hydro-thermal venting. In September 1985, an interdisciplinary 

 group of scientists aboard the R/V Hakuho Maru conducted a 

 comprehensive hydrographic and geochemical study of the water 

 column above the summit of Loihi Seamount. During that cruise a 

 hydrothermal plume rich in dissolved methane, helium (with a high 

 ^He/^He ratio), excess carbon dioxide and high concentrations of 

 Fe, Mn, Ni, and Co was detected (Sakai, Tsubota, Nakai, 

 Ishibashi, Akagi, Gamo, Tilbrook, Igarashi, Kodera, Shitashima, 

 Nakamura, Fujioka, McMurtry, Malahoff, and Ozima 1977). These 

 results indicated the presence of an active hydrothermal system 

 at or near the summit of Loihi. 



In February 1987, two active hydrothermal vent fields were 

 located, mapped and sampled by scientists aboard the deep-sea 

 research submersible Alvin . On that expedition, we obtained 

 several water and bacterial mat samples from the larger of the 

 two fields in an area that we named Pele's Vent. In August 1987 

 we returned to Pele's Vent during a NOAA-NURP sponsored research 

 expedition using the deep-sea research submersible Pisces V. 



Sample Collection 



Hydrothermal fluids flowing from Pele's Vent were collected 

 on Alvin dives #1797-1800 and Pisces V dives #25-29 using either 

 a small volume (approximately 700 ml) piston-driven titanium 

 sampler specially-designed for hydrothermal vent research (Von 

 Damm, Edmond, Grant, Measures, Walden, and Weiss 1985), standard 

 5-1 Niskin bottles attached to the basket of the respective 

 submersible or a 2-1 Niskin baggie sampler adapted with a 2-m 

 tygon tube for directional sampling. Bacterial mats were also 

 sampled using either a standard Alvin tube core or the suction 

 produced by the opening of the 2-1 Niskin baggie samplers. The 

 complete sample log is given in Table 1. 



Sample Processing and Chemical Analyses 



Dissolved methane and total dissolved inorganic carbon 

 measurements 



Gas sampling was routinely the first of several subsampling 

 procedures. Water samples for the measurement of dissolved 

 methane and total dissolved inorganic carbon were transferred 

 into 60 ml ground glass stoppered bottles immediately and 

 poisoned with 0.5 ml of a saturated HgCl2 solution. During 

 sampling, care was taken to avoid gas exchange or trapping 

 bubbles in the sample bottles during sealing. The samples were 

 returned to the laboratory for analysis. 



Dissolved methane was determined using a modification of the 

 gas stripping method described by Swinnerton, Linnenbom, and 

 Cheek (1962) and Swinnerton and Linnenbom (1967). Dissolved 

 gases were purged from a 23 ml subsample with methane-free 



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