expansion rate and geometry of floating vegetation mats on the margins of thermokarst lakes,...

EARTH SURFACE PROCESSES AND LANDFORMSEarth Surf. Process. Landforms 36, 1889–1897 (2011)Copyright © 2011 John Wiley & Sons, Ltd.Published online 16 August 2011 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/esp.2210

Expansion rate and geometry of floating vegetationmats on the margins of thermokarst lakes, northernSeward Peninsula, Alaska, USAAndrew D. Parsekian,1* Benjamin M. Jones,2,3 Miriam Jones,4 Guido Grosse,2 Katey M. Walter Anthony4 and Lee Slater11 Department of Earth and Environmental Sciences, Rutgers University, Newark, New Jersey2 Permafrost Laboratory, Geophysical Institute, University of Alaska, Fairbanks, Alaska3 Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska4 Water and Environmental Research Center, INE/IARC, University of Alaska, Fairbanks, Alaska

Received 19 April 2011; Revised 20 June 2011; Accepted 5 July 2011

*Correspondence to: A. D. Parsekian, Department of Earth and Environmental Sciences, Rutgers University, Newark, New Jersey. E-mail: [email protected]

ABSTRACT: Investigations on the northern Seward Peninsula in Alaska identified zones of recent (<50years) permafrost collapsethat led to the formation of floating vegetation mats along thermokarst lake margins. The occurrence of floating vegetation mat fea-tures indicates rapid degradation of near-surface permafrost and lake expansion. This paper reports on the recent expansion of thesecollapse features and their geometry is determined using geophysical and remote sensing measurements. The vegetation mats wereobserved to have an average thickness of 0.57m and petrophysical modeling indicated that gas content of 1.5–5% enabled floatationabove the lake surface. Furthermore, geophysical investigation provides evidence that the mats form by thaw and subsidence of theunderlying permafrost rather than terrestrialization. The temperature of the water below a vegetation mat was observed to remainabove freezing late in the winter. Analysis of satellite and aerial imagery indicates that these features have expanded at maximumrates of 1–2myr-1 over a 56year period. Including the spatial coverage of floating ‘thermokarst mats’ increases estimates of lake areaby as much as 4% in some lakes. Copyright © 2011 John Wiley & Sons, Ltd.

KEYWORDS: permafrost; ground penetrating radar; fen; floating peat; thermokarst; lake expansion

Introduction

Thermokarst lakes are widespread landscape features occurringin the Arctic and boreal zones of Alaska, Canada and Siberiathat may account for 22–48% of the landscape in some regions(Zimov et al., 1997; Côté and Burn, 2002; Hinkel et al., 2005;Grosse et al., 2006; Walter et al., 2007b). Recently, thermokarstlakes have been hypothesized to be an important part of theglobal carbon cycle as they release methane (CH4) to the atmo-sphere (Zimov et al., 1997; Walter et al., 2006). Upon thaw inanaerobic lake bottoms, microorganisms produce CO2 andCH4 by consuming organic matter previously sequestered inpermafrost. With 25 times the global warming potential ofCO2 on a 100-year time span (Boucher et al., 2009), the releaseof CH4 from thermokarst lakes is of concern to climate warm-ing. Thermokarst lakes that occur in regions of Pleistocene-aged yedoma-like permafrost, which is characterized by loessand loess-related sediments, ice-supersaturation, includingmassive ice wedges, and high organic matter content (Zimovet al., 1997), are particularly important because the thickyedoma sediments contain large amounts of labile, Pleistocene-aged organic carbon. As thermokarst lakes expand intosurrounding ice-rich permafrost, they tap into carbon that hasbeen sequestered in the frozen matrix for thousands to tens ofthousands of years, allowing greenhouse gas production to

continue with ongoing thaw bulb (i.e. ‘talik’) development be-neath these lakes (Walter et al., 2008).

Thermokarst-lake-dominated landscapes are dynamic sys-tems with a variety of processes influencing lake expansionand drainage. A thermokarst lake is an expanding water bodythat forms in a depression initiated when ice-rich permafrostthaws (van Everdingen, 1998). Once a pond of sufficient sizeforms, adjacent permafrost is subject to thaw, adjacent icewedges melt and the open water expands laterally (Black,1969). Ponding also results in the warming and degradationof surrounding permafrost (Lachenbruch, 1962). Thaw bulbscreated by long-term permafrost thawing beneath lakes havebeen identified as a source of CH4 gas derived from latePleistocene-aged (>10ka) organic carbon deposited in perma-frost soils and sediments (Zimov et al., 1997; Walter et al.,2006, 2007a, 2008). Depending on volume of the thaw bulb,large point-source ebullition seeps can occur in these lakes thatserve as an efficient pathway for Pleistocene-aged carbon to theatmosphere (Walter et al., 2006).

