Search the data

Discrete dissolved inorganic carbon (DIC) and total alkalinity (TA) from CTD bottles from RRS James Cook cruise 31 (JC031)

Originator's Protocol for Data Acquisition and Analysis

Sampling strategy

Water was collected from depth using a stainless steel CTD frame fitted with a Sea-Bird SBE 32 twenty-four way carousel (s/n 32-45661-0621) equipped with 24, twenty litre OTE external spring water samplers. However, due to the complete loss of the CTD package at station 12, water was subsequently collected using a stainless steel CTD frame fitted with a Sea-Bird SBE 32 twenty-four way carousel (s/n 32-19817-0243) equipped with 20, ten litre OTE external spring water samplers (rosette positions 1-20) and 4, twenty litre OTE external spring water samplers (rosette positions 21-24). DIC (also denoted as TCO ₂ ) was sampled and analysed using methods similar to those described by Bakker et al. (2007). Water samples for the determination of DIC were drawn from the 20 and 10 litre niskin bottles on the CTD rosette and collected in 500 mL glass bottles in the usual way to avoid gas exchange with the air. Depths were distributed evenly over the water column with a bias towards the upper 1000 m. Samples were kept cold and stored in the dark. Samples from the upper 500 m were analysed within 8 hours of collection, with deeper samples following within 20 hours of collection. If such rapid analysis was not possible, the samples were poisoned with mercuric chloride (100 µmol L ^-1 per 500 mL of saturated HgCl ₂ solution).

DIC analysis

Samples for DIC were analysed using coulometry. Two different instruments were used for this analysis. The first instrument (ID #1) used an extractor unit built after the design by Robinson and Williams (1992). For the first instrument, samples were kept at about 4 °C prior to analysis to prevent bubble formation in the extractor tubing. However, analysis was typically carried out at approx. 8 °C. The second instrument (ID #7) was a Vindta combined DIC/alkalinity instrument (#7, version 3C) and was operated at 25 °C (Mintrop, 2004). Two replicate analyses were made on each sample bottle. Also replicate samples from the rosette were drawn. The DIC concentration was determined by the coulometric method after the method of Johnson et al. (1987). Generally, all samples from one station were run using the same coulometer cell. At least two CRMs (Certified Reference Materials) (batches 90 and 92) were used per coulometric cell and station. Certified Reference Material was provided by Andrew Dickson's lab (Scripps Oceanography Centre) as described in the Dickson et al. (2007) Handbook for inorganic carbon measurements (available on CDIAC website). More information on analysis can be found from p53 of the cruise report .

DIC Field replicate information

Analysis of 2 replicate analyses (per bottle). For the stand-alone instrument, values from the 1st and the 2nd analysis did not show a systematic offset and both values were used. For the Vindta instrument, only values for the 1st analysis were used, as DIC of the 2nd analysis was systematically slightly lower than that of the 1st analysis due to outgassing. Nonetheless the repeat analyses on the Vindta provided a useful quality control on the performance of the instrument.

DIC accuracy and precision

The accuracy was estimated as 3.0 µmol kg ^-1 . The precision was estimated as 2 µmol kg ^-1 . The repeatability of 447 repeat analyses on the stand-alone instrument 1 was 1.4 µmol kg ^-1 (following SOP 23 in Dickson et al., 2007). The repeatability of DIC on the Vindta V7 was 2.4 µmol kg ^-1 for 28 replicate samples.

TA analysis

Alkalinity measurements were made by potentiometric titration with two Vindta instruments (#4 and #7, version 3C, Mintrop, 2004). The acid consumption up to the second endpoint was equal to titration alkalinity. The systems used a highly precise Metrohm Titrino for adding acid, an ORION-Ross pH electrode and a Metrohm reference electrode. The pipette (volume approximately 100 mL), and the analysis cell had a water jacket around them. The titrant (0.1 mol L ^-1 hydrochloric acid, HCl) was made in the home laboratory. Samples on one Vindta (#4) were run after analysis on the stand-along DIC extractor and warming to 25 °C. Samples on the second Vindta (#7) were run for both DIC and alkalinity at 25 °C. Replicate analyses were run for all samples on the Vindta #7 and for most samples on the Vindta #4. At least two Certified Reference Materials (CRM) of batch 92 or 90 were run per station. Certified Reference Material as provided by Andrew Dickson's lab (Scripps Oceanography Centre) as described in the Dickson et al. (2007) handbook for inorganic carbon measurements (available on CDIAC website). TA was calculated in Matlab code by Steven van Heuven, version 33, based on code by Ernie Lewis in QB (1996). More information on the analysis used can be found from p53 of the cruise report .

