TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52 (No. 4), pp. 317-333, 2018

Mineral nitrogen isotope signature in clay minerals formed under high ammonium environment conditions in sediment associated with ammonium-rich sediment-hosted hydrothermal system

Jaeguk Jo,1* Toshiro Yamanaka,1 Tomoki Kashimura,1 Yusuke Okunishi,1 Yoshihiro Kuwahara,2 Isao Kadota,1 Youko Miyoshi,3,4 Jun-ichiro Ishibashi3 and Hitoshi Chiba1

1Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan
2Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
3Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
4Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8567, Japan

(Received September 5, 2017; Accepted January 17, 2018)

Abstract: The shallow-seafloor Wakamiko hydrothermal field in Kagoshima Bay, Japan, has been characterized by high ammonium contents (∼17 mM) in the discharge fluid. The sediment layer covering the field also contained a high concentration of ammonium in the pore fluids. The fluid chemistry suggests that the high concentration was attributed to whether mixing with hydrothermal fluid or the mineralization of organic matter. The occurrence of sediments such as smectite, illite and mica has been reported, and is associated with hydrothermal activity. Those 2:1 clay minerals can fold ammonium as mineral nitrogen (MN) in their interlayers. Extracted clay fractions from the sediment core samples obtained from the field were mainly composed of smectite. It contained a small amount of MN and its δ15N values reflected that of pore fluid ammonium in the case of a high ammonium (>4 mM) environment. On the other hand, δ15N values of MN in the low ammonium (≤4 mM) environment were similar to those of the associated adsorbed organic nitrogen. The high ammonium environment associated with hydrothermal activity is expected to enhance formation of ammonium-bearing clay minerals, and then the δ15N value of MN fixed in the clay minerals is possibly preserved a signal of pore fluid ammonium.
Key words: ammonium-bearing clay mineral, ammonium nitrogen isotope, seafloor hydrothermal system, nitrogen isotope fractionation

*Corresponding author E-mail: paeh44em@s.okayama-u.ac.jp

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