Soils were collected from the frozen permafrost layer (> 60 cm below the surface) at four 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 three 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 light exposure experiments.
The radiocarbon (14C) and stable carbon (13C) isotopic compositions of dissolved inorganic carbon (DIC) produced following exposure of DOC to UV and visible light were quantified for each permafrost leachate at WHOI. Each permafrost leachate was equilibrated to room temperature and then placed in up to four precombusted (450 °C; 4 hrs) 600 mL quartz flasks with ground glass stoppers and no headspace. The flasks were exposed to custom-built light-emitting diode (LED) arrays consisting of ten high-powered (≥ 100 mW), narrow-banded (± 10 nm) 309 or 406 nm chips alongside one or two foil-wrapped dark control flasks. Exposure times ranged from 8 to 25 hours to achieve similar concentrations of DIC produced from each permafrost DOC sample and at each wavelength. After LED exposure, foil-wrapped light-exposed and dark control flasks were immediately transferred to foil-wrapped, precombusted 500 mL borosilicate glass bottles (450 °C; 4 hrs) in a N2-filled glove bag, preserved with saturated mercuric chloride, and plugged with gas-tight ground glass stoppers (McNichol et al., 1994). Those bottles were stored in the dark at room temperature for ≤ 1 week until preparation for carbon isotope analyses at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at WHOI (McNichol et al., 1994). Bottles were kept foil-wrapped while each water sample was acidified with trace-metal grade phosphoric acid (85%) to pH < 2 and stripped of DIC with high-purity N2 gas. The resultant carbon dioxide (CO2) was trapped and purified cryogenically, and then its concentration was quantified manometrically. 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 DIC 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 DIC had an instrumental precision of 1-2‰ (Longworth et al., 2015; McNichol et al., 2001).
The Δ14C and δ13C of CO2 produced from the photomineralization of DOC were calculated as follows:
Δ14C-CO2 λ = [(Δ14C-DICLight,λ * [DIC]Light,λ) – (Δ14C-DICDark * [DIC]Dark)] / ([DIC]Light,λ – [DIC]Dark)
δ13C-CO2 λ = [(δ13C-DICLight,λ * [DIC]Light,λ) – (δ13C-DICDark * [DIC]Dark)] / ([DIC]Light,λ – [DIC]Dark)
The Δ14C and δ13C of CO2 produced in each light-exposed flask were calculated relative to one or two dark controls and are reported as the average ± 1 standard error (SE) of replicate values for the experiments conducted alongside two dark controls. The concentration, Δ14C, and δ13C of DIC in the dark controls are reported as the average ± 1 SE of replicate flasks (n = 2). This approach to quantify the Δ14C and δ13C of CO2 produced from photomineralization of organic carbon was previously described in detail for polystyrene (Ward et al., 2019). In this previous study, experimental reproducibility of Δ14C and δ13C of CO2 produced from photomineralization was 5‰ and 0.1‰, respectively (± 1 SE; n = 3).
References:
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.
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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., G. A. Jones, D. L. Hutton, A. R. Gagon. 1994. The rapid preparation of seawater ΣCO2 for radiocarbon analysis at the National Ocean Sciences AMS facility. Radiocarbon, 10.1017/S0033822200040522
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