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 d15N values reflected that of pore fluid ammonium in the case of a high ammonium (≫4 mM) environment. On the other hand, d15N 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 d15N value of MN fixed in the clay minerals is possibly preserved a signal of pore fluid ammonium.
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