Simultaneous measurement of concentration and isotope ratio of dissolved gases by using a continuous flow isotope ratio mass spectrometry and its application to oceanography

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We have constructed a continuous flow isotope ratio mass spectrometric system by combining a gas chromatograph and an isotope ratio mass spectrometer equipped with differential pumping, in order to measure concentration and stable isotope ratio of dissolved gases simultaneously. The main targets are dissolved oxygen and hydrogen in seawater. The system was connected with vacuum lines which were prepared for extraction of these gases from water samples and their purification and pre-concentration. Seawater samples were taken with a CTD-Carousel water sampling system during the R/V Hakuho Maru KH-04-5, KH-05-2, KH-06-2 cruises in the western Pacific Ocean and the RAT Hansei Maru KT-05-11 cruise in the Japan Sea. We successfully estimated both the rate of O_2 consumption by bacterial organic matter decomposition (1.2 μmol kg^<-1> y^<-1> in the Japan Sea bottom water and 0.11 μmol kg^<-1> y<-1> in the Philippine Sea deep water) and isotopic fractionation factor of dissolved O_2 consumption (0.988 to 0.990). These values will be important standards to compare with those of future measurements for global change monitoring. As for dissolved hydrogen, we are in progress to achieve analytical precision high enough to analyze dissolved H_2 in natural seawater. Alternatively, we have measured dissolved H_2 and its δD values for submarine hydrothermal fluids from the Indian Ridge and back arc basins in the western Pacific for the first time. We have found uniquely deuterium depleted H_2 (δD: -800 to -700‰) for high temperature fluids from Southern Mariana Trough, in contrast with those (δD: -400 to -300‰) observed at Central Indian Ridge and Okinawa Trough, which are consistent with isotope fractionation equilibrium between H_2 and H_2O at high temperatures (300-400℃). The discovery of the low δD in the Southern Mariana Trough demonstrates other H_2 sources such as chemical reaction between H_2O and crushed rocks.