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Home , Ice segregation, Ice wedge, Palsa

, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

Ground-ice features and depth of across a chronosequence, NW Alaska

L.J. Plug Department of Earth Sciences, Dalhousie University, Halifax,

ABSTRACT: Morphology and accumulation rates of peat vary over 102–103 y in permafrost peatlands because of interactions between vegetation, permafrost and geomorphology. Measurements of peat depth, active-layer depth, and distributions of ground-ice features are reported here for a 4400 y peatland chronosequence in swales of the Espenberg beach-ridge plain, NW Alaska, with swale ages constrained by 55 radiocarbon assays. Permafrost aggrades to 1.5 m deep within 500 years after swales are isolated from the sea. Continuous peat layers form by 600 y, followed by initially rapid and then slower rates of peat accumulation, reaching depths of 1.2 m by 1500 y but only 1.9 m at 4400 y. Frost blisters occur in peatlands 600–1200 y old, where peat does not extend the full depth of the , facilitating injection of water under hydrostatic pressure through unfrozen sand during early winter. and peat ridges first occur in large numbers after approximately 1200 y and persist thereafter. Peat ridges at Espenberg are sinuous, elongate palsas whose planform and initiation is attributed to cooling-induced tension fractures.

1 INTRODUCTION

Ground-ice features, a broad class of geomorphic forms composed of accumulations of ice in the ground, are common in level terrain in the Arctic and and display dynamics over time scales rang- ing from vertical heave of 3 my1 at the ground sur- face over expanding bodies of intrusive massive ice (e.g. van Everdingen 1982) to 1000 y life-cycle of Figure 1. Location of the Espenberg beach-ridge plain mounds underlain by segregation ice (Seppälä 1986). (inset) and aerial photograph and ground transects Because ground-ice features commonly occur in oth- described in the text. erwise flat peatland terrain where small differences in elevation drive relatively significant changes in moisture and microclimate, dynamics of ground-ice features can drive variations in vegetation and ground- the assumption that this represents evolution of these water properties. properties over thousands of years for a single surface. Most measurements of ground-ice features address Three geomorphic features are considered here: 1) time-scales of less than one year (e.g. Pollard and Frost blisters, sometimes called hydrolaccolith mounds, French, 1984), to a maximum of several decades (e.g. are up to several meters high with cores of one or more Mackay and Burn, 2002) because this is as long as layers of intrusive ice which presumably form by injec- instrumentation, remote sensing, or direct observation tion of water ahead of rapidly descending freezing have been possible. Longer time-scale evolution has fronts (Outcault et al. 1986; Pollard & French 1984). 2) been inferred from features at varying stages of devel- mounds are similar in size to frost blisters, but opment, but without absolute age constraints (Matthews have cores rich in segregation ice which form over many et al., 1997). Here, the development of three ground-ice years by repeated migration of water towards freezing features – frost blisters, palsa mounds and peat ridges – fronts where cooling is locally enhanced (Seppälä over thousands of years is investigated primarily using 1982). Although characteristically found in areas of dis- a chronosequence of peatland surfaces underlain by continuous permafrost, palsas also occur in areas where permafrost at Espenberg, NW Alaska (Figure 1). A permafrost is continuous (Washburn 1983). 3) Unlike chronosequence is a series of contemporary surfaces of the previous two features, peat ridges at Espenberg do different age which together depict the probable history not correspond to an existing classification. These are of a single site over time; for Espenberg, contemporary linear features up to 1.5 m high, 3 m across and hun- properties including depth of peat, depth to permafrost, dreds of meters in length, with ice-rich cores of peat and the frequency of occurrence of ground-ice features similar to those that underlie palsas. The origin of peat are described for age-constrained swale surfaces under ridges at Espenberg is considered later.

