High alkalinity due to sulfate reduction in the CLAM hydrothermal field, Okinawa Trough

High alkalinity ( > 1.8 mmol kg^-1), pH (4.7-5.3), and ammonium ( ∼ 5 mmol kg^-1) in hydrothermal fluids from the JADE and CLAM sites in the Mid-Okinawa Trough back-arc basin demonstrate organic matter decomposition during high-temperature fluid-sediment interaction. The endmember fluid from the JADE site ( ∼ 320°C black smoker), which is free of Mg and SO₄, has similar chemical characteristics as those at the Escanaba Trough, one of the representative sediment-hosted ridge systems in the eastern Pacific. On the other hand, fluid samples from the CLAM site ( ∼ 100°C) show remarkably high alkalinity (10.3 mmol kg^-1) comparable to the highest alkalinity of 10.6 mmol kg^-1 (Guaymas Basin) yet observed in hydrothermal fluids, and significantly higher δ³⁴S of SO₄ ( > 25‰) than ambient seawater (21‰). It is suggested that sulfate reduction plays a dominant role in decomposing organic matter to increase alkalinity in the CLAM site. Variation of the δ³⁴S values of H₂S (from -0.2 to +3.0‰) in the CLAM fluids taken from different vents suggests that the CLAM site should consist of multiple local hydrothermal systems in different evolutionary stages with regard to sulfate reduction.