Variation in thermokarst lake expansion is largely related tolake surface area, bluff height along lake margins, and bathym-etry (Jones et al., 2011). Thermokarst-lake expansion hasbeen traditionally recognized by changes in open water areasof lakes through benchmark measurements on the ground orwith optical remote sensing (Lewellen, 1970; Jorgenson and

1890 A. D. PARSEKIAN ET AL.

Shur, 2007; Arp et al., 2011; Jones et al., 2011). This study fo-cuses on a previously understudied mode of thermokarst lakeexpansion, which is the degradation of permafrost along lakemargins covered by floating mat vegetation. While these featuresare not geologically new, their role in thermokarst lake expansionhas not been investigated until this point. Currently these ‘thermo-karst mats’ are observed to form along the lowland margins ofmany thermokarst lakes on the Seward Peninsula. Hopkins(1949) was the first to specifically describe ‘floating turf, asmuch as 50 feet in diameter, still covered with living vegeta-tion’ on the Seward Peninsula in the vicinity of Imuruk Lake.Kane and Slaughter (1973) revealed the thermal insulating abil-ities of floating vegetation mats during a hydrologic investigationof a peat-covered lake situated in a zone of discontinuous perma-frost near Fairbanks, Alaska.Although understanding mechanisms and rates of lake-

margin expansion is essential to predictions of greenhousegas emission from thermokarst lakes (Walter et al., 2007a), theimplications for greenhouse gas release from thermokarst matfeatures and the entire lake system they belong to are largelyunknown so far. The aim of this study was to characterize thespatial characteristics (thickness, extent, and expansion rate)of the floating vegetation mats in thermokarst lakes on thenorthern Seward Peninsula, investigate the mechanism of theirformation, and determine their potential overall importance inthermokarst lake systems. In addition, we determined the ex-tent of thawed permafrost obscured by floating vegetation matsat the study site and provided a basis for larger scale estimatesof bioavailable organic carbon in the thaw bulbs below lakeportions covered by floating vegetation mats. We used high-resolution remote sensing imagery to map the spatial extent ofthe collapse features across the study site, ground-penetratingradar (GPR) to noninvasively map the thickness of the floatingvegetation mats and ground temperature measurements to ver-ify the presence of a talik despite very shallow water depths.

Field Site

The floating vegetation mats studied were located on LakeRhonda and Lake Owl (informal names), typical later-generation thermokarst lakes (Plug and West, 2009) located ap-proximately 27km west-southwest of Cape Espenberg on thenorthern Seward Peninsula, Alaska (Figure 1). Lake Rhonda isapproximately 1.3km across on its longest axis and irregularlyshaped. Lake Owl, located approximately 11.3km SE of LakeRhonda, is approximately 1km across and is also irregularlyshaped. On the northern Seward Peninsula of Alaska, bluff typecan typically be differentiated into upland and lowland shorebanks. Upland bluffs ranging in height from 7–20m representareas where lakes erode into ice-rich permafrost deposits con-taining large syngenetic ice wedges (ice wedges that formsimultaneously with sedimentation and actively grow season-ally through cracking and expansion) and organic carbon-rich deposits that date to the Late Pleistocene to Holocene(MacKay and Black, 1973). Lowland bluffs typically range inheight from 0.2m to 3m and largely are formed by frozen lacus-trine deposits and terrestrial peat formed during the Holocene.Lowland bluff types represent areas of the landscape that havebeen previously reworked by the formation and subsequentdrainage of a thermokarst lake. While these deposits are alsoice-rich, at least in the upper horizons, the epigenetic icewedges found there tend to be fairly narrow (1m) and shallow(within upper 2m), and the organic carbon is of Holocene ormixed Pleistocene–Holocene age (Hopkins and Kidd, 1988).Low-relief shorelines comprise 80% of the Lake Owl perimeter,and are located along the north, east and southern lake

Copyright © 2011 John Wiley & Sons, Ltd.

margins. Steep, upland bluff margins (20% of lake perimeter)occur on the western side rising >20m above the water level.Both lakes are largely flat-bottomed with depths between 1and 2m and freeze to the bottom as indicated by synthetic ap-erture radar satellite imagery from late winter. Exceptions are anarrow zone offshore along the high upland margins where wa-ter depths drop sharply to 3–5m possibly due to active thermo-karst subsidence and melt-out of old ice wedges.

