Dataset: dissolved cobalt
Deployment: NBP0608

Sampling and Analytical Methodology:

From Saito et al. (2010)
Mak A. Saito, Tyler J. Goepfert, Abigail E. Noble, Erin M. Bertrand, Peter N. Sedwick, Giacomo R. DiTullio. (in press) A Seasonal Study of Dissolved Cobalt in the Ross Sea, Antarctica: Micronutrient Behavior, Absence of Scavenging, and Relationships with Zn, Cd, and P. Bigeosciences, 7, 4059-4082, 2010. Open Access:  http://www.biogeosciences.net/7/4059/2010/bg-7-4059-2010.html  doi:10.5194/bg-7-4059-2010

Sampling Methodologies
Seawater samples for trace metals analysis were collected using either 5 L Teflon-coated external spring Niskin-X samplers (General Oceanics Inc., described as NX samples) or 2.5 L and 10 L Go-Flo samplers (General Oceanics Inc.). The Niskin-X and 10 L Go-Flo bottles were deployed on a non-metallic line (Spectra) and closed using solid PVC messengers, whereas the 2.5 L Go-Flo samplers were mounted on an 11-position rosette unit modified for trace metal sampling, which was deployed on the Spectra line. The rosette unit was programmed to close the samplers at pre-determined times on the upcast, with the precise depth of sampler closure determined using an integrated pressure sensor. The NX vertical profiles typically consisted of samples obtained from 9 Niskin-X samplers and two additional 10 L Go-Flo samplers; two vertical profiles (Station 54 and JR23) were obtained using only the Go-Flo samplers. Upon recovery, seawater samples were filtered through either 0.2 micro-m Supor Acropak filter cartridges (Pall Corp., for NX samples) pre-rinsed with filtered surface seawater, or 0.4 micro-m 144mm polycarbonate membrane filters (GE, Osmonics, for Go-Flo samples) pre-cleaned with hydrochloric acid and Milli-Q deionized water. Cobalt was also measured in samples from three surface transects during CORSACS-1, collected using a trace-metal clean towed fish sampler deployed at ~3 m depth while underway at ~5 knots, with samples filtered using in-line 0.2 µm Supor Acropak filters (Bruland, 2005). All sample bottles were washed by soaking in 1% citranox overnight, rinsing thoroughly with Milli-Q, soaking in 10% Instra-analyzed HCl (Baker, Inc) for two weeks, followed by thorough Milli-Q rinsing and soaking with dilute (pH 2) Instra-analyzed HCl overnight. Bottles were rinsed with seawater prior to filling, and samples were refrigerated in darkness until analysis.

