Debyser, Margot; Tuerena, Robyn E; Francis, Adam; Mahaffey, Claire; Norman, Louisa; Ganeshram, Raja S; Semiletov, Igor P: Stable dissolved and particulate nitrogen isotopes measured on CTD and underway samples during AMK73 in the Laptev Sea [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.995172 (dataset in review)

Dissolved and particulate stable nitrogen isotopic ratios were determined in seawater samples collected during the 73rd expedition onboard RV Akademik Mstislav Keldysh (AMK73) in the Laptev Sea in October 2018 under ice free conditions. Seawater samples were filtered inline from Niskin bottles or the ship's underway system, and frozen at -20˚C until analysis on land at the University of Edinburgh laboratories. Particulate samples were collected via filtering through combusted GF/F filters (0.7μm pore size) until the filter was evenly loaded. The filters were stored at -80°C until analysis at the University of Liverpool. Seawater samples for nitrate isotope analysis were collected following the GEOTRACES protocols (Cutter et al., 2010, 2020) and analysed using the traditional denitrifier method protocols (Casciotti et al., 2002; Sigman et al., 2001; Weigand et al., 2016) at the Oceanography laboratories of the University of Edinburgh (Debyser et al., 2022; Tuerena, Hopkins, Buchanan, et al., 2021). Samples with low concentrations (<2 μM) were analysed at 5 nmol instead on an Elementar Isoprime precision mass spectrometer coupled with an Elementar greenhouse gas device. Measurements were corrected using international reference standards IAEA-N3 and USGS-34 in each run with standard reproducibility of ± 0.2‰ and ± 0.6‰ for δ15N-NO3 & δ18O-NO3 respectively. The isotopic composition of total dissolved N (δ15N-TDN) was measured in N isotope samples by peroxidation of TDN to NO3 and subsequent N isotope analysis through the denitrifier method. This follows the method described in (Knapp et al., 2005) and used for the Bering Sea shelf in Granger et al. (2011). For PON isotopes, Prior to analysis the filters were freeze-dried, wrapped in tin foil cones (OEA Laboratories) and pelletised. δ15N-PN was determined by EA-IRMS using a Costech Instruments Elemental Analyser coupled to Thermo Scientific Delta V Advantage mass spectrometer fitted with Conflo IV gas handling system. The instrumentation was operated using ISODAT 3.0 isotope ratio MS software. L-glutamic acid standards USGS40 and USGS41a (US Geological Survey, Reston Stable Isotope Laboratory) were run alongside the samples and were used both for data processing and to assess the performance of the instrumentation. An internal standard consisting of freeze-dried, finely powdered prawn was also used as a further 'check standard'. A 10-point calibration using standard USGS40 was measured to provide the linear regression equation which was used to derive PON concentrations from the respective peak areas. The concentration (μg L-1) was calculated using the concentration obtained from the whole filter and volume of seawater filtered. The detection limit for PON was 10 μg. This dataset includes salinity, nitrogen concentrations and stable nitrogen isotope signatures of seawater, which provides useful information on the nitrogen biogeochemical cycle of the Laptev Sea, as influenced by the Lena river delta.