The irregularly shaped vegetation mats bordering LakeRhonda and Lake Owl are concentrated on the low-relief shor-elines that would have been in the middle of the earlier-generation thermokarst lake that once occupied these basins.In contrast, the western shoreline of Lake Owl and the southernshoreline of Rhonda contain yedoma-like permafrost with steepsides and no measureable floating vegetation. We selectedfloating vegetation mats for study on the east side of LakeOwl and the northern side of Lake Rhonda (Figure 1) becausethey were accessible in relation to field camps and becauseLake Rhonda and Lake Owl have several additional datasets(discussed later) associated with them to enhance our interpre-tation. The mat surfaces were covered primarily with vegeta-tion typical of a sedge meadow: Carex aquatilis and otherspecies such as Potentilla palustris, Eriophorum angustifolium,Eriophorum scheuchzeri, Arctophila fulva, Hippurus vulgarus,and Sphagnum sp. The soil in which the living vegetation wasrooted was saturated, poorly decomposed peaty organic mat-ter. In general a ‘moat’ of open water typically <1m wide withsome emergent vegetation (Hippurus vulgaris, Arctophila fulva,Calliergon giganteum) separated the floating mat from the adja-cent lowland tundra/lake edge, though a direct connection tothe tundra was present in some places (Figure 2). The surfaceof the mat was soft and unstable; during field data collectionthe weight of the instrument operators would cause the mat tostart sinking after a few moments of standing still. Parts of themats could not be visited because of poor stability for walking.The mat on Lake Rhonda has several open water holes, patchesof brown dead plants and, in some places close to the moat, dy-ing or dead small shrubs (Betula nana and Salix sp.) were ob-served. Small areas of open water were observed within themats on Lake Owl, though no dead vegetation was noted onthose mats.

Methods

Remote sensing measurements

In order to understand how these permafrost collapse featuresevolved over time we used high-spatial resolution historic ae-rial photography and contemporary satellite imagery to recon-struct change in areal extent of the features adjacent to LakeRhonda and Lake Owl between 1951 and 2007 (1m horizontalpixel resolution, RMSE of transformation=�2.0m and �1.5mfor 1951 and 1978 images, respectively, relative to the 2003aerial mosaic (Manley et al., 2007a, 2007b), RMSE=�1.4mfor the 2006 image (Jones et al., 2011). We used a computer-based semi-automated classification technique to extract thewaterline of the respective water bodies. This was accom-plished with an object-oriented classification scheme withinthe eCognitionW image processing package (Frohn et al., 2005),in which water bodies were delineated based on their surfacereflectance characteristics as well as their overall object-shape.The automated delineation was then supplemented with man-ual correction of misclassified objects due to terrain shadowor flooding (following Jones et al., 2011). The floating mat areawas subsequently manually delineated by digitizing theirvisual boundaries since it is difficult to extract these features

Earth Surf. Process. Landforms, Vol. 36, 1889–1897 (2011)

Figure 1. (a) Study site location on a regional map. (b) Locations of both Lake Owl and Lake Rhonda shown on an aerial photograph. Inset (c) showsdetail of Lake Rhonda and inset (d) shows detail of Lake Owl. The white arrows indicate the lake margins with floating vegetation. Thin black linesindicate the location of ground-penetrating radar profiles on Mat 1 and Mat 2 at Lake Owl. (© SpotImage Planet Action)

1891EXPANSION RATE AND GEOMETRY OF THERMOKARST VEGETATION MATS

using automated techniques due to the spectral similarity be-tween the floating vegetated surface and adjacent tundra vege-tation. The open-water moat between the mats and tundra washelpful for their manual delineation. We made direct compari-sons between change in surface area over time and determinedthe degradation rate of the adjacent permafrost from imageryacquired in 1951, 1978, and 2006. Expansion rates were deter-mined with the Digital Shoreline Analysis System tool (Thieleret al., 2005) at 10m increments around the perimeters of lakesand floating mat features, similar to methods used to examinelake expansion rates in Jones et al. (2011).

Ground-penetrating radar (GPR) and field surveys

GPR surveys were used to determine the thickness of the float-ing peat mats at Lake Owl. The GPR method is well suited toshallow subsurface investigations in materials that have lowelectrical conductivity (Davis and Annan, 1989; Neal, 2004),which was observed to be ~0.01 mS cm-1 at Lake Owl. Whencollecting data, an electromagnetic wave propagates into theground from the transmitting antenna and is reflected from

Figure 2. (a) An oblique aerial photograph showing a floating vegetation mthe bottom of the frame, and the floating mat is the lighter shaded area adjac(b) Transition from tundra (left) to floating vegetation mat (right) with an open-the right side is about 2m tall. This figure is available in colour online at wil


interfaces in the subsurface and the receiving antenna thensenses the returned energy. The primary physical property ofsoil or rock that governs the strength of the returned signal isits dielectric permittivity – a materials’ ability to support propa-gation of an electric field. In turn, dielectric permittivity is pri-marily determined by water content (Topp et al., 1980) – thestrong contrast in dielectric properties between the mat, waterand lake bottom is a critical factor enabling us to observe aque-ous environments effectively.

