Effects of hypoxia on benthic organisms in Tokyo Bay, Japan: a review

Bottom hypoxia (dissolved oxygen concentration ≤2 ml l(-1)) from anthropogenic eutrophication is a growing global concern. Here, we summarized characteristics of hypoxia and its effects on benthic organisms in Tokyo Bay. Despite recent decreases in nutrient inputs, hypoxia has been increasing in duration and spatial extent, suggesting that the substantial loss of tidal flats from reclamation is contributing to a decrease in the ability of Tokyo Bay to recycle nutrients. Hypoxia develops in the central to northern part of the bay and persists from spring to autumn, causing defaunation of benthic organisms. After the abatement of hypoxia in autumn, the defaunated area is recolonized, either through migration or larval settlement. Some megabenthic species with a spawning peak in spring and summer experience failure of larval settlement, which is probably due to hypoxia. The adverse effects of hypoxia are an impediment to recovery of benthic organisms in Tokyo Bay.     

Characterizing atmospheric circulation signals in Greenland ice cores: insights from a weather regime approach

Greenland ice cores offer seasonal to annual records of δ¹⁸O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ¹⁸O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ¹⁸O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO? regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ¹⁸O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ¹⁸O variance explained of similar magnitude for the whole set of ice core records. Greenland δ¹⁸O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region.     

Density and undrained shear strength of bed sediment from ND-CPT

The density and the undrained shear strength (s _u) of bed sediments at either side of the Isahaya Bay dike in the Ariake Sea, Japan, were investigated using nuclear density cone penetration tests (ND-CPTs). The nuclear density cone penetrometer (ND-CP) was operated from a boat, conducting 71 ND-CPTs. Furthermore, 26 undisturbed samples were obtained for soil density and s _u measurements to calibrate the ND-CPT data. The results show that the density and s _u obtained from in situ with the ND-CPTs agree well with those from the laboratory tests on undisturbed samples, and the obtained density profiles show good repeatability. The vertical variation in density and s _u of the bed sediment can be obtained from the ND-CPT results. The relations between the density and s _u show that s _u increases with an increase in density, but that this relation is site specific. The values of s _u show an exponentially growing trend and the values of log s _u show a linearly increasing trend with density. The vertical distribution of the bed sediments can be described using the density values obtained with the ND-CPT.     

Characterization of total suspended solid dynamics in a large shallow lake using long‐term daily satellite images

Large, shallow‐water lakes located on floodplains play an important role in creating highly productive ecosystems and are prone to high concentrations of suspended solids due to sediment resuspension. In this study, the aim was to determine the dominant processes governing the total suspended solid (TSS) concentration at the water surface in Tonle Sap Lake, Cambodia, which is a large, shallow‐water lake. Satellite remotely sensed daily reflectance data from 2003 to 2017 were used. Seasonal changes in TSS concentration indicated that bottom sediment resuspension during dry seasons was mostly caused by wind and the TSS concentration was closely correlated with the water depth of the lake. The TSS concentration during flood periods was controlled by both wind and inflow currents from the Tonle Sap River. Additionally, we confirmed that surface/subsurface flow with a low TSS concentration from forests on the floodplain lowered the TSS concentration year round, except during August and September. This fact implied that the floodplain forest area decrease may increase the lake TSS concentration. An analysis of the long‐term changes in TSS indicated that a decrease in the water level during flood periods resulted in the high TSS concentrations observed during the subsequent dry periods. Therefore, climate change and water resource development, which are likely to cause water level reductions in the Mekong River during flood periods, may increase the TSS concentration in Tonle Sap Lake, particularly during the dry season.     

Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. δ¹⁵N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.     

Field measurements of absolute gravity in East Antarctica

Abstract This paper reports the results of field-based absolute gravity measurements aimed at detecting gravity change and crustal displacement caused by glacial isostatic adjustment. The project was initiated within the framework of the 53rd Japanese Antarctic Research Expedition （JARE53）. Absolute gravity measurements, together with GPS measurements, were planned at several outcrops along the Prince Olav Coast and S6ya Coast of East Antarctica, including at Syowa Station. Since the icebreaker Shirase （AGB 5003） was unable to moor alongside Syowa Station, operations were somewhat restricted during JARE53. However, despite this setback, we were able to complete measurements at two sites： Syowa Station and Langhovde. The absolute gravity value at the Syowa Station IAGBN （A） site, observed using an FG-5 absolute gravimeter （serial number 210; FG-5 #210）, was 982 524 322.7＋0.1 ktGal, and the gravity change rate at the beginning of 2012 was -0.26 gGal.a-1. An absolute gravity value of 982 535 584.2~0.7 ktGal was obtained using a portable A-10 absolute gravimeter （serial number 017; A-10 #017） at the newly located site AGS01 in Langhovde.     

