Dataset: GP16 Dissolved Organic Carbon

Carlson Lab Process:
Analytical runs were assessed daily based on internal standards, blanks and reference calibrations as described Carlson et al. 2010. Briefly, DOC samples were analyzed via high temperature combustion using a Shimadzu TOC-V or TOC-L in shore based laboratory at the University of California, Santa Barbara. The operating conditions of the Shimadzu TOC-V have been slightly modified from the manufacturer’s model system. The condensation coil has been removed and the headspace of an internal water trap was reduced to minimize the system’s dead space. The combustion tube contains 0.5 cm Pt pillows placed on top of Pt alumina beads to improve peak shape and to reduce alteration of combustion matrix throughout the run. CO2 free carrier gas is produced with a Whatman® gas generator (Carlson et al. 2004, 2010). Sample are drawn into 5 ml injection syringe and acidified with 2M HCL (1.5%) and sparged for 1.5 minutes with CO2 free gas. Three to five replicate 100 µl of sample are injected into combustion tube heated to 680 degrees C. The resulting gas stream is passed though several water and halide traps, including an added magnesium perchlorate trap. The CO2 in the carrier gas is analyzed with a non-dispersive infrared detector and the resulting peak area is integrated with Shimadzu chromatographic software. Injections continued until the at least three injection meet the specified range of a SD of 0.1 area counts, CV ≤2% or best 3 of 5 injections.

Extensive conditioning of the combustion tube with repeated injections of low carbon water (LCW) and deep seawater is essential to minimize the machine blanks. After conditioning, the system blank is assessed with UV oxidized low carbon water. The system response is standardized daily with a four-point calibration curve of potassium hydrogen phthalate or glucose solution in LCW. All samples are systematically referenced against low carbon water and low and high reference sea waters typically deep Sargasso Sea reference waters (2600 m) and surface Sargasso Sea water every 6 – 8 analyses (Carlson et al. 2010).  The standard deviation of the deep and surface references analyzed throughout a run generally have a coefficient of variation ranging between 1-3% over the 3-7 independent analyses (number of references depends on size of the run).

Daily reference waters were calibrated with DOC CRM provided by D. Hansell (University of Miami; Hansell 2005).

DOC calculation:
µMC  =  (average sample area – average machine blank area) / (slope of std curve)

Data from each DOC run were then streamed and summarized into a master file (excel based spreadsheet) which was merged with sample collection metadata (bottle & CTD data from ODF and GT-C bottle files). Data were visualized using Ocean Data View (Schlitzer, R., Ocean Data View, http://odv.awi.de, 2016), assessed by individual station and profile, and WOCE quality flags added to identify outliers. Any questionable samples or profiles are first re-analyzed. If reruns were repeatable within limits of instrument precision (1-2 uM C), an average of the two runs is reported. For samples that remain outliers (i.e. a sudden shift of 3 uM C or greater within an oceanic profile), WOCE flags are left in place to designate "bad" or "questionable". 

Data output from the TOC-V instrument is in micromoles carbon per liter. Analytical temperature was also reported for each run and used for accurate conversion to micromoles carbon per kg using supporting sample salinity data from the ODF-bottle file.

A final review of the data was then conducted by both the PI and data manager before data submission.

Additional GEOTRACES Processing by BCO-DMO:
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.