The Cryosphere, 9, 465–478, 2015 www.the-cryosphere.net/9/465/2015/ doi:10.5194/tc-9-465-2015 © Author(s) 2015. CC Attribution 3.0 License. Geophysical mapping of palsa peatland permafrost Y. Sjöberg1, P. Marklund2, R. Pettersson2, and S. W. Lyon1 1Department of Physical Geography and the Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden 2Department of Earth Sciences, Uppsala University, Uppsala, Sweden Correspondence to: Y. Sjöberg ([email protected]) Received: 15 August 2014 – Published in The Cryosphere Discuss.: 13 October 2014 Revised: 3 February 2015 – Accepted: 10 February 2015 – Published: 4 March 2015 Abstract. Permafrost peatlands are hydrological and bio- ing in these landscapes are influenced by several factors other geochemical hotspots in the discontinuous permafrost zone. than climate, including hydrological, geological, morpholog- Non-intrusive geophysical methods offer a possibility to ical, and erosional processes that often combine in complex map current permafrost spatial distributions in these envi- interactions (e.g., McKenzie and Voss, 2013; Painter et al., ronments. In this study, we estimate the depths to the per- 2013; Zuidhoff, 2002). Due to these interactions, peatlands mafrost table and base across a peatland in northern Swe- are often dynamic with regards to their thermal structures and den, using ground penetrating radar and electrical resistivity extent, as the distribution of permafrost landforms (such as tomography. Seasonal thaw frost tables (at ∼ 0.5 m depth), dome-shaped palsas and flat-topped peat plateaus) and talik taliks (2.1–6.7 m deep), and the permafrost base (at ∼ 16 m landforms (such as hollows, fens, and lakes) vary with cli- depth) could be detected. Higher occurrences of taliks were matic and local conditions (e.g., Sannel and Kuhry, 2011; discovered at locations with a lower relative height of per- Seppälä, 2011; Wramner, 1968). This dynamic nature and mafrost landforms, which is indicative of lower ground ice variable spatial extent has potential implications across the content at these locations. These results highlight the added pan-Arctic as these permafrost peatlands store large amounts value of combining geophysical techniques for assessing spa- of soil organic carbon (Hugelius et al., 2014; Tarnocai et al., tial distributions of permafrost within the rapidly changing 2009). The combination of large carbon storage and high po- sporadic permafrost zone. For example, based on a back-of- tential for thawing make permafrost peatlands biogeochem- the-envelope calculation for the site considered here, we es- ical hotspots in the warming Arctic. In light of this, predic- timated that the permafrost could thaw completely within the tions of future changes in these environments require knowl- next 3 centuries. Thus there is a clear need to benchmark cur- edge of current permafrost distributions and characteristics, rent permafrost distributions and characteristics, particularly which is sparse in today’s scientific literature. in under studied regions of the pan-Arctic. While most observations of permafrost to date consist of temperature measurements from boreholes, advances in geophysical methods provide a good complement for map- ping permafrost distributions in space. Such techniques can 1 Introduction provide information about permafrost thickness and the ex- tent and distribution of taliks, which can usually not be ob- Permafrost peatlands are widespread across the Arctic and tained from borehole data alone. As the spatial distribution cover approximately 12 % of the arctic permafrost zone and extent of permafrost directly influences the flow of wa- (Hugelius et al., 2013; Hugelius et al., 2014). They often ter through the terrestrial landscape (Sjöberg et al., 2013), occur in sporadic permafrost areas, protected by the peat adding knowledge about the extent and coverage of per- cover, which insulates the ground from heat during the sum- mafrost could substantially benefit development of coupled mer (Woo, 2012). In the sporadic permafrost zone, the per- ◦ hydrological and carbon transport models in northern lat- mafrost ground temperature is often close to 0 C, and there- itudes (e.g., Jantze et al., 2013; Lyon et al., 2010). This fore even small increases in temperature can result in thawing may be particularly important for regions where palsa peat- of permafrost. In addition, permafrost distribution and thaw- Published by Copernicus Publications on behalf of the European Geosciences Union. 466 Y. Sjöberg et al.: Geophysical mapping of palsa peatland permafrost lands make up a large portion of the landscape mosaic and 0’0”N regional-scale differences exist in carbon fluxes (Giesler et ° 0 70°0'0"N 7 N al., 2014). 1 1 Geophysical methods offer non-intrusive techniques for 0’0”N measuring physical properties of geological materials; how- T2 ° T2 60°0'0"N 60 ever, useful interpretation of geophysical data requires other 6 10°0'0"W 20°0'0"E 40°0'0"E 2 7 2 7 types of complementary data, such as sediment cores. 