Fluid–Sediment Interaction in a Marine Shallow‐Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan

Abstract

The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow-water hydrothermal activity associated with fumarolic gas emissions at the crater sea fl oor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05-13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper-Dolphine (Japan Agency for Marine-Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fl uid chemistry were studied. The pore fl uids of these sites showed a drastic change in chemical profi le from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end-member composition. Occurrence of illite/smectite interstratifi ed minerals in the North site sediment is attributed to in situ fl uid -sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the signifi cant low potassium concentration of the hydrothermal end-member, the abundant occurrence of low-temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fl uid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fl uid and alteration minerals in the North site sediment. This difference may refl ect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.

Geochemical and Visual Indicators of Hydrothermal Fluid Flow through a Sediment-Hosted Volcanic Ridge in the Central Bransfield Basin (Antarctica)

“…The concentration-depth profiles presented here (Figure 8) suggest that at Hook Ridge, pore fluid composition is affected by both hydrothermal fluid flow and organic matter diagenesis and, in this respect, are similar to other sediment-hosted hydrothermal systems such as the Guaymas Basin, Gulf of California [38] and the Wakamiko Crater, Kagoshima Bay [39], [40]. In contrast, concentration-depth profiles at Middle Sister and the background site provide a record of organic matter diagenesis but no indication of hydrothermal influence.…”

Archaeal Diversity and Distribution along Thermal and Geochemical Gradients in Hydrothermal Sediments at the Yonaguni Knoll IV Hydrothermal Field in the Southern Okinawa Trough

“…In addition to the hydrothermal fluid and ambient seawater, interstitial water was extracted from the sediments after the method described by Nakaseama et al (37). Geochemical analyses of water samples were conducted following Gieskes et al (9) with slight modifications (37). Silica and ammonia concentrations were determined onboard by colorimetric techniques (9), and sulfate concentrations were determined by ion chromatography after a 300-fold dilution.…”

“…The filtered water samples described above were subjected to geochemical analysis. In addition to the hydrothermal fluid and ambient seawater, interstitial water was extracted from the sediments after the method described by Nakaseama et al (37). Geochemical analyses of water samples were conducted following Gieskes et al (9) with slight modifications (37).…”

New evidences of hydrothermal fluids circulation at the Devonian Kess Kess mounds, Hamar Laghdad (eastern Anti‐Atlas, Morocco)

“…The hydrothermal fluids could have migrated through fractures and fissures in the rocks of Hamar Laghdad Ridge (Mounji et al, 1998), and pores of sediments below the sediment-water interface. Then, as reported from other hydrothermal settings (Marumo and Hattori, 1999;Nakaseama et al, 2008;Brand et al, 2010), the mixing of warm hydrothermal fluids with the cold bottom seawater could have drastically changed the chemical conditions and temperature in the pore spaces of the shallow buried sediments. Probably, the clinochlore, anatase, quartz and goethite minerals observed at Hamar Laghdad Ridge precipitated in the sediments in which fluid migration occurred.…”