Deepest and hottest hydrothermal activity in the Okinawa Trough: the Yokosuka site at Yaeyama Knoll

Abstract

Since the initial discovery of hydrothermal vents in 1977, these 'extreme' chemosynthetic systems have been a focus of interdisciplinary research. The Okinawa Trough (OT), located in the semi-enclosed East China Sea between the Eurasian continent and the Ryukyu arc, hosts more than 20 known vent sites but all within a relatively narrow depth range (600-1880 m). Depth is a significant factor in determining fluid temperature and chemistry, as well as biological composition. However, due to the narrow depth range of known sites, the actual influence of depth here has been poorly resolved. Here, the Yokosuka site (2190 m), the first OT vent exceeding 2000 m depth is reported. A highly active hydrothermal vent site centred around four active vent chimneys reaching 364°C in temperature, it is the hottest in the OT. Notable Cl depletion (130 mM) and both high H₂ and CH₄ concentrations (approx. 10 mM) probably result from subcritical phase separation and thermal decomposition of sedimentary organic matter. Microbiota and fauna were generally similar to other sites in the OT, although with some different characteristics. In terms of microbiota, the H₂-rich vent fluids in Neuschwanstein chimney resulted in the dominance of hydrogenotrophic chemolithoautotrophs such as Thioreductor and Desulfobacterium. For fauna, the dominance of the deep-sea mussel Bathymodiolus aduloides is surprising given other nearby vent sites are usually dominated by B. platifrons and/or B. japonicus, and a sponge field in the periphery dominated by Poecilosclerida is unusual for OT vents. Our insights from the Yokosuka site implies that although the distribution of animal species may be linked to depth, the constraint is perhaps not water pressure and resulting chemical properties of the vent fluid but instead physical properties of the surrounding seawater. The potential significance of these preliminary results and prospect for future research on this unique site are discussed.

Keywords: biodiversity; chemosynthetic ecosystem; fluid chemistry; hydrothermal vent; microbial composition; sulfide deposit.

Figure 1.

Seafloor topography of (a) entire southern Okinawa Trough and (b) Yaeyama Graben. Grey lines and a filled red circle in (b), respectively, represent surface ship track for geophysical observation including MBES-based hydrothermal site survey and the suggested location of hydrothermal activity. Seafloor topography in (b) was obtained by YK14-16, KR15-16 and YK16-07 cruises while partly sourced from a public database provided by Japan Coast Guard.

Figure 2.

Images acquired by MBES equipped on (a) R/V Yokosuka and (b) AUV URASHIMA. Anomalous reflections elongating vertically in water column over the western end of the Yaeyama Knoll suggest some compounds which have physical properties distinct from seawater such as CO₂ bubbles/hydrates and hydrothermal fluid.

Figure 3.

Event map for the Yokosuka site, large area map showing all relevant dive tracks is presented on the top with an expanded map of the hydrothermally active area (indicated by a solid box) on the bottom. Indications of each symbol are shown in the legend.

Figure 4.

Pressure–temperature plot for the global hydrothermal sites (dataset used from [15,32]). An open star and open circles, respectively, represent the Yokosuka site and other OT sites, whereas black dots represent global hydrothermal sites. A broken curve and the grey hexagon, respectively, represent the two-phase boundary and critical point for 3.5% NaCl solution [44]. A horizontal broken-dot line represents the deepest strait in the East China Sea.

Figure 5.

Major vent chimneys in the Yokosuka site. (a) Neuschwanstein chimney (max temp. = 356.9°C), (b) Hohenschwangau chimney (max temp. = 364.1°C), (c) Heidelberg chimney (max temp. = 349.9°C) and (d) Shisa chimney.

Figure 6.

Megafaunal communities of the Yokosuka site. (a) Overview of shrimp aggregations on the top of the Neuschwanstein chimney. (b) Close-up of a Shinkaicaris leurokolos alvinocaridid shrimp aggregation on the Hohenschwangau chimney. (c) Shinkaia crosnieri squat lobsters near the base of the Hohenschwangau chimney. (d) Large aggregations of scale worms (black dots) on the surface of the Shisa chimney. (e) A typical animal colony around the base of chimneys of the Yokosuka vent field dominated by Alvinocaris longirostris shrimps, Munidopsis ryukyuensis squat lobsters, and Provanna clathrata snails. (f) Peripheral tubeworm (Lamellibrachia sp. and Alaysia sp.) bush 50 m east of the Neuschwanstein chimney. (g) Overview of a peripheral community visually dominated by poecilosclerid sponges near the Heidelberg chimney. (h) Close-up of the sponge-dominated peripheral community, most notable animals being Lamellibrachia sp. and Alaysia sp. tubeworms and B. aduloides mussels.

Figure 7.

Mg diagrams for Yokosuka site fluids. Open circles represent the chemical composition of the Hohenschwangau fluids and ambient seawater. The Neuschwanstein and Heidelberg fluids are shown by open squares and filled circles, respectively. Connecting lines from ambient seawater to each low-Mg fluid (dotted line for Neuschwanstein fluid) represent their extrapolation to Mg = 0 for estimating endmember fluid composition. All the concentrations are presented in the unit of millimolar.

Figure 8.

A plot of H₂ and CH₄ concentrations in high-temperature hydrothermal fluids. Both axes are shown with a logarithmic scale. Grey-coloured and open squares, respectively, represent sediment-involved and sediment-not-involved hydrothermal sites. After Kawagucci et al. [54] with modification and additional data shown in this study (table 1).

Figure 9.

Composition of the microbial community based on taxonomic grouping (order level) of 16S rRNA gene amplicon reads. DNA was extracted from a single pulverized chimney sample from each vent. OTUs with greater than 3% frequency in either sample are presented, and the rest and unassigned taxa are indicated as ‘others’.

Figure 10.

Nonmetric multidimensional scaling (nMDS) plot visualizing similarity in megabenthos composition among the different habitat types identified, based on Jaccard's index of similarity calculated using species presence/absence data. The overlaid contours are based on results from group-average cluster analysis.