Water vapor δD dynamics over China derived from SCIAMACHY satellite measurements

This study investigates water vapor isotopic patterns and controls over China using high-quality water vapor δD data retrieved from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography(SCIAMACHY) observations.The results show that water vapor δD values on both annual and seasonal time-scales broadly exhibit a continental effect,with values largely decreasing northwestward from coastal lowlands to high-elevation mountainous regions.However,region-specific analysis reveals spatially distinct patterns of water vapor δD between seasons.In the monsoon domain(e.g.,China south of 35°N),depletion in D in the summer and fall seasons is closely tied to monsoon moisture sources(the Indian and Pacific oceans) and subsequent amount effect,but higher δD values in winter and spring are a result of isotopically-enriched continental-sourced moisture proceeded by less rainout.In contrast,farther inland in China(non-monsoon domain),moisture is derived overwhelmingly from the dry continental air masses and local evaporation,and δD values are largely controlled by the temperature effect,exhibiting a seasonality with isotopically enriched summer and depleted winter/spring.The observation that the spatial pattern of water vapor δD is the opposite to that of precipitation δD in the summer season also suggests that partial evaporation of falling raindrops is a key driver of water vapor isotope in the non-monsoon domain.This study highlights the importance of non-Rayleigh factors in governing water vapor isotope,and provides constraints on precipitation isotope interpretation and modern isotope hydrological processes over China.     

Thermal Evolution of the Phaethon–Geminid Stream Complex

The thermal evolution of the Geminid meteor stream and the Phaethon–Geminid stream Complex (PGC) are summarized. Sodium contents of Geminid meteor streams are altered thermally, perhaps during orbital motion in interplanetary space due to the short perihelion distance of the orbit (q ～ 0.14 AU). However, the temperature of meteoroids is less than the sublimation temperature of Na in alkali silicates, suggesting that the parent body 3200 Phaethon itself might have suffered from the thermal processing. On the other hand, a breakup event on PGC parent is suggested by the existence of dynamically associated asteroids (Phaethon, 2005 UD and 1999 YC) sharing pristine features (C, B types). A possible mechanism behind the breakup is the sublimation of ice inside the PGC parent due to its thermal evolution. It is tempting to guess that the PGC parent might be evolved dynamically from the outer part of the main asteroid belt where the residence of ice-rich asteroids (main belt comets) into current PGC-like orbit.     

Synoptic and mesoscale controls on the isotopic composition of precipitation in the western United States

We present a new event-scale catalog of stable isotopic measurements from 5?years of storm events at 4 sites in southern California, which is used to understand the storm to storm controls on the isotopic composition of precipitation and validate the event-scale performance of an isotope-enabled GCM simulation (IsoGSM) (Yoshimura et?al. 2008). These analyses are motivated to improve the interpretation of proxy records from this region and provide guidance in testing the skill of GCMs in reproducing the hydrological variability in the western US. We find that approximately 40% of event-scale isotopic variability arises from the percentage of precipitation that is convective and the near surface relative humidity in the days prior to the storms landfall. The additional isotopic variability arises from the fact that storms arriving from different source regions advect moisture of distinct isotopic compositions. We show using both field correlation and Lagrangian trajectory analysis that the advection of subtropical and tropical moisture is important in producing the most isotopically enriched precipitation. The isotopic catalog is then used along with satellite-derived δD retrievals of atmospheric moisture to benchmark the performance of the IsoGSM model for the western US. The model is able to successfully replicate the observed isotopic variability suggesting that it is closely reproducing the moisture transport and storm track dynamics that drive the large storm-to-storm isotopic range. Notably, we find that an increase in moisture flux from the central tropical Pacific leads to a convergence of isotopically enriched water vapor in the subtropics and consequently an increase in δ¹⁸O of precipitation at sites along the entire west coast. Changes in poleward moisture flux from the central Tropical Pacific have important implications for both the global hydrological cycle and regional precipitation amounts and we suggest such changes can be captured through instrumental and proxy-reconstruction of the spatiotemporal isotopic patterns in the precipitation along the west coast of the US.     

Seismic velocity discontinuities in the crust and uppermost mantle beneath the Kanto district, central Japan, identified from receiver function imaging and repeating earthquake activity

We constructed vertical cross-sections of depth-converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the Kanto district, central Japan. Repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting Philippine Sea plate and the subducting Pacific plate. As a result, we present images of some major seismic discontinuities. The upper boundary of the Pacific plate dips to the northwest in northern Kanto and to the west–southwest in southern Kanto with some undulations. On the other hand, the upper boundary of the Philippine Sea plate as a whole dips to the northwest. However, it is concave to the northeast in the southern Boso peninsula. We suggest that the low-velocity mantle wedge may be indicated on the top of both subducting plates. Plate thickness gradually decreases to the northeast. The northeastern end of the Philippine Sea plate is interpreted to be at depths of 45–90 km. The Moho discontinuity in the overriding plate is deeper than 25 km in the northern Kanto. It contacts the subducting Philippine Sea plate in the southwestern part near 35.8°N.