Late-Quaternary paleoenvironmental record from a ... - IARC Research

Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
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 14 C yr BP, indicating that the palsa contains organic
sediments encompassing the entire Holocene.
1 INTRODUCTION
“Palsa-scale frost mound” is a generic term used to
describe a variety of morphologically similar aggradational
features developed within the active layer above
permafrost. Although several formative processes have
been identified, the positive relief of all frost mounds
is ascribable to accumulation of ice in the substrate
(Nelson et al. 1992, Gurney 2001).
The periglacial literature contains a great deal of
controversy about the term palsa. At its root lies
disagreement about whether classification schemes
should be based on morphology, genesis, constituent
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
strictly morphological sense with reference to mediumscale
frost mounds, independent of genesis, constituent
material, or location in the hierarchy of permafrost
continuity.
Terminological issues aside, palsa-scale frost
mounds can be important sources of paleoenvironmental
information (e.g., Vorren 1972, Vorren & Vorren
1975, Lavoie & Payette 1995). Once a palsa is formed,
its microclimate changes and new plant associations,
very different than those of surrounding low areas,
occupy palsa summits (Sjörs 1961, Railton & Sparling
1973, Seppälä 1990). Vegetation differences atop and
around frost mounds can play a useful role in applying
14 C techniques to date the inception of mound growth.
Vorren (1972, 1979a, 1979b) and Vorren & Vorren
(1975) first demonstrated that radiocarbon dating, in
combination with pollen analysis, could be used to estimate
timing of the onset of frost mound growth.
This paper reports results of a palsa coring program.
Patterns of pollen, soil organic carbon, and soil
texture from a core collected from a palsa summit are
analysed. The palsa is located in an area that has undergone
significant landscape evolution, with consequent
impact on local hydrology and vegetation. These local
patterns are superimposed on a long-term signal reflecting
regional climate and vegetation changes in the late
Quaternary. Our primary goal is to use pollen and soil
analysis to test the feasibility of separating the signals
and reconstruct local landscape processes.
2 SITE DESCRIPTION AND FIELD METHODS
During the summer of 1997 we investigated a frost
mound located west of the White Hills on Alaska’s
North Slope (N 69°29.34, W 150°05.27). The site is
near the boundary between the Arctic Coastal Plain and
Arctic Foothills physiographic provinces and lies within
the zone of continuous permafrost. The mound meets
Washburn’s (1983b) morphological definition of a
palsa: 6 m in height, it has a basal diameter of 12 m and
contains abundant peat. It is developed near the edge
of a series of nested, partially drained lake basins
(Fig. 1). A prominent outer beach scarp is associated
with an older lake basin. A less pronounced beach
scarp, formed by a younger lake, lies basinward and is
highlighted in Figure 1. The younger basin has been
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 measured,
but snowdrifts on the palsa’s flanks are likely to
be substantial. Vegetation atop the palsa and flanking
slopes is a shrub assemblage consisting of Betula
nana (dwarf birch), Salix glauca (willow), and Ericad
(heath), with some mosses and grasses. Thaw depth,
measured in mid-August in two years, averaged 45 cm.
By contrast, vegetation in the bog surrounding the
palsa is dominated by Carex aquatilis (sedge), and the
thaw depth averaged 28 cm.
A sediment core was obtained from the bog surrounding
the palsa, at a distance of 15 m from it, using
a 7.62 cm (3 inch) diameter coring barrel designed by
USA CRREL, driven by a Little Beaver © power auger.
A second core, 128 cm in length, was obtained from a
position atop the palsa. Peat was present throughout
the core, indicating that we had not penetrated into the
massive ice core. Although equipment limitations prevented
drilling deeper in the palsa in 1997, we returned
to the site the following year and exactly re-occupied
the borehole. We used extension rods and core extractors
to penetrate an additional 54 cm, to a total depth
of 182 cm. The palsa core can therefore be considered
continuous. The extended section of the core showed
that peat continues downward another 50 cm, and is
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
segregation ice in the palsa core.
Cores were sampled at 10–15 cm intervals for soil
bulk density and texture, pollen, and microfossil content.
The organic carbon content was discontinuously
sampled in the upper 40 cm, but regularly sampled at a
5-cm interval from 40 to 130 cm depth. Oven-dried soils
(dried at 65°C for 48 hr) were passed through a 2-mm
230
Figure 2.
analysis.
White Hill core textural and compositional
sieve and characterized using methods established by
the Soil Survey Staff (1996). These analyses included
easily oxidizable organic C by dichromate digestion
(6A1a) and particle-size distribution with sand and silt
fractionation by sieving and pipette, respectively (3A,
3A1). The results are summarised in Figure 2.
