Late-Quaternary Paleoenvironmental Record from a Palsa-Scale Frost Mound in Northern Alaska

Late-Quaternary paleoenvironmental record from a palsa-scale frost mound in northern Alaska

W.R. Eisner & K.M. Hinkel Department of Geography, University of Cincinnati, Cincinnati, OH, USA F.E. Nelson Department of Geography, University of Delaware, Newark, DE, USA J.G. Bockheim Department of Soil Science, University of Wisconsin, Madison, WI, USA

ABSTRACT: A peat-covered, palsa-scale frost mound was discovered near the White Hills on the North Slope of Alaska, in the continuous permafrost zone. The palsa is situated within a partially drained lake basin and currently supports willow-birch shrub vegetation very dissimilar to that of the surrounding bog. In 1997 and 1998, the palsa was cored to a depth of 1.83 m; the upper 1.78 m contains undisturbed (non-cryoturbated) layers of frozen peat while the lower 5 cm is lacustrine silt containing thin ice lenses. The organic sediments were analyzed for pollen, fungi, algae, and other microfossils. Combined with sediment textural analysis, the record demonstrates local changes in hydrology, mound formation, and vegetation succession in response to landscape processes. In situ wood from the core from a depth of 123 cm yielded an age of 9610 60 14C yr BP, indicating that the palsa contains organic sediments encompassing the entire Holocene.

1 INTRODUCTION combination with pollen analysis, could be used to esti- mate timing of the onset of frost mound growth. “Palsa-scale frost mound” is a generic term used to This paper reports results of a palsa coring pro- describe a variety of morphologically similar aggrada- gram. Patterns of pollen, soil organic carbon, and soil tional features developed within the active layer above texture from a core collected from a palsa summit are permafrost. Although several formative processes have analysed. The palsa is located in an area that has under- been identified, the positive relief of all frost mounds gone significant landscape evolution, with consequent is ascribable to accumulation of ice in the substrate impact on local hydrology and vegetation. These local (Nelson et al. 1992, Gurney 2001). patterns are superimposed on a long-term signal reflect- The periglacial literature contains a great deal of ing regional climate and vegetation changes in the late controversy about the term palsa. At its root lies Quaternary. Our primary goal is to use pollen and soil disagreement about whether classification schemes analysis to test the feasibility of separating the signals should be based on morphology, genesis, constituent and reconstruct local landscape processes. materials, or combinations of these parameters. In this paper we follow conventions established by Washburn (1983a, b) and Nelson et al. (1992), using “palsa” in a 2 SITE DESCRIPTION AND FIELD METHODS strictly morphological sense with reference to medium- scale frost mounds, independent of genesis, constituent During the summer of 1997 we investigated a frost material, or location in the hierarchy of permafrost mound located west of the White Hills on Alaska’s continuity. North Slope (N 69°29.34, W 150°05.27). The site is Terminological issues aside, palsa-scale frost near the boundary between the Arctic Coastal Plain and mounds can be important sources of paleoenvironmen- Arctic Foothills physiographic provinces and lies within tal information (e.g., Vorren 1972, Vorren & Vorren the zone of continuous permafrost. The mound meets 1975, Lavoie & Payette 1995). Once a palsa is formed, Washburn’s (1983b) morphological definition of a its microclimate changes and new plant associations, palsa: 6 m in height, it has a basal diameter of 12 m and very different than those of surrounding low areas, contains abundant peat. It is developed near the edge occupy palsa summits (Sjörs 1961, Railton & Sparling of a series of nested, partially drained lake basins 1973, Seppälä 1990). Vegetation differences atop and (Fig. 1). A prominent outer beach scarp is associated around frost mounds can play a useful role in applying with an older lake basin. A less pronounced beach 14C techniques to date the inception of mound growth. scarp, formed by a younger lake, lies basinward and is Vorren (1972, 1979a, 1979b) and Vorren & Vorren highlighted in Figure 1. The younger basin has been (1975) first demonstrated that radiocarbon dating, in partially infilled by sediments and vegetation. The palsa

