Fracture-filling and fracture-lining calcites may preserve a signature of the chemical evolution process of groundwaters. To develop a methodology for assessing the paleo-hydrogeological evolution of groundwater systems, especially with respect to understanding variations in redox conditions, calcite fracture mineralization was studied using two deep boreholes (ca. 500 m and 1,000 m) drilled in the Tono area of central Japan. Calculated Eh values based on the variations of Uand Fe concentration in calcite were plotted in the range from -180 mV to -275 mV, from -104 mV to -268 mV, respectively in the case of pH=8. By combining the content of U and Fe of calcite, based on the cathodluminescence characteristics, it is suggested that the redox conditions have varied with a range of 90 mV as the calcite grew in the evolving groundwater in each studied grains. Thus, the present study suggests that redox conditions in this area may have changed little. Therefore, redox-condition of groundwater will be kept in reduction condition during calcite precipitation. It is shown that the methodology carried out in this study is useful for assessing the long-term hydrochemical evolution process.

View full abstract

Hydroperoxides such as hydrogen peroxide (H₂O₂) and organic hydroperoxides (ROOH) were measured in environmental samples in Hiroshima prefecture, Japan, during 1993-2002. Hydroperoxides were determined by fluorescence detection following an enzymatic separation of H₂O₂ from ROOH using a flow injection analysis (FIA-FD) or following a HPLC separation of individual hydroperoxides (HPLC-FD). Clear diurnal variation of H₂O₂ concentration (0.02〜1.58 ppbv) in the air was observed at Higashi-Hiroshima and H₂O₂ was correlated with O₃, solar radiation and temperature. Mean concentrations of H₂O₂ in the air were 0.67, 0.73, 2.0 and 0.81 ppbv at the south slope and summit of Mt. Gokurakuji, Hiroshima Bay and Iyo-Nada of the Seto Inland Sea, respectively, while total ROOH (ROOHs) concentrations were 1.42, 0.72, 1.4 and 0.6 ppbv, respectively. H₂O₂ in rainwater collected at Higashi-Hiroshima showed the concentration range in n.d. 〜56,400 nM (ave. 4,730 nM), with highest in summer and lowest in winter. No individual ROOH was detected in the rain waters. Dew samples contained <3,600, <353 and <179 nM of H₂O₂ in Higashi-Hiroshima, the south slope and summit of Mt. Gokurakuji, respectively, whereas ROOHs were contained <3,500, <998 and <1,300 nM, respectively. H₂O₂ concentrations (17〜301 nM) in river water samples tended to be higher (ave. 185 nM) in summer and lower (ave. 87.3 nM) in winter while ROOHs exhibited almost same concentration ranges (33〜2,000 nM) despite almost no seasonal change. Commercial natural mineral waters contained n.d. 〜1,420 and 79〜375 nM of H₂O₂ and ROOHs, respectively, whereas no peroxides were detected from tap waters. H₂O₂ could be generated by HO₂/O₂^-, which were formed by the reaction of dissolved organic matter with nitrate-induced OH radicals in natural mineral waters while no H₂O₂ would be generated in tap waters due to quick scavenging of OH with excess HClO/ClO^-, which are used for sterilization of tap waters.

View full abstract

I started a study of polar organic compounds in the atmosphere of Los Angeles and its vicinity in early 1980s, when acid rain problems became an important environmental issue. In late 1980s, another two problems were recognized to be serious; i.e., global warming due to the increased greenhouse gases and depletion of stratospheric ozone discovered over Antarctica. I have developed GC and GC/MS methods to determine various organic compounds in the atmosphere including water-soluble mono- (C₁-C₁₀) and di-carboxylic (C₂-C₁₀) acids. Using those methods, formic and acetic acids were found as major monoacids in rainwater and in gas phase, followed by propionic acid. Total monoacids were found to account for up to 12% of total organic carbon in the Los Angeles rainwater. Their concentrations were equal to 1/4 to 1/2 of those of inorganic acids (sulfate and nitrate). Similarly, oxalic acid (C₂) was found as the most abundant diacid species followed by malonic (C₃) and succinic (C₄) acids in aerosols and rainwater. Studies of diacids in the atmosphere of urban Tokyo showed a diurnal variation with daytime maximum, indicating that secondary photochemical production of small diacids is more important than the primary emission from fossil fuel combustion (motor exhausts). Although diacids (C₂-C₁₀) account for <2% of total aerosol carbon in the urban atmosphere, the values increased up to 9% in the Arctic and 26% in the marine aerosols from the Pacific. Polar sunrise experiments conducted at Alert in the high Arctic further confirmed a significant secondary production of small diacids in early spring when sun returns on the horizon. Based on the spatial distributions of diacids over the Pacific, photochemical transformation (aging) of organic aerosols was characterized by an enrichment of oxalic acid in the marine atmosphere. Because dicarboxylic acids are water-soluble and can act as cloud condensation nuclei (CCN) in the atmosphere, they likely contribute to the formation of cloud droplets and thus affect indirectly on radiation balance of the Earth and the climate changes. Recent advances in the organic aerosol studies are also reviewed together with the studies on their hygroscopic properties (CCN activity) with a scope for the future study.

View full abstract

The non-traditional lithium (Li) isotopic tracer has a great potential to provide a major breakthrough in the investigation of the material cycle in the terrestrial mantle. Using a developed multiple-collector ICP mass spectrometry method, we revealed Li isotopic systematics of mantle-derived samples. The main significance in our results is the finding of extremely low ⁷Li/⁶Li values in several mantle-derived samples. Based on earlier results for eclogites, it had been proposed that subducted highly altered oceanic crust would have extremely low ⁷Li/⁶Li values. The significantly low ⁷Li/⁶Li values, however, had never been observed in any mantle-derived samples before our finding. We have also proposed that the enrichment of isotopically light Li may be general property of the enriched mantle type 1 end-member component (EM 1). In this scenario, the Li in the EM 1 source mainly originates from Li in the highly altered basalt of the uppermost part of subducted oceanic crust. Thus, the Li isotopic signature is sensitive to the degree of alteration experienced by the basaltic crust and can thus be used to distinguish what part of the basaltic crust was recycled.

View full abstract

Oxygen isotope ratio and other chemical tracers are used to distinguish water mass modification processes in the Sea of Okhotsk and the Arctic Ocean. Distributions of tracers show that formation of the dense water during sea ice formation is a key process in both regions. This process freshens, cools, and ventilates water below the surface. In the Sea of Okhotsk, however, diapycnal mixing in the Kuril Straits is found to contribute more than the dense water formation to form the Okhotsk Sea Intermediate Water. This process will also affect the properties of North Pacific Intermediate Water, which receives anthropogenic CO₂ and freshwater by mixing with Okhotsk Sea Intermediate Water. In the Arctic Ocean, formation of dense water and distribution of fresh water determine the salinity distribution in the upper part of the ocean. A newly investigated tracer, alkalinity, reveals that Siberian river runoff and meteoric water from the Bering Sea are the main freshwater sources, even in the Canada Basin. Historical alkalinity and oxygen isotope ratio data from the past 70 years are combined to draw distributions of sea ice meltwater/brine and other freshwaters for the entire Arctic Ocean. Accumulation of freshwater with brine in the Canada Basin indicates that this is the region where massive amount of freshwater enters, and a large portion of freshwater is converted into sea ice.

View full abstract