Previous   |    Next

doi:10.2204/iodp.proc.344.102.2013

Geochemistry

The Costa Rica Seismogenesis Project (CRISP) was designed to help us understand the processes that control fault zone behavior during earthquake nucleation and rupture propagation at erosional subduction zones. The first phases of this project focused on sampling sediments, fluids, and crustal rocks to fully characterize the material before subduction. Fluids and associated diagenetic reactions are a key component of this study, as they affect hydrological parameters (e.g., permeability and pore pressure) and may regulate the mechanical state of the plate interface at depth. The concentration of dissolved species and their isotopic composition provide critical data for the identification of fluid sources, fluid-rock interaction, pathways of fluid migration, and plumbing of the system. In addition, geochemical data can help characterize the subsurface biosphere and aid in constraining the mass balance inventories operating in this subduction zone.

Sampling protocol

The majority of geochemistry samples were obtained from two general procedures. First, pore fluid whole-round samples were collected at a frequency of approximately three samples per core in the first three cores and, subsequently, two samples per core to the bottom of the hole, with higher resolution across zones suspected to be fluid conduits. The length of the whole rounds was 10 cm in the shallow cores and increased with depth, depending on pore fluid recovery, to a maximum of 50 cm. Second, dedicated whole-round samples were acquired at each site for He isotope analysis at depths across fluid conduits and/or above oceanic basement. Detailed sampling protocols for each site are given in the site chapters.

For headspace analyses of gas concentrations, two sediment plugs were routinely collected adjacent to each interstitial water sample; one was used for standard shipboard hydrocarbon concentration monitoring and the other for stable-isotope measurements at onshore laboratories. When present, void gases were sampled at the catwalk using a syringe attached to a stainless steel tool used to puncture the core liner, as described by Kvenvolden and McDonald (1986).

Pore fluid collection

For pore fluid analyses, whole-round cores were cut on the catwalk, capped, and taken to the laboratory for processing. In general, samples collected at all sites, between the seafloor and 50 mbsf, were processed inside a nitrogen bag to avoid oxidation of redox-sensitive elements; at Site U1381, whole rounds from the entire sediment section were processed inside a nitrogen bag. All other cores were processed under normal atmospheric conditions.

During high-resolution sampling or shallow-water coring, when the capacity to process pore fluid cores immediately after retrieval was exceeded, capped whole-round core sections were wrapped with electrical tape and stored under a nitrogen atmosphere at 4°C until they were squeezed, which occurred within 24 h of core retrieval.

After extrusion from the core liner, the surface of each whole-round interstitial water core sample was carefully scraped with a spatula to remove potential contamination from seawater and sediment smearing in the borehole. In APC cores, ~0.5 cm of material from the outer diameter and the top and bottom faces was removed, whereas in XCB and RCB cores, where borehole contamination is higher, as much as two-thirds of the sediment was removed from each whole round. The remaining sediment (~150–300 cm³) was placed into a titanium squeezer, modified after the stainless steel squeezer of Manheim and Sayles (1974). Gauge forces up to a maximum 30,000 lb were applied using a laboratory hydraulic press to extract pore water. Most samples were squeezed at <20 MPa.

The squeezed pore fluids were filtered through a prewashed Whatman No. 1 filter placed in the squeezers, above a titanium screen. The squeezed pore fluids were collected in precleaned plastic syringes attached to the squeezing assembly and subsequently filtered through a 0.45 µm Gelman polysulfone disposable filter. In deeper sections, fluid recovery was as low as 2 mL after squeezing the sediment for as long as ~2 h.

Sample allocation was determined based on the pore fluid volume recovered and analytical priorities based on the objectives of the expedition. Shipboard analytical protocols are summarized in the following section.

