Hiroshi A. Takahashi

Balance And Behavior Of Carbon Dioxide At An Urban Forest Inferred From The Isotopic And Meteorological Approaches

Diurnal variations in δ 14 C, δ 13 C and the concentration of atmospheric carbon dioxide in an urban forest were measured on 9 February 1999 to discriminate and quantify contributions from different CO 2 sources. The biogenic CO 2 concentration remained relatively constant throughout the day. However, anthropogenic CO 2 concentration fluctuated with the atmospheric CO 2 concentration, and seemed to be controlled by wind velocity and the amount of exhaust gases from fossil fuel burning. The vertical profiles of anthropogenic, biogenic, and total CO 2 showed a constant concentration within forest during daytime because of the large vertical CO 2 influx, strong winds, and neutral atmospheric condition. The biogenic contribution at night decreased from the forest floor upwards with a smooth gradient, while the anthropogenic contribution showed a direct mirror because of the location of respective CO 2 sources-the vertical gradient of wind velocity and the horizontal CO 2 supply.

A distinct freshwater‐adapted subgroup of ANME‐1 dominates active archaeal communities in terrestrial subsurfaces in Japan

Anaerobic methane-oxidizing archaea (ANME) are known to play an important role in methane flux, especially in marine sediments. The 16S rRNA genes of ANME have been detected in terrestrial freshwater subsurfaces. However, it is unclear whether ANME are actively involved in methane oxidation in these environments. To address this issue, Holocene sediments in the subsurface of the Kanto Plain in Japan were collected for biogeochemical and molecular analysis. The potential activity of the anaerobic oxidation of methane (AOM) (0.38-3.54 nmol cm⁻³ day⁻¹) was detected in sediment slurry incubation experiments with a (13) CH(4) tracer. Higher AOM activity was observed in low-salinity treatment compared with high-salinity condition (20‰), which supports the adaptation of ANME in freshwater habitats. The 16S rRNA sequence analysis clearly revealed the presence of a distinct subgroup of ANME-1, designated ANME-1a-FW. Phylogenetic analysis of the mcrA genes also implied the presence of the distinct subgroup in ANME-1. ANME-1a-FW was found to be the most dominant active group in the archaeal communities on the basis of 16S rRNA analysis (75.0-93.8% of total archaeal 16S rRNA clones). Sulfate-reducing bacteria previously known as the syntrophic bacterial partners of ANME-1 was not detected. Our results showed that ANME-1a-FW is adapted to freshwater habitats and is responsible for AOM in terrestrial freshwater subsurface environments.

Gas pathways and remotely triggered earthquakes beneath Mount Fuji, Japan

Large earthquakes sometimes trigger local seismicity that is distal to their rupture zones. Various mechanisms for this triggered seismicity have been proposed, based on either the static stress change or ground shaking from seismic waves, but local geological structure is rarely studied to discern why this seismicity is remotely induced. We present the results of a joint three-dimensional resistivity and isotopic analysis of the groundwater system surrounding Mount Fuji, Japan, where increased seismicity was observed following the A.D. 2011 Tohoku-Oki megathrust earthquake. An electrically conductive zone and high concentrations of magmatic gases (He and CO2) correspond to the zone of triggered seismicity. In contrast, a contribution of magmatic water is not suggested from 2H (deuterium, D) and 18O isotope ratios. These results suggest that the earthquakes were triggered within a fractured zone through which magmatic gases preferentially migrated. We hypothesize that the upwelling of gas-rich hydrous fluids and/or gas bubbles occurred along this fracture pathway, causing an increase in the pore pressure and triggering the resultant earthquake sequence.

Pathways for escape of magmatic carbon dioxide to soil air at Unzen Volcano, SW Japan.

