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doi:10.2204/iodp.proc.348.102.2015

Geochemistry

Interstitial water geochemistry for core samples

Sample preparation

When core recovery and quality allowed, 10–41.5 cm long whole-round core (WRC) samples were collected from Hole C0002P cores. Samples were not collected from Core 348-C0002P-1R due to low core recovery. Squeezed IW and ground rock interstitial normative determination (GRIND) pore water (GW) (Wheat et al., 1994) were sampled for the typical suite of shipboard measurements. In addition, IW was sampled from cores collected during testing of the SD-RCB in Hole C0002M for an experiment examination of the effect of high-pressure squeezing of clay minerals on Cl^– concentrations and stable isotopes. Before sampling for IW/GW whole-round sections, the core sections were scanned by XRCT to check for the presence of an intact interval of homogeneous sediment and to avoid structurally or lithologically important features. The section was immediately cut, capped, and then delivered to the geochemistry laboratory for processing. The sample was placed into a nitrogen-filled glove bag and removed from the core liner. Sediment along the outer surface of the WRC was scraped off, as well as along any internal fractures that came in contact with seawater or drilling fluid or had experienced smearing or oxidation. About 5 cm³ of presumably clean sediment from the inside of the core was transferred to a preweighed glass vial and immediately capped. This sample was analyzed by gas chromatograph–electron capture detector (GC-ECD) to assess the drill mud contamination by a perfluorocarbon (PFC) introduced into the drilling mud. Each cleaned WRC was then stored in a N₂-filled bag at room temperature until used for squeezing or GRIND methods.

Squeezing method

The squeezing method was used on all IW samples from Hole C0002M and on the first available IW sample from Hole C0002P (Sample 348-C0002P-2R-3, 96–137.5 cm). Indurated samples were crushed into small fragments inside the glove bag, making them easier to put into the squeezer. The portion of the cleaned WRC to be squeezed was placed in a Manheim-type titanium squeezer (Manheim, 1966).

The squeezers used during Expedition 348 were modified to work under higher squeezing pressures to increase IW extraction from lithified or low-porosity samples. The inner diameter of these squeezers was 55 mm. A newly developed “water-gathering plate” was used in the lower filtration assembly (Fig. F15). The water gathering plate is 55 mm in diameter, 3 mm thick, and includes 32 holes (1 mm diameter each) and 8 grooves to funnel extracted water to a central 3.6 mm diameter hole for flow down and out toward a syringe. Initial experiments conducted before drilling revealed that some mud was passing around the edges of the filter assembly and into the exit port. The final filtration assembly that successfully solved this problem was, from bottom to top, a bottom dish squeezer plate, a rubber disk, a titanium dish, a disk of filter paper, the water-gathering plate, a titanium mesh disk, a second filter paper disk, the sediment sample, and a third filter paper disk (Fig. F15A). The outer portions of the water-gathering plate and the titanium mesh disk were covered with Teflon tape to improve the seal with the squeezer jacket. This assembly was used in the squeezing method for Cores 348-C0002M-1R through 4R.

To allow more extraction of pore water from low-porosity sediment during the squeezing of Section 348-C0002P-2R-3, a squeezer with a top and bottom syringe port was used (Fig. F15B); this assembly was composed of the same filtration set as for squeezing samples from Hole C0002M, on both the top and bottom of the squeezer assembly.

Both assemblies were successfully tested to an applied load of 60,000 lb for >12 h during experiments with Hole C0002M samples. Following the procedures of the Expedition 319 Scientists (2010b), the samples were presqueezed using a manually operated squeezer until a few drops of water came out. This allowed a maximum amount of IW to be collected. Then, a 25 mL acid-washed (12 M HCl) syringe was installed into the IW sample port(s) of the squeezer, and the samples were subjected to automatic squeezing.

The squeezing sequences (Table T10) comprised 6 steps for Hole C0002M and 4 steps for Hole C0002P samples. The following applied loads, with calculated internal pressures and duration of squeezing time in parentheses, were used for IW extraction from Sections 348-C0002M-1R-2 and 2R-2 on hydraulic press Number 2 (calibration of the device was made on 24 December 2013). IW was collected after preprogrammed hydraulic press recipes 1–3 (aliquot A), preprogrammed hydraulic press recipes 4–6 (aliquot B), and four larger applied forces from 30,000 to 60,000 lb (aliquot C and C′/C”) using the following parameters:

• Recipes 1–3: 15,000 lb (5 min), 17,000 lb (7 min), and 20,000 lb (10 min).
• Recipes 4–6: 21,500 lb (10 min), 23,000 lb (10 min), and 25,000 lb (10 min).
• Applied forces 30,000 lb (10 min), 40,000 lb (10 min), 50,000 lb (10 min), and 60,000 lb (720 min).