901 2 ESPENBERG SWALE-PEATLAND of and poor fen vegetation dominated by sedge CHRONOSEQUENCE and cotton grass persists, interrupted by patches of vegetation such as and ericaceous The Espenberg beach-ridge plain is a 30 2 km, east- shrubs on the raised, drier surfaces of palsa mounds west trending sandy spit attached to the northern coast and peat ridges. of the Seward Peninsula, Alaska (Figure 1), located at 66.6°N in the zone of continuous permafrost (Brown et al. 1997). Mean annual temperature is 5.8°C 3 METHODS (NOAA 2000). The beach-ridge plain formed during late-Holocene periods of steady wave climate and Frequency of occurrence for frost blisters and palsas reduced storm intensity in the Chukchi Sea (Mason et (the number of individual features per square meter of al. 1997). Series of shore-parallel ridges approximately swale peatland) and the linear meters of peat strings 0.5–2 m high and 3–20 m wide, separated by 10–100 m (meters of peat string per square meter of swale peat- wide swales, were added to the beach ridge plain during land) were measured along five aerial-photograph progradational periods from approximately 4500 to transects and two ground transects (Figure 1), each 3300 BP and 1700 to 1200 BP, whereas shoreline trans- with orientations perpendicular to local beach ridges. gression and dune building occurred along most of the Seven to fifteen sample plots of area 2500 m2 were dis- Espenberg coastline from approximately 3300–1700 tributed along each transect; the number and dimen- BP and 1200–0 BP (Mason et al. 1997). The ages of sions of plots was varied because the number and ridges and swales at Espenberg are constrained by width of swales is not constant along the beach-ridge fifty-five radiocarbon assays on samples from archaeo- complex (Mason et al. 1997). Near-infrared 1:6000 logical contexts (n 32), basal swale and buried scale images (National Park Service, 1987), which humic horizons in ridges (n 14), buried driftwood have resolution sufficient to resolve individual fea- (n 4), and marine shells (n 5). Dates on marine tures approximately 0.5 m across (Figure 3), were used materials are adjusted by 400 y to compensate for for aerial-photograph transects. Frost blisters, palsas, regional reservoir effects due to old oceanic carbon and peat ridges were identified on aerial-photographs (Mason & Ludwig 1990; Stuiver et al. 1993). A com- through vegetation- and moisture-dependent colour plete listing of all radiocarbon assay samples and their signatures and plan-view morphology. Two ground location and stratigraphic context is given in Mason transects located along the path of aerial-photograph et al. (1997). Here, these age determinations are used transects were used to test these interpretations. to estimate ages of swale peatlands (Figure 2). Surface elevation, depth of peat, and depth to perma- Vegetation at Espenberg is broadly characterized frost were measured along one shared ground/aerial as “coastal meadows-dwarf shrub mosaic” transect (transect A, Figure 1), using a closed survey (Anderson et al. 1974). Vegetation varies with swale age with a theodolite, motorized hollow-auger corer, and (Plug, unpublished data), ranging from disturbance- steel probe, respectively. Depth to permafrost was favored xeric species such as Elymus arenarius mollis measured in early September, near the close of the (rye grass) along the modern beach and youngest thaw season at Espenberg. Peat cores were located swales, followed in the next swales by the introduction midway between ridges that bound swales where peat of shrub-tundra species including Empetrum nigrum, depth is presumably greatest. Vaccinium uliginosum, and Salix spp. (willow). From To investigate the origin and characteristics of the 8th swale (ϳ1200 y old) onward, a mesic community ground-ice features, a core was taken from one frost

Figure 2. Chronology and altitude across the Espenberg beach ridge plain, measured along transect A (Figure 1). Altitudes are meters above low tide. Ridge numbering scheme and the 55 radiocarbon assays that constrain the ages of swales are out- lined in Mason et al. (1997). The oldest swale at Espenberg (formed at approximately 4500 BP) is absent from the region crossed by this transect because of localized at the lagoon bluff.