All field GPR datasets were collected during August 2008 us-ing a Malå GPR system equipped with unshielded 200MHz an-tennas determined to be an appropriate frequency toinvestigate the desired depth for the subsurface medium. TheGPR system used has separable transmitting and receiving an-tennas to enable multiple offset measurements as well as profil-ing when connected using a wooden frame. Since verticalresolution is largely controlled by the dielectric permittivity(er) of the medium and the frequency of the 200MHz antennas,GPR can be expected to resolve changes on the scale of about0.05m in saturated peat (er�80). For the profiling surveys a par-allel broadside common-offset mode was used. The antennaswere held at a fixed distance of 0.6m and the operator moved

at on the periphery of a thermokarst lake. Unaltered tundra is present atent to the lake (15–20m polygonal patterning in foreground for scale).water moat dividing two distinct vegetation communities. The person oneyonlinelibrary.com/journal/espl



the frame along a predetermined line at a step interval of 0.1m.Care was taken to ensure that both the transmitter and receiverwere in contact with the ground surface after each movementso that maximum energy was passed into the ground. Depthdeterminations on common offset profiles are calculated byconverting the travel time to depth using the materials’ velocity.In order to determine the velocity of the electromagnetic en-ergy through the floating vegetation mats, we utilized the com-mon-midpoint survey method (Greaves et al., 1996). In thissurvey, the transmitting and receiving antenna are moved apartstepwise from a center point at an interval of 0.1m and tracesare collected at progressively larger offsets. The radar wavescontinue to reflect from the point directly below the center ofthe survey line with increasing offset, and the velocity can becalculated by determining the relationship between offset andtravel time. Uncertainty associated with the velocity calcula-tion was estimated based on the method described by Jacoband Hermance (2004) that uses standard statistics to define95% confidence intervals. All GPR data were processed usinga dewow filter (frequency optimized high-pass (Neal, 2004))to remove low frequency noise and a gain function to compen-sate for signal loss at longer travel times. A background removalwas performed to eliminate horizontal ringing features associ-ated with instrument noise.The electrical properties of the floating peat determined from

the common mid-point GPR data were used to estimate thephysical properties of the peat using the complex refractive in-dex model (CRIM). This is a multiphase dielectric mixing modelthat has been used previously to determine the free-phase gascontent of peat soils (Comas et al., 2008; Parsekian et al.,2010). The free-phase gas calculations were made using theCRIM with an assumed range of porosity of 0.93–0.97 thatwould be representative of poorly decomposed peat (Lettset al., 2000; Beckwith and Baird, 2001). This dielectric mixingmodel is based on the bulk permittivity (determined with GPR),the permittivity of the individual components (gas phase=1,water ~86, peat ~2.5), the porosity of the material (0.93–0.97)and a fitting factor (0.35 for peat). Comas et al. (2008) providea detailed review of the application of the CRIM model to gasestimation in peat soils.

Additional methods

Additional field surveys were conducted in April 2009, June2010, and August 2010 to acquire information on ground ther-mal regimes of permafrost beneath and adjacent to the floating

Figure 3. Peat mat expansion rates shown for (a) Lake Rhonda and (b) Lakaerial photo). Areas with the highest expansion rates are indicated with arrowvious generation thermokarst lakes. Note the similar color spectra of the floattomated classification difficult. Grid spacing is 100m. This figure is available


mats, to delineate the current mat edge and to observe vegeta-tion composition. Temperature loggers were deployed in loca-tions on the mat, in the moat and at a control location 200mfrom the mat within the permafrost. The temperature data timeseries was truncated due to the sensors being damaged, proba-bly due to animal tampering, but is still useful for our interpre-tation. A steel probe was pushed into the ground under thefloating mat along a transect perpendicular to the moat to con-firm the presence of a talik below the floating mat. A survey-grade differential global positioning system (GPS) receiver(LEICA VIVATM) was used to record the current location of thelandward edge of the moat in August 2010 and expansion rateswere calculated in relation to the 2003 aerial photograph forselect locations.

Results and Interpretation

Remote sensing

Based on remote sensing of erosion rates of a widening isth-mus, the morphometry of Lake Rhonda suggests the likelihoodof recent coalescence of two adjacent thermokarst lakes about75 to 100years ago: 75% of Lake Rhonda has formed withinthe confines of a series of drained thermokarst lake basins. LakeOwl also probably represents the coalescence of numerousprevious thermokarst lakes. Time series of remotely sensed im-agery covering Lake Rhonda and Lake Owl showed that theopen water area of thermokarst lakes expanded by shore ero-sion between 1951 and 2007 (Figure 3) – a change that we in-fer to be a result of permafrost degradation. Lake Rhondaincreased from 69ha to 79ha, while Lake Owl expanded from60ha to 70ha, which amounts to roughly an increase in area of17% for both lakes. Uncertainty in lake and floating mat areameasurements is assumed to be small given the high-resolutionaerial photographs used for the calculations. Jones et al. (2011)also found that when using the waterline to delineate thelake margin, Lake Rhonda expanded at a rate of 0.51myr-1,while Lake Owl expanded at a rate of 0.56myr-1 over the ob-served 56year time period, with a calculated uncertainty of0.09myr-1. The floating mats occupy 3.9% of Lake Rhondaand 3.6% of Lake Owl, though visual observations of otherlakes in the area clearly indicate larger percentages of floatingmat cover are common. The total area of the floating vegetationmat features at the lake margins also increased over time, butthermokarst expansion occurred substantially faster along themat margins than along the open-water lake margins. The

