USGS Home
Contact USGS
Search USGS

Methods

Sampling of water included both underway continuous and discrete water samples. These samples were analyzed either shipboard, onshore, or both, as noted in the descriptions.

Shipboard Activities

Underway Continuous Measurements

Continuous measurements of pH, pCO₂, TCO₂, salinity, and temperature were collected from 22:35 UTC on August 3, 2010 to 20:55 UTC on September 5, 2010 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 in 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 de-bubbler and a multiport 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 60 seconds except during a MICA flushing cycle, which occurred for approximately 10 min each hour. 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 were 0.002 for pH, 2 parts per million (ppm) pCO₂, and 2 micromoles per kilogram (μmol/kg) for TCO₂. Quality of the data as determined by Xuewu Liu (USF) are reported on the HLY1002_MICA.csv spreadsheet.

Discrete Surface Samples

Discrete water samples were collected while underway following protocols outlined in Dickson and others (2007). Surface water samples were collected every 2 hours (h) for measurement of pH and carbonate ion concentration. Additional discrete samples were collected three times per day for post-cruise laboratory analysis of total alkalinity and total carbon, nutrients (NH₄, silica, PO₄, and NO₂+N), carbon and oxygen isotopes, and elemental analysis. Water samples were removed from the manifold sampling port of the vessel's flow-through seawater system downstream from the de-bubbler, thermosalinograph, and fluorometer.

Shipboard pH Analyses

Approximately 30 milliliters (mL) of seawater was collected directly into cylindrical optical glass cells for pH on the total scale (pH_T) measurements on the total hydrogen ion scale following the procedure of SOP6b (Dickson and others, 2007). Once filled, cuvettes for pH were rinsed with distilled (DI) water, then 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

pCO₂ of air was measured using a Li-Cor LI-7000 infrared gas analyzer affixed to the jack staff at the bow of the vessel approximately 10 m above the sea surface. The gas analyzer was calibrated daily using standardized gases of concentration 402 ppm. Data were logged every 2 min. The precision of air pCO₂ measurements is ±1 ppm.

Discrete Vertical Profile Samples

Discrete samples from vertical profile casts were collected at 10 locations (see Maps section). For 6 of these casts, a 24-bottle Niskin rosette (12-L bottle volume) with an electronic trigger was fitted with a Sea-Bird SBE 911plus CTD and altimeter. 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. During two deployments, a 12-bottle rosette (30-L bottles) was fitted with a CTD and an altimeter. Water samples were collected from the Niskin bottles for the full suite of discrete analyses shipboard and ashore as described above for surface samples. Shipboard TA was measured spectrophotometrically using a method under development by Dr. Xuewu Liu of USF. These TA values were derived from an experimental procedure and are thus not definitive. Detailed procedures will be published.

Geographic and Other Data

Geographic data was 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 the Healy yardarm ultrasonic anemometer (or the jackstaff ultrasonic anemometer).

Discrete Surface Samples Analyzed Ashore

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 continuously prior to analysis.  Analyses were completed at the USF Department of Geology Stable Isotope Laboratory using a Thermo-Finnigan Delta V 3-kiloelectron volt (keV) Isotope Ratio Mass Spectrometer coupled to a Finnigan GasBench II preparation device. Separate analyses of δD and δ¹⁸O of H₂O were completed by equilibrating 200 µL of sample with approximately 12 mL headspace of H₂ (approximately 1% in balance He for δD) and CO₂ (approximately 0.3% in balance He for δ¹⁸O) in septum-capped vials. After equilibration, the isotopic composition of the headspace gas was measured (methods following Epstein and Mayeda, 1953; Prosser and Scrimgeour, 1995).  Analyses of δ¹³C of dissolved inorganic carbon (DIC) was completed by injecting 1 mL of sample into an approximately 12-mL vial that was pre-flushed with He and pre-filled with 1 mL of 85 percent H₃PO₄ (methods following Assayag and others, 2006).  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 are the ¹⁸O/¹⁶O, D/H ¹³C/¹²C ratios of the sample and standard, respectively for δ¹⁸O, δD and δ¹³C.  The standards used as a reference for the δ-scale are VSMOW for H₂O and VPDB for DIC.  Internal standards were used in the calibration to the VSMOW and VPDB scales (VEEN and HTAMP waters with δ¹⁸O = -13.17 per mil (‰) and +15.05‰; δD = -96.8 and +40.5‰; Carrara Marble and NBS-18 calcites with δ¹³C = +2.01‰ and -5.04‰). Analytical precision (2σ) on these standards was better than 0.15, 1.0 and 0.2‰ for δ¹⁸O, δD, and δ¹³C, respectively. Analytical precision (2σ) on the samples was better than 0.2‰ for δ¹⁸O and 1‰ for δD and δ¹³C.

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 those created or modified by Dickson (1990), Lueker and others (2000),and Ho and others (2006).

Table 1 indicates number of samples taken, field preparation, and location of analysis. Table 2 describes the sensors on the sampling Rosette.