Dataset: Alkalinity and dissolved inorganic carbon (DIC) from water samples collected at coral reefs in the equatorial central Pacific in 1973, 2012, 2015, and 2018

Location Information

Five coral reef islands in the equatorial, central, South Pacific.

1. Kanton Atoll (2.8°S, 171.7°W),  is a 200 – 500 m wide barrier island surrounding a lagoon that is 14.5 km long and ~5 km wide. A single, 500 m wide channel connects the lagoon to the ocean. The complex pattern of shallow (~ 1 m deep) line (reticulate) reefs span the central portion of the lagoon with pools between the line reefs that are 10 – 26 m deep.
2. Enderbury Island (3.1°S, 171.1°W) is about 4.7 km long and 1.2 km wide, with coral reefs extending 50  m to 400 m offshore.
3. Rawaki Island (3.7°S, 170.7°W) is about 1.2 km long and 0.8 km wide with coral reefs surrounding much of the island.
4. Nikumaroro Island (4.7°S, 174.5°W) is a 300 – 800 m wide barrier island surrounding a lagoon that is about 5 km long and 0.8 km wide. A single, 170 m wide channel connects the lagoon to the ocean.
5. Orona (Hull) Atoll (4.5°S, 172.2°W) is a ~300 m wide barrier island surrounding a lagoon that is about 8 km long and 3 km wide.  

For a more complete description of these islands, see Obura, D. (2011). 

Samples Collected in 2012, 2015, and 2018

Samples from 2012, 2015, and 2018 were collected on three cruises to Kanton Island:

1. June 7-18, 2012:  Pangaea Exploration, Emily Penn, vessel Seadragon, PI Anne Cohen
2. September 5-26, 2015: Vessels Nai'a and Hanse Explorer, cruise led by New England Aquarium, PIs Dr Randi Rojan, Dr Simon Thorrold, Dr Stuart Sandin
3. May 5-23, 2018: Pangaea Exploration, Captain Eric and Shanley Loss, vessel SeaDragon, PI Anne Cohen

Discrete water samples were collected using Niskin samplers, transferred to the appropriate bottles on site, shipped to Woods Hole Oceanographic Institution and analyzed to determine salinity, total alkalinity (TA), dissolved inorganic carbon (DIC), and in some cases nitrate, ammonium, phosphate and silicate concentrations. Unfiltered 250 mL bottle samples for carbonate chemistry were fixed with 100 µL of a saturated HgCl₂ solution immediately upon collection to inhibit biological activity. TA analyses were performed with a Marianda Versatile Instrument for the Determination of Total inorganic carbon and titration Alkalinity (VINDTA 3C). The VINDTA 3C uses an open cell potentiometric titration for TA analysis. The TA measurements were standardized using in-house laboratory seawater standards calibrated using certified reference materials obtained from Andrew Dickson at Scripps Institution of Oceanography (Dickson et al. 2007). Analyses of replicate samples and of replicate independent working standards each day yielded a mean TA precision of approximately ± 1.5 μmol kg⁻¹. Some water samples were also analyzed to determine nitrate, ammonium, phosphate, and silicate concentrations at the Woods Hole Oceanographic Institution Nutrient Analytical Facility using a four channel segmented flow AA3 HR Autoanalyzer. Bottle salinity was measured on a Guildline Autosal salinometer. Simultaneous water temperatures were measured using either a HOBO Temp Pro data logger or a Sontek Castaway CTD.

Samples Collected in 1973

From Smith and Jokiel (1978), the 1973 water samples were collected from a small skiff or by wading from shore. They were captured in 250-ml polyethylene bottles and maintained near collection temperature until they could be analyzed (average time ~ 6 hours). A Corning model 101 pH meter and a combination electrode were used for all pH and alkalinity measurements. Salinity was determined using a Plessey model 6230N laboratory conductivity salinometer.