e Owl between 1951 and 2006 (data plotted on the most recent 2006s and occur on the low-relief shorelines that were lake bottoms of pre-

ing mat compared with the surrounding terrestrial tundra that makes au-in colour online at wileyonlinelibrary.com/journal/espl



expansion rates of the floating mats at both studied lakes rangefrom 1–2myr-1 for the 56year observation period, exceedingthe rates of expansion along other shore types and the mean ex-pansion rates of the lake by several times (Figure 3). The LakeRhonda floating mat area increased by 55% from 1.4ha in1951 to 3.1ha in 2006. While the mat area adjacent to LakeOwl also increased during the same period, it was to a muchlesser degree (16%, from 2.1ha to 2.5ha). It should be noted thatwhile these mats expand in area on the landward side of the fea-ture as lowland permafrost degrades, they are destroyed on thelakeward side of the feature presumably due to wave action,which tears pieces of the floating mat away after which they sub-sequently drift into the lake and begin to sink. Since the matsgrew over the 56year observation period, mat destruction pro-cesses are probably slower than mat landward expansion.

Vegetation mat thickness

The results of the GPR surveys used to determine mat thicknessare provided in Figure 4. Both profiles (Mat 1 and Mat 2, loca-tions shown on Figure 1 and Figure 3) have similar reflection sig-nals driven by variations in water content with the most notableevent being a gently curved reflection 0.9–1.4m below the sur-face corresponding to the lake bottom. The variable returned sig-nal at approximately 0.5m below the surface corresponds to theinterface between the vegetation mat and the water column. Thevegetation mat was clearly identified by the presence of sub-parallel chaotic reflections commonly associated with strati-graphic variations in organic soils between the surface and ap-proximately 0.5m (Jol and Smith, 1995). Variations in the peatstructure during development result in minor heterogeneitiesin water content thus controlling the permittivity and causingthese reflections in the radargram. The water column is expectedto have homogeneous electrical properties and there are noreturned signals recorded in this portion of the subsurface. Thelake water–sediment interface showed a sharp change in watercontent and fully attenuated the electromagnetic signal due tothe high electrical conductivity of the pore fluid (s>1 mS cm-1

observed in the basal sediments of a nearby similar lake)and surface conduction of fine texture sediment. It is notablethat there was a phase change in the returned signal wherethe interface at the bottom of the organic material returneda positive phase and the lake sediment returned a reversed

Figure 4. Common-offset ground-penetrating radar data. Vegetation Mat 1 (edge of Lake Owl. Both profiles show chaotic reflections between 0m and 0.61.2m that is open water, and then the basal reflector of the lake bottom. The wmid-point survey.


negative phase. This is due to the presence of water (the highestpermittivity material commonly found in nature) between twomaterials with lower permittivity and further confirms our modelof the structure of the subsurface. Due to high signal attenuationat the lake sediment interface, probably a result of surface con-ductance and high pore water conductivity, it was not possibleto image the talik thickness using the GPR method.

The sites are similar and both vegetation mats investigatedhave approximately the same mean thickness at 0.57�0.04m, though the lake bottom below vegetation Mat 1 has a max-imum depth 0.11m deeper than vegetation Mat 2 (see Mat 1and Mat 2 locations on Figure 1 and Figure 3). The water col-umn below the mats was observed to average 0.55�0.07mand 0.59�0.11m for Mats 1 and 2, respectively, however, itis possible that this water column varies seasonally or over longperiods. Comparing the standard deviations (SD) of the reflec-tors, the interface between the vegetation mats and the watertends to be less variable (SD=0.04) than the lake bottom (SD=0.07–0.11m), possibly because the vegetation mats are nearlyhorizontal while the lake bottom curves up near the shoreline.

Using the relationship between material velocity and wave-length, we calculate the maximum theoretical vertical resolu-tion of the 200MHz antennas used is 0.043m. Coringapproaches can be used to calibrate GPR velocities to confirmdepth estimates, though in this case we have instead used com-mon mid-point data to accomplish this task. The non-invasivedetermination of thickness may be more accurate than coresor probe measurements because GPR is sensitive to a changein physical properties whereas direct probes can compressand deform the mat leading to errors in the thickness estimates(Jol and Smith, 1995; Slater and Reeve, 2002). The GPR dataacquisition caused less destructive disturbance to the mat thandirect coring would have, while obtaining a high level of spa-tial continuity and simultaneously obtaining measurements ofthe physical properties of the subsurface materials. We did,however, make measurements of mat thickness on LakeRhonda in August 2010 and found the average thickness tobe 0.6m along a 40m transect.

