigate the evolution of these SST anomaly poles, heat budget analysis of mixed-layer using outputs from an ocean general circulation model is conducted. The warming of the mixed-layer by the climatological shortwave radiation is enhanced (suppressed) as a result of negative (positive) mixed-layer thickness anomaly over the positive (negative) SST anomaly pole. This contribution from shortwave radiation is most dominant in the growth of SST anomalies. In contrast to the results reported so far, the contribution from latent heat flux anomaly is not so important. The discrepancy in the analysis is explained by the modulation in the contribution from the climatological heat flux by the interannual mixed-layer depth anomaly that was neglected in the past studies.