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Methods

Field Methods

Sonde

A YSI 6600 CTD sonde was deployed to provide temperature, salinity, and seawater scale pH values for water chemistry calculations during 11CEV01 and 11CEV02. The sonde was located in the wet lab of the R/V Weatherbird II. The lab is equipped with a seawater flowthrough system which siphons water from about 2 feet below the Gulf's surface and pumps the water into the lab. Water flowed continuously to the sonde through tubing from the lab's flowthrough nozzle to a bucket containing the sonde. Data values were logged at 5 minute intervals.

Water Samples

Two types of water sampling, Surface and Station, occurred during 11CEV01 and 11CEV02. During sample collection, both types were treated similarly; once the samples were obtained, container stoppers were coated with Apiezon L grease to ensure an airtight seal and prevent contamination. Upon arrival at the USGS Carbon Analytical Laboratory in Saint Petersburg, Florida, the samples were analyzed for alkalinity and total carbon. The Trackline page contains a map of the sampling locations (no station samples were taken during 11CEV01).

Surface sater samples were collected hourly while underway during 11CEV01 and 11CEV02 using a 20 liter sampling container to retrieve a bulk water sample from the sea surface. From this container, an aliquot was collected into a 500-mL borosilicate bottles and poisoned with 100 μL of saturated mercuric chloride solution. In situ readings of temperature and salinity were made on the water remaining in the bucket.

Water samples were also taken from a specially chosen location, as opposed to the underway samples which were taken at regular intervals as the ship moved between sites. Eight stations were selected prior to the start of 11CEV02 for on-site sampling.

Laboratory Methods

Water samples were analyzed for pH (±0.002), and total alkalinity(±1.0 μmol/kg SW) at the USGS Carbon Analytical Laboratory in Saint Petersburg, Florida, using Ocean Optics spectrophotometers and the methods of Zhang and Byrne (1996) and Yao and Byrne (1998), respectively. Total carbon (3 μmol/kg SW) was measured for a subset of samples using a UIC, Inc. carbon coulometer using methods of Dickson et al. (2007). Salinity measurements from the YSI 6600 sonde were verified in the lab against a YSI 3200 conductivity instrument.