Estimation of the magmatic contribution to soil air at Unzen Volcano, SW Japan, was carried out using carbon isotopes, both (super 14) C and (super 13) C, and a mixing model of isotopic mass balance in order to assess the spatial variation of magmatic influence from the volcano. The advantage of using soil air samples is that a wide range of gas sampling sites can be selected. Magmatic CO (sub 2) contributed mostly in the eastern region from Unzen Volcano. The high magmatic contribution to soil air appeared along the Akamatsudani fault zone located southeast of the volcano. Our observations across the fault also showed remarkable peaks of CO (sub 2) concentration and delta (super 13) C values, suggesting that magmatic fluid comes up along the fracture zone as for the normal fault system of the graben.

Seasonal fluctuation of stable carbon isotopic composition in Japanese cypress tree rings from the last glacial period; possibility of paleoenvironment reconstruction.

Seasonal variations of delta (super 13) C were analyzed for two Japanese cypress trees (Chamaecyparis obtusa), one buried and one living. Both trees were different in age but sampled in areas geographically close to each other in central Japan. A buried cypress with 394 annual rings was excavated from Old Fuji mudflow, the last glacial strata of the dormant Mt. Fuji volcano. The accelerator mass spectrometry (AMS) radiocarbon date of this glacial sample was 18,600+ or -120 BP (NUTA-4884). A living tree stem, which has 192 rings, was cut from the Izu Peninsula in 1986. In order to measure the seasonal delta (super 13) C fluctuation, the tree rings were divided equally into three earlywood and one or two latewood consecutive sections. The delta (super 13) C value within an annual ring generally increased from the first to the third or fourth sections then decreased in the last section. This pattern of the variation was similar in the glacial and modern samples. The delta (super 13) C value within an annual ring seems to be controlled by environmental factors (not plant physiological ones), since there was no isotopic shift in the seasonal delta (super 13) C variation at the earlywood-latewood boundary, which was controlled by plant physiology. The result suggests the potential to reconstruct the paleoenvironment within a year using the seasonal delta (super 13) C variation, though site-specific conditions such as soil characteristics would also affect to its fluctuation.

TREE-RING WIDTH AND STABLE CARBON ISOTOPE COMPOSITION OF JAPANESE CYPRESS IN THE LAKE BIWA AREA, CENTRAL JAPAN, AND THEIR HYDROLOGIC AND CLIMATIC IMPLICATIONS

Chronologies of tree-ring width and stable carbon isotope composition of Japanese cypress were developed to help reconstruct a 300-year record of past hydrologic and climatic environments in the Lake Biwa area, central Japan. Site chronologies were built with 37 trees for ring width and four trees for carbon isotope composition, respectively. Correlation analysis with monthly climatic data revealed that radial growth of the trees is related to temperature in early spring, precipitation (or number of precipitation days) in early summer and precipitation in previous-year summer to autumn. Tree-ring cellulose carbon isotopic composition is correlated most significantly with the number of precipitation days in early summer months. Consequently, a chronology of the number of precipitation days in May was reconstructed by multiple regression analysis with ring-width and carbon-isotope predictors and was validated by comparison with the recent observed record.

Radiocarbon Dating of Groundwater in Granite Fractures in Abukuma Province, Northeast Japan

Knowledge of the groundwater age is indispensable for understanding groundwater flow in crystalline rocks. The present study is the first to discuss the radiocarbon ages of groundwater in Abukuma granite, Fukushima Prefecture, northeast Japan. The vertical profiles of 14C dates and 13C are obtained from 3 boreholes (depths of 140, 230, and 306 m). Chemical and carbon isotopic compositions suggest that dead-carbon contamination of groundwater occurred during groundwater storage in the fractures. 14C concentration was corrected by using isotopic mass balance in which dead-carbon contamination of the groundwater was considered. The 14C dates ranged from modern to ~16 ka. The relationship between tritium and 14C data in 1 borehole suggests the simultaneous inflow of shallow groundwater to deeper levels occur for the depths between 60 and 100 m. The vertical profiles of 14C dates indicate a relatively constant age of 10–16 ka for groundwater deeper than 100 m, which may have been influenced by rapid sea-level changes after the glacial period.