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Expedition 336 CORKs: sensors and sampling

A variety of sensors, samplers, and sampling ports were deployed in the CORKs in Holes 395A, U1382A, and U1383C. Some of the sensors and samplers resided on the CORK wellheads, whereas others were placed internally (i.e., downhole) within the CORK casing. Wellhead configurations prior to deployment for Holes U1382A and U1383C are shown in Figures F5 and F9. A compilation of downhole deployed sampling systems and sensors is shown in Figure F16.

Pressure sensors

Pressure monitoring in the North Pond CORKs is accomplished with wellhead-based pressure loggers monitoring hydrological horizons of interest via screened umbilicals. Pressure data from Expedition 336 CORKs can be downloaded during CORK servicing visits with a submersible or ROV using an underwater mateable connector.

Pressure measurement and logging systems deployed during Expedition 336 were built into frames designed to slide onto mounts within one of the three measurement and sampling bays in the CORK wellheads (Fig. F12). These systems share characteristics with those deployed following CORK installation during Expedition 327 (Fisher et al., 2011). 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 336 include absolute pressure gauges (Paroscientific Model 8B7000-2) to monitor conditions at multiple depth intervals and the seafloor. The installation in Hole U1382A includes two gauges to monitor pressures at the seafloor and in the single formation zone; the installation in Hole U1383C includes four gauges to monitor pressures at the seafloor and in the three formation zones isolated downhole. 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 327 CORK systems (Fisher et al., 2011). Loggers were configured to sample at a 2 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 using an RS-422 protocol at speeds of up to 230 kbps. Several days prior to each Expedition 336 CORK deployment, pressure-monitoring lines were tested for hydraulic integrity from below the CORK seafloor seal up to the data loggers (similar testing was conducted for all lines and valves on the CORK body). 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 ⅛ 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 the CORK deployment after all monitoring line connections were complete. Screw-cap purge valves were installed at high points for each pressure line (when the CORK was positioned vertically), located behind the control valves in the wellhead. 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 either at the formation screens or the seafloor.

Temperature sensors

Temperature measurements within the borehole are accomplished by autonomous temperature sensors and logger units placed within the CORK instrument string. Temperature data are acquired upon recovery of the instrument string (i.e., every few years).

Autonomous temperature sensors and data loggers deployed downhole within Expedition 336 CORKs are similar to those deployed during IODP Expeditions 301 and 327 and described in detail by Fisher et al. (2011, 2005). We elected to use autonomous loggers rather than a preinstrumented thermistor cable because (1) the Expedition 336 plan for multiple fluid and microbiological sampling zones would have required extensive expenses and minimal freedom in choosing zone location with a single thermistor cable and (2) we wanted to monitor temperatures at the depths of the OsmoSamplers because their sampling rates depend on the temperature-dependent viscosity of seawater.

As was used for the Expedition 301 and 327 CORKs, we deployed in the Expedition 336 CORKs a mix of two commercially available marine temperature logging tools. One was built by Antares Datensysteme GmbH (, with a 5 y lithium battery and thermistor type chosen to optimize resolution over the 0°–30°C temperature range expected at the North Pond sites. The other set was constructed by Onset Computer Corporation (, HOBO Model U12-015-3, modified with a titanium pressure case and long-life battery for use to full ocean depth. The Onset tools have a working range of –40°–100°C but a lower resolution (0.02°–0.1°C) because of their greater working range and because they use Antares tools which use 12-bit rather than 16-bit analog to digital conversion. Both manufacturers provide factory calibrations for their temperature loggers, and we plan to recalibrate the loggers after the eventual recovery of the downhole sensor strings.

All of the temperature tools deployed in CORKs during Expedition 336 were housed inside polyvinyl chloride (PVC) housings containing OsmoSamplers and microbiological growth systems (Fig. F16). The distribution of individual temperature tools on CORK instrument strings is described in the “Site 395,” “Site U1382,” and “Site U1383” chapters (Expedition 336 Scientists, 2012b, 2012c, 2012d).


Oxygen measurements within the borehole are being taken by autonomous oxygen sensors and loggers placed within the borehole on the CORK instrument string. The oxygen sensor is a standard Aanderaa oxygen optode 4330. The sensor is connected to a custom RBR, Inc., data logger housed in a titanium body. The logger will power the sensor for 5 y, collecting data every 12 h. The logger and sensor package is housed in a stainless steel strength member (2⅞ inch OD) to allow the sensor to align with the OsmoSampler packages while being thin enough to pass through the 3 inch gravity seal. Oxygen concentration data will be acquired upon recovery of the instrument strings (i.e., in a few years).

