Soils were collected from the frozen permafrost layer (> 60 cm below the surface) at five sites underlying moist acidic tussock or wet sedge vegetation, and on three glacial surfaces on the North Slope of Alaska during summer 2018. Soil cores were collected at Imnavait wet sedge tundra using a SIPRE corer, and the permafrost layer (1.0 – 1.3 m below the surface) was separated from the soil core using a knife. At the other four sites, 1 m x 1 m x 1 m soil pits were excavated using a jack hammer, shovels, and pickaxe. Soil sampling at each site took place over the course of one day. From each site, an equal mass of soil (~2.5 kg) was placed in four Ziploc bags (1 gallon) and then each soil sample was quintuple-bagged. Following collection, soil samples were immediately transferred to coolers in the field and then stored in freezers at the Toolik Field Station for ≤ 4 weeks until overnight shipment on dry ice to the Woods Hole Oceanographic Institution (WHOI). All soil samples were frozen upon arrival at WHOI and immediately placed into freezers until leachate preparation.
Dissolved organic carbon (DOC) was leached from each permafrost soil at WHOI as described in the following five steps. First, frozen soil in one or two Ziploc bags was broken into smaller pieces inside the bag using a clean chisel. Second, 0.8 to 3.3 kg of frozen soil was transferred to a new Ziploc bag (1 gallon) and then thawed in a chest cooler at 4 °C for up to 20 hours. Third, the thawed permafrost soil was added to five liters of MilliQ water (Millipore Simplicity ultraviolet, UV, system) in a MilliQ-rinsed high density polyethylene (HDPE) bucket (5 gallon). Each bucket was covered with a HDPE lid and allowed to leach at 4 °C for 24 hours. Fourth, the permafrost leachate was filtered through a sieve with 60 mm nylon mesh screening (Component Supply) into a new, MilliQ-rinsed 5 gallon HDPE bucket and then placed in the chest cooler at 4 °C for ≤ 1 day to allow suspended particles to settle before additional filtration. Fifth, the 60-mm filtered leachate was filtered through 10 mm (Geotech Environmental Equipment, Inc.) and then finally through 0.2 mm (Whatman), MilliQ-rinsed high-capacity cartridge filters. Four liters of the final 0.2-mm filtered permafrost leachate (now referred to as permafrost leachate) were then transferred to a precombusted (450 °C; 4 h) 4 L glass amber bottle and kept at 4 °C prior to carbon isotope analyses.
The radiocarbon (14C) and stable carbon (13C) isotopic compositions of DOC were analyzed from each permafrost leachate at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at WHOI following Beaupré et al. (2007). Each permafrost leachate was diluted with UVC-oxidized MilliQ water (1.5 hrs; 1200 W medium pressure mercury arc lamp) to achieve a total carbon (C) mass greater than 800 μg and less than 2000 μg. The diluted permafrost leachate was acidified with UVC-oxidized trace-metal grade phosphoric acid (85%) to pH < 2 in a precombusted quartz reactor (450 °C; 4 hrs) and the dissolved inorganic carbon (DIC) was purged with high-purity helium gas in the dark. The DOC was then oxidized with UVC light to DIC for 4 hours (1200 W medium pressure mercury arc lamp), and the resultant carbon dioxide (CO2) was extracted cryogenically (Beaupré et al., 2007). On average, 1370 ± 240 μg of C were extracted cryogenically from each permafrost leachate (± 1 standard error, SE; n = 6). A subsample of the CO2 was analyzed for 13C using a VG Prism-II or Optima stable isotope ratio mass spectrometer (instrumental precision of 0.1‰; Coplen et al., 2006), and the δ13C (‰) was calculated as follows:
δ13C = (13Rsample/13Rstandard – 1)
where 13R is the isotope ratio of a sample or standard (VPDB), as defined by:
13R = (13C/12C)
The remaining CO2 was reduced to graphite with H2 and an iron catalyst, and then analyzed for 14C isotopic composition using an accelerator mass spectrometer at the NOSAMS facility (Longworth et al., 2015). The Δ14C (‰) of DOC was calculated from the fraction modern as previously described (Stuiver & Polach, 1977; McNichol et al., 2001) using the oxalic acid I standard (NIST-SRM 4990). Δ14C analyses of DOC had an instrumental precision of 2-6‰ (Longworth et al., 2015; McNichol et al., 2001).
DOC leached from one permafrost soil (Toolik moist acidic tundra) was prepared and analyzed for 14C and 13C twice to quantify the standard error of duplicate analyses. Δ14C and δ13C analyses of DOC had standard errors of 1‰ and 0.1‰, respectively (1 SE; n = 2). A procedural blank was quantified by oxidizing MilliQ water with UVC light in a precombusted quartz reactor (450 °C; 4 hrs) for 1.5 hours, acidifying to pH < 2, and purging the DIC as described above. Residual CO2 was extracted cryogenically and its concentration was quantified manometrically. The procedural blank was 4 μg of C, which was < 0.5% of the total carbon masses extracted from the permafrost leachates. When available, Δ14C and δ13C are reported as the average ± 1 SE of experimental replicates (n = 2).
References:
Beaupré, S. R., E. R. M. Druffel, S. Griffin. 2007. A low-blank photochemical extraction system for concentration and isotopic analyses of marine dissolved organic carbon. Limnol. Oceanogr.: Methods, 10.4319/lom.2007.5.174
Bowen, J.C., C. P. Ward, G. W. Kling, R. M. Cory.. Arctic amplification of global warming strengthened by sunlight oxidation of permafrost carbon to CO2. In review.
Coplen, T. B., W. A. Brand, M. Gehre, M. Gröning, H. A. J. Meijer, B. Toman, R. M. Verkouteren. 2006. New guidelines for δ13C measurements. Anal. Chem., 10.1021/ac052027c
Longworth, B. E., K. F. von Reden, P. Long, M. L. Roberts. 2015. A high output, large acceptance injector for the NOSAMS Tandetron AMS system. Nucl. Instr. Meth. Phys. Res. B, 10.1016/j.nimb.2015.04.005
McNichol, A. P., A. J. T. Jull, G. S. Burr. 2001. Converting AMS data to radiocarbon values: Considerations and conventions. Radiocarbon, 10.1017/S0033822200038169
Stuiver, M., H. A. Polach. 1977. Discussion: Reporting of 14C data. Radiocarbon, 10.1017/S0033822200003672
