Carbon dioxide, nutrients, dissolved oxygen and transient tracers dynamics

Work at sea

In all survey regions and hydrographic transects, water samples were taken from the CTD rosette sampler at depths all through the water column, but with a bias towards the upper layers. Total CO₂ (TCO₂; also known as DIC) and total alkalinity was determined in discrete water samples taken from the rosette sampler. Both TCO₂ and total alkalinity are measured together with a VINDTA instrument (MARIANDA, Kiel), which combines the two measurements. The accuracy is set by internationally recognized and widely used certified reference material (CRM) obtained from Prof. A. Dickson at Scripps (USA). TCO₂ is the sum of all dissolved inorganic carbon species and is determined by a precise coulometric method. For every coulometric cell that was used in the coulometer, at least two CRMs were measured in duplicate at the beginning and the end of the analyses. The alkalinity measurements were made by potentiometric titration with a strong acid (HCl) as a titrant. The acid consumption up to the second endpoint is equivalent to the titration alkalinity. The system uses a highly precise Metrohm Titrino for adding acid, a pH electrode and a reference electrode. In addition to the CRMs, some sample bottles were measured on both VINDTAs to check the internal consistency of the data. The measurement temperature for both TCO₂ and total alkalinity was 25°C. Measurements of the water were carried out immediately after sampling.

Generally, this means that the samples did not have to be stored for longer than 12 hours. In a very few cases, the time before measuring was somewhat longer, and then the samples were stored in the dark.

Preliminary results

A total of 110 stations were sampled for the CO₂ system with about 2,000 analyses.

In addition, surface water partial pressure of CO₂ (pCO₂) was collected from the ship’s seawater supply continuously during the cruise (Fig. 5.1.1). Sea surface pCO2 is obtained with a General Oceanics system with an infrared analyser (LiCOR), both for seawater using a water-air equilibrator and for the atmosphere, the air being pumped from the crow’s nest.

Fig. 5.1.1: Sea surface CO₂ (parts per million, ppm) along the ANT-XXVIII/3 cruise track

Data management

The TCO₂, alkalinity and pCO₂ data will be largely processed after the cruise. The final data will be submitted to data centers, as has been done with all data of previous cruises with Polarstern. The usual data center for carbon research is the Carbon Dioxide Information and Analysis Center (CDIAC; Boulder, USA) together with CCHDO. In the past, data have also been transferred to Pangaea, and they should be published within two years after the end of the cruise.

Other variables that are essential to biogeochemical studies involving the CO₂ system are oxygen and major nutrients.

Fig. 5.1.2: Vertical distribution of the vertical diffusivity preliminarily derived from the MSS measurements at the indicated stations

5. Carbon dioxide, nutrients, dissolved oxygen and transient tracers dynamics

Sample water was obtained from the rosette sampler from all depths. All samples were collected in 125 ml polypropylene bottles directly after the trace gases, oxygen and TCO sampling. In the lab container the nutrient samples were transferred into 5 ml polyethylene vials, covered with parafilm against evaporation, and placed in the sampler after rinsing three times. All analyses were done within 15 hours on the auto-analyzer, a Technicon TRAACS 800 Auto-analyzer. Calibration standards were diluted from stock solutions of the different nutrients in 0.2 μm filtered low nutrient seawater (LNSW) and were freshly prepared every day. The LNSW is surface seawater depleted of most nutrients; it is also used as baseline water for the analysis between the samples. Each run of the system had a correlation coefficient of at least 0.9999 for 10 calibration points, but typical 1.0000 for linear chemistry. The samples were measured from the lowest to the highest concentration in order to keep carry-over effects as small as possible, i.e. from surface to deep waters. Prior to analysis, all samples and standards were brought to lab temperature of 22°C in about two hours; concentrations were recorded in μmol per liter at this temperature. During every run a daily freshly diluted mixed nutrient standard, containing silicate, phosphate and nitrate (a so-called nutrient cocktail), was measured in triplicate. Additionally, a natural sterilized Reference Material Nutrient Sample (JRM Kanso, Japan) containing known concentrations of silicate, phosphate, nitrate and nitrite in Pacific Ocean water, was analyzed in triplicate every 2 weeks in a run. The cocktail and the JRM were both used to monitor the performance of the analyzer. Finally, the nutrient cocktail (referred to in the results as ANTCOCK98x100) was used to adjust all data to the level of the known concentrations of the cocktail by means of a correction factor. The final data set is thus referenced to the same cocktail values, which makes data comparable and consistent. From every station the deepest sample bottle was sub-sampled for nutrients in duplicate, the duplicate sample-vials were all stored dark at 4°C, and measured again in the next run with the upcoming stations, this being for statistical purposes. More than 2,200 samples were analyzed for phosphate, silicate, nitrate and nitrite in total, of which 1,807 at CTD stations.

