Many studies addressed the global cycle of methane, an important greenhouse gas. Yet variations of atmospheric methane concentration over the last 40 years have not been well explained. Synthesis analysis, which combines both bottom-up and top-down methodologies, is the current activity of the international research community. Measurements of isotope ratios of atmospheric methane have been made for the last decades. However, the research background has changed dramatically due to advances such as increasing number of data aided by progress of measurement techniques, more detailed survey of isotope signatures of various methane sources, and the aforementioned synthesis analyses. This presentation will cover the current understanding of the global methane budget and challenges in isotope measurement.

In Japan there is an ongoing plan to launch the “Global Observing SATellite for Greenhouse gases and Water cycle (GOSAT-GW)”, with an objective to observe carbon dioxide, methane, and nitrogen dioxides, providing the data at a horizontal resolution of 3 km or less. The missions of GOSAT-GW include (1) monitoring of whole atmosphere-mean concentrations of GHGs, (2) validation of nationwide anthropogenic emissions of GHGs, and (3) detection of GHGs emissions from large sources. We will brief an overview of its objectives and current status, with a focus on scientific targets and approaches, and implications for international climate change abatement strategies.

Environmental research institutes belonging to prefectural governments are collaborating to create publically open database of nation-wide wet deposition. We are continuing sampling and analysis of inorganic ions in wet deposition in Tsugaru, Aomori area. Wet deposition in Tsugaru, Aomori area shows higher nitrate/non-sea salt sulfate ratio than the national average. To identify the cause of different nitrate/non-sea salt sulfate ratio, sampling and analysis of deposited snow in the mountainous area near Hirosaki city. The nitrate/non-sea salt sulfate ratios in the deposited snow samples were similar to the wet deposition samples of the urban sampling sites, which suggests an existence of local emission source of nitrate. Deposited snow samples at Mt. Iwaki showed the supply of mineral source having higher Mg/Ca ratio than the national average.

Saline lakes represent 23% of the area of all lakes on Earth and located mostly in arid and semi-arid climates. Olgoi and Orog lakes are located in semi-arid climates of the Valley of the Gobi lakes in Mongolia. It is the desert area that one of the sources of Asian dust (kousa) comes to Japan. Moreover, lake shrinkage has occurred in recent decades if the lake dried up it is possible to be a source of trace elements contamination. It can affect the environment and possible to move to a greater distance beyond the continent by the westerly winds. Then we need to clarify the geochemical behavior (solubility and mobility) of trace elements especially As and U in saline lake water and sediment in this area. Olgoi lake has no inflow and outflow river. Orog lake has one inflow river but no outflow. The depth of both lakes is around 1m. The water samples were collected from both lakes from 2015 to 2020. The major cations and anions of the water sample were analyzed using inductively coupled plasma optical emission spectroscopy and high-performance liquid chromatography. The trace elements were analyzed by using an inductively coupled plasma mass spectrometer. Sediment cores were taken by a gravity corer in August 2018 from Olgoi and Orog lake. Sediment samples were analyzed by using X-ray diffraction (XRD), and synchrotron-based X-ray absorption spectroscopy (XAFS), and 2 step chemical extraction for chemical speciation of As and U using 1M HCl for extractable and using a mixture of 48%HF, 60%HNO3, and 30%HCl, for a residual fraction.Olgoi and Orog lakes exhibited pH >9 and salinity above 1300 mg/kg. The major components of the lake's water are Na⁺, Cl^- and SO₄^2-. The high Na and Cl concentrations are the results of the evaporation of the lake waters. Also, the results showed that high concentrations of As (~3019ppb) and U (~590ppb) in lake water, which exceeded the WHO standards of As (10ppb) and U (30ppb). We found a positive correlation between As and U with Na and Cl. The total As and U concentrations of Olgoi lake sediment was 14.5ppm and 0.9ppm, respectively. The total As and U in Orog lake sediment ranged from 2.4 to 3ppm and 2.2 to 2.7ppm, respectively. The partial extraction results showed that As and U in the sediments are distributed among extractable fraction and residual fraction almost equally. XRD analysis showed the presence of authigenic minerals including calcite and monohydrocalcite. The results of XAFS analysis revealed As in Orog lake sediment was As (V) and primarily coprecipitated with calcite and secondarily adsorbed on ferrihydrite. As in Olgoi lake sediment was mainly As (V) and 20-30% was As (III) and coprecipitated with calcite. As and U possibly are removed by the authigenic carbonates. An extractable fraction of partial extraction is including ferrihydrite and calcium carbonates. This study showed that high concentrations of As and U in saline lake water and low in sediment. Therefore, little amount of As and U were adsorbed into sediments but mostly accumulated in water due to high evaporation. This study suggests that the As and U may be precipitated as salts after drying out of the lake water, which must be highly soluble.

