Nitrate Isotopes (CalCOFI Cruise)

Methods

Sample collection

Samples were collected during CalCOFI cruises from 2010-2016. Samples for nitrate and nitrite isotopes were filtered through GF/F filters (0.7 mm pore size) directly from Niskin bottles mounted on a standard conductivity-temperature-depth rosette system (CTD), and frozen until analysis. 

Nitrate isotope analysis

Nitrogen and oxygen isotopes of NO3- were analyzed using the denitrifier method (Casciotti et al., 2002; Sigman et al., 2001). Samples from 2010-2012 were measured according to Rafter & Sigman 2016. Those from 2013-2015 were measured according to Buchwald et al. 2018. Briefly, sample N2O was purified using a customized purge and trap system and analyzed on a continuous flow IsoPrime 100 isotope ratio mass spectrometer (IRMS). NO2- concentrations greater than 2% of NO3-+NO2- were removed by addition of sulfamic acid prior to injection (Granger & Sigman, 2009). Corrections for drift, size and fractionation of O isotopes during bacterial conversion were carried out using NO3- reference materials USGS 32, USGS 34 and USGS 35 (McIlvin & Casciotti, 2011). Typical reproducibility for d15NNO3 and d18ONO3 measurements was ±0.2‰ and 0.4‰, respectively. Samples with less than 1 uM nitrate were not analyzed.

Nitrite isotope analysis

Nitrogen isotopes of NO2- were analyzed using the sodium azide method, in which NO2- is quantitatively converted to N2O by addition of an acetic acid buffered sodium azide solution (McIlvin & Altabet, 2005). Reported d15NNO2 values were normalized against internal nitrite isotope reference standards WILIS 10 (-1.7‰) and WILIS 11 (+57.1‰) (Wankel et al., 2017). Typical reproducibility for d15NNO2 is ±0.1‰.