Sedimentary geochemistry of a clearwater Arctic lake

Surface area analysisBrunauer-Emmett-Teller (B.E.T.) specific surface area (Brunauer et al., 1938) was obtained by a 90-point N2 (g) adsorption/desorption isotherm on a TriStar 3000 surface area and porosity analyzer (Micromeritics). For surface and bottom samples, we used the Barrett-Joyner-Halenda (B.J.H.) model (Barrett et al., 1951) to estimate cumulative pore volume and average size.Thermogravimetric analysis (TGA)The thermal stability of bulk sediments was determined by TGA (Q5000, TA Instruments), where the mass loss was measured in relation to the initial mass (~10 mg). Samples were heated in the range of 100ºC–1000ºC at a rate of 10ºC min-1, starting with a constant temperature at 100ºC for 5h, under a constant stream of N2 (40 mL min-1).Elemental analysis and stable isotope analysisElemental and stable isotope analyses (δ13C, δ15N) of C and nitrogen (N) were performed at Stable Isotope Laboratory (Sveriges lantbruksuniversitet, Umeå, Sweden) using elemental analyzer (Flash EA 2000) coupled to a continuous flow isotope ratio mass spectrometer (DeltaV, both Thermo Fisher Scientific).ExtractionsWe used alkaline extraction that allows for desorption of OC from goethite. Approximately ~0.06 g of the sample was mixed with 12 mL Milli-Q water and put in an ultrasonic bath for 10 min, followed by an addition of NaOH until reaching pH ~10.5. The suspension was left for 24 h in an end-over-end sample rotator and was then centrifuged for 10 min at 4000 rpm. The supernatant was filtered through a 0.45 µm syringe filter (Filtropur S, Sarstedt), then analyzed using a FORMACS HT-I TOC analyzer (Skalar Analytical).