Whole-round samples designated for He isotopic analysis were also cut on the catwalk and immediately transferred into a plastic sealable bag initially flushed with ultrahigh-purity N₂. Samples were taken to a special processing and squeezing station set up in the refrigerated core storage repository located on the lowermost deck of the JOIDES Resolution. This refrigerated repository is a He-free environment kept at 4°C, whereas the shipboard Chemistry Laboratory uses He as a carrier gas for the gas chromatographs, ion chromatograph, and CHNS elemental analyzer. The samples were cleaned in a glove bag, squeezed, transferred into copper tubing samplers that were previously flushed with ultrahigh-purity N₂, and crimped.

Shipboard pore fluid analyses

Pore fluid samples were analyzed on board following the protocols in Gieskes et al. (1991), Murray et al. (2000), and the IODP user manuals for new shipboard instrumentation, which were updated during this expedition.

Salinity, alkalinity, and pH

Salinity, alkalinity, and pH were measured immediately after squeezing, following the procedures in Gieskes et al. (1991). Salinity was measured using a Fisher temperature-compensated handheld refractometer. The pH was measured with a combined glass electrode, and alkalinity was determined by Gran titration with an autotitrator (Metrohm 794 basic Titrino) using 0.1 N HCl at 20°C. Certified Reference Material 104 obtained from the laboratory of Andrew Dickson, Marine Physical Laboratory, Scripps Institution of Oceanography (USA), was used for calibration of the acid. International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater was used for calibration and was analyzed at the beginning and end of a set of samples for each site and after every 10 samples.

Chloride

High-precision chloride concentrations were acquired using a Metrohm 785 DMP autotitrator and silver nitrate (AgNO₃) solutions that were calibrated against repeated titrations of an IAPSO standard. In Hole U1381C, where fluid recovery was ample, a 0.5 mL aliquot of sample was diluted with 30 mL of a HNO₃ solution (92 ± 2 mM) and titrated with 0.1015 N AgNO₃. At all other sites, a 0.1 mL aliquot of sample was diluted with 10 mL of 90 ± 2 mM HNO₃ and titrated with 0.1778 N AgNO₃. Repeated analyses of an IAPSO standard yielded a precision better than 0.05%. Additionally, chloride concentrations were measured by IC using the third-party (Scripps Institute of Oceanography) Metrom 861.004 Advanced Compact ion chromatograph as described bellow for sulfate and bromide. These data have a precision of 0.15% and are included in the tables for comparison purposes.

Sulfate and bromide

Sulfate and bromide concentrations were determined with a third-party (Scripps Institute of Oceanography) Metrom 861.004 Advanced Compact ion chromatograph. The ion chromatograph included an MSM II suppressor module 46, 853 CO₂ suppressor, a thermal conductivity detector, and a Metrosep A Supp column. The eluent solutions used were 3.2 mM Na₂CO₃ and 1.0 mM NaHCO₃. Samples were diluted 1:50 with deionized water, using specifically designated pipettes. The analytical protocol was to run a standard after five samples for six cycles, after which three extra standards were analyzed. The standards used were based on IAPSO dilutions of 50×, 80×, 150×, 250×, 500×, 750×, and 1000×. Sample duplicates were analyzed during each run for reproducibility. Reproducibility was also checked based on the interspersed standard samples run throughout the expedition. Analytical precision was 0.3% for sulfate and 2% for bromide. Concentrations were based on peak areas.

In the uppermost ~50 m at each site, pore fluid aliquots for SO₄ and Br analyses were bubbled with He for as long as 2 min to remove sulfide that could oxidize to sulfate. The bubbled solutions were subsequently analyzed by the protocol described above. At each site, samples were run continuously as they were collected to monitor the location of the sulfate-methane transition zone (SMTZ) and any potential drill fluid contamination. Below the SMTZ, sulfate is depleted in the pore fluids, and any sulfate present in a sample is a result of contamination with surface seawater that was pumped down the hole while drilling. Based on the sulfate concentration of each interstitial water sample, we used the chemical composition of the surface seawater to correct each analysis for contamination using the following equations:

and

where f_SW is the fraction of a pore fluid sample that is contaminated with surface seawater and f_PW is the fraction of uncontaminated pore water in a sample. The subscripts SW, PF, and meas denote surface seawater, pore fluid, and measured, respectively. [X]_corr is the corrected value of a solute (e.g., Cl, Ca, Sr, etc.), [X]_meas is the measured concentration of that solute, and [X]_SW is the concentration of the solute in surface seawater.