Estimate of trapped gas content

The resulting common mid-point GPR dataset (Figure 5) indi-cated that the velocity of the electromagnetic energy within

a) is approximately 50m north of Vegetation Mat 2 (b) along the easternm corresponding with peat, an area of no reflections between 0.6m andhite triangle in the center of Line 2 indicates the location of the common


Figure 5. Schematic of the relevant ray paths showing direct transmis-sions and reflection from interfaces in the subsurface as recorded in acommon mid-point radargram with the corresponding signal responseshighlighted. Velocities reported are calculated directly from the shapeof the hyperbola.


the floating vegetation mat is 0.0353m/ns, similar to other pub-lished values for peat (e.g. 0.036m/ns (Comas et al., 2008)).The velocity to the deepest interface between the water andlake sediment is 0.0346mns-1 which, when corrected usingthe Dix equation (Dix, 1955) to account for the overlying mate-rial, results in an interval velocity 0.0339mns-1, which corre-sponds with the expected velocity of radio waves in water(Buchner, 1999). We used the CRIM model to convert theGPR velocity data into physical properties of the peat. TheGPR velocity calculation for the organic material of the vegeta-tion mat has an associated uncertainty of 0.001mns-1 (Jacoband Hermance, 2004). Propagating velocity uncertaintythrough the CRIM model, we estimate that the vegetation matat Lake Owl contained between 1.5 �1% and 5�2.5% gascontent by volume. Given that the surface area occupied byfloating vegetation mats on Lake Owl is 2.5ha and the meanmat thickness is 0.57m, the volume of gas entrapped withinthe floating mats per unit area is estimated to be between 8.5Lm-2 and 29Lm-2. Uncertainty in this estimate stems from therange of possible porosity values used in the CRIM model(0.93–0.97 as previously defined), possible error in the GPR ve-locity estimates, and possible error in the remote-sensing calcu-lation of the surface area of the mats.

Other results

The thermal data from locations in the floating mats and in thesurrounding tundra reveal spatial variations in temperature.Notably, the temperature 1.5m below the surface of the floatingvegetation mat on Lake Rhonda is slightly above 0�C in themiddle of April (late winter) indicating (1) that the site doesnot freeze to the bottom, and (2) a talik is present underneaththe floating mat. In contrast, the adjacent lake surface was fro-zen to the bottom at a depth of 1.2m. On the landward marginof the mat, temperature measurements 1m below the surface inthe moat area were �3�C, and 1.5m below the surface, 1mfrom the mat edge it was �4�C. A temperature of �7.25�Cwas recorded at the control site located 200m from the mat1.5m below the tundra surface. Direct probe measurements


made through the floating mat on Lake Rhonda indicated thatthere was at least 3m of unfrozen sediments below the lake bot-tom, although a frozen boundary was not encountered. Finally,comparison of DGPS measurements from August 2010 at oneof the Lake Rhonda floating mats allowed comparison withthe mat outlines from aerial photography from July 2003 anddetermination of expansion rates within the most recent time.This investigation revealed that the average expansion rate ofthe floating mat on the northwestern end of Lake Rhonda(Figure 3(a)) was 1.79myr-1 between July 2003 and August 2010,confirming the persistence of expansion at high rates.

Discussion

Formation and distribution of floating mats

Floating vegetation mats can be found within the southern two-thirds of Alaska in the form of terrestrializing lakes or on theedges of open water (Viereck et al., 1992). In these conditions,peat typically develops outward from a shoreline through theprocess of terrestrialization and may form a border of floating or-ganic material or eventually completely fill a basin (Schwintzerand Williams, 1974; Haraguchi, 1991; Kuhry and Turunen,2006). Our observations of shoreline retreat on Lake Rhondaand Lake Owl for the past 60years (as detailed in Figure 3)demonstrate that the floating mats on these thermokarst lakesdo not indicate a process of terrestrialization but the opposite,which is lake expansion. We suggest the following mechanismto explain the formation of thermokarst mats at our study sites.First, terrestrial peat formed in low-center ice wedge polygonson permafrost adjacent to the lakes to an approximate uniformthickness. The vegetation in these low center polygons alreadycontained water-adapted species such as Carex sp. andEriophorum sp. As permafrost thawed and ground ice meltednext to the lake, the sediments underlying the terrestrial peat sub-sided and the structurally interconnected peat started to float(Figure 2). The buoyancy of the mats is attributed to trappedgas in the tissues of aquatic plants and in the form of bubbleswithin the pore space. Once the peat mat is detached fromthe underlying sediment a rapid selection process defineswhich plant species survive and thrive in the mats versus thosethat die off due to increased water stress, further enhancing thebuoyancy of the mat. This is similar to the process proposed byHopkins (1949) where the permafrost thaws horizontally intothe lake edge (formation of a thermo-erosional niche) and thensubsides en masse leaving only the buoyant vegetation on thewater surface. Furthermore, in the classic model for terrestriali-zation, the floating peat is wedge-shaped with the edge towardsthe lake center being thinner than the edge supported by theground (Clymo, 1991). Based on the uniform horizontal struc-ture observed with GPR (as seen in Figure 4), and the remotesensing-based observations of rapid expansion at these loca-tions, these vegetation mats probably did not form accordingto the classic terrestrialization model. As indicated by Irelandand Booth (2011), peat can form a floating mat if given an ap-propriate substrate for initiation, and then the hydraulic regimeis subsequently altered. In our case, the substrate is Holocenepermafrost on the periphery of the lake, and the change in hy-draulic regime would occur when this permafrost thaws andsubsides leaving the peat on the lake surface. A similar forma-tion mechanism has been described for mat formation aroundlakes on the North Slope of Alaska (Hinkel et al., 2003), butthe mats in that study shrunk after formation rather thanexpanding as those in our study do. Although insufficient infor-mation is available to conclusively determine why the vegeta-tion mats on the North Slope may differ from those on the