An intermediate aliquot (C′) of IW was collected after 50,000 lb from Sections 3R-1 and 4R-1.

For Section 348-C0002P-2R-3, the following sequence was applied: 5,300 lb (10.9 MPa; 5 min), 10,700 lb (20.0 MPa; 10 min), 16,000 lb (30.0 MPa; 30 min), and 21,300 lb (39.9 MPa; 720 min). The maximum squeezing pressure value was chosen to avoid expulsion of interlayer water from hydrous clay minerals, according to the tests conducted on Hole C0002M core samples. Extracted water was kept at 4°C prior to analysis.

GRIND method

Squeezing applied on the first available sample from Hole C0002P (Sample 348-C0002P-2R-3, 96–137.5 cm) did not yield IW. Therefore, the GRIND method was used on all core samples from Hole C0002P, including samples from Section 348-C0002P-2R-3. The sample from Section 6R-2 was divided in two aliquots, and both were prepared identically to assess the reproducibility of the method. However, Section 6R-2 was squeezed longer to extract more water. Scraped samples were crushed to 1 cm pieces. For each GW analysis, <30 g of solid sediment was used. Fragments <1 cm in diameter of the IW core were weighed in a glass dish and dried for 24 h at 60°C in a ventilated oven to measure pore water content. A second aliquot of 80 g of the IW core was weighed in a glass beaker and transferred to an agate mill bowl with 5 agate balls. Milli-Q water (10 mL [2 × 5 mL]), purged for 24 h with nitrogen gas, was pipetted into the sediment aliquot. The sediment was crushed at 400 rpm for 5 min (Section 6R-2) or 10 min (Sections 2R-3, 3R-2, 4R-2, and 5R-2) until all hard chunks were converted to paste. The paste was then transferred into the squeezing jacket. Some drying of the paste occurred during this transfer step.

The filtration assembly for squeezing the GW samples was the same as described above. The paste was carefully transferred from the milling bowl into the squeezer jacket, and the sediment was squeezed at 5,300 lb (10.9 MPa) to 10,700 lb (20 MPa) for at least 1 h each. An additional step at 30 MPa was used on both subsamples of Section 6R-2 to recover more GRIND water.

For both the squeezing method and the GRIND method, the squeezed water was filtered with a 0.45 µm disposable filter and stored in glass or high-density polyethylene (HDPE) sample vials, previously prepared by immersion in 55°C 10% trace metal–grade 12 M HCl for at least 24 h, rinsed with Milli-Q water, and dried in a Class 100 laminar flow clean hood. An aliquot of IW/GW was stored in a 4 mL HDPE bottle for analysis of pH, alkalinity (when water volume allowed), major anions (sulfate and bromide), and nutrients (phosphate and ammonium ions). Another aliquot of IW was stored in a 4 mL HDPE bottle acidified with 0.4 vol% 6 M HCl for analysis of major (Na, K, Ca, and Mg), minor (Ba, Si, B, Li, Mn, and Sr), and trace (V, Cu, Zn, As, Rb, Mo, Cs, Pb, and U) elements.

Assessing drilling mud contamination

Along with PFC assessment in IW/GW samples, mud water and liner-core liquid (LCL) were also analyzed to determine if IW sample contamination occurred.

Mud water

During drilling in Hole C0002N, 10 samples of drilling mud were collected from the active mud circulation pit. Mud was also collected during drilling and coring of Hole C0002P. Mud water (LMW) extracted from the drilling mud was analyzed for carbonates, pH, alkalinity, salinity, chlorinity, major anions and cations, and minor and trace (Hole C0002P only) elements. Mud samples were collected simultaneously with cuttings, and the lag time from drill bit to the surface was evaluated with a CaC₂ tracer (Strasser et al., 2014a).

Because of high viscosity, mud-water samples were diluted 10 times. A 1 mm aliquot of mud-water sample was pipetted in a Falcon tube with 9 mL of ultrapure water and then sonicated for 1 h in an ultrasonic bath. After centrifugation at 9500 rpm (4°C) for 1 h, the supernatant was filtered at 0.45 µm and analyzed. Water content in the drilling mud was determined by drying in a vacuum oven for 48–65 h. Results are listed in Table T17 in the “Site C0002” chapter (Tobin et al., 2015).

