Dissolved Inorganic Carbon (DIC, TCO2) and Total Alkalinity (TA) discrete samples from CTD bottles during RRS James Cook cruise 71
Originator's Protocol for Data Acquisition and Analysis
Sampling Strategy
Samples were collected at up to 12 depth levels at 7 CTD stations.
Sampling Method
Samples were collected from 10L CTD Niskin bottles into special ground glass topped glass bottles using a small length of narrow bore tubing inserted into the bottom of the bottle. Care was taken to ensure that no gas bubbles were present in the sample by first inverting the bottle and revolving it slowly while returning it to an upright position. Once filled the bottle was flushed from the bottom with three times its volume before inserting the glass stopper. The sample was then opened in a fume hood and 2.5 mL of seawater removed and 50 µL of mercuric chloride added by gently placing the pipette tip below the surface and expelling the liquid. A small amount of silicone grease was placed on the stopper before it was replaced and sealed into the bottle with tape.
TCO 2 Analysis Method
The analysis of DIC was undertaken in National Oceanography Centre, Southampton using VINDTA 3C (Marianda, Germany, unit 11 and unit 24). By reaction with phosphoric acid, dissolved inorganic carbon of the sample was converted to CO 2 . The CO 2 gas was carried by N 2 into the coulometer cell and reacts with monoethanolamine to form a titratable acid which causes the fading of the blue indicator. Responding to the colour change, a current flow generates base to remove the acid and restore the indicator to the original colour. The amount of CO 2 can be estimated from the required coulombs of the current required (corrected for blank), and DIC concentration can then be calculated given the known sample volume.
Standardization Technique
In order to standardize the results, Certified Reference Materials (CRM) from A.G. Dickson, Scripps Institution of Oceanography were analysed as standards to calibrate the instrument at the beginning and end of each day of analysis. A daily correction factor was applied to all measured values according to Millero et al. (1998).
Sample Volume
19.1681 mL
Field Replicate Info
Repeated measurements on the same batch of seawater (n>=3) in the lab gave consistent results (precision for the whole dataset estimated as 2.0±0.7 µmol kg -1 ).
Poisoning Correction Description:
Saturated HgCl 2 (0.05 mL) is added to the collected sample to prevent the sample from biological modification. Since we use a large sampling volume (250 mL), the dilution effect of adding HgCl 2 is neglectable (0.02 %).
Poison Volume:
0.05 mL
Accuracy Info:
Repeated measurements on the same batch of seawater (n>=3) were undertaken every day prior to sample analysis in order to assess the precision of the method. The precision was estimated for the whole dataset to be 2.0±0.7 µmol kg -1 for DIC and 1.2±0.5 µmol kg -1 for TA. The accuracy of the measurement is assured by the standardization using the CRM from A.G. Dickson, Scripps Institution of Oceanography on daily basis.
Alkalinity (TA) Analysis method
Alkalinity was resolved using the non-linear curve fitting (least-squares) approach of Dickson et al. (2007).
Type of Tritration
Titration with HCl (~0.10 mol L -1 ) uses a closed cell procedure with an open cell, with a pH half cell electrode (glass bodied Orion 8101SC Ross, USA) and an Ag/AgCl reference electrode (model 6.0729.100, Metrohm, Switzerland).
Cell Type
open cell
Sample Volume
97.409 mL
Accuracy Info
Repeated measurements on the same batch of seawater (n>=3) were undertaken every day prior to sample analysis in order to assess the precision of the method. The precision was estimated for the whole dataset to be 2.0±0.7 µmol kg -1 for DIC and 1.2±0.5 µmol kg -1 for TA. The accuracy of the measurement is assured by the standardization using the CRM from A.G. Dickson, Scripps Institution of Oceanography on daily basis.
References
Dickson A.G., Sabine C.L., and Christian J.R., 2007. Guide to best practices for ocean CO 2 measurements. PICES Special Publication 3, 191 pp.
Hartman S.E., Dumousseaud C. and Roberts A., 2011 Operating manual for the Marianda (Versatile INstrument for the Determination of Titration Alkalinity) VINDTA 3C for the laboratory based determination of Total Alkalinity and Total Dissolved Inorganic Carbon in seawater. Southampton, UK, National Oceanography Centre, 66 pp. (National Oceanography Centre Internal Document, 01).
BODC Data Processing Procedures
The data arrived at BODC in one Excel ( .xls ) file containing discrete samples collected from the underway non-toxic supply and CTD deployments. Data received were loaded into the BODC database using established BODC data banking procedures. Data were loaded into BODC's database without any changes. Originator's variables were mapped to appropriate BODC parameter codes as follows:
| Originator's variable | Originator's units | Description | BODC Code | BODC units | Comments |
| DIC CRMd | µmol kg -1 | Dissolved inorganic carbon (DIC) or TCO 2 | TCO2MSXX | µmol kg -1 | |
| TA CRMd | µmol kg -1 | Total alkalinity (TA) | MDMAP014 | µmol kg -1 | |
| salin | Salinity (CTD sensor) | Not transferred as part of this dataset | |||
| temp | Temperature (CTD sensor) | Not transferred as part of this dataset | |||
| NO3 | µmol L -1 | Inorganic nitrate (nitrate+nitrite) | Not transferred as part of this dataset | ||
| SiO4 | µmol L -1 | Inorganic silicate | Not transferred as part of this dataset | ||
| PO4 | µmol L -1 | Inorganic phosphate | Not transferred as part of this dataset |
Data Quality Report
None (BODC assessment)
Problem Report
None (BODC assessment)