229 Figure 1. Aerial view of palsa, bog, and pond; inner beach scarp is noted with dotted line. is located near the outlet of the remnant lake, where it drains into an adjacent basin through a narrow strait. The climate of the site is cold and semi-continental, with estimated mean annual temperature of 10.7°C and annual monthly range of 41.4°C (Willmott & Matsuura 2001). Although the thickness of the permafrost is unknown, it is estimated to exceed 300 m based on an assumed geothermal gradient of 3°C/ 100 m. Average snowcover depth has not been meas- Figure 2. White Hill core textural and compositional ured, but snowdrifts on the palsa’s flanks are likely to analysis. be substantial. Vegetation atop the palsa and flanking slopes is a shrub assemblage consisting of Betula sieve and characterized using methods established by nana (dwarf birch), Salix glauca (willow), and Ericad the Soil Survey Staff (1996). These analyses included (heath), with some mosses and grasses. Thaw depth, easily oxidizable organic C by dichromate digestion measured in mid-August in two years, averaged 45 cm. (6A1a) and particle-size distribution with sand and silt By contrast, vegetation in the bog surrounding the fractionation by sieving and pipette, respectively (3A, palsa is dominated by Carex aquatilis (sedge), and the 3A1). The results are summarised in Figure 2. thaw depth averaged 28 cm. Subsamples were prepared for pollen analysis A sediment core was obtained from the bog sur- by standard procedures including treatment with rounding the palsa, at a distance of 15 m from it, using hydrofluoric acid and acetolysis (Berglund & Ralska- a 7.62 cm (3 inch) diameter coring barrel designed by Jasiewiczowa 1986). A known amount of Lycopodium © USA CRREL, driven by a Little Beaver power auger. clavatum spores was added in tablet form to each A second core, 128 cm in length, was obtained from a sample to determine pollen concentrations and as a position atop the palsa. Peat was present throughout processing control. Samples were mounted in silicon the core, indicating that we had not penetrated into the oil and identified at 400 magnification. Pollen massive ice core. Although equipment limitations pre- accumulation rates were not calculated. Identification vented drilling deeper in the palsa in 1997, we returned of algal, fungal, and zoological remains was carried to the site the following year and exactly re-occupied out using an extensive collection of photographs and the borehole. We used extension rods and core extrac- reference material (Van Geel 1978, 1986, Eisner & tors to penetrate an additional 54 cm, to a total depth Peterson 1998a, b). Photographs of the microfossil of 182 cm. The palsa core can therefore be considered types mentioned in the text, as well as further continuous. The extended section of the core showed information on type ecological significance, can be that peat continues downward another 50 cm, and is found online (Eisner 2000). derived from aquatic plants. The peat is underlain by ice-rich lacustrine silts typical of thaw lake bottom sediments. Thin (3 mm) ice lenses grade into massive 3 STRATIGRAPHY AND DATING segregation ice in the palsa core. Cores were sampled at 10–15 cm intervals for soil Cored sediment from the palsa showed little evidence of bulk density and texture, pollen, and microfossil con- mixing or deformation, except near the base where tent. The organic carbon content was discontinuously some cryoturbation was apparent. Discreet macrofossils sampled in the upper 40 cm, but regularly sampled at a included twigs from willow shrubs and complete leaf 5-cm interval from 40 to 130 cm depth. Oven-dried soils and seed remains. These were found throughout the (dried at 65°C for 48 hr) were passed through a 2-mm palsa core, and are ideal for identification and dating. 