Subsamples were prepared for pollen analysis
by standard procedures including treatment with
hydrofluoric acid and acetolysis (Berglund & Ralska-
Jasiewiczowa 1986). A known amount of Lycopodium
clavatum spores was added in tablet form to each
sample to determine pollen concentrations and as a
processing control. Samples were mounted in silicon
oil and identified at 400 magnification. Pollen
accumulation rates were not calculated. Identification
of algal, fungal, and zoological remains was carried
out using an extensive collection of photographs and
reference material (Van Geel 1978, 1986, Eisner &
Peterson 1998a, b). Photographs of the microfossil
types mentioned in the text, as well as further
information on type ecological significance, can be
found online (Eisner 2000).
3 STRATIGRAPHY AND DATING
Cored sediment from the palsa showed little evidence of
mixing or deformation, except near the base where
some cryoturbation was apparent. Discreet macrofossils
included twigs from willow shrubs and complete leaf
and seed remains. These were found throughout the
palsa core, and are ideal for identification and dating.

Carbon stored in the palsa amounts to 135 kg C m 3 ,
greater than that reported from other sites on Alaska’s
North Slope (Michaelson et al. 1996).
Results from AMS dating are reported in Table 1,
and confirm that the palsa contains organic sediments
encompassing at least the entire Holocene. The consistency
of the dates demonstrates stratigraphic integrity
throughout the 9,000 yr record.
We also obtained dates from plant material in the
lower core (128–183 cm), as indicated in Table 1.
Although marginally younger than the sediments immediately
above, this unit was interspersed with plant
roots, making precise dating problematic. The dates,
and the composition of the material, indicate that these
sediments represent a basal detrital unit composed of
eroded, reworked, and redeposited peat, organic silts,
and roots of aquatic plants (Hopkins & Kidd 1988). This
unit was deposited in a lacustrine environment, and has
little stratigraphic integrity. The pollen and microfossils
were derived from older reworked materials, or could
have been transported from elsewhere in the watershed.
Because the pollen and microfossils represent a variety
of environments, we concluded that this layer
(128–183 cm) was unsuitable for detailed analysis.
Initially we hypothesized that a comparison of pollen
stratigraphy from the palsa summit and the adjacent
bog sediment, combined with dating at the point of
divergence between the two pollen signals, should
Table 1. Radiocarbon and calibrated 14 C ages 1
CAMS Lab nr. Depth (cm) 14 C age Cal age ranges
43556 3–8 modern 1954 AD
43557 40–45 1420 60 596–665 AD
42609 65–70 4720 100 3633–3556;
3540–3496;
3465–3375*
42610 120–123 9610 60 9175–9111;
9008–8888;
8883–8821*
53255 156–161 9190 50 uncalibrated**
53256 176–182 9440 50 uncalibrated**
1 CALIB rev. 4.3 (Stuiver & Reimer 1993); * BCE.; ** second (lower) core.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Depth (cm)
Picea/Pinus
Alnus
Betula
Ericaceae
Salix
Populus
Juniperus
Artemisia
Cyperaceae
Gramineae
Equisitum
indicate the timing of frost mound incipience. However,
the core collected from the adjacent lake margin bog
was found to contain modern aquatic plant roots in the
upper 26 cm, and this was immediately underlain by
lacustrine silts. Lacking a thick sequence of peat to
date, we chose to discontinue further analysis of the bog
core and focus exclusively on the palsa.
The pollen and microfossil percentage diagram (Fig.
3) has been divided into local Zones A (base at 128 cm)
to D (near-surface). Zone discrimination is based on
qualitative analysis of pollen, microfossils, radiocarbon
dates, and sediment texture. Fungi, algae and other
microfossils are presented as percentages of the pollen
sum. Fossils are represented as percentages of the total
pollen sum, although they are not part of the total fossil
pollen. More than 30 fungal, algal, and rhizopod types
were identified during the analysis, although only those
discussed here are depicted in the diagram.
Zone A (128–98 cm) comprises most of the early and
middle Holocene, beginning around 9600 14 C yr BP.
This zone represents a period of several thousand years,
and the pollen reflects a series of major vegetation fluctuations.
The local environment of Zone A is characterized
by birch (Betula), fluctuating between 3 and 28%,
heaths (Ericaceae) and willow (Salix). Dominant herbs
are sedge (Cyperaceae) and grass (Gramineae), which
also fluctuate within the zone, becoming dominant during
the period in which shrub pollen decreases. The
major non-pollen microfossil, type 55A, is a fungal
spore that favors mesotrophic (moderately nutrient-rich)
conditions and peaks concurrently with grass pollen.
