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Methods

Sampling of water

During the 6-week (August 15-September 28, 2011) UNCLOS (United Nations Convention Law of the Sea) Extended Continental Shelf Project cruise on the U.S. Coast Guard Cutter Healy (Co-Chief Scientists: Larry Mayer and Andrew Armstrong), discrete and continuous underway water samples were collected and, when possible, analyzed to document the carbonate chemistry of the Arctic waters. These data are being used to test the saturation state of the seawater with respect to calcium carbonate. These data are critical to refine existing models, which fail because of a lack of baseline data.

Underway Continuous Measurements

Approximately 9,000 continuous measurements of pH, pCO₂, TCO₂, salinity, and temperature were collected from August 15, 2011, to September 28, 2011, using a flow-through Multiparameter Inorganic Carbon Analyzer (MICA) and Sea-Bird SBE49 CTD attached to the flow-through system of the USCGC Healy. Geographic, salinity, temperature, and fluorometric data were also collected using a shipboard Ashtech ADU5 GPS system, a Sea-Bird SBE45 Thermosalinograph, and a Seapoint Chlorophyll Fluorometer (SCF). A complete description of these can be found at Chayes and others (2010). The intake of the shipboard flow-through system was located approximately 8 meters (m) below the sea surface on the port side of the vessel. Water entered the sampling baffles at depth, was pumped to an ice chest for separation of ice, and was then pumped to a multi-port sampling manifold located in the ship's laboratory. Filtered seawater was then fed to a custom made PVC de-bubbler containing a Sea-Bird SBE49, prior to being transported to the intake port of the MICA. Measurements were taken and logged approximately every 7 min except during a MICA flushing cycle which occurred for approximately 10 min each hour (h). The MICA was calibrated using Certified Reference Material from Professor Andrew Dickson of the University of California at San Diego. Precision and accuracy for each channel was 0.002 for pH, 2 parts per million (ppm) pCO₂, and 2 micromiles per kilogram (μmol/kg) for TCO₂. Quality assurance and quality control of data were performed using a custom Python script that identified and flagged data that deviated significantly from previous values. Spectrophotometric values that fell outside of acceptible intensity ranges were flagged separately. Quality of the data is reported on the spreadsheet using numeric values as described in the metadata.

The Autonomous Flow-through (AFT) System was also used to collect pH and pCO₂ (seawater data). However, issues with data drift and periodic equipment failure, probably due to the extremely cold water, made running this machine impractical for the duration of the cruise. The AFT System was shut down at the end of August.

Discrete Surface Samples

Discrete water samples were collected while underway following protocols outlined in Dickson and others (2007). Surface water samples were collected for measurement of pH, total alkalinity/total carbon, nutrients (NH₄, silica, PO₄, and NO₂+N), stable carbon and oxygen isotopic composition, elemental analysis, dissolved organic carbon (DOC) and particulate organic carbon (POC). Water samples were removed from the sampling port of the vessel's flow-through seawater system in the main laboratory. More than 515 pH discrete samples were measured underway. Generally, when the ship was traveling at 2-4 knots (kn), pH samples were collected every 2 h and analyzed. The remainder of the samples were taken every 6 h. When the ship traveled faster (~12 kn), pH samples were taken every hour and the rest of the suite of samples were taken every 4 h. Particulate organic carbon samples were collected twice daily. The results of the land-based analyses of nutrients (NH₄, silica, PO₄, and NO₂+N), stable carbon isotopic composition, elemental analysis, DOC, and POC will be reported when analyses are completed.

Shipboard pH Analysis

Approximately 30 milliliters (mL) of seawater was collected directly into cylindrical optical glass cells for pH measurements on the total hydrogen ion scale (pH_T) following the procedure of SOP6b (Dickson and others, 2007). Cuvettes for pH were placed into an aluminum cell warmer attached to a water bath at 25°C for approximately 30 min. Shipboard pH measurements were performed using an Agilent 8453 spectrophotometer, purified metacresol purple indicator dye, and equations modified by Liu and others (2011).

Measurement of pCO₂ in Air

A Picarro Isotopic CO₂ Analyzer was set up in the anchor chain room in the bow. The intake tube for the analyzer was attached to the flagstaff on the forecastle, approximately 3 m above the main deck, and was fed through a standpipe on the deck down into the anchor chain room. The analyzer recorded air pCO₂ and carbon isotopic composition of the pCO₂ every 2 seconds. These data were averaged/condensed down to match the recorded times of the MICA and provide a baseline of atmospheric pCO₂ concentration. The precision of air pCO₂ is ±1 ppm.