Because of the scientific importance of sulfate concentrations, pore fluid samples were also taken for shore-based analyses, and these were spiked with 100 µL 4 M Cd(NO₃)₂ solution to precipitate the sulfide as CdS.

Ammonium and phosphate

Ammonium concentrations were determined by spectrophotometry using an Agilent Technologies Cary Series 100 UV-Vis spectrophotometer with a sipper sample introduction system following the protocol in Gieskes et al. (1991). Samples were diluted prior to color development so that the highest concentration was <1000 µM. Phosphate was measured using the ammonium molybdate method described in Gieskes et al. (1991), using appropriate dilutions.

Major and minor elements

Major and minor elements were analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) with a Teledyne Prodigy high-dispersion ICP spectrometer. The general method for shipboard ICP-AES analysis of samples is described in ODP Technical Note 29 (Murray et al., 2000) and the user manuals for new shipboard instrumentation, with modifications as indicated. Samples and standards were diluted 1:20 using 2% HNO₃ spiked with 10 ppm Y for trace element analyses (Li, B, Mn, Fe, Sr, Ba, and Si) and 1:100 for major constituent analyses (K, Ca, Mg, and SO₄). Each batch of samples run on the ICP spectrometer contains blanks and solutions of known concentrations. Each item aspirated into the ICP spectrometer was counted four times from the same dilute solution within a given sample run. Following each instrument run, the measured raw-intensity values were transferred to a data file and corrected for instrument drift and blank. If necessary, a drift correction was applied to each element by linear interpolation between the drift-monitoring solutions.

Standardization of major cations was achieved by successive dilution of IAPSO standard seawater to 120%, 100%, 75%, 50%, 25%, 10%, 5%, and 2.5% relative to the 1:100 primary dilution ratio. Analytical precision based on repeated analyses of the 100%, 75%, 50%, 25%, and 10% dilution standards over the 2 month expedition was Ca < 0.6%, Mg < 0.8%, Na < 2%, and K < 1.5%. Average accuracies of the analyses based on repeated analyses of 100% IAPSO run as an unknown throughout each batch of analyses were Ca < 1.5%, Mg < 1.5%, Na < 2.5%, and K < 2%.

For minor element concentration analyses, the interstitial water sample aliquot was diluted by a factor of 20 (0.5 mL sample added to 9.5 mL of a 10 ppm Y solution). Because of the high concentration of matrix salts in the interstitial water samples at a 1:20 dilution, matrix matching of the calibration standards is necessary to achieve accurate results by ICP-AES. A matrix solution that approximated IAPSO standard seawater major ion concentrations was prepared according to Murray et al. (2000). A stock standard solution was prepared from ultrapure primary standards (SPC Science PlasmaCAL) in a 2% nitric acid solution. The stock solution was then diluted in the same 2% ultrapure nitric acid solution to concentrations of 100%, 75%, 50%, 25%, 10%, 5%, and 1%. The calibration standards were then diluted using the same method as the samples for consistency. The final matrix-matched 100% standard solution contained the following concentrations of elements: B = 1388.9 µM, Li = 288.2 µM, Si = 1186.7 µM, Mn = 54.6 µM, Fe = 17.9 µM, Sr = 228.1 µM, and Ba = 36.4 µM. The 100%, 75%, 50%, 25%, 10%, and 5% standards were repeatedly analyzed with each batch and over the 2 month expedition as a check of analytical precision. The average precision of the minor element analyses were B < 1%, Ba < 1%, Mn < 1%, Li < 1.5%, Si < 1.5%, and Sr < 1%. Because values of many of these elements in IAPSO standard seawater are either below detection limits (e.g., Fe and Mn) or variable, the average accuracy of the analyses were determined by repeated analysis of the 75%, 25%, and 10% check standards and were B < 1.5%, Ba < 1.5%, Mn < 1.5%, Li < 1.5%, Si < 1.5%, and Sr < 2%. The precision and accuracy of the Fe analyses were deemed to be of insufficient quality, and Fe concentrations are not reported for this expedition.