Most of the pollen and microfossil fluctuations represent
the influence of local vegetation and landscape
processes. However, alder (Alnus) and spruce (Picea)
pollen are present through most of the core in small
amounts (5–10%), suggesting that these are products
of long-distance transport. The poplar (Populus) peak
(12%) at 107 cm may indicate climate change. The
poplar rise occurred in northern Alaska and northwestern
Canada between ca. 11,000 and ca. 8000 14 C yr BP
and is generally described as a period of increased solar
insolation (Ritchie 1984, Anderson & Brubaker 1994,
Sphagnum
Nuphar
Forbs
Unidentifiable
type 3
type 16
type 20
Type 46
type 55A
Type 79-hyphae
Type 332
352 Arcella
353
Testate amoeba
Algae
Rhizopods
Fungal remains
20 20 40 60 80 20 20 20 20 40 60 20 20 20 20 20 20 40 60 20 20 20 20 40 60 20 20 40 60 80
Zone D
Zone C
Zone B
Zone A
Figure 3.
White Hills pollen and microfossil percentage diagram.
231

Bartlein et al. 1995). Poplar pollen does not have a wide
dispersal range (Edwards & Dunwiddie 1985), and the
percentages found at this level may indicate establishment
of a localized poplar grove some time after 9600
14 C yr BP and before 4720 14 C yr BP.
Sediments in Zone A (Fig. 2) contain a large component
of organic material (25–35%) and silt (35–45%),
with 15–25% sand and clay sized particles. These proportions
are similar to those near the surface (Zone D),
and the mineral fraction may be of aeolian origin.
The pollen, microfossils, and sediment texture/composition
evidence from Zone A indicate that a terrestrial,
mesotrophic shrub/graminoid tundra, consisting
mainly of low willow, birch, sedges and grasses, existed
during this entire period.
The transition from Zone A to Zone B (98–65 cm) is
marked by a sharp increase in horsetail (Equisitum) to
23%, which may be indicative of surface disturbance. An
abrupt rise in birch pollen occurs at 98 cm (87%), followed
by an abrupt return to moderate levels. The birch
spike probably reflects the expansion of local birch, or
possibly even birch catkins incorporated into the sediment,
since much of the sample was composed of birch
pollen clusters. The remainder of Zone B (98–65 cm) is
marked by high frequencies of Cosmarium turpini (Type
332B), a pioneering species of green algae preferring
low-nutrient aquatic environments and typically flourishing
in shallow, sandy pools. Rapid growth of this
algae could have been encouraged when the supply of
nutrients from groundwater was cut off and the primary
nutrient source was meteoric water. Several fungal
remains and rhizopods (or Thecamoebe) are notable:
Centropyxis aculeata Ehrenberg (Type WH8) is an
invertebrate adapted to living in highly acidic environments
of wet mosses and Sphagnum. Sphagnum percentages
rise concurrently with Type WH8 in Zone B. An
abundant fossil in this zone is Type 353B, eggs of aquatic
flatworm Microdalyellia armigera, typical of eutrophic
environments. Conversely, Type 352 (Arcella), which
shows up in the early part of the zone, is typical of nutrient-poor
environments. Despite their conflicting signals
in terms of nutrient levels, both rhizopods are indicative
of an aquatic environment. The vascular plants also
reflect this change, with sedges dominant throughout the
period and dry-ground vegetation such as birch, willow,
and grass at relatively low levels.
Textural and compositional analysis (Fig. 2) demonstrate
an increase in the coarse (sand) and very fine
(clay) mineral fraction, with a concurrent reduction in
the silt component. The increased presence of sand
indicates that Zone B formed in a higher energy environment
than Zone A. The organic component remains
high, but decreases steadily upsection. Based on the
various lines of evidence, there appears to have been
a shift to an aquatic environment. We interpret this
period as one of inundation from rising lake levels and
increased peat accumulation along the Sphagnum/
sedge bogs of the lake margin.
Zone C (65–20 cm) shows a long period of increasingly
terrestrial conditions, indicated by the prevalence
of birch (15–22%), heath (1–10%), and willow
(2–5%). Sage (Artemisia) pollen increases for the first
time (to 10%). The zone is again introduced by a rise,
although smaller, of Equisitum. Zone C also contains
large amounts of fungal hyphae (Type 79) which is
typically an indication of increased biotic activity and
soil formation. Most of the aquatic species (C. turpin,
C. aculeate E., and M. armigera) that were present in
Zone B decline dramatically.
Zone C shows a pronounced decline in organic matter.
The coarse mineral component virtually disappears,
silt dominates (70%), and the clay fraction increases
to around 25%. These trends indicate a period of partial
lake drainage around 4700 14 C yr BP, with a consequent
lowering of the lake’s water level. The site, located near
the margins of the smaller basin, was at least partially
drained. Soil formation was initiated at this time. The
fine mineral fraction was delivered by aeolian processes
or during episodic inundation events.
