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doi:10.2204/iodp.proc.327.107.2011

Expedition 327 CORK sensors and sampling

Pressure

Pressure measurement and logging systems deployed during Expedition 327 were built into frames designed to slide onto mounts within one of the three measurement and sampling bays in the CORK wellheads (Fig. F8). These systems share characteristics with those deployed following CORK installation during Expedition 301. Electronics and batteries are housed in a cylindrical pressure case, and communication with a computer for programming and data download is accomplished using an underwater mateable connector (Teledyne ODI). Instruments deployed during Expedition 327 include three absolute pressure gauges (Paroscientific model 8B-7000) to monitor conditions at multiple depth intervals and the seafloor. The loggers can sample at time intervals as short as 1 s, and sensors have temperature-compensated pressure resolution on the order of 2 Pa (0.2 mm water), similar to those deployed on Expedition 301 CORK systems (Fisher et al., 2005). Loggers were configured to sample at a 1 min interval on deployment. Pressure gauges are connected with inch OD stainless steel lines to hydraulic couplers attached near the base of the system frame, and the couplers are inserted into matching receptacles mounted near the base of the pressure bay. The receptacles are connected with ⅛ inch stainless steel lines to three-way valves mounted in the same wellhead bay, then down through the seafloor CORK seal and to ¼ inch stainless steel lines that extend to depth.

CORK data loggers communicate with a RS-422 protocol at speeds of up to 230 kb/s. Data from pre–Expedition 327 CORKs that use the same connector and communications systems were downloaded and the loggers were reprogrammed using the submersible DSRV Alvin and ROVs ROPOS and Jason. As described earlier, the pressure logger on the CORK installed in Hole 1026B is currently connected to the NEPTUNE Canada network (www.neptunecanada.ca/). Expedition 327 and other Expedition 301 CORKs are similarly “NEPTUNE ready,” and hopefully more installations will go online in the near future.

Several days prior to each Expedition 327 CORK deployment, pressure monitoring lines were tested for hydraulic integrity from below the CORK seafloor seal up to the data loggers. For each CORK, the pressure measurement frame was installed in the wellhead, placing the hydraulic couplers in the receptacles. A fluid injection and pressure monitoring manifold was connected to tubing pigtails below the CORK seal. The fittings connecting inch monitoring lines to the pressure gauges were loosened, and the lines were de-aired by pumping freshwater through the lines for 10–20 min at 40–50 psi. The fittings were tightened, pressure was shut in at the testing manifold, and conditions were monitored using the data logger to verify that a complete seal was sustained. Once system integrity was verified, the data logger remained installed in the wellhead until the CORK was deployed.

Additional de-airing of pressure monitoring lines was completed during the final stages of CORK deployment, after all monitoring line connections were complete. Screw-cap purge valves were installed at high points for each pressure line (when CORK was positioned vertically), located behind the control valves in the wellhead (Fig. F8D). After each CORK wellhead was lowered through the rig floor, it was held over the moonpool and the purge valves were loosened; the wellhead was then lowered several meters below the water surface. After waiting 10 min for air in the pressure lines to escape, the wellhead was raised and the purge valves were tightened. Three-way valves in the wellheads are configured for each pressure gauge so that it can be set to monitor conditions at either depth or at the seafloor. Monitoring intervals and three-way valve configurations for Expedition 327 CORKs, as deployed, are described later.

Temperature

Autonomous temperature sensors and data loggers deployed in Expedition 327 CORKs are similar to those deployed during Expedition 301 (Fig. F9). We elected to use autonomous loggers rather than a preinstrumented thermistor cable because (a) we could not be certain as to the desired final space-out of thermistor sensors and other components until the holes were completed to their final depths; (b) thermistor cables are considerably more expensive than autonomous loggers; (c) thermistor cables require electrical pass-throughs for plugs and pressure cases, adding to the system’s complexity and opportunity for failure; and (d) use of a braided Spectra line makes it easier to attach plugs, weights, and fluid and microbiological sampler systems. We also needed to monitor temperatures at the depths of the osmotic samplers because their sampling rates depend on the temperature-dependent viscosity of seawater, and this requires use of autonomous temperature loggers.

We purchased modified versions of two commercial temperature logging tools for use with the Expedition 327 CORKs (Fig. F9A). Two sets of instruments were built by Antares Datensysteme GmbH (www.antares-geo.de/), the manufacturer of tools used in CORKs deployed during Leg 205 and Expedition 301 (Pfender, 2002, number 1481). These tools differ from standard commercial instruments in being slightly wider and having double O-ring seals and a 5 y lithium battery. Antares tools used during Expedition 301 (purchased in 2004) had a working range of ~41°–100°C, which allowed us to maintain nominal tool resolution of 1 mK across the range of temperatures anticipated in basement (≤60°C). Five of these tools, recovered from Holes 1026B and U1301A following a 4 y deployment, were refurbished at the factory and recalibrated in preparation for Expedition 327. In addition, 18 of a newer generation of Antares instruments having a working range of ~1°–100°C were purchased and calibrated in 2009. All of these tools are programmed and operated using a through-case serial connection, with operating software run on a PC, so that the pressure cases need never be opened to operate the tools, check battery power, or access data.

