Dissolution of silica accompanied by oxygen consumption in the bottom layer of Japan’s central Seto Inland Sea in summer

Distribution of silicic acid (Si(OH)₄) in bottom water was investigated in the central Seto Inland Sea under stratified conditions in summer. Water samples were collected at 10 stations on April 24 and 25 and July 7 and 8, 2012. In July, stratification progressed, and a cold water mass (dome) of <20 °C appeared. In response to the cold dome, low oxygen content was observed in the bottom layer of the eastern part of Hiuchi-Nada. In this water mass site, apparent oxygen utilization values calculated from dissolved oxygen (DO) concentrations increased, coinciding with increase of Si(OH)₄ concentrations from April to July. This suggests that increase of Si(OH)₄ [dissolution of biogenic silica (diatom frustules)] was accompanied by DO consumption due to degradation of organic matter such as plankton soft tissue. These findings suggest that a bacterial degradation of the organic matrix that covers diatom frustules could accelerate the dissolution of biogenic silica in bottom water under stratified conditions.     

Seasonal changes of the South American monsoon system during the Mid-Holocene in the CMIP5 simulations

The South American Monsoon System (SAMS) is a major climatic feature of South America, and its domain extends from Amazon to La Plata basin. The SAMS region is vulnerable to variations of climate and precipitation patterns, which could impact economic activities and lead to potential societal consequences. In the face of a warming future scenario, the importance of the study of the past climate with numerical simulations is to evaluate the climate models and to assure the reliability of future projections. Here we investigate the Mid-Holocene SAMS, evaluating changes in strength, life cycle and associated dynamical mechanisms in ten Earth System Models simulations. Our results show that the SAMS was weaker in the Mid-Holocene than in the pre-industrial climate in December–January–February (DJF), but stronger in September–October–November (SON). This is probably a consequence of insolation variations in the Mid-Holocene, which contributed to changes in the moisture flux from the Atlantic Ocean to the continent, the strength of the upper-level atmospheric circulation, and the amount of precipitation over the SAMS region. Moreover, we suggest that the life cycle of the SAMS was altered during the Mid-Holocene, with an earlier onset and demise. Our results also indicate that Mid-Holocene SAMS changes are connected to precipitation variations near Northeast Brazil, in a dipole configuration of precipitation between western Amazon and Northeast Brazil, due to the influence of the Walker cell. Finally, this study highlights a need for improvement of the numerical models to better simulate the amount of precipitation over South America and the upper-level circulation over western Amazon in SON, which are crucial factors for a more realistic representation of the SAMS.     

Solar vector magnetograms of the Okayama Astrophysical Observatory

The title instrument is mounted on the 65 cm solar Coudé telescope at the Okayama observatory. Observation is usually of the Fei 5250 Å line. The data obtained are briefly described.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

Ultraviolet absorbers in the Venus clouds

Some absorption features in the ultraviolet spectrum of Venus observed by the OAO-2 cannot be interpreted in terms of H₂SO₄. Carbon suboxide polymer has a yellow colour and absorption at 2000 Å. Fine graphite grains have an absorption band at about 2175 Å as is well known in the case of the interstellar extinction curves. A mixture of these substances which is inevitably formed in the Venus atmosphere by photochemical reactions is the best candidate for explaining the Venus absorption features in the ultraviolet.     

Ion chemistry in the cometary atmosphere

Dirty ice of a second kind (major components, H₂O, CO, and N₂; minor components less than several percents, NH₃, CH₄, and other organic substances such as HCN, CH₃CN etc.) is assumed for the composition of volatiles in the cometary nucleus. The consistency with the observations of molecular ions and daughter molecules in the cometary atmosphere is argued by taking into account various ion-molecular reactions and dissociative recombinations. There is a satisfactory agreement for the second kind of dirty-ice model, but the presence of large amounts of CH₄ and NH₃ is found to be rather in contradiction with observational evidence. A velocity of 8 km s^?1 for the hydrogen atoms, derived from analysis of the hydrogen Lyman-alpha corona around comets, is found from the dissociative recombination of H₃O⁺, the dominant constituent of cometary ionosphere, in accordance with H₃O⁺+e ^?→OH+H+H.     

Long-term precipitation and stream discharge records at seven forested experimental watersheds along a latitudinal transect in Japan: Jozankei,Kamabuchi, Takaragawa,Tsukuba, Tatsunokuchi-yama,Kahoku and Sarukawa

This data note introduces a database of long-term daily total precipitation and stream discharge data for seven forested watersheds in Japan that have been continuously monitored by the Forestry and Forest Products Research Institute. Three of the watersheds started data collection in the 1930s. Forest cover across the sites ranges from cool to warm temperate regions with the latitude spanning from 31 to 44° N and annual precipitation ranging from 1200 to 3000 mm yr⁻¹. The effects of vegetation change via clearcutting, thinning and forest fire (among other stressors) on stream discharge can be analysed from the long-term observation sites. Moreover, this multi-site dataset allows for inter- and intra-site comparisons of annual water loss (difference of annual precipitation and stream discharge). These long-term datasets can provide comprehensive insights into the effects of climate change and other stressors on forested ecosystems, not only in Japan but across a spectrum of forest types, if combined with other long-term records from other forested watersheds across the world.     

