Fig. 2 Schematic diagram of onboard gas-extraction system showing pressure gauge ( P ) and
sample bottle ( S )
septum for subsequent shore-based analysis of dissolved CH 4 and total carbon
dioxide (SCO 2 ) for reference of gas samples. The remaining fluid was preserved in
a polypropylene bottle for measurements of major cations and anions as well as
isotopic analyses of pore water. All samples were refrigerated in the dark and kept
at 4°C until analysis.
Immediately after collection, the pore-water samples in the polypropylene bottles
were analyzed for silica, ammonium, and alkalinity (Gieskes et al. 1991 ). For the
silica determination, the samples were diluted fivefold and analyzed by the silicomo-
lybdate colorimetric method (Strickland and Parsons 1968 ). Ammonium content was
determined by spectrophotometry of phenol blue (Solorzano 1969 ). Alkalinity was
measured by potentiometric titration (Dyrssen and Sillen 1967 ; Edmond 1970 ).
Concentrations of major elements including cations (Na, Mg, and Ca) and anions
(Cl and SO 4 ) were measured with an ion chromatograph after appropriate dilutions
at Hokkaido University (Tsunogai and Wakita 1995 ). Isotope analyses of carbon
were made on a Finnigan MAT-252 mass spectrometer at Hokkaido University. For
measurements of isotopic composition and concentration of CH 4 , the samples were
prepared by cryogenic removal of CO 2 and oxidation of CH 4 with cupric oxide at
960°C, then introduced into the mass spectrometer (Tsunogai et al. 2002 ). Analytical
precision was 6.5% for concentration and 0.3‰ for isotopic determinations.
Hydrogen concentration was measured with a reduction gas analyzer at
Hokkaido University (Konno et al. 2006 ). Hydrogen isotopic composition of H 2
was determined using a continuous flow isotope ratio mass spectrometer system at
the Ocean Research Institute, University of Tokyo (Kawagucci et al. 2010 ). Gas
samples were injected into the analytical system through a pre-evacuated stainless
steel line and carried by ultrapure He through gas chromatograph columns. H 2 in
the injected gas was separated from the other gas species and introduced into the
isotope ratio mass spectrometer (Thermo Finnigan, DELTA XP ). Analytical precision
was estimated to be 5‰.