The other temperature tools prepared for Expedition 327 were constructed by Onset Computer Corporation (www.onsetcomp.com/), HOBO model U12-015, modified with a titanium pressure case and long-life battery for use in the deep sea. The Onset tools have a working range of –40°–100°C but a lower resolution (0.02°–0.1°C across their working range) than the Antares tools because they use 12-bit rather than 16-bit analog/digital (A/D) conversion. Like the Antares tools, the Onset tools are programmed and data are accessed using software on a PC, but the end of the pressure case for the Onset tools must be opened to allow communication with a computer.

All of the Expedition 327 temperature tools were calibrated in 2009 (Fig. F10). We used a stirred-fluid temperature calibration bath (Hart Scientific model 7011) capable of maintaining temperature stability of ±0.001°–0.01°C over a temperature range of –5°–100°C and a National Institute of Standards and Technology (NIST)-traceable resistance-temperature device (RTD) probe (Hart Scientific model 1521) with an accuracy of ±0.001°C. The RTD and digital readout were recalibrated to factory specifications in 2008. Calibration of the Onset and newer Antares tools was completed at 11 temperatures across a range of ~2°–80°C. The older Antares tools were calibrated at seven temperatures across a range of ~42°–80°C.

Calibration in 2009 improved the accuracy of all tools relative to factory values. Onset tools have typical root-mean-squared (RMS) errors of 0.01°C during calibration (Fig. F10A), based on a third-order polynomial fit of apparent (factory) temperatures to reference probe temperatures. However, this is for a stable bath with temperatures measured every 10 s for 30–40 min, which allows averaging of 180–240 measurements to improve the effective resolution. In practice, the digital resolution of individual measurements with these tools varies from 0.02°C near 2°C to 0.1°C near 65°C. Both generations of Antares tools have typical calibrated RMS errors of 0.001°–0.003°C (Fig. F10B, F10C) and similar resolutions across their full range of operation. For field deployment in CORKs, temperature tools were programmed to record data every 90–120 min, so as to preserve battery power and memory for 4–5 y. If borehole conditions are relatively stable, tool resolution may be improved by averaging values measured over several days.

Most of the temperature tools deployed in CORKs during Expedition 327 were housed inside perforated sections of hydraulic hose, which was spliced to the Spectra cable and covered with electrical tape prior to instrument string deployment (Fig. F9B, F9C). This hose helps to protect the temperature instruments from damage during string deployment and recovery. Additional temperature tools were deployed inside polyvinyl chloride (PVC) housings containing OsmoSamplers and microbiological growth systems. The distribution of individual temperature tools on CORK instrument strings is described in “Configuration of Expedition 327 CORKs.”

Continuous fluid sampling

Instrument strings deployed within Expedition 327 CORKs included multiple downhole OsmoSampler systems (Jannasch et al., 2004) built around a common design (Wheat et al.) (Fig. F11). CORK systems deployed during earlier expeditions also included wellhead OsmoSamplers, as described later. No wellhead OsmoSamplers were deployed on Expedition 327 CORKs because these new CORKs use ½ inch sampling lines that are intended to flow freely (discharging much more fluid than is captured by OsmoSamplers). We wished to leave these CORKs sealed to allow recovery following drilling and assessment of regional pressure conditions prior to the start of a long-term flow experiment in summer 2011.

The borehole OsmoSampler systems were designed to pass through drill pipe and the inner CORK casing, and thus could have an outer diameter no larger than the smallest restriction imposed by the lowermost plug seat (3⅜ inch ID) in the casing string. The outer housing of the pumps was made of clear PVC, whereas most of the other pump parts were made of standard gray PVC. Each of the pump pieces was sealed with double O-ring seals, and the membranes were held in place with a single O-ring and a two-part epoxy (Hysol ES1902, www.henkelna.com/). Membrane configuration differed depending on the application: pumps contained either one or two Alzet 2ML1 membranes or one Alzet 2ML4 membrane (www.alzet.com/). The salt gradient across the membrane was created using excess noniodized table salt (NaCl) supersaturated in water on one side of the membrane and deionized (18.2 MΩ) water on the other side (Fig. F11B).

The pumps were mounted on a central ½ inch OD stainless steel rod used to provide strength for the assembly, which consisted of threaded end-caps, the central rod, pump(s), sample coils, PVC connectors for stability, and in some cases temperature loggers, as described earlier. Each component fit within a 2⅞ inch OD clear PVC tube, used to protect the assembly during deployment and recovery. The OsmoSampler assemblies were connected in a series using ½ inch galvanized shackles with PVC isolators.