Liner-core liquid

As soon as the cores from Hole C0002P were recovered and brought on the core deck, the LCL was collected in a 45 mL centrifuge tube. From this tube, 2.5 mL was transferred into a 20 mL preweighed glass vial and heated at 80°C for 30 min. Then, 300 µL of the liberated gas was extracted through the vial septum with a microsyringe and injected into the GC-ECD for analysis of PFC. Results are listed in Table T22 in the “Site C0002” chapter (Tobin et al., 2015).

Interstitial water analyses

The standard IODP procedure for IW analysis was modified according to the availability and functionality of onboard instruments (Expedition 319 Scientists, 2010b). However, because of the limited amount of extracted pore water, not all of the standard IODP measurements were conducted. When IW/GW volumes were too low, standard pH and alkalinity measurements were not made because they consume 3 mL of IW. In this case, only pH was measured using the LAQUAtwin B-712 compact pH meter, with reported accuracy of 0.1 pH.

Concentrations of numerous major and minor components in the IW/GW were analyzed. Chlorinity was measured on a 100 µL aliquot by potentiometric titration using a Metrohm autotitrator and silver nitrate (AgNO₃) as a titrant in 30 mL of 0.2 M sodium nitrate (NaNO₃) solution. Relative standard deviation (RSD) for chlorinity was better than ±0.2%, based on repeated analyses of International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater, which were conducted after every five IW samples. Bromide and sulfate concentrations were measured with a Dionex ICS-1500 ion chromatograph with an anion column. An aliquot was diluted to 1:100 (10 µL in 990 µL) with Milli-Q water. IAPSO standard seawater aliquots (2.5, 5, 7.5, and 10 µL in a total of 1000 µL) were analyzed at the beginning and end of each run for quality control and to monitor potential drift in sensitivity throughout a particular run. RSDs are ±3% for bromide and ±1% for sulfate. An ion chromatograph was used to determine the concentrations of major cations such as Na⁺, K⁺, Mg²⁺, and Ca²⁺. Aliquots of IW/GW samples acidified with 6 M HCl (Tamapure-AA-100 grade) were used for this measurement. These samples were diluted to 1:200. The diluted samples were placed in an autosampler together with five calibration solutions and two blank solutions (Milli-Q water). For quality control, a 1:200 solution of diluted IAPSO standard seawater was measured after every eight IW samples.

To determine the concentration of each element, standard solutions were analyzed to construct calibration curves from the measured peak area and the known concentration. For the calibration, an IAPSO standard seawater solution containing Na⁺ (480 mM), K⁺ (10.4 mM), Mg²⁺ (54 mM), and Ca²⁺ (10.6 mM) was diluted to four standard solutions (25%, 50%, 75%, and 100%). The resulting RSD for the measurements was less than ±0.5% for Na⁺, ±1% for K⁺, ±0.7% for Mg²⁺, and ±0.5% for Ca²⁺.

For PO₄^3– and NH₄⁺, colorimetric methods were applied. Both dissolved solids must be analyzed within 24 h because they are quickly degraded by biological activity. Ammonium adsorption of indophenol blue at 640 nm wavelength was measured with a spectrophotometer (Shimadzu UV-2550PC), with an aliquot of 100 µL of sample IW used as the minimum volume. Standard, blank, 2, 4, 6, 8, and 10 mM NH₄⁺ solutions were prepared in the same manner as the sample solutions and analyzed within 5 h. Phosphate (an aliquot of 100 µL IW) was analyzed using adsorption of molybdate blue at 885 nm wavelength with the spectrophotometer used for NH₄⁺. Standard, blank, 0.5, 1, 2.5, 5, and 7.5 mg/L solutions were prepared in the same manner as that of sample solutions. RSDs of repeated analyses of both components are <1%.

Minor element (B, Ba, Fe, Li, Mn, Si, and Sr) concentrations were determined on 500 µL aliquots using an inductively coupled plasma–atomic emission spectrometer (ICP-AES) (Horiba Jobin Yvon Ultima2). Aliquots of IW/GW acidified with 6 M HCl (Tamapure-AA-100 grade) were diluted to 1:20 with 0.15 M HNO₃. Ultrapure primary standards (SPC Science PlasmaCAL) were prepared with a matrix solution of sulfate-free artificial seawater to fit the sample matrix, and 10 ppm Y solution was added as an internal standard. A matrix solution that approximated IAPSO standard seawater major element concentrations was prepared by mixing the following salts in 1 L of Milli-Q water acidified with 4 µL of Tamapure-AA-100–grade 6 M HCl: 26.9 g NaCl, 3.81 g MgCl₂, 1.0 g CaCO₃, and 0.75 g KCl. A stock standard solution was prepared from ultrapure primary standards (SPC Science PlasmaCAL) in the 1% HNO₃ solution and then diluted in the same 1% ultrapure HNO₃ solution used for IW samples to concentrations of 100%, 50%, 25%, 10%, 5%, and 1%. A 10 ppm Y solution diluted as 1% HNO₃ solution was prepared as a blank. A series of standards were made by adding 1.25 mL of each stock solution to 8.75 mL of matrix solution. The matrix-matched 100% standard solution contained the following concentrations of elements:

