Dataset: GP16 Molybdenum dissolved

Water column samples were taken from the GEOTRACES carousel and filtered through pre-cleaned, 0.2 um Pall Acropak Supor filter capsules as described elsewhere (e.g., Cutter et al., 2012; Hatta et al., 2015). Near surface water samples were collected using an underway towed-fish pumped seawater system (Bruland et al., 2005) with samples filtered through sequential 0.45 um Osmonics and 0.2 um Polycarbonate (PCTE) cartridge filters. Filtered water was collected in 125 mL HDPE bottles (Nalgene) that had been precleaned by soaking in hot 1.2 M HCl (reagent grade) for at least 8 h with subsequent thorough rinsing with ultrapure distilled deionized water (Barnstead E-pure). Samples were acidified in a laminar flow bench aboard ship using 0.5 mL of ultrapure HCl per 125 mL sample.

Molybdenum, was measured using a ThermoFisher Element 2 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) using an isotope dilution method similar to the one described for Ba by Jacquet et al. (2005). Aliquots (50 uL) of each sample were spiked with 25 uL of a 95Mo-enriched solution (~770 nM) and then diluted 30-fold with 0.2 um ultrapure filtered water. A sample of 96.8% enriched 95Mo was obtained from Oak Ridge National Laboratories for use as the enriched isotope spike. The ICP-MS was operated in low resolution and both 95Mo and 98Mo were determined. The samples were bracketed every 10 samples with a blank and the spike 95Mo solution. The volumes of the spikes, samples, and dilution water were accurately assessed by calibrating each pipette by weight. The reproducibility error of this method was estimated by comparing samples collected at the same depths on different casts at the same station. For 31 pairs of these replicate samples, the average absolute deviation was 2.3 nmol/kg or typically 2.1%. Repeated runs of runs of US GEOTRACES intercalibration samples and in-house reference solutions suggest a precision of ±2.3%; the limit of detection for molybdenum was ~1 nmol/kg.

Additional Information (including references) can be found in GEOTRACES Intercalibration Document

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.

BCO-DMO Processing:
- molybdenum concentration MO_D_CONC_BOTTLE and MO_D_CONC_FISH rounded to one decimal place
- modified parameter names to conform with BCO-DMO and GEOTRACES naming conventions
- split GeoFISH data and bottle sample data into separate columns so GEOTRACES naming conventions of _BOTTLE and _FISH would be appropriate
- replaced blanks (missing data) with 'nd' ('no data')
- New data version 10 Nov 2016: Removed a duplicate line for sample number 3494
- 04 March 2019: removed embargo on dataset.