Site U1382 | Site U1383 | Holes U1383D, U1383E, U1382B, U1384A
IODP Expedition 336:
Mid-Atlantic Ridge Microbiology
Site 395 Summary
PDF file is available for download.
Investigating coupled geochemical and
microbial processes in active aquifers within the upper oceanic crust is the
main science goal of Expedition 336. The primary objective of the expedition is
initiating multilevel subseafloor borehole observatories (CORKs). One
observatory was planned to be installed in the 664-m deep Hole 395A in the
southeastern part of North Pond. Hole 395A was drilled during DSDP Leg 45 in
1975-1976, was logged repeatedly, and equipped with a first-generation CORK in
1997 during Leg 174B (Becker et al., 1998). Hole 395A is located in an area of
exceptionally low conductive heat flow (Langseth et
al., 1992). The low conductive heat flow is due to cooling of uppermost
basement by cold seawater that recharges basement and is inferred to flow
underneath the sediment cover in a northerly direction.
At the beginning of our operations at
Site 395, the old Leg 174B CORK, including the entire 603-m internal string with
thermistors, a data logger and pressure sensors, was successfully pulled out of
Hole 395A and secured on board. The pressure and temperature data were
downloaded, the thermistors cut out of the string, and sections of the string
were sampled for microbiologic analyses. Further microbiological samples were
obtained from the CORK ROV platform and CORK wellhead elements. The hole was
then logged with a new in situ deep
UV fluorescence tool for detecting microbial life in ocean floor boreholes -
the Deep Exploration Biosphere Investigative tool (DEBI-t). Other logging data
obtained include spectral gamma ray, and temperature. A rock ledge in the
borehole at around 180 m below seafloor (mbsf) had to be bridged by lowering
the logging bit to ~198 mbsf, but then an open hole section of 405.7 m was
logged (total depth reached was 603.5 m). The lowermost ~50 m of the hole was
not logged as Leg 174B found it filled with rubble. The logging results
are consistent with the data obtained by Bartetzko et
al. (2001) and allow distinguishing the distribution of massive basalt, pillow
basalts, altered lava flows, and rubble zones (sedimentary breccia and hyaloclastite).
A 530-m long, multi-level CORK
observatory was assembled to perform long-term coupled microbiological,
biogeochemical, and hydrological experiments. Assembling the observatory
entailed preparing osmotically-driven fluid samplers, microbial incubation experiments, seven temperature, and two oxygen sensors. Packers at 111, 149, and 463 mbsf were
installed to isolate the borehole into three intervals that are characterized
by different thermal and fluid flow regimes. Umbilicals containing fluid sampling
lines attached to the outside of the CORK casing were designed to reach depths
of 122, 220, 430, and 506 mbsf. OsmoSamplers for fluid geochemistry and microbiology were lowered on spectra rope inside the slotted
or perforated CORK casing to sample four intervals: 118-140, 240-261, 415-438,
and 499-527 mbsf. The CORK wellhead was instrumented with sensors for
monitoring pressures in the four zones isolated by packers and with OsmoSamplers for retrieving fluid samples from the lowermost zone.
The assembly and installation
proceeded well until the CORK head broke off during the final step of releasing
the CORK running tool. The CORK head experienced forces that bent the wellhead
and severed its 5 inch pipe about 4 m below the top of
the reentry cone. This also parted the spectra line and the umbilicals, leaving
the downhole tool string in place. Based on the portion of the CORK wellhead
recovered, the upper end of the remaining 5 inch diameter cup packer
subassembly near the seafloor (5 inch pipe mandrel) is not completely rounded, but may be open enough to allow recovery of the internal downhole
samplers, sensors, and experiments in the future. Several stainless steel tubes
likely extend above the cup packers and the top of the 5 inch
casing. Damage on stabilizing fins above the
cup packers suggests that they may have been too large in diameter to enter the
throat of the reentry cone (DSDP documentation indicated a 24 inch diameter;
but now thought to be less). This may have been the root cause of the
installation failure – similar damage was observed on the Leg 174B CORK
that was recovered (it too did not fully land). The CORK pressure logging system was recovered along with
the broken-off wellhead. The recorded data do not definitively resolve whether
or not the downhole CORK packers actually inflated, however, no data are
otherwise available to suggest that the packers did not inflate as intended
either. A plan is being formulated to recover the downhole instrument string in
four years with a remotely operated vehicle (ROV).
Bartetzko, A., Pezard, P., Goldberg, D., Sun, Y.-F.,
Becker, K., 2001. Volcanic stratigraphy of DSDP/ODP Hole 395A: An
interpretation using well-logging data. Mar. Geophys. Res. 22, 111-127.
Becker, K., Malone, M.J., et al., 1998. Proc. ODP, Init.
Repts., 174B: College Station, TX (Ocean Drilling Program).
Langseth, M.G., Becker, K., Von Herzen, R.P., Schultheiss,
P., 1992. Heat and fluid flux through sediments on the western flank of the
mid-Atlantic Ridge: a hydrogeological study of North Pond. Geophys.
Res. Lett. 19, 517-520.