Magnitude of TA blank correction

Acid factor instrument V4: 0.0970 ± 0.0004 mol L ^-1 ; instrument V7: 0.0976 ± 0.005 mol L ^-1 for the hydrochloric acid. Both instruments used sub-samples of the same acid stock solution. An acid factor of 0.1 mol L ^-1 HCl was aimed for when preparing the solutions. The accuracy of the calibration of the TA pipette would have contributed to the difference in the average acid factor between both instruments.

TA accuracy

The accuracy of the TA data (WOCE flag 2) was estimated as 3 µmol kg ^-1 for the cruise. The accuracy was estimated from the average standard deviation of CRMs per acid batch, which was 1 µmol kg ^-1 for the western A21 (SR1) section and 3 µmol kg ^-1 for the eastern SR1b section. The precision of the TA sample averages (WOCE flags 2 and 3) was estimated as 3 µmol kg ^-1 for the cruise from the repeatability of 674 pairs of duplicate samples (Dickson et al., 2007, SOP 23). The repeatability was slightly better for Vindta V4 (2.3 µmol kg ^-1 , 372 samples) than for Vindta V7 (2.7 µmol kg ^-1 , 302 samples). TA data were calibrated with the acid factors, whenever possible per acid batch. Preliminary data showed substantial offsets between neighboring stations, in particular for the second eastern SR1b section for Vindta V4, with Vindta V4 TA data often lower than those for V7. Such offsets between stations were deemed to be unrealistic and an artifact of the analytical procedure. The acid factor of such outlier stations on Vindta V4 was adjusted in order to remove the offset. This correction concerns stations 11 (+5 µmol kg ^-1 ), 38 (+4 µmol kg ^-1 ), 39 (+3 µmol kg ^-1 ), 43 (+2 µmol kg ^-1 ), 50 (+5 µmol kg ^-1 ), 54 (+5 µmol kg ^-1 ), 58 (+8 µmol kg ^-1 ), 66 (+3 µmol kg ^-1 ), 70 (+11 µmol kg ^-1 ), 71 (+3 µmol kg ^-1 ), 74 (+3 µmol kg ^-1 ), 78 (+3 µmol kg ^-1 ), 82 (+3 µmol kg ^-1 ), and 83 (+3 µmol kg ^-1 ). These corrections increased TA and, if incorrect, would have biased the TA data high. The TA data from these stations were given WOCE flag 3 (questionable, probably good). Samples with a difference between repeat analyses exceeding 5 µmol kg ^-1 have been given flag 3.

References Cited

Bakker D.C.E., Nielsdóttir M.C., Morris P.J., Venables H.J. and Watson A.J., 2007. The island mass effect and biological carbon uptake for the subantarctic Crozet Archipelago. Deep-Sea Research II, 54: 2174-2190, doi:10.1016/j.dsr2.2007.06.009

Robinson C. and Williams P.J.leB, 1992. Development and assessment of an analytical system for the accurate and continual measurement of total dissolved inorganic carbon. Marine Chemistry 34: 157-175

Mintrop L., 2004. VINDTA, Versatile Instrument for the Determination of Titration Alkalinity. Manual for versions 3S and 3C. Version 2.0. MARine ANalytics and Data (MARIANDA), Kiel, Germany, 45pp.

Johnson K.M., Williams P.J. LeB., Brändström L. and Sieburth J.McN., 1987. Coulometric total carbon dioxide analysis for marine studies: automatization and calibration. Marine Chemistry, 21: 117-133

Dickson A.G., Sabine C.L. and Christian J.R. (Eds.) 2007. Guide to best practices for ocean CO ₂ measurements . PICES Special Publication 3, 191 pp.

BODC Data Processing Procedures

Data arrived at BODC in one Microsoft Excel format file. The file contained CTD sensor (from the CTD upcast at bottle firing depths) and discrete dissolved inorganic carbon (DIC) and total alkalinity (TA) measurements from CTD bottles. The data received were loaded into the BODC database using established BODC data banking procedures. The following changes to the data originator's data were made:

• Measurement temperatures: Channels for the temperature of the DIC and TA measurement were generated at BODC as this depended on which analytical instrument was used. The analytical temperatures were obtained from the originators. The DIC measurement temperature was approx. 8 °C for instrument #1 and 25 °C for #7. The TA measurement temperature was 25 °C for both instruments (#4 and #7).

• Absent data values: Absent data values were removed.

• Quality control flags: WOCE flags were converted to the appropriate BODC flag (2 = good, 3 or 4 = 'L').

The data were screened in-house prior to loading. Data were then loaded into BODC's database without any further changes. Originator's parameters were mapped with BODC codes as follows:

The following variables were created at BODC:

Data Quality Report

Data quality good (BODC assessment)

Problem Report

No problems to report (BODC assessment)