Fluid organic geochemistry

Routine analysis of hydrocarbon gas in sediment cores is a part of standard IODP shipboard monitoring to ensure that the sediments being drilled do not contain greater than the expected amount of hydrocarbons that is safe to operate with. The most common method of hydrocarbon monitoring used during IODP expeditions is the analysis of gas samples obtained from either sediment samples (headspace analysis) or from gas expansion pockets visible through clear plastic core liners (void gas analysis), following the procedures described by Kvenvolden and McDonald (1986).

When gas pockets were detected, free gas was drawn from the sediment void using a syringe attached to a hollow stainless steel tool used to puncture the core liner. For headspace analyses, a 3 cm³ bulk sediment sample was collected from the freshly exposed top end of a core section and next to the interstitial water sample immediately after core retrieval using a brass boring tool or plastic syringe. The sediment plug was sealed with an aluminum crimp cap with teflon/silicon septa. The vial was then heated to 70°C for ~30 min to evolve hydrocarbon gases from the sediment plug. When consolidated or lithified samples were encountered, chips of material were placed in the vial and sealed. For gas chromatographic analysis, a 5 cm³ volume of headspace gas was extracted from the sealed sample vial using a standard gas syringe and analyzed by gas chromatography.

The standard safety gas analysis program was complemented with additional headspace samples taken at the same resolution described above, to measure the stable carbon and hydrogen isotope composition of hydrocarbons at onshore laboratories. The sampling method was the same as that used for the safety analysis, except that the sediment plug was extruded into a 20 cm³ headspace glass vial filled with 10 cm³ of a 1 M potassium chloride (KCl) solution containing borosilicate glass beads and immediately sealed with an aluminum crimp cap with teflon/silicon septa. These vials had been flushed with helium and capped within 1 h prior to sampling, in order to remove air from the headspace and ensure the sample was preserved anaerobically. After the sample was sealed in the vial, it was vigorously shaken to help dissociate the sediment. Potassium chloride is toxic and was thus used to stop all microbial activity in the sediment. The glass beads (3 mm in diameter) were used to help break up the sediment plug during shaking and liberate gas trapped in sediment pore space or adsorbed on particles.

Headspace samples were directly injected into the gas chromatograph fitted with a flame ionization detector (GC3-FID) or the natural gas analyzer (NGA). The Agilent/HP 6890 Series II gas chromatograph (GC3) is equipped with 8 ft, 2.00 mm inner diameter (ID) × ⅛ inch outer diameter stainless steel column packed with 80/100 mesh HayeSep R (Restek) and an FID set at 250°C. The GC3 oven temperature was programmed to hold for 8.25 min at 80°C, ramp at 40°C/min to 150°C, hold for 5 min, and return to 100°C postrun, for a total of 15 min. Helium was used as the carrier gas. The GC3 system determines concentrations of methane (C₁), ethane (C₂), ethene (C₂₌), propane (C₃), and propene (C₃₌). For hydrocarbon analysis, the NGA is outfitted with an Agilent/HP 6890 Series II GC equipped with an Agilent DB-1 dimethylpolysiloxane capillary column (60 m × 0.320 mm diameter × 1.50 µm film thickness) fitted with an FID and using He as carrier gas (constant flow of 21 mL/min). The NGA oven temperature was programmed to hold for 2 min at 50°C, ramp at 8°C/min to 70°C, and then ramp at 25°C/min to 200°C, with a final holding time of 5.1 min. The FID temperature was 250°C. For nonhydrocarbon gases, thermal conductivity detector separation used three columns: a 6 ft × 2.0 mm ID stainless steel column (Poropak T; 50/80 mesh), a 3 ft × 2.0 mm ID stainless steel molecular sieve column (13X; 60/80 mesh), and a 2.4 m × 3.2 mm ID stainless steel column packed with 80/100 mesh HayeSep R.