Seward Peninsula, we speculate that increased wave action(due to higher winds), contrasting vegetation communities orvariations in the peat properties may be responsible. Our con-ceptual model describing mat formation (Figure 6) suggests thatthe permafrost thaws at a lake edge allowing the mineral com-ponent to slump as the material volume is reduced with the re-moval of the ice matrix while the organic component remainsafloat. At the final perpetually stable stage, gas emissions fromthe talik may help to increase buoyancy of the floating mat.Additional information about the age of the floating organic

material may be useful for improving this model. For example,it is unclear if the oldest peat separates from the underside ofthe floating mat and falls to the lake bottom leaving only rela-tively young material in place, or if the mat remains entirelyintact over time. Accelerated mass spectrometry (AMS) radio-carbon dating of the base of the terrestrial peat (0.52m deep)in the nearby drained basin (north of the floating mats indicatedin Figure 1(c) revealed an age of 1110 �65cal yBP (Joneset al., ) suggesting that the age of mat material should rangefrom approximately that point in time to present.The distribution of floating vegetation mats on the eastern

and northeastern side of Lake Owl may be attributed to topog-raphy and wave action (based on estimated wind direction(Shulski and Wendler, 2007 p.103)) and the presence of a smalloutlet stream. The energy derived from flowing water may helpto propagate mat expansion, as active layer thickening and per-mafrost thaw are accelerated in areas where water is activelyflowing on the surface or through active layer seeps. While itis difficult to predict the exact prevailing wind direction at LakeOwl given limited historical data, summer winds on the SewardPeninsula generally blow towards the east (data from Kotzebue,AK (West and Plug, 2008)) or northeast (data from Nome, AK(Shulski and Wendler, 2007 p.103]). Therefore, when the lakeis not ice-covered, wave action perpendicular to the prevailing

Figure 6. A conceptual model for the formation of the floating vege-tation mats showing permafrost thaw at a lake edge and slumping ofthe mineral component while the organic component remains afloat.By the final stage, gas emissions from the talik may help to increasebuoyancy of the floating mat.


wind direction would preferentially erode the northeasternshoreline. This appears to be the case as seen in Figure 3,though it is difficult to attribute the spatial distribution of thevegetation mats exclusively to wind direction given the vari-able shoreline topography.

Expansion rate of floating vegetation mats

The pattern of expansion rates around these two thermo-karst lakes suggests that floating vegetation mats promoterapid degradation of surrounding permafrost that is a triggerfor mat growth, expanding at rates as high as 2myr-1 asmeasured from historic aerial photographs and corroboratedin recent years using high resolution DGPS technology. This ex-pansion appears to act as a positive feedback to further lakesidepermafrost degradation. Though it is challenging to relate thisexpansion rate to past studies given the limited existing re-search on the topic, it is safe to say these mats advance far morerapidly than those expanding only due to terrestrialzation at0.025myr-1 (Kratz, 1988). When accounting for the presenceof floating mats in a calculation of net lake expansion, themean expansion rate for Rhonda Lake increases from 0.51myr-1 to 0.55myr-1, whereas in Lake Owl it increases from0.56myr-1 to 0.58myr-1. For ground truth comparison pur-poses, the floating vegetation mat on Lake Rhonda is expand-ing at a maximum rate of 1–2myr-1 according to the remotesensing analysis (the north side of the largest mat, Figure 3(a)),while the GPS survey determined a similar expansion rate of1.79myr-1.

