We use the Optimum Multiparameter Analysis (OMPA) method to determine water mass percentages along the GP16 transect. This involves the use of hydrographic measurements from GP16 (temperature, salinity, oxygen, and nutrients), which can be found in a separate BCO-DMO dataset (see CTD - ODF Bottle). For samples having potential density anomalies greater than 27 kg m-3, the OMPA was conducted according to the procedure outlined by Tomczak (1981), Tomczak and Large (1989), and others. For samples in the thermocline, defined as those samples having densities between 26 and 27, we use the 'thermocline array' method to determine the water mass fractions, according to the procedure outlined by Jenkins et al. (2015). Samples having densities less than 26 kg m-3 were not included in the water mass analysis. The uncertainty in water mass percentage is calculated using a Monte Carlo routine, where the defined properties of each water mass are varied to encompass the reported range of properties in that water mass. Lastly, the signal to noise ratio is calculated by dividing the percentage of given water mass in a each sample by the uncertainty of that water mass.
BCO-DMO 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:
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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.
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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.