902 blister and trenches were excavated across two palsas three years, to complete collapse for two approxi- and eleven peat ridges. To determine whether peat mately 0.5 m tall frost blisters over a period of two ridges at Espenberg are associated with a nearby ice- years (Table 1). In 1993, prior to their collapse, cracks wedge, 10 cores with spacing 0.5 m were drilled in a 10 cm wide and several meters long in unfrozen sur- straight line across and perpendicular to one large face peat were observed in the two frost blisters that peat ridge. Fixed markers on the crests of three frost thawed. Frost blisters at Espenberg first occur in peat- blisters, three palsas, and two strings were surveyed lands ϳ400 y old (Figure 4), coinciding with the first over the four successive summers of 1993–1996 to measurable accumulation of peat (Figure 5). No frost measure rates of vertical displacement through frost blisters occur in swale peatlands 1500 y old along heaving or thaw-induced subsidence. For interannual any transect. comparisons, benchmarks of 1.5 m rebar rod were Palsa mounds at Espenberg are generally 2–4m driven into sandy beach ridges which are well-drained across and 0.5–1 m high, reaching up to 6 m in diame- and therefore not susceptible to , The ter and 1.8 m high. Permafrost in palsas is rich in inter- estimated error of survey results is ϳ1 cm, based on stitial ice and lenticular ice, together approximately results of repeated closed surveys. Table 1. Measured rates of annual vertical displacement for frost blisters (FB), palsas (Pa) and peat strings (Str). 4 RESULTS 1994 1995 1996 Interpretation (m/y) (m/y) (m/y) Ground-ice features at Espenberg display the following general properties: FB I* 0.02 0.00 0.03 stable, or slow collapse Frost blisters are lobate in shape and up to 20 m in FB II* 0.11 0.30 0.02 collapsed in 2 years * diameter and 1.4 m high. Most are approximately FB III 0.02 0.40 0.09 collapsed in 2 years * 10 m across and 1 m high. The cored frost blister Pa I 0.01 0.03 0.01 stable or slow heave * showed 3 discrete layers of clear pure ice 20 cm, 8 cm Pa II 0.06 0.04 0.00 intermittent? Pa III* 0.00 0.02 0.01 stable and 12 cm thick respectively – the lower two layers Str I †a 0.03 0.03 0.02 heaving separated by a 3 cm layer of sedge peat and the upper b 0.06 0.05 0.02 rapid heaving two differentiated by an abrupt change in the orienta- Str II†a 0.01 0.00 0.00 stable tion and number of intra-ice bubbles. Measured rates b 0.00 0.01 0.00 stable of vertical displacement vary widely between speci- † mens, from apparent stability for one frost blister over *Measurements taken at feature crest. Measurements taken at two points (a,b) separated by 5 m along feature crest.

0.0012

0.0008 ) -2 0.0004 (m

0 Blister frequency 0 500 1000 1500 2000 2500 3000 3500 4000 4500 -0.0004 Surface Age (y)

0.008 0.006

) 0.004 -2

(m 0.002 0 Palsa frequency 0 1000 2000 3000 4000 5000 -0.002 Surface Age (y)

0.1

) 0.07 -1

0.04

meters (m 0.01 Figure 3. Monochrome reproduction of a region from a Peat ridge linear 0 500 1000 1500 2000 2500 3000 3500 4000 4500 ϳ -0.02 colour-infrared aerial photograph of a 3500 year old Surface Age (y) swale peatland surface at Espenberg. Black and dark gray areas are standing water and wet sedge vegetation. Light Figure 4. Frequency of occurence of ground-ice features regions of palsas (long arrow) and peat strings (short (top – frost blisters; middle – palsa mounds; bottom – peat arrow) stand 20–80 cm high and support herbaceous and ridges) as functions of swale surface age for 5 transects. shrub vegetation. Error bars denote 1 S.D.