Total Dissolved Cobalt Analyses
Total dissolved cobalt (dCo), which is defined as total Co determined in filtered UV-irradiated seawater samples, was analyzed using a cathodic stripping voltammetry (CSV) protocol modified from our previous studies (Saito and Moffett, 2001; Saito et al., 2004; Saito et al., 2005; Saito and Moffett, 2002), with the addition of an automated titration system and a slight decrease in reagent usage. Briefly, seawater samples were digested in quartz tubes for 1 hour at ambient pH using a Metrohm UV digestor cooled with a Brinkmann water chiller. After this treatment, 9.25 mL of sample were pipetted into a Teflon sample cup that had been pre-rinsed with a small aliquot of sample; to this was added 50 microL of 0.5 M N-(2-hydroxyethy)piperazine-N-(3-propanesulfonic acid) buffer (EPPS, Sigma), 30 microL of 0.1 M dimethylglyoxime (DMG; Sigma-Aldrich), and 0.75 mL of 1.5 M sodium nitrite (Fluka). These three reagents were purified as previously described (Saito and Moffett, 2001; Saito et al., 2004; Saito et al., 2005). Instrumentation consisted of two Metrohm 663 hanging mercury drop stands, each interfaced to an Eco-Chemie mAutolab III, IME interface device, and portable Windows XP computer running GPES Electrochemical software (Eco-Chemie). Samples were purged for 3 minutes with filtered ultra high purity nitrogen gas, and voltammetric scans were performed at stirrer speed 5 using the large drop size. The CSV analysis involved deposition for 180 seconds at -0.6 V followed by a high speed scan from -0.6 V to -1.4 V at 10V s^-1. Total dissolved cobalt was quantified by standard additions with a Metrohm Dosimat, using a 5 x 10^-9 M CoCl2 standard solution prepared from a 1000 ppm Co atomic absorption standard (Fisher Scientific). A GPES script was programmed for three replicate scans of the sample prior to adding Co, followed by a scan after each of four automated 25 pM Co additions. Cobalt concentrations were calculated by dividing the mean of the triplicate sample scans by the standard addition slope determined by linear regression analysis, followed by reagent dilution and blank corrections for each set of reagents: 5.5 +/- 1.0 pM for the CORSACS-1 dCo profiles, 13.4 pM for the CORSACS-1 dCo surface transects and labile Co measurements, and 8.3 ± 1.3 pM for the CORSACS-2 dCo and labile Co measurements, as determined by running blank seawater as previously described (Saito and Moffett, 2001). Triplicate analysis of a surface sample from the Ross Sea using this automated method yielded 27.8 pM ± 0.7 pM (2.6% RSD).  Linear regressions of the standard additions data typically yielded r2 values of 0.99 or greater, with samples being reanalyzed if r2 < 0.99. Analyses were primarily conducted at sea, although some refrigerated samples from CORSACS-2 were analyzed in our shore-based laboratory within two months after the cruise. This analytical method is identical to that used for  our submitted analyses of the intercalibration samples and vertical profile from the US GEOTRACES intercalibration program (Bruland, 2010), with the exception that these CORSACS samples were run ‘fresh’ at ambient pH, while GEOTRACES samples were acidified, stored, and neutralized prior to analysis.

Cobalt Chemical Speciation: Labile Cobalt
The chemical speciation of dissolved cobalt was determined using a high-throughput labile Co method as previously described (Saito et al., 2004; Saito et al., 2005). Briefly this approach involves the analytical system described above for total dissolved cobalt, now with overnight equilibration of filtered seawater with 3x10-4 M DMG, which allows for quantification of ligand within the detection window for KCoL of 1013.7 to 1015.7, calculated using an estimated 50 pM natural organic cobalt ligand concentration and 10-fold range above and below a side reaction coefficient of aCoHDMG2 of 28460 (Saito et al., 2005). Conditional stability constants are not calculated in this high-throughput method. After equilibration, EPPS and nitrite are added and the sample is titrated with a Co standard as described for the dCo determinations above, but without the UV-irradiation treatment. The result is an estimation of what we refer to as labile cobalt, which is the dissolved cobalt that is not bound by the natural strong organic ligands following equilibration.

References: 

Bertrand, Erin M.; Sa, Mak A.;Lee, Peter A.; Dunbar, Robert B.; Sedwick, Peter N.; DiTullio, Giacomo R. (2011) "Iron limitation of a springtime bacterial and phytoplankton community in the Ross Sea: implications for vitamin B12 nutrition" Frontiers in Microbiology. vol 2 no. 00160. ISSN=1664-302X. DOI: 10.3389/fmicb.2011.00160  http://journal.frontiersin.org/Journal/10.3389/fmicb.2011.00160/abstract

Bruland, K. W. 2010. GEOTRACES Co Intercalibration Results, http://www.geotraces.org/documents/SAFeReferenceSample-Co.pdf

Saito, M. A., and Moffett, J. W. 2001. Complexation of cobalt by natural organic ligands in the Sargasso Sea as determined by a new high-sensitivity electrochemical cobalt speciation method suitable for open ocean work, Mar. Chem., 75, 49-68.

Saito, M. A., and Moffett, J. W. 2002. Temporal and spatial variability of cobalt in the Atlantic Ocean, Geochim. Cosmochim. Acta, 66, 1943-1953.

Saito, M. A., Moffett, J. W., and DiTullio, G. 2004. Cobalt and Nickel in the Peru Upwelling Region: a Major Flux of Cobalt Utilized as a Micronutrient, Global Biogeochem. Cycles, 18.

Saito, M. A., Rocap, G., and Moffett, J. W. 2005. Production of cobalt binding ligands in a Synechococcus feature at the Costa Rica Upwelling Dome, Limnol. Oceanogr., 10 50, 279-290.