• B = 145 µM.
• Ba = 11.4 µM.
• Fe = 2.80 µM.
• Li = 22.5 µM.
• Mn = 2.84 µM.
• Si = 55.7 µM.
• Sr = 17.8 µM.

Because values of many of these elements in IAPSO standard seawater are either below detection limits (e.g., Fe and Mn) or variable, a standard prepared in the 10% matrix-matching solution was repeatedly analyzed to calculate the precision of the method. RSDs determined by repeated analyses of the 10% matrix-matching solution were ±2.5% for B, ±1.5% for Ba, ±3.5% for Fe, ±4.1% for Li, ±2.5% for Mn, ±2.5% for Si, and ±2.0% for Sr; As, V, Cu, Zn, Rb, Mo, Cs, Pb, and U were quantified on 500 µL IW samples using ICP–mass spectrometry (ICP-MS) (Agilent 7500ce) equipped with an octopole reaction system to reduce isobaric interferences from polyatomic and double-charged ions. We used the remainder of the same aliquot after determining major and minor elements using an ion chromatograph and ICP-AES.

To correct for interferences among some of the transition metals (V, Cu, and Zn) and some major element oxides, solutions containing the metals with concentrations similar to IAPSO standard seawater values were prepared. These solutions were then analyzed at the beginning of each measurement, and an interference correction was applied based on the average ion counts per second measured on the standard solutions divided by the abundance of the interfering elements. A 500 µL aliquot of sample IW/GW was diluted with 500 µL of 500 ppb In internal standard solution and 4 mL of 1% HNO₃ based on the previous determination of detection limits and low concentrations of the elements of interest.

A primary standard solution was made to draw the calibration lines matching the maximum range of predicted concentrations based on published results of deep-sea pore fluid compositions in a variety of settings. The concentrations of the standard are as follows:

• As = 40 ppb.
• V = 40 ppb.
• Cu, Mo, Pb, and U = 40 ppb.
• Zn = 140 ppb.
• Rb = 540 ppb.
• Cs = 40 ppb.

This primary standard was diluted with 1% HNO₃ solution to relative concentrations of 100%, 50%, 25%, 10%, 5%, 1%, and blank. A 500 µL split of these standards was then further diluted by addition of the In solution, 3.5 mL of 1% HNO₃ solution, and 500 µL of a 560 mM NaCl solution to account for matrix suppression of the plasma ionization efficiency.

The 200% and 400% standard solutions were also prepared using 100% solution changing dilution rate (i.e, instead of combination of 500 µL 100% standard, 500 µL In, and 4 mL HNO₃ solutions, 1000 and 2000 µL of 100% standard solution were diluted with 500 µL In solution and 3.5 and 3.0 mL HNO₃ solutions, respectively). The 25% standard was diluted accordingly and analyzed together with eight samples throughout every analysis series for precision and to check the drift during measurements. Blanks were also analyzed every eight samples, and detection limits were determined to be three times the standard deviation of a procedural blank of Milli-Q water acidified with 4 mL of Tamapure-AA-100 grade 0.15 M HNO₃. The average precision of multiple determinations of the 25% ICP-AES standard was

• ±4.7% for ⁷⁵As,
• ±3.6% for ⁵¹V,
• ±1.7% for ⁶⁵Cu,
• ±4.5% for ⁶⁵Zn,
• ±1.6% for ⁸⁵Rb,
• ±4.5% for ⁹⁵Mo,
• ±0.6% for ¹³³Cs,
• ±1.96% for ²⁰⁸Pb, and
• ±2.0% for ²³⁸U.

Organic geochemistry

Gas analysis in core samples

Headspace analysis

For headspace analysis, ~5 cm³ or half the volume of a 20 mL glass vial of sediment was taken from the core using a cork borer; in the case of highly consolidated sediment, pieces of sediment were crushed with a chisel or in a tungsten mortar. The sample was placed in a glass vial (20 cm³) that was immediately sealed with a silicon septum and crimped metal cap. The exact mass of the wet sample was determined after gas analysis was finished.