The δ13C–δ18O variations in marble in the Hida Belt,Japan

Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ¹³C [VPDB] and δ¹⁸O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ¹³C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO₂ releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ¹³C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ¹⁸O values of calcites are remarkably lower than the marine-carbonate values. The large δ¹⁸O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ¹⁸O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (⁸⁷Sr/⁸⁶Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H₂O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.     

Petrographic and mineralogical study of hydrothermal alteration of the Rokko Granite at Hakusui-kyo and Horai-kyo along the Arima-Takatsuki Tectonic Line in western Japan

Field, hand specimen, and microscopic investigations alongside X-ray diffraction analyses revealed four types of hydrothermal alteration (Type-A, -B, -C, and -D) based on the mode of occurrence of altered rocks and alteration mineral assemblage at Hakusui-kyo and Horai-kyo along the Arima-Takatsuki Tectonic Line (ATTL) in western Japan. Type-A alteration locally occurred as gray alteration halos with sulfide minerals. Type-B and -C alterations were confined to fault gouge veins and occurred as greenish-gray veins and brown veins, respectively. Type-C alteration crosscut Type-B alteration. These alterations were associated with a number of granitic fragments including cohesive breccia and micrographic facies. Type-D alteration occurred locally in brown sediments. Different mineralogical features in the four alterations are summarized as (Type-A) illite; (Type-B) chlorite; (Type-C) limonite (Fe³⁺ hydroxides and goethite) and calcite; and (Type-D) limonite. We propose that the alterations can be broadly divided into Paleocene hydrothermal alteration (Type-A) and post-Late Miocene hydrothermal alteration (Type-B, -C, and -D): Type-A alteration occurred at approximately 200 °C during hydrothermal activity after a granitic intrusion in Late Cretaceous; Type-B, -C and -D alterations occurred under hydrothermal activity accompanying deep fluids with repeated ascents invoked by the seismicity of the ATTL after the Late Miocene. The fluids may have been the “Arima-type thermal waters” (i.e., mixtures of convective groundwater and Na-Ca-Cl-HCO₃-type fluids). Type-B alteration occurred in fractures at depths where the temperature was ≥150 °C. Type-C alteration overprinted Type-B alteration as a result of mixing of new deep fluids and descending oxidized meteoric water near the surface. Fe³⁺ hydroxides and calcite precipitated from the fluids due to the oxidation of Fe²⁺ and the degassing of CO₂, respectively, at ambient to near-boiling temperatures. When the ascending fluids gushed out from the fractures, they generated Type-D alteration at the surface under similar temperature conditions due to the oxidation of Fe²⁺.     

Seasonal variation of volatile organic iodine compounds in the water column of Funka Bay,Hokkaido, Japan

Volatile organic iodine compounds (VOIs) emitted from the ocean surface to the air play an important role in atmospheric chemistry. Shipboard observations were conducted in Funka Bay, Hokkaido, Japan, bimonthly or monthly from March 2012 to December 2014, to elucidate the seasonal variations of VOI concentrations in seawater and their sea-to-air iodine fluxes. The bay water exchanges with the open ocean water of the North Pacific twice a year (early spring and autumn). Vertical profiles of CH₂I₂, CH₂ClI, CH₃I, and C₂H₅I concentrations in the bay water were measured bimonthly or monthly within an identified water mass. The VOI concentrations began to increase after early April at the end of the diatom spring bloom, and represented substantial peaks in June or July. The temporal variation of the C₂H₅I profile, which showed a distinct peak in the bottom layer from April to July, was similar to the PO₄ ^3? variation profile. Correlation between C₂H₅I and PO₄ ^3? concentrations (r = 0.93) suggests that C₂H₅I production was associated with degradation of organic matter deposited on the bottom after the spring bloom. CH₂I₂ and CH₂ClI concentrations increased substantially in the surface and subsurface layers (0–60 m) in June or July resulted in a clear seasonal variation of the sea-to-air iodine flux of the VOIs (high in summer or autumn and low in spring).     

Disturbance of recruitment success of mantis shrimp in Tokyo Bay associated with effects of hypoxia on the early life history

We investigated effects of severe hypoxia (dissolved oxygen <1 ml l⁻¹) on recruitment of mantis shrimp Oratosquilla oratoria in Tokyo Bay. Ten-year field surveys were conducted to examine quantitative relationships in annual mean densities of larvae and juveniles, and spatial distribution of juveniles and severe hypoxia. There was no significant correlation between annual mean densities of larvae and juveniles, suggesting that mortality during larval or juvenile stages varies among years, which might have regulated abundance of young-of-the-year juveniles. Juvenile density was low in the severely hypoxic area, implying that hypoxia could affect survivals and spatial distribution of juveniles. Meanwhile, there are yearly fluctuations in juvenile density in normoxic areas of both northern and southern part of the bay. This evidence suggests that abundance of post-settled juveniles might have been determined by not only effects of hypoxia, but also other factors influencing mortality during the early life stages.