Dataset: GP16 Selenium iodine and arsenic species from Super GeoFish

Samples analyzed in triplicate and means computed. Standard additions of selenite were used for daily calibration on a representative sample and its slope applied to subsequent samples.

Iodide concentrations were determined using cathodic square wave stripping analysis following Luther at al. (1988) and Tian and Nicolas (1995). A BASi model CGME electrode was used as follows: 10 mL sample and 100 mL 0.1% Triton, 5 min purge with oxygen free nitrogen, 0.2 mL addition of 1 M sodium sulfite to remove any remaining oxygen, followed by 1 min of purging. The voltammogram was recorded three times under the following conditions: drop size 11, deposition time 90 s, quiet time 5 s, scan increment 2 mV, scan range –120 mV to -700 mV, frequency 125 Hz and amplitude 25 mV. Precision was 1.9% RSD (n=10) and sensitivity 4.07 nA/nM. The method was calibrated using standard additions, with standards made from AnalR grade potassium iodide.

Iodate was determined with the Wong and Brewer (1974) method using a spectrometer (Shimadzu 160 UV). In brief, a sample in a 10 cm quartz cuvette is acidified with sulfuric acid containing sulfanilamide to remove nitrite interference (Chapman and Liss 1977; Rue, Smith et al. 1997) and iodide added to form tri-iodide that is then determined at a wavelength of 535 nm. Standard addition of potassium iodate (Analar grade) was used for daily calibration on a representative sample and its slope applied to subsequent samples. Duplicate samples were run for every depth; triplicate samples were determined for the 0 addition seawater to allow an estimate of precision. Precision was better than 5% RSD at concentration above 100 nmol/L.

Dissolved selenite, selenite+ selenate, and total dissolved selenium concentrations were determined in triplicate using selective hydride generation/atomic absorption spectrometry following Cutter (1978), Cutter (1983), and Cutter and Bruland (1984). The standard addition method of calibration was used to assure accuracy (one sample on an analysis day was used and its slope applied to the other samples on that day). Selenate was calculated as the difference between selenite+selenate and selenite determinations, while organic selenide was the difference between total dissolved selenium and selenite+selenate determinations. Precision was always less than 10% RSD, but typically less than 5% RSD at the observed concentrations. Detection limits were 0.02 nmol/L for all selenium species.

 Particulate elemental selenium concentrations were determined using a sulfite leach following Velinsky and Cutter (1990) and analyzed like selenite+selenate following Cutter (1978).  Precision was ca. 10% RSD (n=3). 

Arsenic speciation was determined using selective hydride generation, liquid nitrogen-cooled trapping, and then revolatilization and determination with gas chromatography/photoionization detection (Cutter et al., 1991; Cutter and Cutter, 2006). Calibration performed daily via the standard additions method, with a minimum of 4 additions of AsIII and AsV. The slope from the linear fit to these data was then applied to all samples for that day. Detection limits were 25 pmol/L for As(III) and As(V). Precision was better than 8% RSD.

DMO notes (for current data version  23 Dec 2016):
* new version of data based on re-submission
* removed column name cruise_id as it was actually the section id (EPZT) and all data came from the same cruise TN303
* did not add a column "SECT_ID" for value "EPZT" as was done in previous data version
* padded time column with leading 0 if hours was less than 2 digits (e.g. 836-> 0836)

DMO notes (for data version 03 Jan 2017):
* changed "Cruise" (value EPZT throughout) to SECT_ID
* added new column "cruise_id" with value TN303
 * DATE,TIME to PI_DATE,PI_TIME as this did not match the event times (~15-20 min difference)
* variable names changed to conform to GEOTRACES conventions
* padded time column with leading 0 if hours was less than 2 digits (e.g. 836-> 0836)

Additional GEOTRACES Processing:

As 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:

cruise_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.

This added information will facilitate subsequent analysis and inter comparison of the datasets.

Bottle 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.

A standardized BCO-DMO method (called "join") was then used to merge the missing parameters to each US GEOTRACES dataset, most often by matching on sample_GEOTRC or on some unique combination of other parameters.

If 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.

See the dataset parameters documentation for a description of which parameters were supplied by the PI and which were added via the join method.

References: 

Chapman, P. and P. S. Liss (1977). "Effect of nitrite on spectrophotometric determination of iodate in seawater." Marine Chemistry 5(3): 243-249. http://dx.doi.org/10.1016/0304-4203(77)90019-6

Cutter, G. A., and K. W. Bruland. "The marine biogeochemistry of selenium: A re‐evaluation." Limnology and Oceanography 29.6 (1984): 1179-1192. http://dx.doi.org/10.4319/lo.1984.29.6.1179

Cutter, G.A. and L.S. Cutter. 2006. The biogeochemistry of arsenic and antimony in the North Pacific Ocean. Geochem. Geophys. Geosystems (G3), 7, Q05M08, http://dx.doi.org/10.1029/2005gc001159

Cutter, Gregory A. "Elimination of nitrite interference in the determination of selenium by hydride generation." Analytica Chimica Acta 149 (1983): 391-94. http://dx.doi.org/10.1016/s0003-2670(00)83200-6

Cutter, Gregory. "Species Determination of Selenium in Natural Waters." Analytica Chimica Acta 98 (1978): 59-66. http://dx.doi.org/10.1016/s0003-2670(01)83238-4

Cutter, L.S., G.A. Cutter, and M.L.C. San Diego McGlone. 1991. Simultaneous determination   of inorganic arsenic and antimony species in natural waters using selective hydride generation with gas chromatography/photoionization detection. Anal. Chem. 63:1138 1142. http://dx.doi.org/10.1021/ac00011a015

Luther, G. W., C. B. Swartz, et al. (1988). "Direct determination of iodide in seawater by cathodic stripping square-wave voltammetry." Analytical Chemistry 60(17): 1721-1724. http://dx.doi.org/10.1021/ac00168a017

Rue, E. L., G. J. Smith, et al. (1997). "The response of trace element redox couples to suboxic conditions in the water column." Deep-Sea Research Part I-Oceanographic Research Papers 44(1): 113-134. http://dx.doi.org/10.1016/s0967-0637(96)00088-x

Tian, R. C. and E. Nicolas (1995). "Iodine speciation in the northwestern mediterranean-sea - method and vertical profile." Marine Chemistry 48(2): 151-156. http://dx.doi.org/10.1016/0304-4203(94)00048-i

Velinsky, D.J., and G.A. Cutter. "Determination of elemental selenium and pyrite-selenium in sediments." Analytica Chimica Acta 235 (1990): 419-425. http://dx.doi.org/10.1016/s0003-2670(00)82102-9

Wong, G. T. F. and P. G. Brewer (1974). "Determination and distribution of iodate in south-atlantic waters." Journal of Marine Research 32(1): 25-36