For C₁–C₄ hydrocarbon gas analysis, the vial was placed in a headspace sampler (Agilent Technologies G1888 network headspace sampler), where it was heated at 70°C for 30 min before an aliquot of the headspace gas was automatically injected into an Agilent 6890N GC equipped with a packed column (GL HayeSep R) and flame ionization detector (FID). The carrier gas was He. In the GC temperature program, the initial temperature of 100°C was held for 5.5 min before the temperature was ramped up at a rate of 50°C/min to 140°C and maintained for 4 min. Chromatographic response of the GC was calibrated against five different authentic standards with variable quantities of low–molecular weight hydrocarbons and checked on a daily basis. Methane concentration in interstitial water was derived from the headspace concentration using the following mass balance approach (Underwood et al., 2009):

where

• VH = volume of headspace in the sample vial,
• V_pw = volume of pore water in the sediment sample,
• χM = mole fraction of methane in the headspace gas (obtained from GC analysis),
• P_atm = pressure in the vial headspace (assumed to be the measured atmospheric pressure when the vials were sealed),
• R = universal gas constant, and
• T = temperature of the vial headspace in Kelvin.

The volume of interstitial water in the sediment sample was determined based on the bulk mass of the wet sample (M_b), the sediment’s porosity (ϕ, which was extrapolated from shipboard moisture and density (MAD) measurements in adjacent samples), grain density (ρ_s), and the density of pore water (ρ_pw) as

where

• M_pw = pore water mass,
• ρ_pw = 1.000–1.024 g/cm³ (adjusted to salinity based on shipboard data), and
• ρ_s = 2.8 g/cm³.

CHNS analysis

Total carbon (TC) and total nitrogen (TN) were analyzed using samples from cuttings and core samples, and total sulfur (TS) was measured using Hole C0002P core samples. Cuttings (10 cm³) were washed with seawater, sieved, freeze-dried under a vacuum, and ground to powder before analysis. Core samples (~60 mg) were freeze-dried under a vacuum and ground to powder. TC, TN, and TS concentrations were determined using a Thermo Finnigan Flash elemental analysis (EA) 1112 CHNS analyzer. Calibration was based on the synthetic standard sulfanilamide, which contains 41.81 wt% C, 16.27 wt% N, and 18.62 wt% S. About 15–25 mg of sediment powder was weighed and placed in a tin container for carbon and nitrogen analyses. The same amount of powdered sediment was weighed for sulfur analysis. This was mixed with an oxidizer (vanadium pentoxide V₂O₅) in a Ti container and then combusted in an oxygen stream at 900°C and 1000°C for carbon and nitrogen and for sulfur, respectively. The sample and container melt, and the tin promotes a violent reaction (flash combustion) in a temporarily enriched oxygen stream. The combustion produces CO₂, SO₂, and NO₂, which are carried by a constant flow of carrier gas. Then, NO₂ is reduced to N₂, and the mixture of N₂, CO₂, and SO₂ is separated using a GC equipped with a thermal conductivity detector (TCD). The accuracy of the analysis is confirmed using soil NCS reference material (Thermo Scientific, Milan, Italy), sulfanilamide standard (Thermo Scientific), and JMS-1 reference material.

Total organic carbon (TOC) is usually estimated by difference between the TC value and inorganic carbon (IC) value. The IC is determined with the same set of samples used for elemental analysis. Approximately 15–25 mg of sediment powder is weighed and acidified with 2 M HCl to convert the carbonate to CO₂. The released CO₂ is titrated, and the change in light transmittance is measured with a photodetection cell. The weight percentage of calcium carbonate is calculated from the IC content, assuming that all evolved CO₂ is derived from dissolution of calcium carbonate:

No correction was made for the presence of other carbonate minerals. Standard deviation for the samples was less than ±0.05 wt%. NIST-SRM 88b and JSD-2 (standard reference materials) were used to check accuracy. TOC contents were calculated by subtracting IC from TC contents as determined by elemental analysis.

Gas analysis

Onboard mud-gas monitoring system

Continuous mud-gas monitoring (CMGM) is a standard method for the qualitative estimation of in situ gas concentrations in real time. In the framework of IODP, CMGM was carried out during Expedition 319 (Expedition 319 Scientists, 2010a, 2010b) using third-party tools, as well as during Expeditions 337 and 338 with onboard instruments (Expedition 337 Scientists, 2013; Strasser et al., 2014a).

