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Methods

Sampling of water included underway continuous (sonde and AFT) and discrete water samples (surface and station). These samples were analyzed either shipboard, onshore, or both.

Shipboard Activities

Underway Continuous Measurements (Sonde)

Using an YSI 6600 CTD, continuous measurements of temperature, salinity and pH (at the seawater scale, pH_T) were collected during 11BHM03 and 11BHM04 using a flow-through system located in the wet laboratory of the R/V Weatherbird II. The lab is equipped with a seawater flow-through system which siphons water from about 1 meter below the seawater surface and pumps the water into the lab. Water flowed continuously to the YSI sonde through tubing from the lab’s flow-through nozzle to a bucket containing the instrument. Data values were logged at 5 min intervals. In addition, navigational, wind, salinity, temperature and other data were logged shipboard by the R/V Weatherbird II.

Autonomous Flow-Through (AFT) pH

The Sunburst AFT pH, which measures pH on the total hydrogen ion scale, was attached to the flow-through system during 11BHM04. This system sampled the seawater approximately every 3 minutes and has a precision and accuracy of 0.001 and ±0.003, respectively.

Discrete Surface Samples

Discrete water samples were collected while underway during 11BHM03 and 11BHM04 following protocols outlined in Dickson and others (2007). Surface water samples were collected approximately every 2 hours (h) in borosilicate glass biochemical oxygen demand (BOD) bottles for measurement of total alkalinity (TA) and total carbon dioxide (TCO₂₎ on-shore (see below).

Discrete Vertical Profile Samples at Stations

Discrete samples from vertical profile casts were collected at station locations selected prior to the start of both 11BHM03 and 11BHM04 (Trackline). For these casts, a 12-bottle Niskin rosette (20-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. Water samples were collected from the Niskin bottles for TA, TCO₂, and pH analyses.

Shipboard pH Analyses

Discrete samples for shipboard analyses of pH were taken only at the stations. Approximately 30 milliliters (mL) of seawater was collected directly into cylindrical optical glass cells for pH on the total scale (pH_T) measurements 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 degrees Celsius (°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).

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 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 L grease. Samples were transported to the USGS Carbon Chemistry Laboratory 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 micromole per kilogram (μmol/kg) for TA and TCO₂.