Birnessite (δ-MnO2) is recognized as an adsorbent for heavy metal ions in soil and aqueous environments. In particular, coexisting metal ions can be incorporated into the crystalline structure of δ-MnO2 when δ-MnO2 is formed under a high Mn oxidation rate. Thus; in this research we studied the mechanism of Zn coprecipitation and the surface complexation associated with the formation of δ-MnO2 under neutral pH conditions (pH 6.0, and pH7.5), considering the results of surface complex modeling and X-ray absorption fine structure (XAFS) analysis. As a result, the coprecipitation mechanism was estimated as the three-dimensional uptake of Zn accompanying the formation of Zn-Mn minerals on the surface of δ-MnO2, in addition to the surface complexation with Zn between the layers of δ-MnO2. In addition, the produced Zn-Mn minerals differed depending on the pH and Zn/Mn molar ratio, and a large difference was confirmed in the amount adsorbed. From the XAFS analysis, it is considered that this could be caused by the transformation of the mineral composition and crystal structure by a slight difference in pH change.

Arsenate is highly toxic and carcinogenic, which can lead to serious health problems on exposure beyond the safe level in drinking water (0.01 mg/L). Several promising adsorbents have been developed to inhibit the mobility of arsenic. Herein, we quantified the effects of common anions including chloride, sulfate, and phosphate on arsenate selectivity and surface complexation using binary separation factor and phenomenological kinetic model. Specifically, the interfering effect of the anions on the adsorbent’s selectivity for arsenate removal is in the order of chloride > sulfate >> phosphate. The results obtained from the kinetic analysis showed that chloride and sulfate have a negligible effect on the kinetics of arsenate surface complexation. Whereas, phosphate co-existence greatly inhibits the kinetics of arsenate surface complexation by competitive adsorption. Besides, the higher contribution of inner-sphere complexation to the total adsorption of arsenate in a free system is similar to that of the co-existing medium. The findings from this research are essential in creating the practical application of promising materials for arsenic sequestration.

Landslide-dammed paleolake existed along upstream of Ane River, eastern Shiga Prefecture, which was formed by a collapse in Mt. Ibuki before about 5000 years ago. The lamination of the sediments comprises annually deposited structures (known as a varve). The µ-XRF mapping with a Horiba XGT-5000 X-ray analytical microscope shows that arsenic (As) is concentrated along the laminations. The sequential extractions indicate that amorphous sulfide phase occupies the second amount of As in the sediments (16%) after silicate phase (69%). In addition, synchrotron µ-XRF mapping and XANES analyses were investigated for As distribution and speciation in the laminations. As a result, on the one hand, a siderite (FeCO₃) lamina in summer is sandwiched by As₂S₃(III) rich layers (about 1 mm thick), suggesting the Fe(III) oxyhydroxide adsorbs arsenite (H₂AsO₃^-(III)) and arsenate (H₂AsO₄^-(V)) in water column during a vertical mixing in spring and autumn, followed by the deposition to lake bottom and then the sulfate reduction to As₂S₃(III) in the pore water. On the other hand, coarse particles (about 0.3 mm diameter) with AsS(II) and As₂S₃(III) are dispersed in the sediments. Their coexistence suggests that AsS(II) resulted from partial reduction of As₂S₃(III) by biogenic H₂S.