Subsurface processes

The buoyancy of floating vegetation mats can be generally at-tributed to the presence of free-phase gas whether in the formof bubbles or trapped in the aerenchyma tissues of shoots, rhi-zomes and roots of plants (Hogg and Wein, 1988a). It has beenshown that the gas trapped within Typha sp. can be a primarycontributor to the buoyancy of floating peat (Hogg and Wein,1988a). Though specific relationships between C. aquatilisand buoyancy are not available for comparison, the Carexaquatilis that dominates the floating mats observed in this studyhas aerenchyma and could hold gas to keep the mats afloat.Large aerenchyma in C. aquatilis have been identified as a sub-stantial factor for plant-mediated CH4 emissions from floodedlow center polygons in Siberia (Kutzbach et al., 2004). It hasalso been found that some organisms can develop aerenchymawhen exposed to flooded hypoxic conditions (Justin andArmstrong, 1987). Using a relationship between the density ofthe vegetation and the required buoyancy to keep the matafloat, Hogg and Wein (1988b) estimated the free-phase gascontent of boreal Typha sp. floating mats in New Brunswick,Canada to be 3.1–8.4% depending on temperature. Similarly,Fechner-Levy and Hemond (1996) estimated that a sub-borealSphagnum dominated mat in Massachusetts had between 3%and 11% gas content in the form of bubbles, depending on sea-son and temperature history. These values compare favorablywith free-phase gas content of 1.5–5% estimated using GPR.Given that our study site is located in a colder subarctic climatewhile Hogg and Wein (1988b) and Fechner-Levy and Hemond(1996) study sites are in warmer boreal locations, it is reason-able to assume the cooler temperature regime of the NorthernSeward Peninsula would yield volumetric gas contents on thelower end of the ranges estimated in previous studies.



The role of floating mats in carbon cycling

Using the total peat volumes of the floating mats as determinedthrough geophysical and remote sensing methods and an esti-mated carbon density of 0.049gC cm-3 (Vitt et al., 2000), wecalculate that there are 27kgC m-2 in the floating vegetationmats on Lake Rhonda and Lake Owl. This estimate would beinfluenced by the accuracy of the assumed carbon densityvalue, which in the case of Vitt et al. (2000) is based on a largevariety of peat deposits in central Canada. Including floatingmats in the overall change in area and expansion for an individ-ual lake may not make a large difference because these featuresare proportionately small in size. However, the floating matsprobably factor into the carbon budget for a particular lake be-cause in lakes that freeze to the bottom (i.e. <1.5m waterdepth) the majority of the benthic sediments in the central por-tion of the lakes are likely frozen solid preventing winter CH4

production while sediments beneath the mats remain thawedand are able to support CH4 production year-round.Although floating vegetation mats store carbon, they may

also be sources of CH4 to the atmosphere. A preliminary CH4

flux dataset was obtained on the floating mat at Lake Rhondaduring the summer of 2010 where 15–35L static aluminumchambers were placed over sample points and four gas sampleswere extracted during incubation periods of 30–40min at eachsite (following the method of Corradi et al. (2005)). The resultsindicated that the moat was emitting CH4 at a rate of more than1g CH4 m-2day-1. Although this dataset was collected at onlyone point in time, the higher fluxes observed on the mat andin the peripheral moat, which were 2–10 times larger than ad-jacent low center polygonal tundra, suggest that the mats couldbe potential strong methane sources but additional research isneeded to fully quantify the gas flux from floating mat land-forms. CH4 fluxes from thermokarst lakes have implicationsfor up-scaling to regional carbon flux calculations. Remotesensing studies assign expected CH4 flux potentials to variouslandforms including lakes, lake margins, polygonal tundraand unvegetated lake margins in order to extrapolate frompoint measurements to regional estimates (Schneider et al.,2009). If CH4 fluxes as high as our preliminary data indicate(1066mg CH4 m-2day-1) can be confirmed in future researchacross floating vegetation mats that occupy ~4% of the surfacearea of a second-generation thermokarst lake, reclassificationof these landforms to a more prominent status as CH4 emittersmay be necessary.

Conclusions

Floating vegetation mats form on the northern Seward Peninsulaafter thawing of the permafrost and surface subsidence innear-lake shore settings, resulting in a process distinctly differ-ent from terrestrialization commonly observed in boreal peatecosystems. Thermokarst mats are persistent over at least de-cadal time-scales, generally increasing in area rapidly alongthe permafrost thaw front, and eroding slowly at the open-waterlakeside margin. The water beneath floating mats does notfreeze to the bottom in winter, allowing year-round talik growthand microbial CH4 production within the talik. Floating vegeta-tion mat expansion rates are higher than erosion rates of vari-ous other lake margin types. Remote sensing estimates of lakearea based on surface water extent may be compromised bythe presence of floating vegetation. We have shown that GPRcan be used to determine the subsurface dimensions of floatingmat and to estimate the trapped gas content responsible forkeeping the mats afloat.


Acknowledgements—We would like to thank M. Smith for assistancewith field collection of geophysical data. We also thank D. Swanson(National Park Service) and two anonymous reviewers for suggestionsthat greatly improved the quality of this manuscript. We thank theNational Park Service for permitting this research in Bering LandBridge National Preserve. This research was funded through generoussupport of NASA under Carbon Cycle Sciences grant NNX08AJ37G andby NSF under OPP IPY grant #0732735.

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