The sampling campaign was conducted during the austral summer in January 2017 as a part of the 58th Japanese Antarctic Research Expedition. There are several ice-free areas on Sôya Coast with hundreds of lakes and several streams, which were formed during the last 7,000 years under the glacier retreat and iso-static uplifts that occurred after the Last Glacial Maximum. The Langhovde ice-free area is located ~25 km away from Japan's Syowa Station. The Yukidori and Yatsude Valleys in Langhovde run from the edge of the continental ice sheet to Lutzow-Holm Bay. At each stream of the valleys, water samples were directly collected into 550-mL volume polyethylene terephthalate bottles after rinsing more than three times with the collected water. The collected waters were filtered in a field laboratory within a few hours after sampling. Samples were filtered through pre-combusted (450dc, >3 h) glass-fiber filters (nominal pore size 0.3mm, GF-75, Advantec, Tokyo, Japan) into pre-combusted glass bottles (see doi:10.1016/J.WATRES.2019.114901). The filtered samples were immediately spiked with sodium azide (final concentration of 0.02%) for preservation, stored dark at 5dc, and shipped back to Japan. DOM was extracted and desalted prior to FT-ICR MS analysis following an established method using cartridges filled with a styrene-divinylbenzene copolymer (Agilent Bond Elut PPL, 100 mg). Depending on the DOC concentration, the volume used for extraction was adjusted such that 4 µmol C was applied to each cartridge. We performed a mass spectrometric analysis of DOM extracts via FT-ICR MS on a 15 Tesla solariX XR Fourier-transform ion cyclotron resonance mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany). The system was equipped with an electrospray ionization source (ESI, Bruker Apollo II) applied in negative ionization mode. Methanol extracts were mixed with ultrapure water (50:50 v/v) for FT-ICR MS analysis and diluted to a final DOC concentration of 2.5 mg C/L. 200 single scans with an ion accumulation time of 0.1 s were recorded over a mass range of m/z 92 to 2,000 Da and added to one spectrum. Data processing, molecular formulae assignments, and compound category classifications were done with the software package ICBM-OCEAN.