230 Carbon stored in the palsa amounts to 135 kg C m3, indicate the timing of frost mound incipience. However, greater than that reported from other sites on Alaska’s the core collected from the adjacent lake margin bog North Slope (Michaelson et al. 1996). was found to contain modern aquatic plant roots in the Results from AMS dating are reported in Table 1, upper 26 cm, and this was immediately underlain by and confirm that the palsa contains organic sediments lacustrine silts. Lacking a thick sequence of peat to encompassing at least the entire Holocene. The consis- date, we chose to discontinue further analysis of the bog tency of the dates demonstrates stratigraphic integrity core and focus exclusively on the palsa. throughout the 9,000 yr record. The pollen and microfossil percentage diagram (Fig. We also obtained dates from plant material in the 3) has been divided into local Zones A (base at 128 cm) lower core (128–183 cm), as indicated in Table 1. to D (near-surface). Zone discrimination is based on Although marginally younger than the sediments imme- qualitative analysis of pollen, microfossils, radiocarbon diately above, this unit was interspersed with plant dates, and sediment texture. Fungi, algae and other roots, making precise dating problematic. The dates, microfossils are presented as percentages of the pollen and the composition of the material, indicate that these sum. Fossils are represented as percentages of the total sediments represent a basal detrital unit composed of pollen sum, although they are not part of the total fossil eroded, reworked, and redeposited peat, organic silts, pollen. More than 30 fungal, algal, and rhizopod types and roots of aquatic plants (Hopkins & Kidd 1988). This were identified during the analysis, although only those unit was deposited in a lacustrine environment, and has discussed here are depicted in the diagram. little stratigraphic integrity. The pollen and microfossils Zone A (128–98 cm) comprises most of the early and were derived from older reworked materials, or could middle Holocene, beginning around 9600 14C yr BP. have been transported from elsewhere in the watershed. This zone represents a period of several thousand years, Because the pollen and microfossils represent a vari- and the pollen reflects a series of major vegetation fluc- ety of environments, we concluded that this layer tuations. The local environment of Zone A is character- (128–183 cm) was unsuitable for detailed analysis. ized by birch (Betula), fluctuating between 3 and 28%, Initially we hypothesized that a comparison of pollen heaths (Ericaceae) and willow (Salix). Dominant herbs stratigraphy from the palsa summit and the adjacent are sedge (Cyperaceae) and grass (Gramineae), which bog sediment, combined with dating at the point of also fluctuate within the zone, becoming dominant dur- divergence between the two pollen signals, should ing the period in which shrub pollen decreases. The major non-pollen microfossil, type 55A, is a fungal spore that favors mesotrophic (moderately nutrient-rich) 14 1 Table 1. Radiocarbon and calibrated C ages conditions and peaks concurrently with grass pollen. CAMS Lab nr. Depth (cm) 14C age Cal age ranges Most of the pollen and microfossil fluctuations represent the influence of local vegetation and landscape 43556 3–8 modern 1954 AD 43557 40–45 1420 60 596–665 AD processes. However, alder (Alnus) and spruce (Picea) 42609 65–70 4720 100 3633–3556; pollen are present through most of the core in small 3540–3496; amounts (5–10%), suggesting that these are products 3465–3375* of long-distance transport. The poplar (Populus) peak 42610 120–123 9610 60 9175–9111; 9008–8888; (12%) at 107 cm may indicate climate change. The 8883–8821* poplar rise occurred in northern Alaska and northwest- 53255 156–161 9190 50 uncalibrated** ern Canada between ca. 11,000 and ca. 8000 14C yr BP 53256 176–182 9440 50 uncalibrated** and is generally described as a period of increased solar 1 CALIB rev. 4.3 (Stuiver & Reimer 1993); * BCE.; ** second (lower) core. insolation (Ritchie 1984, Anderson & Brubaker 1994,

