Microbial carbon isotope fractionation to produce extraordinarily heavy methane in aging hydrothermal plumes over the southwestern Okinawa Trough

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Abstract

Within neutrally buoyant hydrothermal plumes derived from the Yonaguni Knoll IV hydrothermal field (24°51´ N, 122°42´ E, D = 1,370-1,385 m) in the southwestern Okinawa Trough back-arc basin, we obtained profiles of dissolved manganese (Mn), helium isotopes (<sup>3</sup>He/<sup>4</sup>He), methane (CH<sub>4</sub>) and its stable carbon isotope ratio (δ<sup>13</sup>C<sub>PDB</sub>) in young to old plume waters. We first mapped the spatial distribution of hydrothermal plumes by towing a fixed array of optical sensors (Miniature Autonomous Plume Recorders: MAPRs) above the field. We then made water column observations and samplings using a CTD (Conductivity, Temperature, and Depth sensors)-Carousel package with a transmissometer and Niskin-X bottles at five locations, with distances between 0.6 and 6.1 km from the hydrothermally active center so far discovered. Vertical profiles of light transmission and chemical tracers indicated triple-layered plumes, the centers of which were at depths of 700-800 m, ∼1,050 m and ∼1,200 m. The CH<sub>4</sub> concentrations and δ<sup>13</sup>C values for the 1,200-m plume ranged between 1,026 and 10 nmol/kg, and between -22.4 and +40.4‰ (the highest δ<sup>13</sup>C value yet reported for oceanic CH<sub>4</sub>), respectively, indicating active microbial CH<sub>4</sub> oxidation accompanied by the δ<sup>13</sup>C increase for residual CH<sub>4</sub>. The δ<sup>13</sup>C(CH<sub>4</sub>) was shown to be useful for tracing such a “microbial plume” whose CH<sub>4</sub> concentration has already fallen to almost the background level. By applying the Rayleigh distillation equation for a closed system, we estimated the kinetic isotope fractionation factor of 1.012 for the CH<sub>4</sub> oxidation process occurring within the hydrothermal plumes deeper than 1,000 m.

Journal

  • GEOCHEMICAL JOURNAL

    GEOCHEMICAL JOURNAL 44(6), 477-487, 2010-12-01

    The Geochemical Society of Japan

References:  36

Cited by:  1

Codes

  • NII Article ID (NAID)
    10027719969
  • NII NACSIS-CAT ID (NCID)
    AA00654975
  • Text Lang
    ENG
  • Article Type
    Journal Article
  • ISSN
    00167002
  • NDL Article ID
    10936277
  • NDL Source Classification
    ZM41(科学技術--地球科学)
  • NDL Call No.
    Z53-R488
  • Data Source
    CJP  CJPref  NDL  IR  J-STAGE 
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