1. No category

Fluid emission along the Nankai Trough: Insights from noble
gases in the sediment pore water
○TOMONAGA Yama（UTokyo; Eawag, Swiss Federal Institute of Aquatic Science and
Technology）， ASHI Juichiro ・ TAKAHATA Naoto ・ YAGASAKI Kazuhiro（UTokyo),
TOYODA Shin (Okayama Univ. of Science), SANO Yuji （UTokyo)
Noble-gas geochemistry is known to change in response to the terrigenic fluid emission
triggered by major seismic events, as observed, for instance, in the recent case of the
Tohoku-Oki earthquake occurred in 2011 (Sano et al., 2014). Therefore, characterizing the
terrigenic fluid emission in tectonically active regions is a prerequisite to understand
future geochemical changes (e.g., Tomonaga et al., 2013).
In the present contribution we report the results of the noble-gas measurements conducted
in water and sediment samples collected along the Nankai Trough using the ROV HyperDolphin
during RV Natsushima cruises NT13-08 and NT14-07.
In general, sediment cores acquired at active cold seeps at the splay fault off shore
Kumano are characterized by the presence of crustal He (Toki et al., 2011) and the
respective He concentration gradients are highly variable. Sediment cores collected at
shallower water depths show relatively low He concentration gradients and high
3
He/4He
ratios suggesting the presence of mantle He.
The fluid transport dynamics in the investigated area are discussed in the light of the
new insights provided by the spatial distribution of the fluxes and the isotope signature
of terrigenic He provided by the present study.
References
Sano, Y., Hara, T., Takahata, N., Kawagucci, S., Honda, M., Nishio, Y., Tanikawa, W.,
Hasegawa, A., Hattori, K. (2014). Helium anomalies suggest a fluid pathway from mantle
to trench during the 2011 Tohoku-Oki earthquake. Nat. Commun., 5.
Toki, T., Maegawa, K., Tsunogai, U., Kawagucci, S., Takahata, N., Sano, Y., Ashi, J.,
Kinoshita, M., Gamo, T. (2011). Gas Chemistry of Pore Fluids from Oomine Ridge on the
Nankai Accretionary Prism. In Y. Ogawa, R. Anma, Y. Dilek (Eds.), Accretionary Prisms
and Convergent Margin Tectonics in the Northwest Pacific Basin (pp. 247-262). Modern
Approaches in Solid Earth Sciences, 8. Springer Netherlands.
Tomonaga, Y., Brennwald, M.S., Maden, C., Meydan, A.F., Kipfer, R. (2013). Noble-gas
geochemistry of the M7.2 earthquake close to Lake Van (Turkey). AGU Fall Meeting. San
Francisco, USA.