Preserved Total Alkalinity and Total Carbon Analyses

Seawater samples were collected from the sampling manifold of the shipboard flow-through seawater system in 300-mL borosilicate glass biochemical oxygen demand (BOD) bottles. Samples were preserved by adding 100 microliters (μL) of a saturated solution of mercuric chloride (HgCl₂) and were sealed with a ground glass stopper lightly coated with Apiezon grease. Samples were transported to the USGS Carbon Chemistry Lab in St. Petersburg, Fla. Total alkalinity samples were analyzed using an Ocean Optics USB 2000 spectrophotometer, bromol cresol purple indicator dye, and the methods of Yao and Byrne (1998). Total carbon was analyzed using coulometric methods of Dickson and others (2007). Precision and accuracy for these methods was 1 μmol/kg for TA and TCO₂.

Nutrient Analyses

A syringe that was pre-rinsed with sample water was used to collect approximately 60 mL of seawater. A Sterivex filter cartridge (pore size 0.22 micrometer, μm) was attached to the Luer-Lock fitting of the syringe and was rinsed with approximately 40 mL of sample. The remaining 20 mL of sample was then collected in a 20-mL glass scintillation vile and frozen. Samples were analyzed for NH₄, silica, PO₄, and NO₂+N at the Woods Hole Oceanographic Institution (WHOI) Nutrient Analytical Facility.

Isotope Analyses

Seawater samples were collected for stable isotopic analysis by rinsing, then filling 125-mL serum bottles such that no headspace remained.  Samples were fixed with 50 microliters (µL) of saturated mercuric chloride, and sealed with Teflon crimp caps containing a thin coating of Apiezon grease.  Samples were refrigerated during storage prior to analysis.  Analyses were completed at the University of South Florida Department of Geology Stable Isotope Laboratory using a Thermo-Finnigan Delta V 3 kiloelectron (keV) Isotope Ratio Mass Spectrometer coupled to a GasBench II preparation device.  Analyses of δ¹⁸O of H₂O was completed by equilibrating 200 µL of sample with approximately 12 mL headspace of H₂ in septum-capped vials. After equlibration, the isotopic composition of the headspace gas was measured (methods following Epstein and Mayeda, 1953; Prosser and Scrimgeour, 1995).  The CO₂ produced by this acid-stripping of the DIC was then measured after 24 h of equilibration.  All stable isotope data are expressed in the conventional delta (δ) notation:

δ = [(R_sample-R_standard)/R_standard] × 1000‰

where R_sample and R_standard is the ¹⁸O/¹⁶O ratio of the sample and standard, respectively, for δ¹⁸O.  The standard used as a reference for the δ scale is VSMOW for H₂O.  Internal standards were used in the calibration to the VSMOW scale (VEEN and HTAMP waters with δ¹⁸O = -13.17 per mil (‰) and +15.05‰). Analytical precision on these standards was better than 0.15‰ for δ¹⁸O.

Discrete Vertical Profile Samples

Discrete samples from vertical profile casts were collected at 8 locations (see Maps section). A 24-bottle Niskin rosette (12-L bottle volume) with an electronic trigger was fitted with a Sea-Bird SBE 911plus CTD and altimeter (Table 1). The CTD provided salinity, temperature, depth, fluorescence, and dissolved oxygen data. The rosette was lowered to just above the sea floor, and bottles were filled at select depths as the rosette was brought to the surface. Water samples were collected from the Niskin bottles for the full suite of discrete analyses shipboard and ashore as described above for surface samples.

Geographic and Other Data

Geographic data were collected using the vessel's Ashtech ADU5 global positioning system. Latitude, longitude, date, and time were recorded at 1-min intervals and reconciled with underway and discrete data after the research expedition. Wind speed was acquired from Healy Yardarm Ultra (or JackStaff Ultra).

Data Analyses

Other carbonate system parameters including seawater calcite and aragonite saturation states were calculated using the carbonate speciation program called CO2Calc (Robbins and others, 2010). Equilibrium constants used for these calculations included Dickson (1990), Lueker and others (2000), and Ho and others (2006).

Table 1. CTD sensors in 24-bottle Rosette configuration, field activity H-01-11-AR. [TSG, thermosalinograph]