Data were collected using the Hewlett Packard 3365 Chemstation data processing program. Chromatographic response is calibrated to nine different gas standards with variable quantities of low molecular weight hydrocarbons (N₂, O₂, CO₂, Ar, and He) and checked on a daily basis. Gas concentrations for the required safety analyses are expressed as component parts per million by volume (ppmv) relative to the analyzed gas. The internal volumes of 15 representative headspace vials were carefully measured and determined to average 21.5 ± 0.18 mL. This volume was taken as a constant in calculations of gas concentrations.

Void gas samples were analyzed on shore using gas chromatography with flame ionization (GC3-FID) and thermal conductivity (GC-TCD) detectors. For hydrocarbon analysis, an Agilent 7890A was equipped with a DB-1 dimethylpolysiloxane capillary column (50 m × 0.320 mm diameter × 5 µm film thickness) fitted with a FID and using helium as carrier gas (constant flow of 1.8 mL/min). The GC oven temperature was programmed to hold for 3 min at 30°C, ramp at 20°C/min to 100°C, and then ramp at 25°C/min to 195°C with a final holding time of 5 min. For detection of hydrocarbons, the FID temperature was 250°C. For nonhydrocarbon gases, the GC was outfitted with a 6 ft × 2.0 mm ID stainless steel column, packed with 80 × 100 mesh carbosphere, and a TCD.

The concentrations of C₁–C₆ hydrocarbons gases were calibrated using five different gas standards and the gas concentrations are expressed as parts per million by volume relative to the analyzed gas. Typical precision, assessed using 10 replicate analyses of a standard, was 1.5% for hydrocarbon.

Sediment geochemistry

For shipboard sediment geochemistry, 5 cm³ of sediment was freeze-dried for ~24 h, crushed to a fine powder using a pestle and agate mortar, and sampled to analyze inorganic carbon (IC), total carbon (TC), and total nitrogen (TN).

Elemental analysis

The TC and TN of the sediment samples were determined with a ThermoElectron Corporation FlashEA 1112 CHNS elemental analyzer equipped with a ThermoElectron packed column CHNS/NCS and a thermal conductivity detector. Approximately 10–15 mg of freeze-dried, ground sediment was weighed into a tin cup, and the sample was combusted at 900°C in a stream of oxygen. The reaction gases were passed through a reduction chamber to reduce nitrogen oxides to nitrogen and were then separated by the gas chromatograph before detection by thermal conductivity detector. All measurements were calibrated to a standard soil reference material (soil standard 33840025) for carbon and nitrogen detection (Thermo), which was run every six samples for verification. The detection limit was 0.001% for TN (instrument limit) and 0.002% for TC (procedural blank, measured as an empty tin cup). Repeated (n = 21) analyses of the standards yielded precisions of TN < 3% and TC < 5%.

Inorganic and organic carbon content

Total inorganic carbon (TIC) concentrations were determined using a UIC 5011 CO2 coulometer. Between 10 and 12 mg of freeze-dried, ground sediment was weighed and reacted with 2 N HCl. The liberated CO₂ was titrated, and the end point was determined by a photodetector. Calcium carbonate (CaCO₃) content, expressed as weight percent, was calculated from the TIC content, assuming that all evolved CO₂ was derived from the dissolution of CaCO₃ by the following equation:

No correction was made for the presence of other carbonate minerals. Accuracy during individual batches of analyses was determined by running a carbonate standard (100 wt% CaCO₃) every 10 samples. Typical precision assessed using 36 replicate analyses of a carbonate sample was 1%. The detection limit for CaCO₃, defined here as three times the standard deviation of the blank (2 N HCl), was 0.1% for 100 mg of pelagic clay. Total organic carbon (TOC) content was calculated as the difference between TC (measured on the elemental analyzer) and IC (measured by coulometry):

Top of page   |    Previous   |    Next