Fluid sampling for geochemical analyses

Fluid samples for geochemical analyses (major and minor ions, trace elements, gases, etc.) are collected continuously using both downhole (Fig. F16) and wellhead (Fig. F12) OsmoSamplers (Jannasch et al. 2004). The downhole OsmoSamplers sample fluid directly from the borehole and are based on a common design (Wheat et al., 2011). These samplers will be recovered in a few years during CORK instrument string recovery. The wellhead OsmoSamplers, which sample fluids pulled through the CORK umbilicals, can be recovered on a yearly basis during CORK servicing operations with a submersible or ROV. Wellhead OsmoSamplers have been used on other CORK systems (Wheat et al., 2011); however, these basic wellhead OsmoSamplers were uniquely connected during Expedition 336 to a new generation of “fast-flow” osmotic pumps (Fig. F17), which are described in more detail below.

In general, each downhole OsmoSampler consisted, in vertical orientation from top to bottom, of an osmotic pump, fluid sampling coils constructed of different materials (i.e., Teflon or copper) depending on the experimental design, and in some cases additional sampling materials, pumps, and coils. For each downhole sampling horizon, six different OsmoSampler packages were connected vertically to achieve fluid sampling and experimental objectives. The exact configurations are discussed in more detail below and in the “Site 395,” “Site U1382,” and “Site U1383” chapters (Expedition 336 Scientists, 2012b, 2012c, 2012d). The borehole OsmoSampler packages 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 gravity plug seat through which it must pass. Two different sizes of OsmoSampler systems were used during Expedition 336. OsmoSampler packages deployed in the shallow horizons have a 2⅞ inch OD whereas the deepest OsmoSampler packages have a 2½ inch OD to pass through the 2⅞ inch gravity plug seat. The outer housing of the pumps and the protective sleeves surrounding the sampling coils was made of clear PVC to allow monitoring of the various components while the sleeve is installed, whereas most of the other pump parts were made of standard gray PVC. Each of the pump pieces was sealed with single O-ring seal, and the membranes were held in place with a single O-ring and a two-part epoxy (Hysol ES1902, Membrane configuration differed depending on the application: pumps contained one, five, or eight Alzet 2ML1 membranes ( to achieve desired flow rates. 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. F16). 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. The OsmoSampler assemblies were connected in series using novel stainless steel connectors with cutouts for line handling and a stainless steel pin/plastic protector system to join the OsmoSampler package to the stainless steel connector.

Six kinds of downhole OsmoSamplers packages were deployed during Expedition 336: standard, gas-tight, acid addition, BioOsmoSampling System (BOSS), microbiological enrichment, and microbiological growth (MBIO). Each OsmoSampler package 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 (Wheat et al., 2011). Configurations and depths of individual instruments deployed during Expedition 336 are described in the “Site 395,” “Site U1382,” and “Site U1383” chapters (Expedition 336 Scientists, 2012b, 2012c, 2012d).

A “standard” OsmoSampler package consists of a pump with either five or eight Alzet 2ML1 membranes for either the 2½ or 2⅞ inch OD packages, respectively. Fewer membranes are need in the deeper section (2½ inch OD) to acquire the same volume of fluid given the warmer conditions at depth (20°C versus 6° or 8°C, respectively) Three sampling coil units, each containing a spool of 305 m length PTFE sample coils (1.2 mm ID) are attached to the pump. Once recovered, fluids from the standard package will be analyzed for major and minor ions. The “gas-tight” package is similar to the standard package but uses three copper sample coils to eliminate diffusional gas exchange.

The “acid addition” package consists of (from bottom to top) a PTFE sample coil filled with deionized water, a single 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 three PTFE sample coils, and either a five or eight Alzet 2ML1 membrane pump. Thus, the lower pump, which has a pump rate of or ⅛ that of the upper pump, forces dilute acid into the T-connector, where it mixes with borehole fluids that are subsequently stored in the three 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 OsmoSampler packages have physical configurations similar to that of the acid addition assembly. The “BOSS” package has a PTFE coil that discharges a biological fixative solution (2 mL of saturated HgCl2 in 75% RNAlater [Ambion]), rather than the dilute acid of the acid addition coil. The BOSS package is designed to arrest microbial metabolic processes while maintaining cell structure for shore-based microbial assays. The microbiological “enrichment” package injects a 1.2 mM nitrate solution in sterile seawater. The injection of nitrate-doped sterile seawater is designed to stimulate nitrate-reducing microorganisms. The enrichment package also contains a single microbial growth chamber, described below, so that microorganisms can be identified and quantified following shore-based analyses. The “MBIO” package consists of a series of microbial growth chambers (Orcutt et al., 2010, 2011), described in the next section, six PTFE sample coils, and two osmotic pumps with either five or eight Alzet 2ML1 membranes depending on where the packages are to be deployed within the borehole. By comparing this fluid to that collected with the standard package, researchers will be able to document compositional changes related to microbial and inorganic reactions within the microbial growth incubators.

