Gas Chemistry of Pore Fluids from Oomine Ridge on the Nankai Accretionary Prism - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 249

2 Sampling

Oomine Ridge is on the Nankai accretionary prism, which is constructed by
accretion of sediment from the subducting Philippine Sea plate (Fig. 1a ). On
Oomine Ridge, deep-sea chemoautosynthesis-based faunal communities, consisting
of tube worms and bacterial mats, have been observed by several explorations at a
depth of 2,530 m. They indicate the existence of seeping fluid containing H 2 S or
CH 4 , and they support prolific benthic communities. Chemical analyses of pore
water have led to the postulation that lateral input of groundwater is the source of
these fluids (Toki et al. 2004 ).
During the KY04-11 cruise (5 Sept to 2 Oct 2004) of R/V Kaiyo (JAMSTEC), a
sediment sample was obtained with a piston corer (4 m length) at the seepage site
(PC08: 33°7.32¢N, 136°28.75¢E), as shown in Fig. 1b . The piston corer was deployed
on a Navigable Sampling System (NSS) belonging to the Ocean Research Institute,
University of Tokyo. The NSS consists of a transponder, four thrusters, a TV cam-
era, and a trigger for a corer and is controlled from the tender ship. The recovered
length of the sediment in the core of PC08 was 268.5 cm. The core was composed
of homogeneous sandy silt containing layers of gravelly sand. The lowest part of the
core, about 265 cm below the seafloor (cmbsf), contained a carbonate clast about 5
cm in diameter.
Subsamples of sediments for gas extraction were taken as quickly as possible
after recovery (to avoid degassing and air contamination) using an onboard vacuum
system illustrated schematically in Fig. 2 . First, 3.4-cm holes were drilled in the
side of the inner tube at 30, 80, 130, and 180 cm from the core bottom. A 50-mL
stainless steel cylinder was inserted through the holes into the core sediment, then
the sediment in the cylinder was transferred into a stainless steel syringe with filter
for squeezing. A plunger was held fixed in the syringe with a double O-ring seal,
and the nozzle of the syringe was sealed by a check valve (Swegelok, SS-4H) with
no headspace. The check valve was connected to a stainless steel line, through
which pore water was transferred to a 54-cm 3 evacuated stainless steel container
with amide sulfuric acid and HgCl 2 by compressing the sediment sample in the
syringe with a stainless steel vice (Manheim 1968 ). The retrieved pore water in the
container was degassed by ultrasonication and reaction with amide sulfuric acid to
convert all dissolved carbonates to CO 2 gas and then reacted with HgCl 2 to deposit
H 2 S as HgS. The gas phase in the container was transferred to an evacuated stainless
steel vessel of 50-mL capacity, where it was stored for analysis on shore. The liquid
phase was filtered and sampled into a 50-cm 3 polypropylene bottle for measurement
of gas concentrations in pore water, which is calculated on the assumption that gas
is dissolved in the liquid phase of pore water.
Pore water from other sediment samples was retrieved into sample bottles by
standard handling described by Tsunogai et al. ( 2002 ) and using procedures and
equipment described by Manheim ( 1968 ). About 2 cm 3 of the retrieved pore water
was immediately transferred to a 3-cm 3 vial with amide sulfuric acid and HgCl 2 as
a preservative against microbial activity, and the vial was capped with a butyl rubber