Sample collection and filtering: Water column samples were collected by the U.S. GEOTRACES sampling team from 24 modified 12-L Teflon-coated GO-FLO samplers (General Oceanics) deployed on the GEOTRACES clean CTD rosette system (Cutter and Bruland 2012). The U.S. GEOTRACES sampling team filtered the samples through 0.2 \u00b5m Supor Acropak filter cartridges (Pall) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface samples (~2 m depth) were collected underway either on approach or leaving each station using the Ken Bruland lab towfish, \u201cGeoFish\u201d, deployed to collect surface samples from outside the wake of the ship. Seawater from the GeoFish was pumped directly into the Bruland lab clean air laboratory van and filtered through 0.2 \u00b5m Supor Acropak filter cartridges.
\nFiltered seawater samples for iron speciation (organic complexation) were collected in 500 mL acid-cleaned Nalgene narrow-mouth fluorinated high-density polyethylene (FPE) bottles that had been filled with Milli-Q for at least two weeks after acid cleaning and rinsed three times with filtered seawater prior to sample collection (Buck et al. 2012). All filtered seawater samples for dissolved iron speciation were frozen at -20 \u00baC shipboard by the sampling team and shipped to the University of South Florida for laboratory-based analyses post cruise.
\nDissolved iron speciation (organic complexation) analyses: Dissolved iron speciation (organic complexation) was analyzed using competitive ligand exchange- adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime as the added competing ligand (Buck et al. 2015), modified from the original Rue and Bruland (1995) method. A borate-ammonium buffer was added to all vials, for a final pH of 8.2 (NBS scale; Buck et al. 2007, 2012). Dissolved iron additions of 0 to 10 nM were used in the titrations, for a total of at least 15 points in each titration. These dissolved iron additions were chosen to match those of the U.S. GEOTRACES North Atlantic dataset and include additional datapoints within this range to allow higher resolution data within titrations (Buck et al. 2016). Deposition times of 90 to 600 s were applied to the analyses, with typically longer deposition times for deeper water column samples. CLE-AdCSV analyses were performed on Bioanalytical Systems (BASi) Controlled Growth Mercury Electrodes (static mercury drop setting, size 14) with Epsilon e2 (BASi) electrochemical analyzers. There are no reference samples available for iron speciation measurements of iron in seawater, though this method has been shown to compare well with other labs and techniques in GEOTRACES intercalibration exercises (Buck et al. 2012, 2016).
\nFLAG:\u00a0The standard Ocean Data View qualifying flags were used (reference all flags at\u00a0https://www.bodc.ac.uk/data/codes_and_formats/odv_format/):
\n1: Good Value: Good quality data value that is not part of any identified malfunction and has been verified as consistent with real phenomena during the quality control process. [Used when data were shown to be reproducible]
\n2: Probably Good Value: Data value that is probably consistent with real phenomena but this is unconfirmed or data value forming part of a malfunction that is considered too small to affect the overall quality of the data object of which it is a part. [Used when the reported value reflects analysis of a single or unverified replicate]
\n3: Probably Bad Value: Data value recognized as unusual during quality control that forms part of a feature that is probably inconsistent with real phenomena. [Not used]
\n4: Bad Value: An obviously erroneous data value. [Not used]
\n5: Changed Value: Data value adjusted during quality control. [Not used]
\n6: Value Below Detection Limit: The level of the measured phenomenon was too small to be quantified by the technique employed to measure it. The accompanying value is the detection limit for the technique or zero if that value is unknown. [We report \u201cnot_detected\u201d rather than zero or a detection limit value for this case]
\n7: Value in Excess: The level of the measured phenomenon was too large to be quantified by the technique employed to measure it. The accompanying value is the measurement limit for the technique. [Not used]
\n8: Interpolated Value: This value has been derived by interpolation from other values in the data object. [Not used]
\n9: Missing Value: The data value is missing. Any accompanying value will be a magic number representing absent data. [Not used]
Dissolved iron-binding organic ligand concentrations and conditional stability constants from surface GeoFish and GO-FLO water column samples collected in 2013 during the U.S. GEOTRACES GP16 cruise in the Eastern Pacific between Peru and Tahiti on the R.V. Thomas G. Thompson, \u00a0cruise TN303.
\nThe FISH and BOTTLE data have been split into separate columns as per GEOTRACES Parameter Naming Conventions.The FISH data are to the far right.
\n* To access the data with the FISH and BOTTLE data merged, see:\u00a0http://data.bco-dmo.org/jg/serv/BCO/GEOTRACES/EPZT/Fe_spec_EPZT_joined.html0%7Bdir=data.bco-dmo.org/jg/dir/BCO/GEOTRACES/EPZT/,info=data.bco-dmo.org/jg/info/BCO/GEOTRACES/EPZT/Fe_speciation%7D
CLE-AdCSV data was interpreted using a freely available interpretation program, ProMCC (Omanovic et al. 2015; https://sites.google.com/site/mccprosece), which employs matrix based optimization of titration data fitting to obtain ligand concentrations and conditional stability constants for each sample. Uncertainties in the fitted parameter are presented as the 95% confidence interval provided by the interpretation program. When replicate titrations were conducted, results were presented as averages of the replicates.
\nDissolved iron (DFe) concentrations determined by inductively coupled plasma-mass spectrometry (ICP-MS) in Peter Sedwick\u2019s lab at Old Dominion University were used in the ProMCC interpretation program to calculate ligand concentrations and conditional stability constants from the sample titrations. The DFe values used for this dataset are provided in the speciation submission for reference, please see Sedwick\u2019s EPZT dataset for the complete dissolved iron data and metadata.