In order to determine the dynamics of marine fluorescent organic matter (FOM) using high-resolution spatial data, in situ fluorometers have been used in the open ocean. In this study, we measured FOM during the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) expedition from early December 2019 to early February 2020, using an in situ fluorometer at 148 stations along the two meridional transects (at 80 and 57 E) in the Indian Ocean, covering latitudinal ranges from 6 N to 20 S and 30 to 65 S, respectively. The FOM data obtained from the fluorometer were corrected for known temperature dependence and calibrated using FOM data measured onboard by a benchtop fluorometer. Using the relative water mass proportions estimated from water mass analyses, we determined the intrinsic values of FOM and apparent oxygen utilization (AOU) for each of the 12 water masses observed. We then estimated the basin-scale relationship between the intrinsic FOM and the AOU, as well as the turnover time for FOM in the Indian Ocean (410 19 years) in combination with the microbial respiration rate in the dark ocean (>200 m). Consistent to previous estimates in the global tropical and subtropical ocean, the FOM turnover time obtained is of the same order of magnitude as the circulation age of the Indian Ocean, indicating that the FOM is refractory and is a sink for reduced carbon in the dark ocean. A decoupling of FOM and AOU from the basin-scale relationship was also observed in the abyssal waters of the northern Indian Ocean. The local variability may be explained by the effect of sinking organic matter altered by denitrification through the oxygen-deficient zone on enhanced abyssal FOM production relative to oxygen consumption.

Water originated from subtropical mode water (STMW*) promotes material/energy exchange and affects ecosystems in the western Pacific boundary current area (WPBCA), where it rises by the topography. However, water mixing blurs the ‘mode’ of STMW*, limiting its observations in the WPBCA. In this study, we provide the first direct evidences of STMW* in the WPBCA and quantify the related vertical mixing using 137Cs. The large retention rate of STMW* in the Tsushima Strait implies this part of STMW* experiences little vertical mixing. However, with strong vertical mixing in the Luzon Strait, less than half of STMW* was retained within a short time. Quantifying the contribution of nutrient-rich water in the subtropical gyre to shallow/euphotic layer in the WPBCA and marginal seas, based on observed diffusivity and chemical tracers, could improve ecosystem models, and be further extended to other global boundary current areas.

Laboratory investigation of the potential for diatoms to obtain silica from feldspars, using aluminum as a tracer. Diatoms were cultivated with and without feldspar powder. Population and concentrations of silica and TEP were measured 2-3 times per week throughout one month of cultivation. Aluminum distribution in the frustule was then measured with EPMA. The presence of feldspars seemed to cause the bloom to be spread over a larger number of days, while also inhibiting diatom growth. The total number of living and dead diatoms at the end of the experiment was approximately the same for all three bottles, implying that these two effects counterbalanced one another. TEP concentration remained almost the same for all three until day 20 of the experiment, when it began to rise. The concentration was found to be far higher for albite, and the lowest for control. TEP concentration did not increase until after the peak of the diatom bloom. Silica concentration dropped quickly in all bottles, but showed a lag in the case of albite. By the time of the diatom population peak, Si concentration was extremely low in all cases. After most diatoms had died, around day 25, it began to rise again. When the diatoms were cultivated in an environment where the dominant source of aluminum was from the powdered feldspars, their frustules revealed a higher aluminum to silica ratio in the outer layer of the frustule. Diatoms grown without added feldspars, where aluminum was therefore minimally present as a background trace element, instead showed low Al/Si values for both inner and outer layers. The mechanisms and function of this double peak remains to be examined.