903 50% of total soil volume, with 2–5 mm thick lenses peat along the transect is 1.9 m, at 4400 y. An anom- predominantly parallel to surface morphology (struc- alous decrease in peat depth with surface age centred tural and lenticular composite cryostructure (Murton & on the 2000 y surface, where peat depth decreases from French 1994)). Non-ice composition is peat with some 1.2 m at 1800 y to 0.95 m at 2000 y, probably is due to sand or sand-rich layers. Peat and sand layers in per- core location coinciding with raised morphology in mafrost are involuted and folded with wavelengths of sand that underlies swale peat. ϳ30 cm. These signs of post-depositional deformation commonly extend upward into palsas’ active layers. Measurements of vertical displacement for palsas are 5 DISCUSSION only slightly greater than the magnitude of instrument error, and therefore are consistent with stability or low 5.1 Validity of the Espenberg chronosequence rates of displacement with interannual change of sign (Table 1). Palsas first appear on peatland surfaces The utility of chronosequences for inferring geomor- approximately 600 y old where they are rare and small, phological or ecological change can be compromised if less than 2 m across and 0.3 m high, and occur in large the assumption of uniform conditions is flawed owing numbers on 1200 y and older surfaces on most tran- to changes in the external environment during site his- sects (Figure 4). tory (Fastie 1995, Mann et al. 1995). At Espenberg, Peat ridges at Espenberg are sinuous features up to however, all swales are formed by beach sand of simi- hundreds of meters long, but are otherwise similar in lar lithology and grain size (Mason et al. 1997), and all morphology, composition, and distribution to palsas. swales fall within a narrow range of elevations and Despite the crude polygonal pattern formed by inter- were probably at similar elevations above sea level at connected peat ridges (e.g. Figure 3), similar to ice- the time of formation (Mason et al. 1997). Imperfec- wedge networks, no ice wedges were found in a tions in the Espenberg chronosequence include tem- transect of cores across a peat ridge. Measurements of poral gaps and unequal representation of surfaces due vertical displacement for peat ridges are variable to time-varying rates of shoreline progradation. between ridges; one peat ridge showed no displace- Changes in storm intensity in the Chukchi Sea are ment whereas another ridge heaved up to 6 cm over hypothesized to have caused varying rates of progra- one year and 5 cm the next and displayed ripped peat dation, and any corresponding significant variations along both its flanks. Peat ridges are absent or infre- in temperature and precipitation might challenge the quent on surfaces 1000 y old but conspicuous on assumption of approximately uniform conditions most surfaces older than 1200 y (Figure 4). because the development of ground-ice features, includ- Peat depth generally increases with surface age at ing palsas, is sensitive to temperature thresholds (e.g. Espenberg (Figure 5). Plant litter first accumulates to Washburn 1980). However, paleoecological reconstruc- form a continuous organic soil horizon after 600 y. Peat tions of climate show distribution in NW Alaska depth increases to 1 m by approximately 1500 y, fol- during the mid-Holocene broadly similar to today lowed by slower rates of peat accumulation on transect (Edwards et al. 2000), suggesting that variations in surfaces from 2000 to 4400 y. The maximum depth of climate at Espenberg have been too small to signifi- cantly affect development of geomorphic features. Measurements of ground-ice feature distribution 200 200 along transects display qualitative consistency between 180 180 transects. For example, frost blisters do occur in swale 160 160 peatlands 1700 y old, but are entirely absent in older swales. However, the frequency of occurrence of 140 140 Peat ground-ice features is inconsistent between transects – 120 120 for example, on ϳ1000 y old swale peatlands, frost 100 Active 100 blisters are abundant on transects D and E but absent on Layer 80 80 A and C, resulting in relatively large standard devia- 60 60 tions (Figure 4). One explanation for this range of vari- a 2 Depth of Peat (cm) 40 40 ation between plots is that the plot size used, 2500 m , is too small to capture stochastic variations in peatland 20 20 surfaces. However, larger plot sizes are not feasible 0 0 at Espenberg because swales less than ϳ1000 BP are 0 2000 4000 narrow or are absent across most of the beach-ridge Surface Age (y) plain. All natural chronosequences are likely imperfect Figure 5. Depth of peat and active layer depth in swales owing to heterogeneity inherent to natural systems. along transect A (Figure 1). The following discussion rests on the assumption that