Zone D (20 cm to surface) is problematic for the
analysis of discreet intervals due to rooting by shrubs
and herbs, which can cause penetration of older
deposits by younger material. Although one water lily
pollen grain (Nuphar) was found at a depth of 16 cm,
it is highly unlikely that the site was under water;
animal importation or bioturbation is more likely. In
general, the pollen closely reflects present-day vegetation
at the crest of the palsa, with very high birch
percentages (68%), some heath, willow, and sage, and
a sharp decline in sedge and grass. Fungal hyphae rise
again in this zone, as does fungal type 55A. The
organic matter component increases from 10% to
40% across the Zone C-D transition, and the proportion
of silt and clay decreases.
It is very likely that the transition between Zones C
and D reflects palsa formation, followed by birch
shrub growth and the initiation of soil formation. The
dates indicate that this event took place at some time
between 1420 14 C yr BP and the present.
4 DISCUSSION
Interpretation of the core data, which varies in
response to local landscape changes, is summarized in
Table 2. The lowermost layer (pre-A) is interpreted as
a basal detrital unit deposited above lacustrine silts.
The environmental setting is typical of shallow lake
margins vegetated with Carex. Over time organics accumulate
and water shallows, and eventually the surface
is at or near the lake water level. A similar setting
is present today along the lake margin, and the core
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
unit (50 cm), it is likely that the existing lake occupied
the larger basin and, during this time, the outermost
beach scarp formed.
The transition to Zone A is interpreted as representing
a lowering of the lake water level, and subsequent
replacement of aquatic with terrestrial vegetation. This
could be triggered by partial lake drainage, by a reduction
in precipitation, or an increase in evapotranspiration.
A mesotrophic shrub/graminoid tundra developed,
similar to that existing today. Local peat accumulation
began at the same time (approximately 9340 14 C yr BP)
as at the Meade River bluffs near Atqasuk (Eisner &
Peterson 1998a), located approximately 200 km west
of this site.
Our findings at the White Hills site indicate relatively
warm, dry conditions accompanied by peat
accumulation, and are consistent with those from other
parts of the Arctic Coastal Plain. Despite dry conditions,
high peat accumulation rates are typical throughout
northern Alaska in the early part of the Holocene,
owing to the accompanying warmer temperatures.
Eisner (1999) suggested that initiation of peat accumulation
in Arctic Alaska was fostered by melting of
ice-rich sediments and increased availability of ground
water. The appearance of poplar at the White Hills site
during this period also indicates that warmer and drier
conditions prevailed.
In Zone B, aquatic microfossil indicators and
increased sand content indicate that standing water
existed at the site. This reflects the effect of rising lake
levels, possibly induced by regional climatic changes.
Although we do not have a date at the beginning of
Zone B, other studies show that the middle Holocene
(8000 to 5000 14 C yr BP) was a period of rapid peat
accumulation on the Arctic Coastal Plain (Marion &
Oechel 1993, Eisner 1999), and also of higher precipitation
and lake levels in northern Alaska (Edwards
et al. 2000). We conclude that lake water level rose
in response to increased precipitation around 8000
14 C yr BP, which is consistent with our core dates.
At this time, a new basin developed and formed the
inner beach scarp apparent in Figure 1.
In time, sedimentation and accumulation of organic
remains infilled the lake margin, and the lake bottom
aggraded toward the surface. Terrestrial vegetation
replaced aquatics around 4700 14 C yr BP, as reflected
in Layer C. Along the lake margin, this condition still
exists. At the palsa site, however, palsa formation did
not occur until after 1420 14 C yr BP, but before 1954
cal A.D. This caused topographic uplift and vegetation
succession on the drier palsa crest. It is likely
that the growth of shrubs helped to limit sediment deflation
and augment peat accumulation. Northern Alaska
experienced cooler conditions around 5000 14 C yr BP.
Ice-wedge formation and a decrease in peat accumulation
occurred at this time (Ritchie 1984, Mackay
1992, Marion & Oechel 1993, Eisner 1999).
5 CONCLUSION
The pollen and microfossil stratigraphy from the White
Hills palsa core should be interpreted primarily as a
record of local changes in the area delineated by the
outer beach scarp of the ancient lake. The major
changes in hydrology, palsa formation, and vegetation
development are all responses to landscape processes
that could occur independently of climate variation.
The climatic interpretations we have made for the timing
of basin flooding and for fluctuations in peat accumulation
should be further corroborated with evidence
from other regional basins of similar age. One climatic
event is clearly recorded – the poplar rise – and indicates
a widespread expansion of poplar in this part of
northern Alaska during the early Holocene.
ACKNOWLEDGEMENTS
This material is based upon work supported by the
National Science Foundation under grants OPP-
9911122, ATM 9416858, and GER 99550382 to
Eisner, OPP-0094769 to Hinkel and OPP-0095088 to
233

Nelson. Any opinions, findings, conclusions, or recommendations
expressed in the material are those of
the authors and do not necessarily reflect the views
of the National Science Foundation. The authors would
like to thank Kim Peterson and Lynn Everett for their
invaluable assistance.
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