We analyzed chemical compositions of Fe-Mn micronodules that were separated from deep-sea sediments, including the extremely REY-rich mud, around Minamitorishima Island in the western North Pacific Ocean. We also implemented k-means cluster analysis on the chemical composition data. Then the micronodules were classified into five groups based on their geochemical features. The stratigraphic variations of the clusters indicated that the characteristics of the micronodules varied from almost purely diagenetic to relatively hydrogenetic across the extremely REY-rich mud layer. This implies the changes in the supply of organic matter to the seafloor and the intensity of a bottom water current, which could be associated with an accumulation of fish debris as a host of REY in the sediments. These results support a previously proposed formation mechanism of the extremely REY-rich mud.

Life on the Earth has been thought to be supported by the primary production through photosynthesis and chemosynthesis. However, electrosynthetic microbial activities that exploit electrical energy for their energy conservation was hypothesized based on the recent discoveries of electrical current generation in environments. Primary production of an ecosystem through electrosynthesis would update our understanding of biogeochemical processes, but electrosynthetic microbes are poorly understood and thus their roles in the environment are unknown. Therefore, I am examining electrosynthetic microbes and their molecular systems in ocean environments. This talk will discuss potential roles of electrosynthetic microbial activities in a biosphere by introducing novel microbes thrived in experiments reproducing electrosynthetic settings and considering the electrical energy available in environments.

Since halogens (F, Cl, Br, and I) are highly volatile and partitioned into the liquid phase, they exist in high concentrations in surface reservoirs such as oceans, sediments, and crust. Their elemental ratios also vary greatly from reservoir to reservoir. Owing to these features, halogens are expected to be good tracers of the material exchange between the Earth's surface and mantle, especially the water recycling in the mantle. Halogens in mantle materials have been difficult to analyze by conventional methods due to their low concentrations, but recently, highly sensitive analysis has become possible by a combination of neutron irradiation and noble gas mass spectrometry. The halogen compositions of basaltic glasses at the mid-ocean ridges and hot spots, mantle peridotites, diamonds, and serpentinites have revealed the volatile (particularly water) recycling in the Earth's interior. In this talk, I will introduce the volatile recycling in the mantle traced by the halogen compositions of peridotites and serpentinites.

Carbonaceous meteorite is relatively enriched in volatile materials including water and organic matter, recording the chemical evolution in the primordial solar system. The spatial distribution of organic compounds was distinct in the carbonaceous meteorite depending on their species and homologues. The result may infer aqueous interaction between mineral and organic compounds in the meteorite parent body. In this study, we examined interactions between the organic compounds and olivine, which is one of the major anhydrous silicates in chondritic meteorites, by preparing a column for liquid chromatography packed with olivine and comparing the retention times of four different N-bearing organic compounds. The results of the experiments showed that organic compounds, which tend to present as cations are retained longer, suggesting that the protonated organic compounds interaction more strongly with the oxyanions of silicic acid in olivine. These observations suggest that olivine of the carbonaceous meteorites could affect the interaction between organic matter and water and might have influenced the spatial distribution of organic compounds on the meteorite parent bodies.

Frictional heat during a coseismic faulting facilitates physico-chemical reactions along the fault and eventually changes in redox conditions in the earth. In order to explore a hypothesis that the redox reactions associated with an earthquake could provide a potential metabolic energy source for subsurface life, we have conducted high-velocity friction experiments on olivine and determined redox change within olivine fault zones by X-ray absorption near edge structure (XANES) analysis. We confirm that (1) the rapid Fe oxidation reaction takes place due to the combination of the formation of nano-scale olivine grains with fresh reactive surfaces and the transformation of liquid water into supercritical state by frictional heat, and (2) the Fe oxidation reaction progresses with increasing input shear energy. If seismic observation data could be available, the proposed correlation between the shear energy (equivalent to quake magnitude) and the oxidation progress enables us to estimate the earthquake-driven redox change that could be utilized as energy for subsurface biosphere.