904 the series of swales at Espenberg form an imperfect due to underlying permafrost at Espenberg (i.e. water but valid geologic chronosequence that is useful for cannot drawn to the palsa from below). The mecha- investigating development of periglacial geomorphic nism that limits diameters to the relatively small range features. of ϳ2–6 m at Espenberg is unclear. One hypothesis is that growing palsas reach these diameters upon achieving heights sufficient to trap blowing , as 5.2 Characteristics and dynamics of has been proposed to limit horizontal expansion of ground-ice features palsas in the discontinuous permafrost zone (Seppälä 1994). These are, however, generally much higher than The distribution of frost blisters at Espenberg pro- Espenberg palsas. Snow is significantly redistributed vides the following constraints on their development. at Espenberg by winds that average 20 km/h during First, frost blisters are found only in swales underlain winter months, and gusts to several times that common by permafrost, consistent with the view that per- (NOAA 2000). Espenberg palsa diameter may be mafrost, which confines free water ejected by down- limited by ideal conditions for drifting snow. ward freezing, is necessary for the development of hydraulic potentials sufficient to rupture and deform a 5.3 Origin of peat ridges at Espenberg frozen overburden (e.g. Pollard & French 1984). Second, the absence of frost blisters from old swale Peat ridges and palsas at Espenberg are similar in peatlands at Espenberg indicates the presence of a composition and distribution, consistent with the view limiting factor to frost blister development. Availabil- that peat ridges are elongate palsa that principally ity of water during freeze-up probably is not this lim- grow in height through accumulation of segregation iter, because altitude, swale morphology, and soil ice. However, this view does not account for the plan- water levels are similar across swale peatlands where form of ridges, some of which are 100s of meters in frost blisters do and do not occur. An alternative length. Sinuous raised peat strings, superficially sim- explanation is that injection of water ahead of enclos- ilar to peat ridges at Espenberg, are found in strang- ing freezing fronts, the key process in frost blister moor, the patterned peatlands found in regions of development, requires the presence of an unfrozen milder climate (e.g., Seppälä and Kouteneimi 1985). layer of mineral soil with high hydraulic conductivity. However, unlike strangmoor, Espenberg peat ridges In this view, frost blisters generally are limited to are not consistently oriented perpendicular to slope. peatlands where active layer depth exceeds the depth Some peat ridges at Espenberg intersect to form crude of peat. networks characterized by orthogonal intersections The distribution of palsas at Espenberg constrains between ridges (e.g. Figure 3), which are broadly sug- general properties of these features. First, substrates gestive of rampart patterns that form over some ice- at Espenberg are medium sand and peat or mixtures wedge networks (e.g. Plug & Werner 2002). However, thereof, indicating that palsa development is not peat ridges often are sinuous at small scales, 2m restricted to substrates with a fine-grained mineral (Figure 2), whereas ice-wedge ramparts are straight or component. Second, palsas at Espenberg occur in more gently curved. Moreover, the absence of mas- 1500 year old swales which lack mineral (sand) sive ground ice in the coring transect across a long horizons in the active layer, in contrast to the view that peat ridge suggests that the peat ridge was formed by palsa formation requires the presence of some mineral another mechanism, or that an ice-wedge was present soil because does not occur in pure but too narrow (0.5 m) to be detected in the transect. peat (e.g.Williams & Smith 1989). One explanation is One explanation consistent with measurements is that that palsas on old surfaces at Espenberg are persistent peat ridges initiate as low ramparts that follow cool- features that formed when peat was shallower than the ing-induced tension fractures, and then grow active layer. However, ice segregation also might vertically and horizontally primarily through accu- occur in pure peat at Espenberg, as occurs in many mulation of segregation ice at the permafrost/active pure-peat palsas in discontinuous permafrost (e.g. layer boundary. Seppälä 1988). The latter view is consistent with the large number of palsas on the oldest surfaces, which suggest that new palsas continue to form in pure peat 5.4 Rates of peat accumulation active layers. Palsas at Espenberg are much smaller than most As inferred from measurements of peat depth along described in the literature, especially those in the zone the core transect, peat accumulates in swales at of discontinuous permafrost (e.g. Seppälä 1988). Espenberg at a rate of 0.8 mm y1 for the first ϳ1500 y Their lesser height may be attributable to reduced after a swale is isolated from the sea, and there- availability of unfrozen water to drive ice segregation after at a decreasing rate that averages approximately

905 0.24 mm y1 for years 1500–4400. In comparison, Alaska, U.S.A. Journal of Coastal Research 13: typical Holocene rates of peat accumulation in north- 770–797. ern , inferred from single peat cores rather than Mason, O.K. & Ludwig, S.L. 1990. Resurrecting beach- ridge archaeology: parallel depositional histories from chronosequence studies, are of order 0.5 mm y1 St.Lawrence Island and Cape Krusenstern, Alaska. (Gorham 1991). The decrease in peat accumulation Geoarchaeology 5: 349–373. rate at Espenberg follows disruption of the smooth Matthews, J.A., Dahl, S-O., Berrisford, M.S. & Nesje, A. mire surface by perennial ground-ice features, palsas 1997. Cyclic development and thermokarstic degrada- and peat ridges. Further work will investigate the tion of palsas in the mid-alpine zone at Leirpullan, effect of palsas and peat ridges on ground water flow Dovrefjell, southern Norway. 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