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Dissolved Cobalt from GEOTRACES GA02 cruises PE319, PE321, PE358 and JC057

Responsible investigator

Gabriel Dulaquais ¹ , Marie Boye ¹

¹ Laboratory of Marine Environmental Science (UMR CNRS 6539)

Originator's Protocol for Data Acquisition and Analysis

Sampling protocol

A total of 47 stations with a vertical resolution of 12-16 depths between 9m and 5930m were sampled for dissolved cobalt analyses (DCo), and 15 stations for total (unfiltered) cobalt determinations (T Co). Samples were taken using the TITAN-CTD frame of NIOZ (Netherlands), with 24 ultra-clean 24.4 L sampling bottles made of PVDF (polyvinylidene fluoride) plastic. The frame was placed in a Class-100 container for sub-sampling (de Baar et al., 2008). Unfiltered samples were transferred into acid-cleaned 250mL Nalgene LDPE (low-density polyethylene) bottles for TCo analyses. The samples for DCo analyses were collected after filtration using 0.2 µm Sartobran 300 (Sartorius) cartridges under pure N ₂ pressure (filtered 99.99 % N ₂ , 0.7 atm) in acid-cleaned 250 mL or 500 mL Nalgene LDPE bottles. All samples were acidified at pH.2 using ultrapure HCl (Merck) immediately after their collection. Then the acidified samples were dark-stored in double bags at ambient temperature in preparation of their analyses in the shore-based laboratory.

Analytical procedure

Prior to the analyses, the samples were UV-digested (Saito and Moffet, 2002; Shelley et al., 2010) for 3 h in acid-cleaned silica tubes using a 600 W high-pressure mercury vapor lamp (Bown et al., 2011), and left for an equilibration time of 48 h. Preliminary tests indicated that 3 h of UV-digestion were required to fully recover Co in surface and deep samples.

Dissolved and total cobalt concentrations were determined by flow-injection analysis (FIA) and chemiluminescence detection following the method adapted from Shelley et al. (2010), as described in Bown et al. (2011).

Analytical performance

The mean reagent blank (based on all blank determinations) was 4.2 ± 2.1 pM (n = 180) of Co in MilliQ (n = 180). The limit of detection of the method estimated as three times the standard deviation of the mean reagent blank was 6.3 pM (n = 180). Each series of samples was calibrated by running one or two samples collected during the 'Sampling and Analysis of iron' (SAFe) program or GEOTRACES program. SAFe and GEOTRACES samples were UV-digested for 3 h prior to analysis and the results of DCo concentrations are reported in the Table below. The DCo concentrations we measured in the SAFe and GEOTRACES reference samples are in excellent agreement with the consensus values (http://www.geotraces.org/science/intercalibration/ 322-standards-and-reference-materials). The DCo value obtained in the S-SAFe sample also falls in the consensus value despite the concentration being lower than the detection limit. The analytical precision of the method was determined from repeated analyses of the surface S-GEOTRACES reference sample, yielding an uncertainty of ±3.8 % expressed as relative standard deviation on the mean (n = 15).

References Cited

Bown, J., Boye, M., Baker, A., Duvieilbourg, E., Lacan, F., Le Moigne, F., Planchon, F., Speich, S., and Nelson, D. M. 2011. The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa, Mar. Chem., 126, 193-206, doi:10.1016/j.marchem.2011.03.008.

Dulaquais, G., Boye, M., Rijkenberg, M. J. A., and Carton, X. 2014 : Physical and remineralization processes govern the cobalt distribution in the deep western Atlantic Ocean, Biogeosciences, 11, 1561-1580, doi:10.5194/bg-11-1561-2014

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

Shelley, R. U., Zachhuber, B., Sedwick, P. N., Worsfold, P.J., and Lohan, M. C. 2010. Determination of total dissolved cobalt in UV-irradiated seawater using flow injection with chemiluminescence detection, Limnol. Oceanogr., 8, 352-362, doi:10.1029/2009JC005880.

BODC Data Processing Procedures

Data were received and were loaded using established BODC data banking procedures. Data were supplied in /l and converted into /kg using standard density conversion of /1.025. This document covers data submitted using the following GEOTRACES parameters: