Conceptual Models of 1200 years of Icelandic Soil Erosion Reconstructed
Using Tephrochronology
Andrew J. Dugmore1, Gudrún Gísladóttir2, Ian A. Simpson3, and Anthony Newton1
Abstract - With reference to 18 tephra isochrones, we present six reconstructions of landscapes in South Iceland at precise
times through the last 1200 years and develop three related models of soil erosion. Before the late ninth century A.D., the
landscapes of Iceland were without people and resilient to natural processes. The initial impact of human settlement in
the ninth century AD was most profound in ecologically marginal areas, where major anthropogenic modifi cations of the
ecology drove geomorphological change. In the uplands, overgrazing contributed to the formation of a dense patchwork
of breaks in the vegetation cover where soil erosion developed and resulted in the rapid denudation of large areas. As the
upland soils were shallow (generally less than 0.5 m), the overall impact of erosion on total aeolian sediment fl uxes before AD 1510
was modest. Later erosion of the deeper lowland soils (generally greater than 2 m) involved a lower spatial density of eroding fronts
and a slower loss of soil cover, but a much greater movement of sediment. Land-management strategies, changes in species
patterns of plant communities, extreme weather events, and climate changes have combined in differing degrees to initiate
and drive rates of soil erosion. Sensitivity to change and the crossing of erosion thresholds has varied through time. The
record of soil erosion has major implications for both archaeology and contemporary land management.
1School of GeoSciences, University of Edinburgh, Edinburgh EH8 9XP, Scotland. 2Department of Geography and Tourism,
and Institute of Earth Sciences, University of Iceland, 101 Reykjavík, Iceland. 3School of Biological and Environmental
Sciences, University of Stirling, Stirling FK9 4LA, Scotland. *Corresponding author - andrew.dugmore@ed.ac.uk.
Introduction
Iceland provides rare opportunities to assess human
impacts on soil erosion and landscape change.
Before the Norse settlement, or Landnám of the ninth
century A.D., there is no evidence of people in Iceland
(Buckland et al. 1995, Fridriksson 1994). As a result,
it is possible to identify environmental records from
long periods of the Holocene, during climates similar
to, warmer, and cooler than today that have no anthropogenic
components (Caseldine 1987; Dugmore
1987, 1989; Stötter 1991). Contrast can be drawn with
records from the last 12 centuries when the changing
climates of the “Medieval Warm Period” (Grove
and Switsur 1994) and “Little Ice Age” (Grove 1988)
have been interwoven with extensive human impacts
on the landscape (Arnalds 1987, Runolfsson 1978,
Thomson and Simpson 2007). At present, however,
chronologically precise and spatially explicit models
of long-term landscape change in Iceland are lacking.
Spatial and temporal patterns of tephra deposition provide
one means of creating detailed models of change
that can be tested and used to develop understanding of
the interplay of different processes over diverse landscapes
through century-millennia timescales. In this
paper, we focus on a district of southern Iceland and
use 18 tephra isochrones to develop six reconstructions
of Icelandic landscapes at precise times through
the last 1200 years.
Landscape change in Iceland
The Norse colonists introduced herbivorous
mammals to Iceland for the fi rst time, rapidly building
up populations of sheep, goats, pigs, cattle, and
horses (Amorosi et al. 1997). Woodland and scrub
were cleared, and fi eld systems established (Vésteinsson
1998). Up to the 20th century, livestock grazed
all year round, and farms had access to common
summer pastures extending up to 500–600 m above
sea level (Fridriksson 1973, Thoroddsen 1919).
Vegetation cover in Iceland (103,000 km2) has
diminished signifi cantly since early Norse settlement.
Today it is about 28% of the island’s area
(LMI 1993) as opposed to much more extensive presettlement
vegetation cover estimated to be between
54% (Ólafsdóttir et al. 2001) and 65% (Thorsteinsson
1986) of total land area. The composition of the
vegetation cover has also greatly changed; estimates
of woodland coverage at the time of settlement
vary from 15,000 km2 (14.5%)–40,000 km2 (39%)
(Bergthorsson 1996, Bjarnason 1974, Einarsson
1962, Olafsdottir et al. 2001, Sigurdsson 1977,
Thorarinsson 1961, Thorsteinsson 1986), whereas
present woodland coverage is 1% (LMI 1993).
Changes in the species composition of plant communities,
their distribution, and overall vegetative
cover, have been related to enhanced soil erosion,
increased aeolian sediment fl uxes, slope instability,
and hydrological changes (e.g., Arnalds 1987; Arnalds
et al. 2001a; Dugmore et al. 2000; Einarsson
1961, 1963; Gísladóttir 1998; Hallsdóttir 1987; Haraldsson
1981; Ólafsdóttir et al. 2001; Thorarinsson
1961; Thorsteinsson 1978, 1986). The changes in
both the extent and nature of vegetation cover have
been attributed to direct or indirect anthropogenic
effects acting in combination with unfavorable climate
and erodable soils.
2009 Journal of the North Atlantic 2:1–18
2 Journal of the North Atlantic Volume 2
Andisols, which are of volcanic origin, cover
78,000 km2 or 86% of Iceland (Arnalds 2004) and
have a high susceptibility to cryoturbation, landslide,
wind, and water transport (Arnalds 1999,
Wada et al. 1992). Recent investigation of soil erosion
in Iceland (Arnalds et al. 2001a) has shown that
almost 41,000 km2 (or about 40% of the country)
is characterized by severe soil erosion, and this has
created a variety of landforms. The largest areas of
severe soil erosion are deserts (as defi ned by Arnalds
et al. 2001a), of which sandy surfaces form nearly
22,000 km2 (Arnalds et al. 2001b). Other erosion
forms are the rofabard type (a bank of eroding soil
that separates areas denuded of soil from surviving
areas of soil and vegetation), which cover about
3600 km2, and erosion spots within vegetated areas,
which are found across about 2700 km2 (Fig. 1).
Based on the erosion rate around rofabards, Arnalds
(1999) has suggested that erosion associated with
present rofabards has denuded 15,000–30,000 km2
of land.
Conceptual models of ecological changes in
Iceland due to grazing have been used to explain
both the susceptibility of the land to soil erosion
and various erosion forms (Aradóttir et al. 1992,
Gísladóttir 2001). Both models identify key changes
in vegetation from continuous covers of palatable
deciduous shrubs, grasses, and broad-leaved
herbs to less-productive heathland
dominated by unpalatable evergreen
dwarf shrubs and narrow-leaved herbs.
Development of heathland communities
is suggested to have occurred at
the expense of woodland and herb
communities, and leads to increased
susceptibility of the plant community
to land degradation. Indeed, it is possible
that the heathlands of Iceland may
mostly represent remnants of the original
homogenous woodlands and their
once fertile soils. Gísladóttir (2001)
has described the effect of micro-scale
patterns of species abundance on soil
erosion, identifying heterogeneous
dwarf-shrub heath as potentially very
susceptible to spot erosion and more
homogeneous grass heath as less susceptible.
In thick soils, erosion spots
can expand and form into rofabards.
Homogenous grassland with a thick
root mat is, however, resilient and does
not easily form erosion spots, but may
be systematically reduced in extent by
rofabard encroachment from the edges
of the plant community (Fig. 2).
Overall, these conceptual models
of changes in plant communities and
related increases in susceptibility to
soil erosion aid explanation of rapid
vegetation decline and accelerated soil
erosion. In Iceland, landscape change
is a product of a complex interaction
between natural environmental processes
and human activities, both of
which are heavily infl uenced by past
events. Through time, the sequence of
changes at a particular place will give
an area a unique character. The spatial
variability caused by accumulated
changes may tend to obscure the function
of fundamental processes. The
Figure 1. A characteristic suite of Icelandic soil erosion forms. In the foreground,
erosion spots have developed. At these sites, wind will remove exposed
sediment (some of which maybe subsequently trapped by the surrounding
vegetation). On the hill slope in the background, rofabards have formed
where eroding soil slopes are cutting into a once ubiquitous vegetation and
sediment cover. Photograph © Guðrún Gísladóttir.
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 3
challenge is to disentangle the sequence of events
and interacting processes. In Iceland, this clarifi cation
of the record may be done with unparalleled
precision because of a very well-developed tephrochronological
system of dating control (Fig. 3).
Approach, Methods, and Data Sources
Tephrochronology
Tephrochronology, based on the identifi cation,
correlation, and dating of layers of volcanic ash
or tephra (Thorarinsson 1944), has many potential
applications in geomorphology (Self and Sparks
1981, Thorarinsson 1981). The great chronological
importance of tephra layers is their rapid formation
and wide dispersal, which means that they can be
used to defi ne extensive, and very precise, timeparallel
marker horizons or isochrons (Sparks et al.
1997). Extreme events have formed tephra horizons
of continental scales (e.g., Cas and Wright 1987,
Fisher and Schminke 1984). At a regional scale,
Icelandic volcanoes have formed >200 Holocene
tephras covering areas from 102–106 km2 (Dugmore
et al. 1995; Einarsson et al. 1980; Larsen 1982,
1984, 1996; Larsen and Eiríksson 2008a, 2008b;
Larsen and Thorarinsson 1977; Larsen et al. 1999;
Thorarinsson 1967, 1975, 1980, 1981). Tephrochronologies
are based on the identifi cation, correlation,
and dating of a number of separate tephra deposits
to defi ne a series of isochrons (Thorarinsson 1944,
1981). Resulting geomorphological applications of
tephrochronology and tephra stratigraphy can be
developed to differing degrees.
At one level of application, tephra deposits may
be used to provide limiting dates on geomorphological
features, such as till units and moraines (e.g.,
Dugmore 1989, Stötter 1991, Thorarinsson 1956).
This use of tephrochronology is very effective in
providing spot dates in particular profi les, but it
does not necessarily use the spatial attributes of
Figure 2. A rofabard in South Iceland (ca. 4 km north of point 251; Fig. 5). The exposed soil slope is eroding to reduce the
area of vegetated deep soil (where the sheep are grazing). Most of the soil depth, which includes visible outcrops of black
tephra layers, prehistoric in age, has accumulated since the deglaciation of this area ca. 8000 years ago. Glacially divided
sediments that mantle this underlying hill slope can be seen across the top of the picture and on the upper left-hand side and
upper right-hand side. As erosion proceeds, sediments are deposited on the vegetation, thereby thickening the surviving soil
profi le. The dense vegetation and thick root mat of the grassland has resisted the development of erosion spots. Photograph
© Andrew J. Dugmore.
4 Journal of the North Atlantic Volume 2
the isochronous horizons defi ned by the tephras. If
tephra layers are identifi ed at a number of sites, they
can be used to both defi ne isochrons and intervals of
time and determine the spatial dimensions of rates
of change (e.g., Dugmore and Buckland 1991; Dugmore
et al. 2003, 2007; Thorarinsson 1961). This
application can allow precise 3-D reconstructions to
be made. The use of tephrochronology can be refi ned
further when the form of the tephra layer and its 3-D
geometry within the stratigraphic sequence are also
used to infer the operation of past geomorphic processes,
such as solifl uction and cryoturbation (e.g.,
Dugmore and Erskine 1994, Kirkbride and Dugmore
2005, Thorarinsson 1961) (Fig. 4). Tephrochronology
can therefore provide a particularly powerful
chronostratigraphic framework that may be used to
develop models of landscape change in general and
geomorphological change in particular.
The study area
The district around Eyjafjallajökull was chosen
as a study area because of the wide range of landscapes
present and the natural barriers that defi ne
two hreppur, or communities organised around
common grazing resources (Fig. 5). The
barriers formed by the Mýrdalsjökull
icecap and the Markarfl jót and Jökulsá
rivers effectively constrain rangeland
grazing, creating a coherent district for
an assessment of anthropogenic impacts.
The hreppur extend from coastal sandur
to the upland glacier margins and include
inland valleys. This topographic range is
important because it contains three key
pre-Landnám habitats: marshy lowland
sandur with occasional stands of trees,
forested areas with deep soils ( greater than 2 m), and
upland heaths and grassland with shallow
soils (less than 0.5 m) (Fig. 6). Palynological
research (Erlendsson 2007) shows that
the wooded lowland at Stóra-Mörk soon
changed after Landnám as the landscape
became increasingly open. In these lowlands,
Betula spp. (birch) had more or
less disappeared by the 12th century, although
some woodland lingered into the
late medieval time. The identifi cation of
charcoal production pits has enabled the
utilization of Betula spp. to be tracked
up valley to Langanes (Dugmore et al.
2006). Precise dating based on a combination
of tephrochronology, sediment
accumulation rates, and radiocarbon
dates combined in a Bayesian analysis,
has revealed two phases of exploitation
between cal AD 870–1050 and cal 1185–
1295 (Church et al. 2007). Charcoal
production based on Betula spp. came to
an end in Langanes by the 14th century.
By AD ca. 1417–1510 in the lowlands
at Stóra-Mörk, heathland expansion
(represented by increasing Empetrum
nigrum [black crowberry]) probably
made the vegetation more susceptible to
Figure 3. A typical soil profi le close to Gígjökull (Fig. 5) showing tephra
layers used in this study (e.g., Fig. 8). In the sediment sequences formed
before Landnám, tephra layers (dark and white layers) make up a greater
proportion of the stratigraphy than aeolian soils (brown sediment). This is
a refl ection of both lower aeolian sediment accumulation rates and thinner
tephra layers. The sedimentss in the lower, central part of the profi le form
even layers; vegetation cover disrupted the layers to the lower left-hand
side, and frost action has disturbed the upper right-hand side of the profi le
(compare to Fig. 4). Photograph © Anthony J. Newton.
Figure 5 (opposite page, bottom). The study
area. The inset shows the location within
southern Iceland. Named hills are located with
a triangle shape, and glaciers and icecaps are
named; soil profi les 201–213 are located at
Kroshóll (see also Fig. 12).
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 5
Figure 4. Frost structures in soils in northeast Iceland (66º3´24˝N, 15º47´20˝W). In the background, cryoturbation has
formed well-developed thufur, or frost hummocks. The internal features of these structures can be revealed by tephrochronology
(Fig. 11). In the foreground, an eroding slope is propagating away from the camera, stripping away the shallow
aeolian soil cover to reveal the underlying glacial sediment. Where the soil cover is shallow (less than 10 cm), stone stripes have
formed that cross the eroding slope at right angles. Photograph © Andrew J. Dugmore.
6 Journal of the North Atlantic Volume 2
land degradation (Erlendsson 2007). Soil erosion is
a signifi cant issue today and has been a signifi cant
issue in the past, as it is likely to have contributed to
the early (pre-14th century) abandonment of farms
sites in the Þórsmörk district (Sveinbjarnardóttir
1982, 1992), particularly as it occurred in combination
with widespread vegetation change and a
loss of woodland that necessitated active woodland
conservation measures to ensure the continued local
production of charcoal (Fig. 7; Dugmore et al.
2006). Rofabard erosion forms are striking in the
area (Fig. 2). They are formed in vegetated areas
of thick but non-cohesive Andisols, which are undermined
beneath the root mat, creating bare soil
escarpments (Arnalds 1999). As these slopes erode,
they create areas stripped of soil, and leave isolated
upstanding islands of surviving vegetation and soil,
surrounded by semi-barren exposures of the subsoil
surface. Various erosive processes are active
on rofabards, but water erosion (especially when
driven by strong winds), direct wind erosion, and
freeze-thaw action are the most effective for moving
sediment. These processes are further aided by
trampling of sheep that use rofabards for shelter.
Other erosion forms common in the area are spot or
localized erosion within vegetated areas, and more
extensive exposures of gravels and sand.
Tephrochronology around Eyjafjallajökull
The study area has been frequently covered by
fallout from nearby volcanic systems, resulting
in the formation of at least 78 discrete tephra layers
(Figs. 8, 9; Dugmore 1987). The tephra layers
exhibit a range of macroscopic features that refl ect
major differences in geochemical composition,
eruption mechanism, total tephra volume, and principal
directions of fallout (Self and Sparks 1981).
Icelandic tephra layers are primarily composed of
vesicular glass shards (Larsen 1981). Layer colours
vary from white through yellows, reds, browns, and
greys to black (Thorarinsson 1967). Tephra layers
may be uniform in colour or composed of characteristic
mixes of different coloured pumices, crystals,
or lithic fragments. Around Eyjafjallajökull, tephra
particle sizes range from gravel grade to silt; particle
shapes include a range of vesicularities and
Figure 6. Soil patches on slopes in the broad upper valley of Seljalandsá at ca. 650 m altitude, 2 km SW of profi le 249
(Fig. 5). In this sheltered area, some soils survive that have benefi ted from profi le thickening as they have received sediment
from neighboring eroding areas. Outside the sheltered areas, the Landnám tephra is generally within 0.5 m of the base of
the soil. Photograph © Andrew J. Dugmore.
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 7
both rounded and elongated grains. The thickness
of individual tephra layers varies from ca. 1–500
mm (Dugmore 1987, Einarsson et al. 1980). The
rapid accumulation of aeolian sediments throughout
post-glacial times has generally produced a clear
stratigraphic separation of individual tephra layers,
including deposits that may differ in age by less than
two decades.
The timing of eruptions or tephra falls can be recorded
in historical sources (e.g., Thorarinsson 1967),
correlated to annually laminated icecore records (e.g.,
Grönvold et al. 1995), or dated using radiocarbon
measurements on associated organic material (e.g.,
Kjartansson et al. 1964). The tephrochronology used
here is based on the regional framework established
by a number of workers (Buckland et al. 1991; Dugmore
1987, 1989; Einarsson et al. 1980; Halfl idason
et al. 1992; Haraldsson 1981; Larsen 1981, 1982,
1984, 1996; Thorarinsson 1944, 1967, 1975). Tephras
from the volcanic systems of Katla, Hekla, Eyjafjallajökull,
Torfajökull, and Veiðivötn have been
identifi ed in the study area, and other layers provide
additional local isochrones even though their source
has not been fi rmly identifi ed, e.g., Layers Hr, Ho,
T, and St of Dugmore (1987). The high quality of the
tephra record is enhanced by the presence of tephra
layers close to the stratigraphic location of important
environmental changes.
Of particular relevance to this study, the Landnám
tephra, with an ice core date of AD 871 ± 2 (Grönvald
et al. 1995), effectively marks the start of Norse settlement.
Further temporal control on the Landnámsöld
or “Age of Settlement” AD 870–930 is given by the
Katla tephra of ca. AD 920 (K 920) (Hafl idason et al.
1992), and the Eldgjá tephra of ca. AD 935 (E 935)
(Zielinski et al. 1995). In addition, other tephra horizons
used in this study have been historically dated
to AD 1300, AD 1341, AD 1357, AD 1500, AD 1510,
AD 1721, AD 1755, AD 1821, AD 1823, AD 1918,
and AD 1947 (Einarsson et al. 1980; Larsen et al.
1999; Thorarinsson 1967, 1975). In prehistory, the
tephra layer SILK-YN has been dated using 22 radiocarbon
samples to give a combined date of 1676 ± 12
14C yr BP, or ca. AD cal. 400 (Dugmore et al. 2000). In
addition, the tephras SILK-UN, Layer L, and Layer
K have all been dated with single radiocarbon dates
and used to constrain prehistoric rates of landscape
change (Dugmore 1989).
Figure 7. A part of the woodland in Þórsmörk (Fig. 5). In the foreground, the grassy mound and meadow marks the site of
the farm of Húsadalur, one of the settlement sites that have been intermittently occupied in the region. In the background,
the dense growth of Betula pubescens was once coppiced for charcoal. Photograph © Guðrún Gísladóttir.
8 Journal of the North Atlantic Volume 2
anthropogenic soil erosion induced by grazing fi rst
developed in upland, ecological marginal areas, and
then spread to lower, initially less marginal areas
because of biomass utilization driven by a combination
of stocking levels, loss of grazing land, reduced
growing seasons, and changing land-management
practice. Different factors are thought to come into
play with differing intensity at different times, but
the net result is continued erosion of a decreasing
total area of soil.
Some key questions remain unanswered. Crucially,
there is the apparent inconsistency between
modern rates of soil-cover loss and the historical
rates needed to explain cumulative soil erosion since
Landnám. Fridriksson (1988, 1995) has measured
modern rofabard retreat rates of 16 cm yr-1 in the
Hekla district of southern Iceland, and extensive
measurements by Fridriksson and Gudbergsson
(1995) of rates of erosion-front movement have
ranged from 1–26 cm yr-1. Arnalds (1999) has estimated
an overall loss of Andosols cover due to
rofabard erosion to be 43,200 km2–53,200 km2 since
the time of settlement. These areas are presently
characterized by rofabard areas, deserts, and areas
where sand encroachment and rofabard retreat are
the major processes. The temporal variations of the
soil cover are, however, missing in those fi gures.
Tephras may be grouped to defi ne broader culturally
and environmentally signifi cant phases and
assess change over clearly defi ned periods of time.
Firstly, the stratigraphy below the Landnám tephra
preserves a record of landscapes without human impact.
Secondly, the Landnám tephra combined with
either K 920 and/or E 935 can be used to assess the
initial Norse colonisation and the fi rst generation
of settlement. Thirdly, the stratigraphy bounded by
K 920 or E 935, and the Hekla tephra of AD 1510
(H 1510) encompasses the changing conditions of
the “Medieval Warm Period” (e.g., Jiang et al. 2005,
Massé et al. 2008, Sicre et al. 2008). Fourthly, H 1510
and the Hekla tephra of AD 1947 (H 1947) bound
the sedimentary record of the major cold phases of
the “Little Ice Age” as defi ned by glacier advances
(Bradwell et al. 2006, Casely and Dugmore 2004).
Finally, H 1947 provides an unambiguous modern
stratigraphic marker that effectively coincides with
the fi rst complete aerial survey of Iceland in 1946.
Models of soil erosion
A wider conceptual framework of historical soil
erosion is provided by the altitudinal model of Dugmore
and Buckland (1991) and conceptual model of
stress, dynamics, and thresholds of the ecosystem
by Gísladóttir (2001). The central argument is that
Figure 8. Post-Landnám tephra stratigraphy and sediment accumulation rates calculated for selected profi les at Kroshóll,
Seljaland (location shown on Figure 5).
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 9
data from individual soil profi les, and closely spaced
groups of profi les to tackle some key questions about
landscape change in Iceland. Firstly, have the rates
of soil-cover loss changed in recent times? Secondly,
have sediment-fl ux rates changed in recent times. Finally,
is it possible to reconcile estimates of changing
soil area and sediment-fl ux data?
Results
Soil profi les and measures of soil erosion
Stratigraphic sections in aeolian soils have been
measured at over 200 sites around Eyjafjallajökull,
and detailed fi eld mapping is supported by geochemical
analyses and radiocarbon dating (Dugmore
1987, Dugmore and Buckland 1991, Dugmore and
Erskine 1994, Dugmore et al. 2000, Larsen et al.
1999). The distribution of tephra has been determined
from exposures >50 cm wide in open sections
where the stratigraphy of profi les up to 12 m deep
was recorded to a precision of ± 2 mm.
Individual soil profi les have been assessed using
tephrochronology to determine four key data sets:
the rates and types of sediment accumulation for
clearly defi ned periods of time (Fig. 8), the geometry
of the tephra layers, and the microtopography
Arnalds (1999) estimated that the 232 ha yr-1 soil
loss around present rofabards had reduced by one
order of magnitude because extensive areas have become
desertifi ed, leaving the rofabards as remnants
of previously soil-covered area. Crucially, this implies
a reducing rate of loss of soil area to erosion
In contrast, tephrochronological studies indicate
temporal change in aeolian activity and intensifi cation
after the 17th century. Sigurbjarnarson (1969)
showed sediment infl ux during the 12th and 13th
centuries and again during the “Little Ice Age” in
the 17th–19th centuries, and Thorarinsson (1981)
showed that total sediment accumulation rates accelerated
into modern times. Peaks of accumulation in
early Settlement times, especially at higher altitude
and interior sites, are locally signifi cant, but in absolute
terms they are signifi cantly less than the later
rates reached elsewhere (Dugmore and Buckland
1991, Dugmore et al. 2000). In addition, offshore records
show a marked increase in the terrestrial sediment
infl ux after the 17th century (Jennings et al.