Standard and MBIO OsmoSampler packages were also connected to a new fast-flow osmotic pumping system attached to stainless steel umbilical sampling lines at the CORK wellheads of Holes U1382A and U1383C (Fig. F17). Unlike regular OsmoSampler packages, which rely on small forward osmosis membranes with relatively low flow volumes (milliliters per day range) to drive fluid flow, the fast-flow osmotic pumps utilize a different forward osmosis membrane that allows larger flow volumes (hundreds of milliliters per day). Here, the membrane is sealed (with gaskets) within a PVC pump head, where one side of the membrane is exposed to seawater (to provide the salt gradient for pumping) and the other side is connected to a large reservoir of distilled water. The distilled water is contained within a PVC bag liner sealed inside a PVC reservoir. As distilled water moves across the membrane toward open seawater, the bag liner collapses, creating low pressure inside the PVC reservoir that enables the pumping of fluid up umbilical lines that are connected at the base of the reservoir.

During Expedition 336, the fast-flow OsmoSamplers consisted of two parallel 17 L volume PVC reservoirs connected jointly to a common umbilical sampling system constructed of ⅛ or  inch polyetheretherketone (PEEK) tubing. The size of the reservoirs during Expedition 336 was limited by the need for the wellhead pump systems to fit within the 28 inch diameter restriction of the wellheads for the VIT camera system. The membrane surface area was tailored to achieve pumping rates on the order of 150 mL/d, which should allow the 34 L pumping reservoir to last 7½ months until the fast-flow pumps would be replaced on a servicing cruise with an ROV. Two three-way T-connectors were added to the umbilical sampling line to allow connection of the standard and MBIO OsmoSampler packages, each consisting of one 12-membrane (Alzet) pump and a 300 m long Teflon fluid sampling coil. After assembly of the pumping systems but prior to deployment, the pump head was placed in seawater kept at 4°C to verify the pumping rate and check the condition of the membrane. The umbilical connector was kept in a bucket of cold distilled water until connection to the CORK wellhead in the moonpool immediately prior to CORK deployment. The system was attached to the wellhead via a network of zip ties and rubber bands so that the package would remain on the CORK during deployment but still be removable for ROV during servicing (by cutting with a knife). Future fast-flow osmotic pumps will have lessened size restrictions (because they will not need to pass under a VIT camera system), allowing for larger reservoirs and hence, faster pumping rates or longer pumping times.

Fluid samples can also be collected in real time using mechanical pumping systems connected to the fluid sampling umbilicals in the CORKs (Cowen et al., 2012; Wheat et al., 2011). This mechanism allows larger volume sampling than can be accomplished with OsmoSamplers but is dependent on battery- or submersible/​ROV-powered pumping systems. The basic procedure is to attach a fluid sampling line from one side of a pump to the receptacle on the wellhead. The valve on the wellhead is opened to allow the pump to withdraw fluids from the umbilical that terminates at a hydrologic horizon of interest. The other side of the pump is attached to a series of sampler, sensors, filters, and inverted funnels to facilitate sampling using additional equipment. Such a mechanism allows collection of large volumes of water (hundreds of liters), measurement of ephemeral properties (e.g., dissolved hydrogen), and “redox” conditions.

The L-CORK design allows the borehole to be opened via a 4 inch ball valve to instruments such as pumps, flow meters, or tracer injection devices without disturbing downhole sampling and experimental systems. Although such experiments were not the focus for these initial downhole packages, the L-CORK design allows for future experimentation. The ball valve was replaced with a cap on the Hole U1382A CORK because of a crack that developed during the deployment procedure. A working ball valve system with attachment ring (e.g., Fisher et al., 2011) is on the Hole U1383C CORK.

Microbial growth chambers

Microbial colonization chambers are another form of sampler deployed in Expedition 336 CORKs, both in wellhead and downhole configurations. The principle of these colonization devices, which consist of defined mineral growth substrates to encourage colonization by native planktonic (i.e., in fluids) borehole microorganisms, and which are referred to as Flow-through Osmo Colonization System (FLOCS), are described in more detail elsewhere (Orcutt et al., 2010, 2011). The wellhead FLOCS were deployed in tandem with the fast-flow OsmoSamplers (Fig. F17), while the downhole FLOCS (Fig. F18) were contained within the MBIO and enrichment OsmoSampler packages described above. The exact spacing of the downhole FLOCS experiments is described in the “Site 395,” “Site U1382,” and “Site U1383” chapters (Expedition 336 Scientists, 2012b, 2012c, 2012d). The wellhead FLOCS can be serviced yearly, whereas the downhole packages are only retrieved when the instrument string is recovered (i.e., every few years).

Like the Expedition 301 and 327 passive colonization experiments and flow cells (Fisher et al., 2005; Orcutt et al., 2011; Smith et al., 2011), FLOCS contain series of presterilized chambers packed with colonization substrates (i.e., mineral coupons and fragments). 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) such as has recently been observed from wellhead experiments conducted on the Juan de Fuca CORKs (Orcutt et al., 2011). Connection of the outflow of the chambers to an OsmoSampler sampling system provides a temporal record of chemical changes during deployment, and comparison of these fluids to those collected with standard OsmoSampler package may elucidate biogeochemical reactions occurring within the chambers. Modified FLOCS experiments were also deployed, coupled to an enrichment OsmoSampler package as described above.