Ninth International Conference on Permafrost ... - IARC Research

Soil Carbon Distribution in the Alaska Arctic Coastal PlainErik PullmanUniversity of Alaska Anchorage, Kachemak Bay Campus, 533 East Pioneer Ave., Homer, AK 99603M. Torre JorgensonABR, Inc. – Environmental Research and Services, P.O. Box 80410, Fairbanks, AK 99708Yuri ShurUniversity of Alaska Fairbanks, Civil and Environmental Engineering, P.O. Box 755900, Fairbanks, AK 99775IntroductionThe accumulation of soil carbon in arctic soils representsa globally significant sink of atmospheric carbon that isboth formed and maintained by development and accretionof permafrost. Much of this near-surface carbon currentlysequestered in permafrost can be incorporated into the activelayer through surface disturbance (Pullman et al. 2007) andthermokarst processes related to climate change (Jorgensonet al. 2006). Most recent estimates sample only the activelayers or top 100 cm of soil (however, see Bockheim &Hinkel 2007) or are based on samples taken from a limitednumber of geomorphological terrains.In this study, we have used a landscape-classificationapproach to sampling in an area of the North Slope CoastalPlain of Alaska covering 850 km 2 (Fig. 1). The terrain units,surface forms, and vegetation communities of the landscapein this area were classified through aerial photo interpretationand extensive field surveys. Samples for soil carbon weredistributed across the major terrain types identified acrossthe study area. Total soil carbon results for each terrainunit were then applied to the terrain map and a total carboninventory was calculated for the study area.MethodsSample collectionSoil cores were collected during the summer in 2001–2003in the NPR-A area. An intact soil plug was removed downto permafrost. Intact 3-inch (7.5 cm) soil cores in permafrostwere collected using a specially-designed soil corer attachedto a portable power head. At each location, we attemptedto core into the underlying sand sheet and locate sedimentswith less than 10% visible ice. We postulate that this mayrepresent the syngenetic/epigenetic boundary within theexisting permafrost. At each soil pit, soil stratigraphy,texture, and color were described from the intact, frozen soilcore. Cryogenic structures were described according to Shurand Jorgenson (1998). Visual ice content was estimated foreach stratigraphic section. Soil textures were grouped intolithofacies classes based on similarities on particle sizedistribution and depositional patterns (Pullman et al. 2007).Frozen soil samples were taken every 10–20 cm (withinstratigraphic sections) along the entire length of the core.Sample volumes were measured in the field by taking threemeasures of core length (at equidistant intervals around thecircumference) and three measures of core circumference(at the ends and midpoint of the core length). Unfrozencore material was removed prior to measurement. Meanlength and circumference values were used to calculate thecylindrical core volume. Core samples were then placed inlabeled ziplock freezer bags for transport. A total of 47 coresand bank exposures were included in this analysis, with totalsample depths ranging from 1.8 to 4 m. Most cores extendedto 2.7 m.Field samples were weighed and thawed at roomtemperature. Excess water was decanted off the sample onceall solids had settled to the bottom of the sample bag. Soilswere oven-dried to a constant weight at 60°C. A subsampleof the homogenized dried soil was submitted for total carbonand nitrogen analysis to the Palmer Soils Laboratory (Palmer,Alaska).Soil carbon calculationsSoil carbon content was calculated for each stratigraphicsection within individual cores, and then summed over 100cm and 200 cm. Stratigraphic sections that extended throughthe 100 cm and 200 cm depths were split for the purposesof calculating total carbon for these specific depths. Sincewe did not sample every stratigraphic section of everycore, estimates of the carbon content in unsampled sectionswere made to obtain total carbon stocks for each core. Astratigraphic method was used to reduce variability thatcould be introduced by a length-weighted mean method. Inthis study a total of 612 stratigraphic sections were identifiedin 46 cores. Of these sections, 476 were sampled for %Ccontent. Measurements on a majority of these sections (400)were made for sample volume, excess ice volume, dry soildensity, and water content. Using data from the sampledstratigraphic sections, we were able to calculate mean carboncontents for stratigraphic sections that were missing datafor either % carbon, dry density, or both. For stratigraphicsections missing % carbon measures, we used the mean%C of the section’s lithofacies class (Table 1). For sectionsmissing dry density measures, sample dry density wascalculated using a regression of dry density based on %C ofactive layer samples multiplied by the % excess ice (fromthe visual field estimate). For the minority of stratigraphicsections with neither density nor %C measures, we used avalue of both %C and dry density, based on calculated meansof the appropriate lithofacies class.Mean carbon stocks for each terrain type were applied to theterrain map of the study area to show the spatial distributionof carbon in the active layer, top 1 m, and top 2 m of soilacross the study area to produce a distribution pattern of total245