Sampling methods
Cores were collected near Drew Point, April 10th-19th 2018, using two coring systems. Near-surface cores (upper 4 to 6 m) were acquired using a SIPRE corer (7.5 cm diameter) and cores at depth were acquired using a JIPRO corer (7.5 cm diameter). To capture variations in near-surface permafrost characteristics, we sampled each of the three dominant geomorphic terrain units present in the Drew Point region: primary surface material that has not been reworked by thermokarst lake formation and drainage, an ancient drained thermokarst lake basin (DTLB) (Hinkel et al. 2003; Jones et al., 2012), and a young DTLB (Jones et al., 2012). Each permafrost core spanned from the tundra surface to below local mean sea level. Cores were collected in air temperatures between -10 °C and -20 °C. They were packed into coolers for transport back to Utqiaġvik, Alaska and then flown frozen to the University of Alaska in Fairbanks where the cores were stored in a -20 °C freezer room prior to shipping them frozen to Sandia National Lab in Albuquerque, NM for processing.
Sections from the frozen cores were cut using a band saw that was cleaned with Milli-Q water and ethanol after each use. Core material was then thawed in acid-washed glass beakers at room temperature in preparation for sampling. Aqueous sampling initiated immediately after the frozen cores were fully thawed. In a few cases, core sections were kept in a refrigerator (4 °C) to thaw overnight and sampled the following day. Rhizon samplers were used to extract porewater from the thawed core samples. The filter pores ranged in size from 0.12 to 0.19 µm with a mean pore size of 0.15 µm. Following porewater extraction, thawed soil/sediment was placed in Whirl-packs and frozen for bulk soil/sediment organic carbon and nitrogen content as well as stable carbon and radiocarbon analysis.
Laboratory analyses
Measurements of total organic carbon (TOC) and total nitrogen (TN) content, stable carbon isotope ratios (δ13C) and radiocarbon (14C) analyses of bulk soils/sediments were conducted on 45 samples at the Woods Hole Oceanographic Institution (WHOI), National Ocean Sciences Accelerator Mass Spectrometer (NOSAMS) facility. Bulk samples were dried at 60 °C then finely ground using a mortar and pestle. Ground samples went through a vapor fumigation acid/base treatment step to remove inorganic carbon. This step involved placing samples in a vacuum-sealed desiccator in a drying oven (60 °C) with a beaker of concentrated HCl for 24 hours. Samples were then removed and placed in another vacuum-sealed desiccator with a dish of NaOH pellets, and again stored in a drying oven at 60 °C for another 24 hours. This latter step neutralized excess HCl. Samples were combusted using an Elementar el Vario Cube C/N analyzer. TOC and TN (% by weight) were quantified during this step. The resulting CO2 was transferred to a vacuum line and cryogenically purified. The purified CO2 gas samples were converted to graphite targets by reducing CO2 with an iron catalyst under 1 atm H2 at 550 °C. Targets were subsequently analyzed for stable and radiocarbon isotopes (δ13C as ‰ and 14C as fraction modern carbon). All Δ14C data (in ‰) were corrected for isotopic fractionation using measured δ13C values that were quantified during the 14C-AMS procedure. We measured δ13C in these samples separately on a VG Prism Stable Mass Spectrometer at NOSAMS. Δ14C and radiocarbon age were determined from percent modern carbon using the year of sample analysis according to Stuiver and Polach (1977).
Dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations were measured from filtered porewater samples without any added preservatives, which were kept frozen until analysis. Due to limited sample volume and anticipated high concentrations of dissolved organic carbon and nitrogen, porewater samples for DOC, TDN, and salinity measurements were diluted either 1:10 or 1:15 with ultrapure (18 MOhm cm-1) water prior to analysis. Concentrations of DOC and TDN were measured at The University of Texas at Austin, Marine Science Institute using a Shimadzu TOC-V CSH analyzer equipped with a TNM-1 total nitrogen detector. Porewater remaining after DOC and TDN analysis was used to measure conductivity. Conductivity was measured using a Myron L Ultrameter II and converted to the practical salinity scale (PSS-78). Porewater anions (chloride Cl-, bromide Br-, sulfate SO4
2-, and nitrate NO3
-) were analyzed by high-performance ion chromatography (HP-ICE) on a Dionex DX-500 ion chromatography system equipped with an AS-1 column at the USGS in Menlo Park, CA. The eluent was a solution of 1.0 mM octanesulfonic acid in 2% isopropanol and using 5.0 mM tetrabutyl ammonium hydroxide as a chemical suppressor. Analyses of ratios of the isotopes strontium-87 (87Sr) to strontium-86 (86Sr) were also done at the USGS laboratories in Menlo Park, California with methods consistent with those reported by Bayless and others (2004). Values are reported as the dimensionless ratio of 87Sr and 86Sr concentrations (87Sr/86Sr). The ratios of 87Sr/86Sr isotopes were measured with a multicollector thermal-ionization mass spectrometer in positive-ion mode (PTIMS; Finnigan MAT 261). Trace element analysis of porewater samples (calcium Ca, manganese Mn, aluminum Al, barium Ba, strontium Sr, silicon Si, iron Fe) was performed using inductively couple plasma mass spectrometry (ICP-MS) at Sandia National Laboratories, Albuquerque, NM. After separating from the solids, each aqueous sample was filtered using a 0.45-micron nylon membrane filter and preserved with 6N ultrapure nitric acid HNO3 prior to analysis by ICP-MS. Depending on the concentration of analyte, some samples were diluted at 100x with 2% ultrapure HNO3. ICP-MS data was acquired using a NexION 350D mass spectrometer (Perkin Elmer) equipped with a collision-reaction cell. Testing for calcium, silicon, strontium, and manganese was done using dynamic reaction cell mode with 0.6 mL/min flow of ammonia gas. Quantitative analyses for iron, aluminum, and barium were done using kinetic energy discrimination mode with helium gas flow set at 5 mL/min. Calibration curves for each element were obtained by running certified standard solutions prior to each analytical run. The estimated measurement error for these trace metal analysis is less than 10%.
Core samples volumes were determined by measuring the water volume displaced by vacuum sealed frozen (~ -20 °C) core samples. These core samples were then weighed before and after drying at 50 °C to determine the frozen bulk density (i.e. density of bulk soil/sediment and water; g/cm3), dry bulk density (ρb; g solids/cm3) and water content (g H2O/cm3) of core sections. Due to uncertainty in sample volume measurements, we estimate that the precision of density measurements is approximately 10-15%. We also calculated gravimetric water content (mass of water per mass of dry soil/sediment; %) and estimated effective porosity (%). Here, we define porosity as the volume fraction of ice assuming pore space was saturated with ice and the density of ice was 0.917 g/cm3. The percent of sand, silt, and clay was determined using a hydrometer method at the Oregon State University Core Analytical Laboratory in Corvallis, OR.
References
Bayless, E.R., Bullen, T.D., Fitzpatrick, J.A. (2004). Use of 87Sr/86Sr and ∂11B to Identify Slag-Affected Sediment in Southern Lake Michigan. Environmental Science and Technology 38: 1330-1337.
Jones, M.C., Grosse, G., Jones, B.M. and Walter Anthony, K. (2012). Peat accumulation in drained thermokarst lake basins in continuous, ice‐rich permafrost, northern Seward Peninsula, Alaska. J Geophys Res: Biogeo 117. Doi: 10.1029/2011JG001766.
Hinkel, K.M., Eisner, W.R., Bockheim, J.G., Nelson, F.E., Peterson, K.M. and Dai, X. (2003). Spatial extent, age, and carbon stocks in drained thaw lake basins on the Barrow Peninsula, Alaska. Arctic, Antarctic, and Alpine Research, 35:3, 291-300.
Stuiver, M. and Polach, H.A., 1977. Discussion: Reporting of 14C data. Radiocarbon, 19:355-363.