In general, formation gas is liberated when the drill bit crushes the sediment or rock and is circulated upward with the drilling mud. Once onboard, gas-enriched drilling mud flows along the flow line to a degasser, where an impeller stirs the mud and a vacuum is applied to separate gases from the drilling mud. During Expedition 348, two different degassers were used and installed at two different locations along the flow line (Fig. F16). For the depth interval from 838 to 2330 mbsf, the degasser and configuration were similar to previous expeditions (e.g., Strasser et al., 2014a; see also position D1 in Fig. F16). The drilling mud passes the flow splitter, and some of the mud is bypassed to the degasser. Gas liberated from the drilling mud then migrates through polyvinyl chloride (PVC) tubing to the mud-gas monitoring laboratory (MGML). On its way to the MGML, the gas has to pass a safety valve, which protects the system against mud entering the PVC tubing in case of mud overflow. Because of continuous air contamination, a new degassing unit was installed, which was positioned in the mud trough next to the degasser from GeoServices (position D2 in Fig. F16A). The height of the new degasser can be manually adjusted to ensure that the impeller can stir the drilling mud. The new degasser was used for drilling and coring Hole C0002P in the 1954–3058 mbsf depth interval. During Expedition 348, a jack was attached to the degasser to speed up lifting and lowering of the instrument. The liberated gas is forwarded to the MGML through PVC tubing without passing a safety valve. Inside the MGML, it was necessary to monitor the pressure of the vacuum applied to the degasser. If the pressure was less than –60 hPa (as low as approximately –73 hPa during this expedition), drilling mud forced the mud trap to close; thus the extracted gas could not enter the PVC tubing. In this case, the mud trap had to be cleaned, which took at least 10 min and caused a data gap in the Rn, methane carbon isotope analyzer (MCIA), process gas mass spectrometer (PGMS), and gas chromatograph–natural gas analyzer (GC-NGA) data. To identify drops in mud level, MCIA data were regularly compared to data from GeoServices. If the two data sets started to significantly deviate (several 100 ppm), the degasser had to be adjusted to the new mud level.

For the first configuration, the traveltime from the degasser to the MGML was estimated to be 6 min (Expedition 337 Scientists, 2013; Strasser et al., 2014a), whereas for the new system, the traveltime was reduced to 2 min. For both, diffusion loss during transportation through PVC tubing is negligible (Wiersberg and Erzinger, 2007). Upon arrival in the MGML, the gas had to pass a dehydration module, after which the dry and clean gas was distributed online to measurement instruments. Sampling was possible along a third-party sampling line and an IsoTube port located upstream of the dehydrator and through an IsoTube port located downstream of the dehydrator.

Online analysis of (non)hydrocarbon gases by gas chromatography

The first instrument along the main gas flow line in the MGML is a GC-NGA (Agilent Wasson ECE 6890N), with a gas sampling port with a multiposition valve. Theoretically, the GC-NGA allows the analysis of hydrocarbon gases (methane, ethane, propane, i-/n-butane, and pentane [i.e., C₁–C₅]), Ar, He, O₂, N₂, Xe, CO, and CO₂. Similar to Expedition 338 (Strasser et al., 2014a), nitrogen had to be used as the carrier gas because helium was one of the target components. H₂ was not used because of baseline problems. Gas analysis starts at a 50 cm capillary column that is able to retain hexane and heavier hydrocarbon components, followed by the separation of lighter hydrocarbon gases in another 49 cm capillary column that connects to a FID. Methane was extracted from the rest of the components by an 8 inch micropack column (Wasson ECE Instrumentation, column Code 2378), whereas CO₂ was separated by a 1.27 cm capillary column (Wasson ECE Instrumentation, column Code S036). Both columns are connected to a TCD that has a detection limit of 400 ppm for CO and 200 ppm for the remaining permanent gases. The detection limit for higher hydrocarbons is <1 ppm.

Although the GC-NGA has good sensitivity, the temporal and spatial resolution of the mud-gas analysis with a GC-NGA is limited because of the long measurement time (FID = 15 min; TCD = 20 min). During Expedition 348, the TCD was only used to detect He, H₂, and Xe, and FID measurements were carried out every 12 h to determine the hydrocarbon compositions. The continuous gas flow rate was set at 50 mL/min.

The GC-NGA was calibrated once in the beginning of the expedition by using two standards. The standard mixture for calibration of permanent gases contained 1% of Ar, CO, Xe, O₂, H₂, CO₂, and He in a balance of N₂. The hydrocarbon standard mixture contained 1% C₁–C₅ in a balance of N₂. Afterward, the same standard gases were used to conduct a condition check every 24 h.

