Seawater carbonate chemistry from CTD bottle samples during the Ryder 2019 expedition

Adam Ulfsbo, Christian Stranne, Johan Nilsson

Depth profiles of seawater pH and total alkalinity (TA) from water samples collected during the Ryder 2019 Expedition from coastal waters along northwestern Greenland, from Thule in the south to Sherard Osborn Fjord in the north.

A total of 479 water samples from 30 (out of 51) stations were analyzed for seawater pH and total alkalinity on board the Swedish icebreaker Oden, August-September 2019. The stations cover Nares Strait, southern Lincoln Sea, Sherard Osborn Fjord (where Ryder Glacier drains), and Petermann fjord (where Petermann Glacier drains). Seawater pH (total pH scale, 25 °C, in situ salinity) was determined spectrophotometrically using purified meta-cresol purple. Total alkalinity was determined by open-cell potentiometric titrations.

Seawater pH and TA are two key variables of the marine carbonate system. From these observations, together with concurrent measurements of salinity, temperature, and pressure, the two other measureable variables of the marine carbonate system, i.e., dissolved inorganic carbon (DIC) and the partial pressure of carbon dioxide (pCO₂), can be calculated through thermodynamic models. Observations of any two of the four measureable variables of the the marine carbonate system are useful in assessing the seawater state with respect to, e.g., air-sea gas exchange, buffer capacity, and calcium carbonate mineral saturation.

Citation

Adam Ulfsbo, Christian Stranne, Johan Nilsson (2021) Seawater carbonate chemistry from CTD bottle samples during the Ryder 2019 expedition. Dataset version 1. Bolin Centre Database. https://doi.org/10.17043/oden-ryder-2019-ctd-bottle-ph-alkalinity-1

References

Jakobsson M, Mayer LA, Farrell F, Ryder 2019 Scientific Party (2020) Expedition report: SWEDARCTIC Ryder 2019, Swedish Polar Research Secretariat, 455 pp, ISBN 978-91-519-5132-4

Data description

The discrete CTD bottle data of seawater pH and TA are organized in a comma-separated values (csv) file and include the following parameters:

• Year
• Month
• Day
• Hour
• Minute
• Second
• Station number
• Niskin bottle number
• Latitude (decimal degrees north)
• Longitude (decimal degrees east)
• Pressure (decibar)
• Salinity (Practical salinity, PSS-78)
• Temperature (°C)
• Potential temperature with reference to sea surface (°C)
• Total dissolved oxygen concentration (µmol/kg)
• Fluorescence (MG/M³)
• Turbidity (NTU)
• Total alkalinity (micromols per kilogram seawater)
• Seawater pH on the total scale, at 25 °C, and 0 db pressure

CTD data were collected with a Seabird 911 CTD. The CTD was equipped with a 24 Niskin bottle (12 liters) rosette and the following sensors: Dual SeaBird temperature (SBE 3), conductivity (SBE 04C), dissolved oxygen (SBE 43), turbidity and fluorescence (WET Labs ECO-AFL/FL) and a Benthos Altimeter PSA-916D. In situ conductivity and temperature have been converted to temperature and practical salinity using the EOS-80 equation of state.

These observations are accompanied by recordings of time, station number, bottle number, latitude, longitude, pressure, practical salinity, temperature, potential temperature, total dissolved oxygen, fluorescence, and turbidity from the concurrent CTD bottle files in the Oceanographic CTD data from the Ryder 2019 expedition.

Comments

Water samples were collected using a rosette system equipped with 24 Niskin bottles, each having a volume of 12 L. The bottles were closed at predefined depth during the return of the CTD-rosette package from the bottom to the surface (5, 10, 20, 30, 40, 50, 75, 100, 125, 150, 200, 250, 300, 350, 400, 500, 700 m, bottom). The surface samples (5⁠ – ⁠10 m) were resolved because of the shallow and strong stratification inside the fjord. It should, however, be noted that the Oden mixes the water down to approximately 10 m depth when drifting on the stations. Pressure, salinity, and temperature were recorded at the closing of the bottles and stored in a CTD bottle file.

Seawater pH was determined on the total scale employing a spectrophotometric method using the indicator m-Creosol Purple (mCP) (Carter et al. 2013). Purified mCP (Liu et al. 2011) was purchased from the laboratory of Eric Achterberg, GEOMAR, Kiel, Germany. A 0.2 mM indicator solution was prepared by dissolving pre-weighed mCP indicator in 0.5 L filtered (0.45 μm) seawater of about 34 salinity. The indicator was adjusted to a pH in the same range as the samples, approximately ± 0.2 pH units, by adding a small volume of concentrated HCl or NaOH. Before running a set of samples, the pH of the indicator was measured using a 0.02 cm cuvette. The measurements were performed on board within hours of sampling and samples were thermostated to 25 °C in a water bath 30 minutes prior to analysis. A LabView-controlled system was used where the sample and indicator were mixed in a syringe (Kloehn) before injected to a 1 cm cuvette of a diode array spectrophotometer (Agilent 8453), where the absorbance was measured at wavelengths 434, 578, and 730 nm, the latter accounting for background absorbance. Indicator corrections were made according to the recommendations by Chierici et al. (1999). The pH values were corrected to 25 °C on the total scale. The accuracy was determined by the pureness of the indicator and by analyzing certified reference material (CRM batch #181). The latter measurements indicated that accuracy should be well below 0.01 pH unit. The precision as determined by replicates from the same sample bottle was in the range of ±0.001 pH units.

Total alkalinity was determined using a semi-open cell potentiometric titration method using a 5-Gran point evaluation (Haraldsson et al. 1997). The system measures alkalinity in μmol/L using the nominal hydrochloric acid (HCl) concentration of 0.05 mol/L and 0.65 mol/L sodium chloride (NaCl). Certified reference material (CRM Batch #181) purchased from A. Dickson, Scripps Institution of Oceanography, was used to determine accuracy (Dickson et al. 2007). For all samples and CRM analysis the alkalinity in μmol/kg was calculated using the salinity (from the CTD bottle file and the certified salinity, respectively) and the temperature measured at the beginning of the titration. Sample results were then multiplied with the factor determined from the CRM measurements at each individual station, and the correction was always below 0.5%. The reported precision was computed as standard deviations of duplicate analyses performed continually during the cruise.

For more details, see the cruise report (Jakobsson et al. 2020).