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

Toshitaka Gamo, Hitoshi Sakai, Eun Soo Kim, Kiminori Shitashima, Jun ichiro Ishibashi

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52 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)328-338
Number of pages11
JournalEarth and Planetary Science Letters
Issue number2
Publication statusPublished - Nov 1991
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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