Some 474 samples were analyzed in support of the biological work of Trimborn et al. (this volume).

Preliminary results Analytical methods

Phosphate reacts with ammonium molybdate at pH 1.0, and potassium antimonyltartrate is used as an inhibitor. The yellow phosphate-molybdenum complex is reduced by ascorbic acid and measured at 880 nm (Riley & Murphy, 1962). Silicate reacts with ammonium molybdate to a yellow complex, after reduction with ascorbic acid; the obtained blue silica-molybdenum complex

is measured at 800 nm. Oxalic acid is added to prevent formation of the blue phosphate-molybdenum (Strickland & Parsons, 1968).

Nitrate plus nitrite (NO₃+NO₂) is mixed with an imidazol buffer at pH 7.5 and reduced by a copperized cadmium column to nitrite. The nitrite is diazotated with sulphanylamide and naphtylethylene-diamine to a pink colored complex and measured at 550 nm. Nitrate is calculated by subtracting the nitrite value of the nitrite channel from the ‘NO₃+NO₂’ value. (Grasshoff et al, 1983) Nitrite is diazotated with sulphanylamide and naphtylethylene-diamine to a pink colored complex and measured at 550 nm. (Grasshoff et al., 1983)

Calibration and standards

Nutrient primary stock standards were prepared at the NIOZ. Phosphate: by weighing potassium dihydrogen phosphate into a calibrated volumetric PP flask to 1 mM PO₄. Silicate: by weighing Na₂SiF₆ into a calibrated volumetric PP flask to 19.99 mM Si. Nitrate: weighing Potassium nitrate into a calibrated volumetric PP flask set to 10 mM NO₃. Nitrite: weighing sodium nitrite into a calibrated volumetric PP flask set to 0.5 mM NO₂.

All standards were stored at room temperature in a 100 % humidified box. The calibration standards were prepared daily by diluting the separate stock standards, using three electronic pipettes, into four 100ml PP volumetric flasks (calibrated at the NIOZ) filled with low nutrient seawater LNSW. The blank values of the LNSW were measured onboard and added to the calibration values to get the absolute nutrient values. Our standards are regularly monitored by participating in inter-calibration exercises from ICES and Quasimeme and even more recently from the RMNS exercise organised by Michio Aoyama MRI/Japan.

Method detection limits

The method detection limits was calculated using the standard deviation of ten samples containing 2 % of the highest standard used for the calibration curve and multiplied with the student’s value for n=10, thus being 2.81. (M.D.L = Std Dev of 10 samples x 2.81), M.D.L.(µM/l) Used measuring ranges µM/l:

PO₄ 0.007 3.51 Si 0.057 159.21 NO₃+NO₂ 0.025 45.51 NO₂ 0.003 0.51

Quality control and statistics

Material, followed by statistics using the in-house diluted cocktail98 over all runs:

5. Carbon dioxide, nutrients, dissolved oxygen and transient tracers dynamics