Delta 15N of nitrate from R/V Knorr cruise KN199-04 & KN204-01 in the Subtropical northern Atlantic Ocean from 2010-2011 (U.S. GEOTRACES NAT project) (NCEI Accession 0277276)

Acquisition Description:
Sampling:
Full depth and super station samples were collected from the ship's rosette. Samples were filtered through 0.8/0.45 um acropak500 filter cartridges into triple-rinsed 60 mL HDPE bottles. Demi station samples were collected from the GEOTRACES 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.

Analysis:
Samples were analyzed for nitrate d15N and d18O using the denitrifier method (Sigman et al., 2001; Casciotti, et al., 2001, with technical updates described by McIlvin and Casciotti, 2011). Nitrate isotope reference materials USGS32, USGS34, and USGS35 were used to calibrate d18O and USGS32 and USGS34 to calibrate d15N. Each sample was analyzed 2-6 times. Mean and standard deviations are reported for each sample. All samples were analyzed on a DeltaPLUS XP isotope ratio mass spectrometer, with custom-built inlet 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-. Some samples contained nitrate concentrations lower than the typical analytical window (300 nM NO3-), but were analyzed nonetheless. Samples analyzed with concentrations below 0.3 uM are were flagged as "low concentration". These samples contained a subset with major ion peak areas (m/z 44) less than 1 volt-second, which are flagged at "low area". Two of these samples are flagged as having elevated d18O-NO3 values relative to their d15N-NO3 values. The flagged samples should be treated with caution.

This version (16 March 2013) has not been thoroughly intercalibrated with data from the Sigman Lab. Currently, there exists no detachable d15N offset between the datasets. However, Casciotti d18O data are approximately 0.6 permil higher than Sigman d18O data.