IODP Expedition 301T:
Transit/Cost Rica Hydrogeology
Site Summary: Sites 1253 and 1255
PDF file is available for download.
September 13, 2004
The Costa Rica hydrogeology operation, which took place
during the transit between Expeditions 301 and 303, was designed to replace
CORK-II downhole instrument strings in holes drilled at Ocean Drilling Program
(ODP) Leg 205 Sites 1253 and 1255 offshore Costa Rica. The CORKs are
instrumented in a fractured horizon in the oceanic section of the incoming
plate (Site 1253) and in the décollement zone (Site 1255) to investigate fluid
geochemistry and flow across the margin and their implications for the
seismogenic zone and subduction factory. This operation was necessary because
during Atlantis cruise 11-8 (27 February7 March 2004) the Alvin was unable to recover and thus redeploy the
downhole osmotic fluid samplers (OsmoSamplers) and miniaturized temperature
loggers. The OsmoSamplers installed during Leg 205 were designed to collect a
time series of samples for fluid and gas analyses over a 2 y period; after 2 y,
samples and the information they contain are progressively lost. The associated
temperature loggers will stop recovering data after 6 October 2004.
After a smooth 3411 nm transit from Astoria, Oregon, the ship arrived at Site 1253 at 11:12 hr on September 4, 2004. The Alvin operations had left the two Site 1253 OsmoSampler packages seated at depth, the tools and ~550 m
of Spectra line attached, and a ring and float attached ~20 m above the CORK
wellhead. Operations commenced with the attempts to retrieve the OsmoSampler
packages. After fishing for the line and float assembly for just over 3
hours, we started pulling out of the hole attached to the Spectra line, which
we had hoped was secured to both OsmoSampler packages and sinker bar. After
tripping out of the hole, we found that we had captured the float, some of the
Spectra line and the metal ring used to tie on the float. The remainder of the
fished components either dropped to the seafloor during the trip out or remain
in Hole 1253A. This was left to be found and hopefully recovered after we
completed operations at Hole 1255A.
completing successful operations at Site 1255, we returned to Site 1253 and
reentered Hole 1253A with a fishing tool to inspect for
the OsmoSampler packages and any remaining hardware. The hole was found to be
completely clear. The OsmoSamplers were not found, thus it was assumed they
had been removed from the hole and dropped onto the seafloor when the Spectra
line parted during initial fishing operations. The new OsmoSampler packages
(upper level within a screen and lower level in the open hole) were assembled
with a new sinker bar and 500m line to a seafloor float and deployed in Hole
1253A without incident. Visual inspection with the vibration isolated TV (VIT)
confirmed that the float assembly was properly deployed just above the CORK.
After unlatching from the CORK, the ship was offset and a systematic seabed
survey was conducted utilizing the VIT and the dynamic positioning system.
After ~7 hours of survey, the samplers were found ~20m SE of the wellhead. A
grappling system was rigged and deployed on the VIT sled. Within 2.25 hours
miscellaneous hardware, the upper OsmoSampler, and Spectra line were recovered.
The whereabouts of the lower OsmoSampler and sinker bar remain uncertain.
Hole 1253A fluids coils from two of the Teflon OsmoSamplers were
subsampled for shipboard measurements of alkalinity, pH, ammonium, and
salinity, and for a wide variety of postcruise analyses. Two further copper
coils will also be analyzed for gases and organic components postcruise.
Initial salinity analyses in the two separate fluid coils have very similar
profiles suggesting two complete years of fluids were obtained.
After the initial attempted retrieval of OsmoSamplers at Site
1253, the ship was offset to Site 1255 in DP mode. Once in position, we latched
onto the CORK, and successfully pulled the OsmoSampler package out of the hole.
The new OsmoSampler package with a 150 m line to a seafloor float was assembled
and deployed into Hole 1255A. After two attempts the coring line was
successfully disconnected from the instrument strings, with the float assembly
properly deployed just above the CORK.
The OsmoSamplers recovered from Hole 1255A were subsampled for
shipboard measurements of alkalinity, pH, ammonium, and salinity and for
further postcruise studies. Inspection indicated the osmotic pumps worked well
throughout the period deployed. In addition, osmotic flow-meter fluids were
subsampled for postcruise analyses to determine relative flow rates and
direction. The temperature loggers recovered are in excellent shape.
Operations for this transit leg required 3.8 days, concluding
September 8, 2004. Recovery of the fluid samplers and temperature loggers will
provide 2 y high-resolution time series of fluid chemistry and temperature, and
at Site 1255 also of fluid flow rates and relative flow direction. The
alkalinity and ammonium concentrations at the two sites are distinct. They are
considerably higher in the décollement fluid, at Site 1255, with maximum values
of 17.1mM and 1,828.1µM, respectively, than in the igneous complex formation fluid, at Site 1253, with maximum values of 0.8mM and 170.2 µM, respectively. Bacterially mediated diagenesis of organic matter is responsible for the high alkalinity and ammonium observed at Site 1255.