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Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan

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ABSTRACT

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

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Depth profiles of turbidity, pH, temperature, helium isotope ratios, concentrations and δ13C values of methane, and concentration and δ13C of dissolved inorganic carbon (DIC) obtained at Daiichi-Amami Knoll, Kotakara Shima and Wakamiko Crater.The depth of the sampling point below the sea surface is used as the vertical axis. Black and open circles represent samples taken at station 1 and station 2, respectively, in the region of Daiichi-Amami Knoll. The results of station 3 and station 4 at Kotakara Shima are represented by black and open squares, respectively. The sampling locations of these stations at Daiichi-Amami Knoll and Kotakara Shima are shown in Fig. 1a,d, respectively. The depth profiles at the Wakamiko Crater are shown as station 5 (black inverted triangles) and station 6 (open inverted triangles).
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f2: Depth profiles of turbidity, pH, temperature, helium isotope ratios, concentrations and δ13C values of methane, and concentration and δ13C of dissolved inorganic carbon (DIC) obtained at Daiichi-Amami Knoll, Kotakara Shima and Wakamiko Crater.The depth of the sampling point below the sea surface is used as the vertical axis. Black and open circles represent samples taken at station 1 and station 2, respectively, in the region of Daiichi-Amami Knoll. The results of station 3 and station 4 at Kotakara Shima are represented by black and open squares, respectively. The sampling locations of these stations at Daiichi-Amami Knoll and Kotakara Shima are shown in Fig. 1a,d, respectively. The depth profiles at the Wakamiko Crater are shown as station 5 (black inverted triangles) and station 6 (open inverted triangles).

Mentions: Bathymetric mapping and dredge sampling were carried out at the Daiichi-Amami Knoll (Fig. 1b). In this region, rhyolite lava and cold seep mussels were recovered by dredging (Fig. 1f), implying that the hydrothermal system is an environment rich in methane and hydrogen sulfide. We collected hydrothermal plume samples via CTD-RMS hydrocasts immediately after the water column image analysis (Fig. 1c). The 3He/4He ratios, CH4 concentrations and δ13CCH4 values of the seawater samples acquired at Daiichi-Amami Knoll fall in the ranges of 1.01 to 1.57 Ra (where Ra is the atmospheric 3He/4He ratio of 1.382 × 10−6)16, 3 to 6738 nM and −48.0 to −27.9‰, respectively. Strong hydrothermal activity was identified at a water depth range between 275 to 300 m based on high turbidity, low pH values, high 3He/4He ratios and significant CH4 concentration anomalies corresponding to more positive δ13CCH4 values (Fig. 2), although no apparent temperature anomaly was detected.


Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan
Depth profiles of turbidity, pH, temperature, helium isotope ratios, concentrations and δ13C values of methane, and concentration and δ13C of dissolved inorganic carbon (DIC) obtained at Daiichi-Amami Knoll, Kotakara Shima and Wakamiko Crater.The depth of the sampling point below the sea surface is used as the vertical axis. Black and open circles represent samples taken at station 1 and station 2, respectively, in the region of Daiichi-Amami Knoll. The results of station 3 and station 4 at Kotakara Shima are represented by black and open squares, respectively. The sampling locations of these stations at Daiichi-Amami Knoll and Kotakara Shima are shown in Fig. 1a,d, respectively. The depth profiles at the Wakamiko Crater are shown as station 5 (black inverted triangles) and station 6 (open inverted triangles).
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5037448&req=5

f2: Depth profiles of turbidity, pH, temperature, helium isotope ratios, concentrations and δ13C values of methane, and concentration and δ13C of dissolved inorganic carbon (DIC) obtained at Daiichi-Amami Knoll, Kotakara Shima and Wakamiko Crater.The depth of the sampling point below the sea surface is used as the vertical axis. Black and open circles represent samples taken at station 1 and station 2, respectively, in the region of Daiichi-Amami Knoll. The results of station 3 and station 4 at Kotakara Shima are represented by black and open squares, respectively. The sampling locations of these stations at Daiichi-Amami Knoll and Kotakara Shima are shown in Fig. 1a,d, respectively. The depth profiles at the Wakamiko Crater are shown as station 5 (black inverted triangles) and station 6 (open inverted triangles).
Mentions: Bathymetric mapping and dredge sampling were carried out at the Daiichi-Amami Knoll (Fig. 1b). In this region, rhyolite lava and cold seep mussels were recovered by dredging (Fig. 1f), implying that the hydrothermal system is an environment rich in methane and hydrogen sulfide. We collected hydrothermal plume samples via CTD-RMS hydrocasts immediately after the water column image analysis (Fig. 1c). The 3He/4He ratios, CH4 concentrations and δ13CCH4 values of the seawater samples acquired at Daiichi-Amami Knoll fall in the ranges of 1.01 to 1.57 Ra (where Ra is the atmospheric 3He/4He ratio of 1.382 × 10−6)16, 3 to 6738 nM and −48.0 to −27.9‰, respectively. Strong hydrothermal activity was identified at a water depth range between 275 to 300 m based on high turbidity, low pH values, high 3He/4He ratios and significant CH4 concentration anomalies corresponding to more positive δ13CCH4 values (Fig. 2), although no apparent temperature anomaly was detected.

View Article: PubMed Central - PubMed

ABSTRACT

Shallow submarine volcanoes have been newly discovered near the Tokara Islands, which are situated at the volcanic front of the northern Ryukyu Arc in southern Japan. Here, we report for the first time the volatile geochemistry of shallow hydrothermal plumes, which were sampled using a CTD-RMS system after analyzing water column images collected by multi-beam echo sounder surveys. These surveys were performed during the research cruise KS-14-10 of the R/V Shinsei Maru in a region stretching from the Wakamiko Crater to the Tokara Islands. The 3He flux and methane flux in the investigated area are estimated to be (0.99–2.6) × 104 atoms/cm2/sec and 6–60 t/yr, respectively. The methane in the region of the Tokara Islands is a mix between abiotic methane similar to that found in the East Pacific Rise and thermogenic one. Methane at the Wakamiko Crater is of abiotic origin but affected by isotopic fractionation through rapid microbial oxidation. The helium isotopes suggest the presence of subduction-type mantle helium at the Wakamiko Crater, while a larger crustal component is found close to the Tokara Islands. This suggests that the Tokara Islands submarine volcanoes are a key feature of the transition zone between the volcanic front and the spreading back-arc basin.

No MeSH data available.


Related in: MedlinePlus