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

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

Correlation diagram between stable carbon isotopes of methane (δ13C) and the CH4/3He ratios in hydrothermal/cold seep plumes.Four-component mixing model for methane origin. The end-members include biogenic, thermogenic, heavy and East Pacific Rise (EPR) type. All data are from Table 1 and Supplementary Table.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5037448&req=5

f4: Correlation diagram between stable carbon isotopes of methane (δ13C) and the CH4/3He ratios in hydrothermal/cold seep plumes.Four-component mixing model for methane origin. The end-members include biogenic, thermogenic, heavy and East Pacific Rise (EPR) type. All data are from Table 1 and Supplementary Table.

Mentions: Figure 4 shows the relationship between the δ13CCH4 values and CH4/3He ratios measured in the samples. There can be four major end members for the origin of methane in shallow marine hydrothermal systems: (1) abiogenic methane produced by chemical reactions, as observed on the East Pacific Rise (EPR); (2) biogenic methane produced by microbial activity utilizing inorganic carbon; (3) thermogenic methane from the thermal decomposition of organic matter; and (4) oxidized methane with heavier carbon isotope values formed through microbial fractionation in old gas plumes9. Most of the data from the Tokara Islands (red circles in Fig. 4) indicate mixing between EPR-type abiogenic methane and thermogenic methane, similar to the data from the Okinawa Trough2829. In these regions, the samples closest to the sea surface are characterized by relatively negative δ13CCH4 values and lower CH4/3He ratios, implying that the end members of methane are not only affected by thermogenic methane and EPR-type methane but also by biogenic influences. The hydrothermal vents in these regions are relatively shallow, suggesting that the presence of thermogenic CH4 in the plumes is caused by magmatic activity, as the methane is carried from the deep lithosphere towards the seafloor by vertical hydrothermal fluid migration. However, the data from the Wakamiko Crater (blue squares) appear to be characterized by a different end-member system from the data at the Tokara Islands in Fig. 4. Previous studies suggested that the origin of the methane at Wakamiko Crater is thermogenic due to volcanic heat interacting with organic matter in sediments and high ammonium concentrations in sediment pore water37. The Wakamiko Crater is located within Kagoshima Bay, where organic matter is supplied by surrounding lands and the seawater is affected by the basin morphology and somewhat stagnant currents. However, as mentioned above, in the hydrothermal plumes at the Wakamiko Crater, more complex processes seem to affect the dissolved gas species in the water column. Considering the geographical settings and the results of previous studies, we conclude that the thermogenic methane carried by the emitted hydrothermal fluids and the hydrothermal plumes experiences methane oxidation, resulting in microbial fractionation and isotopically heavy methane.


Helium and methane sources and fluxes of shallow submarine hydrothermal plumes near the Tokara Islands, Southern Japan
Correlation diagram between stable carbon isotopes of methane (δ13C) and the CH4/3He ratios in hydrothermal/cold seep plumes.Four-component mixing model for methane origin. The end-members include biogenic, thermogenic, heavy and East Pacific Rise (EPR) type. All data are from Table 1 and Supplementary Table.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037448&req=5

f4: Correlation diagram between stable carbon isotopes of methane (δ13C) and the CH4/3He ratios in hydrothermal/cold seep plumes.Four-component mixing model for methane origin. The end-members include biogenic, thermogenic, heavy and East Pacific Rise (EPR) type. All data are from Table 1 and Supplementary Table.
Mentions: Figure 4 shows the relationship between the δ13CCH4 values and CH4/3He ratios measured in the samples. There can be four major end members for the origin of methane in shallow marine hydrothermal systems: (1) abiogenic methane produced by chemical reactions, as observed on the East Pacific Rise (EPR); (2) biogenic methane produced by microbial activity utilizing inorganic carbon; (3) thermogenic methane from the thermal decomposition of organic matter; and (4) oxidized methane with heavier carbon isotope values formed through microbial fractionation in old gas plumes9. Most of the data from the Tokara Islands (red circles in Fig. 4) indicate mixing between EPR-type abiogenic methane and thermogenic methane, similar to the data from the Okinawa Trough2829. In these regions, the samples closest to the sea surface are characterized by relatively negative δ13CCH4 values and lower CH4/3He ratios, implying that the end members of methane are not only affected by thermogenic methane and EPR-type methane but also by biogenic influences. The hydrothermal vents in these regions are relatively shallow, suggesting that the presence of thermogenic CH4 in the plumes is caused by magmatic activity, as the methane is carried from the deep lithosphere towards the seafloor by vertical hydrothermal fluid migration. However, the data from the Wakamiko Crater (blue squares) appear to be characterized by a different end-member system from the data at the Tokara Islands in Fig. 4. Previous studies suggested that the origin of the methane at Wakamiko Crater is thermogenic due to volcanic heat interacting with organic matter in sediments and high ammonium concentrations in sediment pore water37. The Wakamiko Crater is located within Kagoshima Bay, where organic matter is supplied by surrounding lands and the seawater is affected by the basin morphology and somewhat stagnant currents. However, as mentioned above, in the hydrothermal plumes at the Wakamiko Crater, more complex processes seem to affect the dissolved gas species in the water column. Considering the geographical settings and the results of previous studies, we conclude that the thermogenic methane carried by the emitted hydrothermal fluids and the hydrothermal plumes experiences methane oxidation, resulting in microbial fractionation and isotopically heavy methane.

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