\nThe detection limit for the electroactive Fe-SA complex has been found to be 0.01 nM (Buck et al. 2007), calculated as three times the standard deviation of measured peak height from a 0.05 nM Fe addition to UV-oxidized and chelexed seawater with a 900 s deposition time. For a 90 s deposition time, the detection limit for Fe-SA is ~0.1 nM. In the U.S. GEOTRACES North Atlantic dataset using the same analytical techniques, samples from two stations were analyzed in triplicate complete titrations, typically two conducted shipboard and one in the laboratory after storing frozen (-20 \u00baC). Over the full water column from both stations, the average standard deviation from the triplicate titrations of each sample was 0.11 nM (n = 36 samples, 108 titrations), and three times this standard deviation estimates a detection limit for ligand concentrations of 0.33 nM. For log K1 values of this ligand class, from the same profiles, the average standard deviation was 0.15 (log units).\u00a0
\nRelated files and references:\u00a0
\nBuck, K. N., L. J. A. Gerringa, and M. J. A. Rijkenberg. 2016. An intercomparison of dissolved iron speciation at the Bermuda Atlantic Time-series Station (BATS): Results from the GEOTRACES Crossover Station A. Frontiers in Marine Biogeochemistry 3: article 262.
\nBuck, K. N., M. C. Lohan, C. J. M. Berger, and K. W. Bruland. 2007. Dissolved iron speciation in two distinct river plumes and an estuary: Implications for riverine iron supply. Limnology and Oceanography 52: 843-855.
\nBuck, K. N., J. W. Moffett, K. A. Barbeau, R. M. Bundy, Y. Kondo, and J. Wu. 2012. The organic complexation of iron and copper: an intercomparison of competitive ligand exchange- adsorptive cathodic stripping voltammetry (CLE-ACSV) techniques. Limnology and Oceanography: Methods 10: 496-515.
\nBuck, K. N., B. Sohst, and P. N. Sedwick. 2015. The organic complexation of dissolved iron along the U.S. GEOTRACES (GA03) North Atlantic Section. Deep-Sea Research 116: 152-165.
\nCutter, G., P. Andersson, L. Codispoti, P. Croot, R. Francois, M. Lohan, H. Obata, and M. Rutgers van der Loeff [eds.]. 2010. Sampling and sample-handling protocols for GEOTRACES cruises version 1.0 ed. GEOTRACES.
\nCutter, G. A., and K. W. Bruland. 2012. Rapid and noncontaminating sampling system for trace elements in global ocean surveys. Limnology and Oceanography: Methods 10: 425-436.
\nOmanovi\u0107, D., C. Garnier, and I. Pi\u017eeta. 2015. ProMCC: An all-in-one tool for trace metal complexation studies. Marine Chemistry 173: 25-39.
\nRue, E. L., and K. W. Bruland. 1995. Complexation of iron(III) by natural organic ligands in the Central North Pacific as determined by a new competitive ligand equilibration adsorptive cathodic stripping voltammetric method. Marine Chemistry 50: 117-138.
\nBCO-DMO Processing Notes:
\n- added conventional header with dataset name, PI name, version date
\n- column names reformatted to comply with BCO-DMO standards
\n- replaced 1\u00a0blank cell\u00a0with nd, 'no data'\u00a0
\n- joined with the\u00a0master file to get columns: GEOTRC_EVENTNO, GEOTRC_INSTR, ISO_DATETIME_UTC_START_EVENT
Additional GEOTRACES Processing
\nAs was done for the GEOTRACES-NAT data, BCO-DMO added standard US GEOTRACES information, such as the US GEOTRACES event number, to each submitted dataset lacking this information. To accomplish this, BCO-DMO compiled a 'master' dataset composed of the following parameters:
\ncruise_id, EXPOCODE,SECT_ID, STNNBR, CASTNO, GEOTRC_EVENTNO, GEOTRC_SAMPNO, GEOTRC_INSTR, SAMPNO, GF_NO, BTLNBR, BTLNBR_FLAG_W, DATE_START_EVENT, TIME_START_EVENT, ISO_DATETIME_UTC_START_EVENT, EVENT_LAT, EVENT_LON, DEPTH_MIN, DEPTH_MAX, BTL_DATE, BTL_TIME, BTL_ISO_DATETIME_UTC, BTL_LAT, BTL_LON, ODF_CTDPRS, SMDEPTH, FMDEPTH, BTMDEPTH, CTDPRS, CTDDEPTH.
\nThis added information will facilitate subsequent analysis and inter comparison of the datasets.
\nBottle parameters in the master file were taken from the GT-C_Bottle and ODF_Bottle datasets. Non-bottle parameters, including those from GeoFish tows, Aerosol sampling, and McLane Pumps, were taken from the TN303 Event Log (version 30 Oct 2014). Where applicable, pump information was taken from the PUMP_Nuts_Sals dataset.
\nA standardized BCO-DMO method (called \"join\") was then used to merge the missing parameters to each US GEOTRACES dataset, most often by matching on GEOTRC_SAMPNO or on some unique combination of other parameters.
\nIf the master parameters were included in the original data file and the values did not differ from the master file, the original data columns were retained and the names of the parameters were changed from the PI-submitted names to the standardized master names. If there were differences between the PI-supplied parameter values and those in the master file, both columns were retained. If the original data submission included all of the master parameters, no additional columns were added, but parameter names were modified to match the naming conventions of the master file.
\nSee the dataset parameters documentation for a description of which parameters were supplied by the PI and which were added via the join method.