5 5 Ground penetrating radar (GPR) has been used extensively Investigated transects Direction of transects in permafrost studies for identifying the boundaries of per- !!! 1 !!!!!!! 10 m borehole T1 !!!!!!! T1 3 3 !!!!!!! !!!!!! !!!!!!! 2 6 m borehole !!!!!! mafrost (e.g., Arcone et al., 1998; Doolittle et al., 1992; !!!!!! !!!!!!! !!!!!!! 4 4 !!!!!!! 3 2 points of 2 m coring !!!!!! Hinkel et al., 2001; Moorman et al., 2003), characterizing 4 2 m coring point ground ice structures (De Pascale et al., 2008; Hinkel et al., 5 2 points of 2 m coring T3 6 CMP profile (saturatedT3 peat) 2001; Moorman et al., 2003), and estimating seasonal thaw 6 0 50 100 Meters depth and moisture content of the active layer (Gacitua et al., 0 50 100 Meters 2012; Westermann et al., 2010). Electrical resistivity tomog- raphy (ERT) has also been widely applied in permafrost stud- Figure 1. Location of the study site (inset), investigated transects, ies (Hauck et al., 2003; Ishikawa et al., 2001; Kneisel et al., existing boreholes (Ivanova et al., 2011, points 1 and 2), coring 2000), the majority of which focus on mountain permafrost. points, and point of CMP measurement (described in Sect. 3.1; By combining two or more geophysical methods comple- aerial photograph from Lantmäteriet, the Swedish land survey, mentary information can often be acquired raising the con- 2012). fidence in interpretations of permafrost characteristics (De Pascale et al., 2008; Hauck et al., 2004; Schwamborn et al., 2002). For example, De Pascale et al. (2008) used GPR and weather parameters have been monitored at the site since capacitive-coupled resistivity to map ground ice in continu- 2005 (Christiansen et al., 2010). Sannel and Kuhry (2011) ous permafrost and demonstrated the added value of combin- have analyzed lake changes in the area and detailed local ing radar and electrical resistivity measurements for the qual- studies of palsa morphology have been conducted by Wram- ity of interpretation of the data. While some non-intrusive ner (1968, 1973). geophysical investigations have been done in palsa peatland Tavvavuoma is located on a flat valley bottom, in piedmont regions (Dobinski, 2010; Doolittle et al., 1992; Kneisel et al., terrain with relative elevations of surrounding mountains 2007, 2014; Lewkowicz et al., 2011), the use of multiple geo- about 50 to 150 m above the valley bottom. Unconsolidated physical techniques to characterize the extent of permafrost sediments, observed from two borehole cores (points 1 and 2 in palsa peatland environments has not been employed. in Fig. 1), are of mainly glaciofluvial and lacustrine origin In this study we use GPR and ERT in concert to map the and composed of mostly sands, loams, and coarser-grained distribution of permafrost along three transects (160 to 320 m rounded gravel and pebbles (Ivanova et al., 2011). The mean long) in the Tavvavuoma palsa peatland in northern Sweden. annual air temperature is −3.5 ◦C (Sannel and Kuhry, 2011), Our aim is to understand how depths of the permafrost table and the average winter snow cover in Karesuando, a mete- and base vary in the landscape and, based on resulting esti- orological station approximately 60 km east of Tavvavuoma, mates of permafrost thickness, to make a first-order assess- is approximately 50 cm, although wind drift generally gives a ment of the potential time needed to completely thaw this thinner snow cover in Tavvavuoma (Swedish Meteorological permafrost due to climate warming. Furthermore, we hope and Hydrological Institute, http://www.smhi.se/klimatdata/ to demonstrate the added value of employing complemen- meteorologi). tary geophysical techniques in such landscapes. This novel Permafrost occurs primarily under palsas and peat plateaus investigation thus helps contribute to our understanding of in Tavvavuoma, where the average thickness of the active the current permafrost distribution and characteristics across layer is typically 0.5 m (Christiansen et al., 2010; Sannel and palsa peatlands, creating a baseline for future studies of pos- Kuhry, 2011). The mean annual temperature in permafrost sible coupled changes in hydrology and permafrost distribu- boreholes is 2 ◦C (Christiansen et al., 2010). However, no ob- tion in such areas. servations of the depth to the permafrost base have been pre- sented for the area. Warming of the air temperature of about 2 ◦C has been observed in direct measurements from the re- 2 Study area gion over the past 200 years (Klingbjer and Moberg, 2003). In light of this warming, winter precipitation (mainly snow) Tavvavuoma is a large palsa peatland complex in northern in northern Sweden shows increasing trends over the past Sweden at 68◦280 N, 20◦540 E, 550 m a.s.l. (Fig. 1) and con- 150 years (Alexandersson, 2002). Furthermore, permafrost sists of a patchwork of palsas, peat plateaus, thermokarst is degrading across the region and northern Sweden (Sjöberg lakes, hummocks, and fens. Ground temperatures and et al., 2013). For example, peatland active layer thickness The Cryosphere, 9, 465–478, 2015 www.the-cryosphere.net/9/465/2015/ Y.