Dataset: GT10-11 - Co Total Dissolved and Labile
Deployment: KN204-01

Samples were collected using the ODU GEOTRACES Carousel (GT-C), and were filtered through 0.2 um Acropak filters in the GEOTRACES clean van and immediately refrigerated. Samples were kept in acid-washed 60 mL LDPE bottles, and either stored for a short time (<7 days) at 4°C and double-bagged prior to analysis, or for a longer time (up to 40 days) at 4°C and in a heat-sealed bag with gas absorbing satchels. Both labile and total dissolved cobalt were analyzed from this sample bottle, and the sample identifier is the allocated GEOTRACES number.

The carousel was used to collect samples from surface to near bottom waters, and an additional sample was collected from a surface towfish at each station. Pre-conditioned, teflon-coated Go-Flo sampling bottles (General Oceanics, Miami, FL) of 12 L capacity were deployed on a polyurethane powder-coated aluminum rosette with titanium pilings and pressure housings (Sea-Bird Electronics, Inc., Bellevue, WA) attached to a Kevlar, non-metallic conducting cable. For more information regarding carousel deployment, please refer to the GEOTRACES Cookbook (www.geotraces.org/science/intercalibration/222-sampling-and-sample-handling-protocols-for-geotraces-cruises), located on the GEOTRACES Program website (www.geotraces.org/). Following the retrieval of the carousel, Go-Flo bottles were moved to the GEOTRACES Program class-100 trace metal clean van, and pressurized with HEPA filtered air for sampling in accordance with published methods (Cutter and Bruland 2012). Surface towfish samples were collected by suspending the towfish off the starboard side with a boom, and sampled water at approximately 2 m depth using a Teflon diaphragm pump following the GEOTRACES Program Cookbook sampling recommendations, and in accordance with previous collection protocols (Bruland et al. 2004).

Sample storage bottles were soaked overnight in the acidic detergent, Citranox, rinsed thoroughly with Milli-Q water (Millipore), filled with 10% HCl to soak for 10 days, rinsed thoroughly with Milli-Q water adjusted to pH 2, and double-bagged, empty.

Concentrations of total dissolved and labile cobalt during the KN199-04 cruise were determined ship-board using a previously described cathodic stripping voltammetry (CSV) method (Saito and Moffett 2001, Saito et al. 2004).  Measurements were made using the Eco-Chemie µAutolabIII systems connected to Metrohm 663 VA Stands equipped with hanging mercury drop electrodes and Teflon sampling cups within 7 days of sampling. Standard additions were carried out with Metrohm 765 Dosimats using a programmed dosing procedure (Noble and Saito et al. 2008).

Concentrations of total dissolved and labile cobalt from the KN204-01 cruise were measured on land between 1 and 6 weeks after the sampling date, using the same protocol as that of the KN199-04 cruise.

For total dissolved cobalt analyses, samples were UV-irradiated for 1 h prior to analysis using a Metrohm 705 UV digester to degrade the organic ligands that bind cobalt and allow binding by the added electroactive cobalt ligand, dimethylglyoxime. Samples were analyzed in 8.5 mL aliquots with the addition of 30 µL recrystalized dimethylglyoxime (DMG, 0.1 mol L-1 in methanol), 1.5 mL purified sodium nitrite (1.5 mol L-1 in Milli-Q water), and 50 µL purified N-(2-hydroxyethyl)piperazine-N-(3-propanesulfonic acid) (EPPS) buffer (0.5 mol L-1 in Milli-Q water).  Reagent purification protocols were identical to those previously published (Saito and Moffett 2001). Cobalt concentrations were determined by the standard additions technique, with initial concentrations measured in triplicate followed by four 25 pmol L-1 cobalt additions. 

The analytical blank was determined by analyzing seawater that had been UV-irradiated for 1 h, equilibrated overnight with prepared Chelex 100 resin beads (Bio-Rad), and UV-irradiated a second time to degrade any leached synthetic ligands. Blanks for each reagents batch (nitrite, DMG, EPPS) were subtracted from the initial sample concentration. Blank analyses for each reagent batch were made at the beginning and end of use to confirm that the blank remained constant during analyses. The averaged blank for all reagent batches for the entire dataset was 4pM +/- 1.2 with a range of 1.7 - 6.3pM (n = 38 for individual blank analyses). For a given reagent batch, the standard deviation was smaller, and we report a detection limit (3 times the standard deviation of the blank) of 1.8pM, representing the average of the detection limits estimates for reagent batches with at least 3 blank analyses (n = 6).
 
For labile cobalt analyses, 8.5 mL of sample were pipetted into acid washed Teflon vials that were preconditioned with a small aliquot of sample water. 30 uL of DMG were added to each vial and allowed to equilibrate overnight in the dark prior to analysis (Saito et al. 2004). Analyses were then performed as described for total concentrations with the addition of the remaining two reagents and use of the standard addition technique. Previously, we determined that natural cobalt ligands in seawater have a conditional stability constant of >10^16.8 (Saito et al. 2005). This suggests that the cobalt is very strongly bound to ligands. Thus, we define labile cobalt as the fraction of total dissolved cobalt that is either bound to weak organic and inorganic ligands in seawater or present as free Co(II), and is then exchangeable with the complexing agent (DMG) used for analysis (Saito et al. 2004, Saito et al. 2005). The difference between the total dissolved cobalt and the labile cobalt can then be used as an estimation of the strong cobalt ligand concentration.

Two full electrochemical systems were utilized for analyses. One electrode was dedicated to total analyses and the other to labile analyses. GEOTRACES standard seawater and internal standard lab seawater were analyzed periodically to ensure that the two electrodes were intercalibrated and functioning properly. GEOTRACES standard seawater was UV irradiated and neutralized using Optima ammonium hydroxide to bring the pH up to 7.5. Standard seawater internal to our lab was UV irradiated as well, but was not previously acidified so received no further treatment prior to analysis. These results demonstrate that the methodologies employed to produce this dataset detect concentrations within the standard deviation of current consensus values for UV irradiated samples, which can be found on the International GEOTRACES Program website (www.geotraces.org/). On occasion, analyses were repeated due to obvious electrode malfunction or to confirm oceanographic consistency of measured values. If the repeated measurement was similar to the initial, the initial value is reported. If the repeated analysis was more oceanographically consistent with adjacent values in the water column, that analysis was used instead.

Our laboratory has participated in the GEOTRACES intercalibration effort using this electrochemical Co method. We report our laboratory values for the GEOTRACES and SAFe standard analyses using this electrochemical method, including those conducted during analysis of the US North Atlantic GEOTRACES Section samples to be: SAFe S1 = 5.4 +/- 2.6 (n=9), SAFe D2 = 48.3 +/- 5.5 (n=7), GEOTRACES GS = 31.4 +/- 4.1 (n=24), GEOTRACES GD = 66.9 +/- 6.2 (n=30). These results are in good agreement with those from the GEOTRACES intercalibration effort for Co using different methods all using UV-oxidation to degrade strong cobalt ligands.