Online analysis of the stable carbon isotopic composition of methane

Methane concentrations and methane carbon isotope ratios were determined at a sampling frequency of 1 Hz using an MCIA (Los Gatos Research, Model 909-0008-0000) on the basis of cavity ring-down spectroscopy technology. The stable carbon isotopic composition of methane is reported in the δ¹³C_CH4 notation relative to the Vienna Peedee belemnite (VPDB) standard and expressed in parts per thousand (per mil) as

where

and

Accuracy is <4‰ for gas concentrations between 200 and 400 ppm but improves to 1‰ for concentrations above 400 ppm. The MCIA comprises a gas dilution system that works with hydrocarbon or “zero air” (i.e., hydrocarbon free standard gas consisting of O₂, N₂, and Ar) and allows measurement of methane concentrations in the 500–10⁶ ppm range (i.e., 100%). Unfortunately, a technical problem with the dilution system precluded the determination of absolute methane concentrations between 1 × 10⁴ and 2 × 10⁵ ppm. Carbon isotope ratios were not affected by this defect. Calibration was carried out once on 26 October 2013, whereas sensitivity was checked daily by manual injection of a gas standard. For both calibration and sensitivity checks, the standard gas (Biso-1) contained 2500 ppm CH₄ and had a δ¹³C_CH4 value of –54.5‰ ± 0.2‰. Monitoring took place at a continuous gas flow rate of 20–40 mL/min.

Online gas analysis by process gas mass spectrometer

Continuous monitoring of He, O₂, Ar, Xe, N₂, CO, CO₂, methane, ethane, propane, and butane (differentiation between n- and i-butane was not possible) was conducted by an AMETEK PGMS. The PGMS does not require a carrier gas, thus contamination of the drilling mud gas is unlikely. The instrument includes a quadropole mass filter by which gases are identified based on the individual molecular masses of the desired compounds. The optimal scanning range of the mass-to-charge ratio (m/z) of the Faraday cup detector is 1–100 but can be extended to an m/z of 1–200 with a mass resolution of 0.5 at 10% peak height. Input gas flow rate was set to 50 mL/min. During Expedition 348, no full-range measurements (m/z = 1–200) were conducted. Instead, the system operated in “trend mode,” in which a predefined set of masses was determined. Similar to Expedition 338 (Strasser et al., 2014a), the reduction of the dwell time to 120 ms allowed a sampling interval of 5 s.

For quality assurance, three different calibrations had to be carried out every 24 h:

1. Binary calibration establishes peak ratios of ion fragments by using a mixture of two gases. The binary calibration should be adjusted to the expected gas composition (e.g., by a combination of a noninterfering balance gas like Ar and a standard gas of similar concentration to the main component in the sample stream). In contrast, for our measurements only CH₄ with a concentration of 100% was used. Masses of 12, 14, and 15 were used for the determination of methane.
2. Blend calibration is necessary to diminish the effect of ionization variations. The calibration was carried out with three different gases:
1. a. Standard gas containing 1% Ar, CO, Xe, O₂, CO₂, and He in a balance of N₂;
2. b. A standard gas consisting of 1% C₁–C₅ in a balance of N₂; and
3. c. Zero air (i.e., O₂, N₂, and Ar). No standard gas suitable for the expected gas concentrations was available.
3. Background calibration is necessary to determine the concentration of atmospheric gases in the vacuum chamber. For this purpose, Ar and N₂ calibration gases were used, each having a concentration of 100%.

During drilling operations, the PGMS ion current became unstable. Consequently, the 1144–1163.5, 1371–1546, 1747.5–1856, and 2036.9 mbsf depth intervals were not used during the data evaluation. In general, the fraction of atmospheric components in the drilling mud gas is assessed using Ar, N₂, and O₂ concentrations. Sources of air contamination are many, including the configuration of the degasser, leaks in the main gas flow line between degasser and MGML, malfunction of the instruments and sampling systems in the MGML, and drilling operations (pipe connection, core recovery, etc.; see also Strasser et al., 2014a). Also, CO₂ concentrations were altered because of the high pH of the drilling mud (Expedition 319 Scientists, 2010b; Strasser et al., 2014a).