Depth (cm) 0 Picea/PinusAlnus Betula EricaceaeSalixPopulusJuniperusArtemisiaCyperaceae GramineaeEquisitumSphagnumNupharForbs Unidentifiabletype type3 type16 Type20 type 46 55AType 79-hyphaeType 332 352 Arcella353 Testate amoebaAlgae RhizopodsFungal remains 10 Zone D 20 30

40 Zone C 50 60 70 80 Zone B 90 100 110 Zone A 120 130 20 20 40 60 80 20 20 20 2040 60 20 20 20 20 20 2040 60 20 20 20 20 40 60 20 20 40 60 80

Figure 3. White Hills pollen and microfossil percentage diagram.

231 Bartlein et al. 1995). Poplar pollen does not have a wide increased peat accumulation along the Sphagnum/ dispersal range (Edwards & Dunwiddie 1985), and the sedge bogs of the lake margin. percentages found at this level may indicate establish- Zone C (65–20 cm) shows a long period of increas- ment of a localized poplar grove some time after 9600 ingly terrestrial conditions, indicated by the preva- 14C yr BP and before 4720 14C yr BP. lence of birch (15–22%), heath (1–10%), and willow Sediments in Zone A (Fig. 2) contain a large compo- (2–5%). Sage (Artemisia) pollen increases for the first nent of organic material (25–35%) and silt (35–45%), time (to 10%). The zone is again introduced by a rise, with 15–25% sand and clay sized particles. These pro- although smaller, of Equisitum. Zone C also contains portions are similar to those near the surface (Zone D), large amounts of fungal hyphae (Type 79) which is and the mineral fraction may be of aeolian origin. typically an indication of increased biotic activity and The pollen, microfossils, and sediment texture/com- soil formation. Most of the aquatic species (C. turpin, position evidence from Zone A indicate that a terres- C. aculeate E., and M. armigera) that were present in trial, mesotrophic shrub/graminoid tundra, consisting Zone B decline dramatically. mainly of low willow, birch, sedges and grasses, existed Zone C shows a pronounced decline in organic mat- during this entire period. ter. The coarse mineral component virtually disappears, The transition from Zone A to Zone B (98–65 cm) is silt dominates (70%), and the clay fraction increases marked by a sharp increase in horsetail (Equisitum)to to around 25%. These trends indicate a period of partial 23%, which may be indicative of surface disturbance. An lake drainage around 4700 14C yr BP, with a consequent abrupt rise in birch pollen occurs at 98 cm (87%), fol- lowering of the lake’s water level. The site, located near lowed by an abrupt return to moderate levels. The birch the margins of the smaller basin, was at least partially spike probably reflects the expansion of local birch, or drained. Soil formation was initiated at this time. The possibly even birch catkins incorporated into the sedi- fine mineral fraction was delivered by aeolian processes ment, since much of the sample was composed of birch or during episodic inundation events. pollen clusters. The remainder of Zone B (98–65 cm) is Zone D (20 cm to surface) is problematic for the marked by high frequencies of Cosmarium turpini (Type analysis of discreet intervals due to rooting by shrubs 332B), a pioneering species of green algae preferring and herbs, which can cause penetration of older low-nutrient aquatic environments and typically flour- deposits by younger material. Although one water lily ishing in shallow, sandy pools. Rapid growth of this pollen grain (Nuphar) was found at a depth of 16 cm, algae could have been encouraged when the supply of it is highly unlikely that the site was under water; nutrients from groundwater was cut off and the primary animal importation or bioturbation is more likely. In nutrient source was meteoric water. Several fungal general, the pollen closely reflects present-day vege- remains and rhizopods (or Thecamoebe) are notable: tation at the crest of the palsa, with very high birch Centropyxis aculeata Ehrenberg (Type WH8) is an percentages (68%), some heath, willow, and sage, and invertebrate adapted to living in highly acidic environ- a sharp decline in sedge and grass. Fungal hyphae rise ments of wet mosses and Sphagnum. Sphagnum percent- again in this zone, as does fungal type 55A. The ages rise concurrently with Type WH8 in Zone B. An organic matter component increases from 10% to abundant fossil in this zone is Type 353B, eggs of aquatic 40% across the Zone C-D transition, and the propor- flatworm Microdalyellia armigera, typical of eutrophic tion of silt and clay decreases. environments. Conversely, Type 352 (Arcella), which It is very likely that the transition between Zones C shows up in the early part of the zone, is typical of nutri- and D reflects palsa formation, followed by birch ent-poor environments. Despite their conflicting signals shrub growth and the initiation of soil formation. The in terms of nutrient levels, both rhizopods are indicative dates indicate that this event took place at some time of an aquatic environment. The vascular plants also between 1420 14C yr BP and the present. reflect this change, with sedges dominant throughout the period and dry-ground vegetation such as birch, willow, 4 DISCUSSION and grass at relatively low levels. Textural and compositional analysis (Fig. 2) demon- Interpretation of the core data, which varies in strate an increase in the coarse (sand) and very fine response to local landscape changes, is summarized in (clay) mineral fraction, with a concurrent reduction in Table 2. The lowermost layer (pre-A) is interpreted as the silt component. The increased presence of sand a basal detrital unit deposited above lacustrine silts. indicates that Zone B formed in a higher energy envi- The environmental setting is typical of shallow lake ronment than Zone A. The organic component remains margins vegetated with Carex. Over time organics accu- high, but decreases steadily upsection. Based on the mulate and water shallows, and eventually the surface various lines of evidence, there appears to have been is at or near the lake water level. A similar setting a shift to an aquatic environment. We interpret this is present today along the lake margin, and the core period as one of inundation from rising lake levels and collected from the bog adjacent to the palsa has a

