DOI: 10.5285/66aaf3c8-fa58-41de-8ef1-480a2e125408
title: ARISE project - Work package 2: Stable nitrogen isotopes of nitrate in sea water, and of bulk tissue and amino-acids of muscle of harp and ringed seals from the Arctic and sub-Arctic

description: This dataset includes stable nitrogen isotopes of 1- nitrate in sea water (d15NNO3) from three sites (i.e. Southern Barents Sea, Northern Barents Sea, Greenland Sea) and 2- of bulk tissue (d15Nbulk) and compound specific stable nitrogen isotopes on amino acids (d15NAA) measured in adult harp seals from five sites (i.e. Southern Barents Sea, Northern Barents Sea, Greenland Sea, Labrador shelf, Baffin Island) and in adult ringed seals from two sites (Baffin Island and Canadian Archipelago) in the Arctic and sub-Arctic. The sea water samples for analyses of d15NNO3 were collected in 2017 and 2018 as part as two ARISE cruises (JR16006 and JR17005). The seal samples were collected from 2015 to 2019 as part of Norwegian commercial sealing and student field courses from the University of Tromso in Norway (Northern and Southern Barents Sea, Greenland Sea) and the Inuit subsistence and commercial harvests in Canada (Labrador Sea, Baffin Island, Canadian Archipelago). Analyses of d15NNO3 were carried out at the University of Edinburgh, UK. Analyses of d15Nbulk and d15NAA of seal muscle tissue were carried out at the Liverpool Isotopes for Environmental Research laboratory, University of Liverpool. Results are reported here in standard delta-notation per-mil relative to atmospheric N2. Funding was provided by the ARISE project (NE/P006035/1 and NE/P006310/1), as part of the Changing Arctic Ocean programme, funded by the UKRI Natural Environment Research Council (NERC).
description: Sample preparation for bulk d15N of seal muscle tissue: 0.5 mg of freeze-dried and homogenized sample was precisely weighed (± 1 µg) and sealed in a tin capsule. Sample preparation for d15N analyses on amino acids of seal muscle tissue: 7 mg of freeze-dried and homogenized sample was hydrolyzed in reaction vessels (6M, mL, 200 µl, 100 °C for 22h). L-Norleucine (Sigma-Aldrich) was added to each sample as an internal standard (80 µl of 5 mg mL-1). On cooling, the samples were transferred into a nanosep centrigal device (45 µm nylon filters) and centrifuged (10 000 rpm for 1 min). Samples were then transferred into clean micro-reaction vessels and lipids were extracted by addition of n-hexane:DCM (3:2 v/v, 0.5 mL). Each sample was shaken by hand for 10 seconds in order for the hydrolysate and solvents to mix. The organic solvents were removed. This was repeated 3 times. Hydrolysates were blown down under N2 for 2 min in order to ensure that all organic solvents were removed and were frozen at -80 °C prior to lyophilization. The amino acids were propylated in 0.25 mL of acidified isopropanol solution (prepared by addition of acetyl chloride to anhydrous isopropanol (1:4 v/v) in an ice bath) at 100 °C for 1 h. The reaction was quenched in a freezer and reagents were evaporated under a gentle stream of N2, DCM was added (3 x 0.25 mL) and evaporated to remove excess reagents. Amino acid methyl esters were then treated with 1 mL of a mixture of acetone:triethylamine:acetic anhydride (5:2:1, v/v), which was added to each sample, and heated at 60 °C for 10 min. Following acetylation, the reagents were evaporated under a gentle stream of N2 and were dissolved in 2 mL of ethyl acetate, to which 1 mL of saturated NaCl solution was added. Phase separation was enabled via mixing and the organic phase was collected; separation was repeated 3 times with addition of 2 mL ethyl acetate. Residual water was removed from the combined organic phases by passing through a Pasteur pipette plugged with glass wool and filled with MgSO4. Finally, samples were evaporated under N2 and the derivatized amino acids were dissolved in DCM and stored at -20 °C until analysis. Sample preparation for d15N analyses of nitrate in sea water (d15NNO3): Seawater for nitrate analysis from the European Arctic (Northern Barents Sea, Southern Barents Sea and Greenland Sea) was collected below the euphotic zone as part of the NERC Changing Arctic Ocean program, from the RRS James Clark Ross in July-August 2017 (JR16006) and May-June 2018 (JR17005). Seawater was collected using a 24-position stainless steel rosette equipped with a an SBE911plus CTD and 20 L OTE bottles. Instrumental analysis for bulk d15N: Samples were analysed using an elemental analyser (Costech) coupled to Delta V isotope ratio mass spectrometer (IRMS; Thermo-Scientific). Isotope values are reported in standard delta-notation per mil relative to atmospheric N2. Instrumental analysis for d15N of amino acids (d15NAA): d15NAA values were determined using a Trace Ultra gas chromatograph (GC) coupled to a Delta V Advantage IRMS with a ConFlo IV interface (Cu/Ni combustion reactor held at 1000 °C, Thermo Fisher). A liquid nitrogen trap was added after the reduction oven to remove CO2 from the sample stream. The separation of amino acids was achieved using an HP Innowax capillary column (30 m x 0.25 mm i.d. x 0.5 µm film thickness, Agilent). Each sample was introduced to the column using a split/splitless injector set at 260 °C. The GC was programmed as follows: held at 50 °C for 2 min, 10 °C min-1 to 180 °C and 6 °C min-1 to 260 °C, and held for 16.7 min. The carrier gas was ultra-high purity helium (flow 1.4 mL.min-1). The ion intensities of m/z 28, 29 and 30 were monitored and the d15N of each amino acid peak were automatically computed (Isodat version 3.0; Thermo Fisher) by comparison with a standard reference N2 gas, which was repeatedly measured (x4) at the beginning and the end of each sample analysis. All results are reported in per mil relative to N2. Instrumental analysis for d15N of nitrate in sea water (d15NNO3): d15NNO3 from the European Arctic (Northern Barents Sea, Southern Barents Sea and Greenland Sea) were determined at the University of Edinburgh, UK, using the denitrifier method (Sigman et al. 2001, Casciotti et al. 2002) and following Geotraces protocols (Schlitzer et al. 2018).
description: Instrumental analysis for bulk d15N: Samples were analysed using an elemental analyser (Costech) coupled to Delta V isotope ratio mass spectrometer (IRMS; Thermo-Scientific). Isotope values are reported in standard delta-notation per mil relative to atmospheric N2. Instrumental analysis for d15N of amino acids (d15NAA): d15NAA values were determined using a Trace Ultra gas chromatograph (GC) coupled to a Delta V Advantage IRMS with a ConFlo IV interface (Cu/Ni combustion reactor, Thermo Fisher). Instrumental analysis for d15N of nitrate in sea water (d15NNO3) d15NNO3 from the European Arctic (Northern Barents Sea, Southern Barents Sea and Greenland Sea) were determined at the University of Edinburgh, UK, using the denitrifier method (Sigman et al. 2001, Casciotti et al. 2002) and following Geotraces protocols (Schlitzer et al. 2018).
description: For details please see the attached text file 'Data_quality'.