Online radon analysis

Radon analysis was performed with a stand-alone radon monitor (Alpha GUARD PQ2000 PRO) provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Institute for Research on Earth Evolution (IFREE). The apparatus was attached to the auxiliary port of the scientific gas monitoring line parallel to other instruments. The radon monitor measures the Rn decay within an ion-counting chamber with a volume of 650 mL (effective volume = ~500 mL). Measurements were carried out every 10 min with 5 counts/min and a sensitivity of 100 Bq/m³ in the concentration range of 2 to 2 × 10⁶ Bq/m³. Internal temperature, pressure, and relative humidity were monitored as well and synchronized with Rn data. The data were not automatically included in the SSX database system but are available in the MGML.

Sampling for shore-based analysis

Samples were collected in IsoTube samplers (Isotech Laboratories, Inc.), copper tubes, and glass flasks. From 838 to 2000 mbsf, only IsoTubes were used for sampling, with a sampling interval of 100 m. Below this depth, the sampling interval was increased to 200 m. Sampling with glass flasks started between 2050 and 3058.5 mbsf with a sampling interval of 150 m. Copper tube samples were taken every 100 m between 2200 and 3050 mbsf. Additional event gas sampling took place when gas peaks significantly above the background concentrations occurred, usually as a consequence of pipe tripping or pipe connection.

The configuration of the third-party sampling line is similar to that during Expedition 338 (Strasser et al., 2014a). Glass flasks and copper tubes were connected to a sampling port at the main gas flow line with PVC tubing. Drilling mud gas can flow from an auxiliary sampling port through the glass flasks, pass the copper tubes, and migrate back to another sampling port at the main gas flow line (Fig. F16).

After sampling, the valves at both ends of the glass flasks were closed and clamps were placed at both ends of the copper tubes. Afterward, both the glass flask and the copper tube were exchanged with empty vials.

Recording online gas analysis and monitoring drilling operations, time, and depth

Gas concentrations determined from mud gas monitoring are easily affected by drilling operations. Therefore, both the drilling parameters and the results from GC-NGA, PGMS, and MCIA measurements were stored in real time in the SSX database, together with gas data and the lag depth determined by technicians from GeoServices (Schlumberger). The SSX system provides a graphic user interface that allows real-time monitoring of the various parameters and helps detect sudden changes in gas concentration. For the PGMS data, real-time monitoring was only possible for two predefined components (O₂ and N₂). The real-time information is stored and can be accessed in the MGML and on the shipboard server. Ship time (UTC + 9 h) was used to synchronize the different parameters. The stand-alone Rn monitor used an internal clock set to UTC + 9 h because its data are not included in the SSX system.

Lag depth is based on the lag time, which includes the time the drilling mud needs to travel from the drill bit back to the ship and the time the gas needs to flow from the degasser to the MGML. Here lag depth, L (as recorded in real time in the SSX database and provided by GeoServices), was used and is calculated based on the lag time, rate of penetration, pump rate, and borehole volume. The lag depth is recorded in meters BRT. Conversion to the mbsf depth scale was done by subtracting water depth (1939 m) and the distance between mean sea level and the rotary table (28.5 m).

Data recording was continuous, even if the mud pumps were turned off because of operational issues and/or gas flow was absent. As a consequence, time periods where lag depth did not change or where gas concentrations were below the detection limit were not included in the data evaluation.

Background control

Before the drilling mud is sent down the drill pipe, it already has a background concentration of atmospheric gases and gases that are not fully removed during gas extraction and mud recycling. In order to assess the background concentrations, drilling mud was sampled from the tank regularly and subject to headspace gas analysis. No sampling was conducted in Hole C0002N. During drilling and coring of Hole C0002P, drilling mud was sampled by the Telnite mud engineers with 50 mL plastic vials, which were completely filled and sealed with a plastic cap. In the laboratory, a fraction of the mud sample was transferred into a 20 mL glass vial, sealed with silicon septum and a metal crimped cap, and analyzed with an Agilent Technologies G1888 network headspace sampler. The sample was heated at 70°C for 30 min before an aliquot of the headspace gas was automatically injected into the GC-FID. Background concentrations of nonhydrocarbon gases could not be determined with the available instrumentation. The results of the background checks are shown in Figure F17 and Table T11. The background gas consisted almost solely of methane with concentrations up to 45.8 ppmv. Ethane and propane were only present in traces, with up to 2.2 and 0.6 ppmv, respectively. It became clear that hydrocarbons are about two orders of magnitude higher than the background concentrations when compared to the hydrocarbon gas concentrations found in the drilling mud gas, (see “Geochemistry” in the “Site C0002” chapter [Tobin et al., 2015]). Consequently, the influence of background gas concentrations in the drilling mud on the real-time measurements is believed to be small.

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