HR: 1340h
AN: H43D-0393    [Abstracts]
TI: Methane and nitrous oxide in submarine groundwater discharges: Toyama Bay, northwestern Japan
AU: * Kameyama, S
EM: skame@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Nishi 8, Kita 10, Kita-ku, Sapporo, 060-0810 Japan
AU: Tsunogai, U
EM: urumu@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Nishi 8, Kita 10, Kita-ku, Sapporo, 060-0810 Japan
AU: Gamo, T
EM: gamo@ori.u-tokyo.ac.jp
AF: Department of Chemical Oceanography Ocean Research Institute, The University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Zhang, J
EM: jzhang@sci.toyama-u.ac.jp
AF: Department of Environmental Biology and Chemistry, Faculty of Science, Toyama University, 3190, Gofuku, Toyama, 930-8555 Japan
AU: Suzuki, M
EM: htms2085@hotmail.com
AF: Department of Environmental Biology and Chemistry, Faculty of Science, Toyama University, 3190, Gofuku, Toyama, 930-8555 Japan
AU: Koyama, Y
EM: aag71180@pop02.odn.ne.jp
AF: Department of Environmental Biology and Chemistry, Faculty of Science, Toyama University, 3190, Gofuku, Toyama, 930-8555 Japan
AB: Submarine groundwater discharge (SGD) in coastal area plays important roles in global hydrologic cycles between land and ocean. Besides, SGD supplies chemical materials such as nutrient and organic carbons directly into the sub-surface ocean, so that it could be also important for discussing global geochemical cycles. The estimated global flux of SGD, however, is highly uncertain. Besides, little is clarified for the influence of the chemical input by SGD to ocean and the residence times of groundwater from recharge on land to discharge on seafloor. Oxic/anoxic condition of groundwater is one of the important parameters that characterize chemical compositions of groundwater. Methane and nitrous oxide in SGD fluids are sensitive tracers for the oxic/anoxic condition of groundwater. In this paper, we studied both SGD fluids and nearby groundwater on land using methane and nitrous oxide as tracers, in the coastal area of alluvial fans formed by Kurobe River and Katagai River in Toyama Pref., northwestern coast of Japan, where extensive distributions of SGD have been reported. Besides to their concentrations, we also determined stable isotopic compositions of methane ($\delta$$^{13}$C ) and nitrous oxide ($\delta$$^{15}$N and $\delta$$^{18}$O ) for additional tracers for discussing origins of the molecules. As for methane, we found that all of the SGD fluids contain less methane than natural waters in equilibrium with atmospheric methane. In proportion to the methane depletion, the $\delta$$^{13}$C values of methane in the SGD fluids vary from -50 $\permil$ almost comparable to atmospheric methane to highly $^{13}$C enriched value of -20 $\permil$. As for nitrous oxide, all of the SGD fluids contain more nitrous oxide than natural waters in equilibrium with the atmospheric nitrous oxide. Both the $\delta$$^{15}$N and $\delta$$^{18}$O values of nitrous oxide are lower than that of the atmospheric nitrous oxide. These results suggest that most of methane in the SGD fluids derived from atmosphere and a part of the methane is oxidized microbially in the course of groundwater flow through the aquifer, while nitrous oxide is produced via nitrification. On the other hand, we found that many of the land groundwaters contain more methane than the SGD fluids. The longer residence times of the land groundwaters than the SGD fluids must be responsible for the methane enrichment. When we categorized the groundwaters into oxic stage and anoxic stage based on the concentration of methane and nitrous oxide, we found that the oxic groundwaters are distributed along the Katagai River. We conclude that the SGD fluids must have flowed through the underground along the Katagai River.
DE: 4825 Geochemistry
DE: 4851 Oxidation/reduction reactions
DE: 4870 Stable isotopes
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting