Sampling: Full depth and super station samples were collected from the ship's rosette. Samples were filtered through 0.8/0.45 um acropak5 00 filter cartridges into triple-rinsed 60 mL HDPE bottles. Demi station samples were collected from the GETORACES Go-Flo rosette. Samples were filtered through 0.2 um acropak capsule filters into triple-rinsed 60 mL HDPE bottles. Each sample was collected in triplicate. Samples were frozen at -20 C immediately and stored frozen until analysis.
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Analysis: Samples were analyzed for nitrate d15N using the denitrifier method (Sigman et al., 2001; Casciotti, et al., 2001, with technical updates described by McIlvin and Casciotti, 2011). In the Casciotti Lab, Nitrate isotope reference materials USGS32 and USGS34 were used to calibrate d15N. Each sample was analyzed 2-6 times. Mean and standard deviations are reported for each sample. Samples were analyzed either on a DeltaPLUS XP or DeltaV PLUS isotope ratio mass spectrometer (IRMS), both with custom-built inlets described in McIlvin and Casciotti, 2011. Where possible (concentrations exceeded 2 uM), samples and standards were analyzed in 20 nanomole aliquots. When NO3- concentrations were below 2 uM, samples were injected in maximum volume (10 mL) and calibrated using interpolations between standard curves of 5 and 20 nano moles NO3-. Samples containing nitrate concentrations lower than 0.3 uM were not analyzed.\u00a0Samples containing measurable NO2- concentrations were treated with sulfuric acid prior to analysis (Granger and Sigman, 2009).
Data Processing: Raw data were calibrated to USGS standards analyzed 6x with each batch of samples. The measured values of these reference materials were used to calibrated the measured d15N\u00a0values to the atmospheric N2 reference scale (McIlvin and Casciotti, 2011).
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BCO-DMO Processing:
\n- modified parameter names to conform with BCO-DMO naming conventions;
\n- replaced blanks (no data) with 'nd';
\n- replaced -999 with 'nd';
\n- joined to EPZT master file (details below).
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Additional GEOTRACES Processing by BCO-DMO:
\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,\u00a0CASTNO, 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, \u00a0BTL_DATE, BTL_TIME, BTL_ISO_DATETIME_UTC, BTL_LAT, BTL_LON, ODF_CTDPRS, SMDEPTH, FMDEPTH, BTMDEPTH, CTDPRS, CTDDEPTH.
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This added information will facilitate subsequent analysis and inter-comparison of the datasets.
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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.
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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.
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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.
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See the dataset parameters documentation for a description of which parameters were supplied by the PI and which were added via the join method.