232 Table 2. Pollen zonation and landscape processes. Zone Landscape process Biotic response Sediments OM mineral D Palsa formation Birch, willow shrubs, OM and silt dominates 20 cm soil development C Partial lake drainage/infilling of lake margin Grasses, shrubs, herbs OM and sand much 65–20 cm by sediments and aquatic plants appear; aquatics disappear reduced, silt dominates B Rising lake level and inundation; Aquatic indicators (vegetation, OM persists, silt much reduced, 98–65 cm inner beach scarp forms algae, zoological remains) increase sand and clay % A Terrestrial; lake level lowered by drainage or Shrub/graminoid tundra OM and silt dominates 128–98 cm reduced input/drought elements Pre-A Infilling of lake margin by Aquatic plant remains, OM to 180 cm, lacustrine silt 183–128 cm sediments and aquatic plants; reworked detritus below with ice lenses, grading outer beach scarp active into ice similar composition. Given the thickness of the pre-A At this time, a new basin developed and formed the unit (50 cm), it is likely that the existing lake occu- inner beach scarp apparent in Figure 1. pied the larger basin and, during this time, the outer- In time, sedimentation and accumulation of organic most beach scarp formed. remains infilled the lake margin, and the lake bottom The transition to Zone A is interpreted as represent- aggraded toward the surface. Terrestrial vegetation ing a lowering of the lake water level, and subsequent replaced aquatics around 4700 14C yr BP, as reflected replacement of aquatic with terrestrial vegetation. This in Layer C. Along the lake margin, this condition still could be triggered by partial lake drainage, by a reduc- exists. At the palsa site, however, palsa formation did tion in precipitation, or an increase in evapotranspira- not occur until after 1420 14C yr BP, but before 1954 tion. A mesotrophic shrub/graminoid tundra developed, cal A.D. This caused topographic uplift and vegeta- similar to that existing today. Local peat accumulation tion succession on the drier palsa crest. It is likely began at the same time (approximately 9340 14C yr BP) that the growth of shrubs helped to limit sediment defla- as at the Meade River bluffs near Atqasuk (Eisner & tion and augment peat accumulation. Northern Alaska Peterson 1998a), located approximately 200 km west experienced cooler conditions around 5000 14C yr BP. of this site. Ice-wedge formation and a decrease in peat accumu- Our findings at the White Hills site indicate rela- lation occurred at this time (Ritchie 1984, Mackay tively warm, dry conditions accompanied by peat 1992, Marion & Oechel 1993, Eisner 1999). accumulation, and are consistent with those from other parts of the Arctic Coastal Plain. Despite dry condi- 5 CONCLUSION tions, high peat accumulation rates are typical throughout northern Alaska in the early part of the Holocene, The pollen and microfossil stratigraphy from the White owing to the accompanying warmer temperatures. Hills palsa core should be interpreted primarily as a Eisner (1999) suggested that initiation of peat accu- record of local changes in the area delineated by the mulation in Arctic Alaska was fostered by melting of outer beach scarp of the ancient lake. The major ice-rich sediments and increased availability of ground changes in hydrology, palsa formation, and vegetation water. The appearance of poplar at the White Hills site development are all responses to landscape processes during this period also indicates that warmer and drier that could occur independently of climate variation. conditions prevailed. The climatic interpretations we have made for the tim- In Zone B, aquatic microfossil indicators and ing of basin flooding and for fluctuations in peat accu- increased sand content indicate that standing water mulation should be further corroborated with evidence existed at the site. This reflects the effect of rising lake from other regional basins of similar age. One climatic levels, possibly induced by regional climatic changes. event is clearly recorded – the poplar rise – and indi- Although we do not have a date at the beginning of cates a widespread expansion of poplar in this part of Zone B, other studies show that the middle Holocene northern Alaska during the early Holocene. (ϳ8000 to 5000 14C yr BP) was a period of rapid peat accumulation on the Arctic Coastal Plain (Marion & ACKNOWLEDGEMENTS Oechel 1993, Eisner 1999), and also of higher precipitation and lake levels in northern Alaska (Edwards This material is based upon work supported by the et al. 2000). We conclude that lake water level rose National Science Foundation under grants OPP- in response to increased precipitation around 8000 9911122, ATM 9416858, and GER 99550382 to 14C yr BP, which is consistent with our core dates. Eisner, OPP-0094769 to Hinkel and OPP-0095088 to

233 Nelson. Any opinions, findings, conclusions, or rec- Marion, G.M. & Oechel, W. C. 1993. Mid- to late-Holocene ommendations expressed in the material are those of carbon balance in Arctic Alaska and its implications the authors and do not necessarily reflect the views for future global warming. Holocene 3(3): 193–200. of the National Science Foundation. The authors would Michaelson, G.J., Ping, C.L. & Kimble, J.M. 1996. Carbon like to thank Kim Peterson and Lynn Everett for their storage and distribution in tundra soils of Arctic Alaska, U.S.A. Arctic and Alpine Research 28(4): 414–424. invaluable assistance. Nelson, F.E., Hinkel, K.M. & Outcalt, S.I. 1992. Palsa- scale frost mounds. In Dixon, J.C. & Abrahams, A.D. REFERENCES (eds), Periglacial Geomorphology: Proceedings of the 22nd Annual Binghampton Symposium in Geo- Anderson, P.M. & Brubaker, L.B. 1994. Vegetation history morphology. 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