Six kinds of OsmoSamplers were deployed during Expedition 327: standard, gas tight, acid addition, BioOsmoSampling System (BOSS), microbiological enrichment, and microbiological growth (MBIO). Each OsmoSampler was intended for a deployment as long as 6 y. These instrument systems are described briefly in the rest of this section and in greater detail elsewhere in this volume (Wheat et al.). Configurations and depths of individual instruments deployed during Expedition 327 are described in “Configuration of Expedition 327 CORKs,” below.

A standard OsmoSampler assembly consists of a pump with a single Alzet 2ML1 membrane and four 305 m long PTFE sample coils (1.2 mm ID). This configuration can sample ~3.5 mL of fluid per week at an anticipated basement temperature of 60°–70°C. Once recovered, fluids from the standard assembly are analyzed for major and minor ions.

The gas-tight assembly is similar to the standard assembly but uses four copper sample coils (Fig. F11C) to minimize diffusional gas exchange. For the Expedition 327 instruments, this will be important both for natural gases and for SF₆, which was one of the tracers injected in Hole U1362B during the 24 h pumping experiment (Fisher, Cowen, et al.).

The acid addition assembly consists of (from bottom to top) a PTFE sample coil filled with deionized water, a single 2ML4 membrane pump, a PTFE coil filled with dilute acid (20 mL of subboiled 6N HCl in 500 mL 18.2 MΩ water), a T-connector with one branch open to borehole fluids and the other connected to four PTFE sample coils, and a single 2ML1 membrane pump. Thus, the lower pump, which has a pump rate one-fourth that of the upper pump, forces dilute acid into the T-connector, where it mixes with borehole fluids that are subsequently stored in the four upper sample coils. This acid addition helps to stabilize redox-sensitive dissolved metals and reactive ions, ideal for shore-based measurements of trace elements in seawater and reacted formation fluids.

The fourth and fifth assemblies have physical configurations similar to that of the acid addition assembly. The BOSS assembly has a coil that discharges 20 μL of saturated HgCl₂ in 500 mL of 18.2 MΩ water, rather than the dilute acid of the acid addition coil. The BOSS assembly is designed to arrest microbial metabolic processes while maintaining cell structure for shore-based microbial assays. The microbiological enrichment assembly injects a fluid having a composition dependent on specific experimental goals. For Expedition 327, the enrichment assembly injects artificial borehole media (based on borehole fluid compositions described by Wheat and Mottl [2000]) with 1 mM NaH¹³CO₃ and 0.2 mM NaH¹²CO₃ to track autotrophic incorporation of ¹³C-labeled substrates. The enrichment assembly also contains a single microbial growth chamber, described below, so that microorganisms that fix dissolved carbon can be identified and quantified during shore-based analyses.

The MBIO assembly consists of a series of microbial growth chambers (Orcutt et al., 2010, 2011), described in the next section, eight PTFE sample coils, and a pump with two 2ML1 membranes. This configuration allows more fluid to pass through the chambers and be recovered than would be provided by the standard assembly. By comparing this fluid to that collected with the standard assembly, researchers will be able to document compositional changes related to microbial and inorganic reactions within the microbial growth incubators.

Microbiological growth incubators

Newly designed Flow-through Osmo Colonization Systems (FLOCS) (Orcutt et al., 2010) were deployed in Expedition 327 CORKs (Fig. F12). Like the Expedition 301 passive colonization experiments and flow cells (Fisher et al., 2005; Orcutt et al., 2011; Smith et al., submitted), FLOCS contain series of presterilized chambers packed with colonization substrate. Some chambers are connected to osmotic pumps that introduce a slow and steady flow of formation fluids, whereas other chambers are passively open to the formation. Microorganisms in the borehole fluids may preferentially colonize various rock and mineral substrates on the basis of favored mineral-fluid redox reactions (e.g., Edwards et al., 2003). Connection of the outflow of the chambers to an OsmoSampler sampling coil provides a temporal record of chemical changes during deployment, and comparison of these fluids to those collected with standard OsmoSampler fluids may elucidate biogeochemical reactions occurring within the chambers.

A modified FLOCS experiment, coupled to an enrichment-injection OsmoSampler, was deployed during Expedition 327 (Fig. F12B, F12E). The enrichment OsmoSampler was configured to deliver a slow and steady supply of ¹³C-labeled bicarbonate to act as a tracer of potential autotrophic reactions that may occur during mineral colonization. Similar to stable isotope probing methods used elsewhere (Boschker et al., 1998; Ginige et al., 2004; Orphan et al., 2001; Radajewski et al., 2000), the incorporation of ¹³C-labeled bicarbonate into biomass during growth will allow autotrophic metabolism to be tracked using molecular techniques in the laboratory. Experiments such as this could help to identify the unknown autotrophic members of microbial communities in oceanic crust that support the base of the food web in this poorly understood biome.

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