HR: 0800h
AN: H11C-0642    [Abstracts]
TI: Spatial Variation of Deep-seated Carbon Contribution in Groundwater, Central Japan
AU: * Takahashi, H A
EM: h.a.takahashi@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Takahashi, M
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Kazahaya, K
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Handa, H
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Hirota, A
AF: Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, 060-0810, Japan
AU: Yasuhara, M
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Morikawa, N
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Ohwada, M
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Inamura, A
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AU: Nakama, A
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1, Higashi, Tsukuba, 305-8567, Japan
AB: Carbon isotopic ratio in groundwater can be used for identifying a contribution of deep source carbon, e.g., magmatic or deep-seated crustal fluids. In the present study, we focused on giving an outline of the spatial variation of deep-seated crustal fluid contribution to groundwater in non-volcanic regions of Japan. We measured the stable carbon isotopic ratio of DIC, chemical compositions and stable isotope (δ¹⁸O and δD) in groundwater or hot-spring samples taken at Kanto, Tokai, and Kinki districts in central Japan. The contribution of deep-seated carbon in groundwater is computed using carbon isotopic mass balance. Spatial variation showed the high concentration of deep-seated carbon in the Arima area, the belt-like area along the Median Tectonic Line (MTL) excepting the Chubu area, the southwest region of the Kii Peninsula and the central region of Shizuoka Prefecture. Especially, the area along the MTL has quite a high concentration of deep- seated carbon. This suggests that the large tectonic line act as a path of deep-seated carbon from the depth grater than ~1500m, which is the deepest sampling depth of groundwater in the present study. Approximately half of such groundwater is indicated to be mixed with the Arima type thermal water or brine from the results of isotopic features and high Cl^- concentration. It is considered that the carbon is supplied with them. The δ¹⁸O and δD relationship proves another half of groundwater showing high concentration of deep-seated carbon is originated from meteoric water. This type of groundwater is characterized by a relatively low concentration of Cl^-. These suggest an existence of an external CO₂ (deep-seated) from underlying crust. Our estimation of deep-seated carbon contribution has a large uncertainty, because of un-counting CO₂ bubble, and estimation errors based on the isotopic fractionation at the bubbling of CO₂ or methanation, and the assumption of δ¹³C values of endmember components. However, we can obtain the general trend of spatial variation of deep-seated carbon contribution using carbon isotopic ratios.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2007 Fall Meeting