2001). Thus, in contrast to modern data on changing
landcover, sediment accumulation rates would seem
to imply increasing rates of soil loss due to erosion.
The Dugmore and Buckland (1991) altitudinal
model of cascading impacts can be refi ned using both
Figure 9. Tephra stratigraphy in upland profi les located on Figure 1. At the time of settlement soil profi les at these sites were
less that 50 cm deep (Fig. 6).
10 Journal of the North Atlantic Volume 2
cate the nature and intensity of erosion (Fig. 10), and
the geometry of the tephras indicate whether sources
of sediment, and hence areas of erosion, are local
(from less than 10 m) or regional (from greater than 1 km) (Dugmore
and Erskine 1994). It is assumed that when sediment
accumulation rates are similar between closely
of the land surface when the tephra was deposited.
In addition, total profi le thickness at Landnám was
determined as an additional factor, with both distinct
geographical variation and important implications
for the development of soil erosion in historical time
(Fig. 9). Accumulation rates and sediment type indi-
Figure 10. Variations in aeolian sediment accumulation at Kroshóll, Seljaland for three time periods: (a) AD ca. 870–920
(ca. 50 yr), (b) AD ca. 920–1510 (ca. 590 yr), and (c) AD 1510–1947 (437 yr). Although the period AD 1510–1947 is less
than three quarters of the period AD ca. 920–1510, sediment accumulation rates and local variability are far greater, a probable
consequence of the development of local sediment sources.
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 11
erosion started at the crest of the slope above profi le
204 soon after AD 1341, then it has propagated ca.
130 m in ca. 650 yr, indicating an overall rate of 20
cm yr–1 (Fig. 10). This rate is consistent with the
modern estimates of Fridriksson (1988, 1995) and
Fridriksson and Gudbergsson (1995).
Increased variability between the 13 profi les
on Krosshóll suggests a second and more extensive
phase of local erosion developed after
1510 (Fig. 10). In this area, modern rofabards are
separated by cross-slope distances on the order of
200 m, which, given widespread local AD vegetation
breaching post 1510, would also suggest an average
eroding face retreat over the following 440 yr of
ca. 20 cm yr–1 (Fig. 10).
Sediment fl uxes and an improved model of landscape
change.
In order to synthesize the data from over 200
profiles, we have refined the altitudinal model
spaced profi les, the sources of aeolian sediment must
lie outside the immediate area. For this situation
to change to localized, large-scale variation in accumulation,
local sources of aeolian sediment must
have developed from which the wind could move
sediment over short distances. Data are presented
in the form of summary diagrams and maps (Figs.
8–11). These focus on both reconstructions of specifi
c years, such as AD 1341 and AD 1821 (Fig. 11),
as well as changes over longer periods, such as the
initial settlement period between AD 870–920, AD
920–1510, and AD 1510–1947 (Fig. 10).
Rates of denudation
The stratigraphic data at Krosshóll indicates two
periods when breaks fi rst developed in vegetation
cover (Figs. 8, 12). The very earliest sign of localized
breaching of the vegetation cover is the presence
of slope-washed gravels in profi le 204 (Fig. 8) that
occurs immediately above the Hekla 1341 tephra. If
Figure 11. Reconstructions of landscape change Seljalandsheiði A.D. 870–1947 in the lowland within the pre-Landnám
woodlands, and in the highland above the natural tree line at Landnám. Each cross section is drawn at the time of a tephra
fall. The land surface at the time of the deposition of the Landnám tephra (ca. AD 870) became a layer below the surface in
ca. AD 920. Prehistoric soils contain numerous tephra layers illustrated by solid horizontal lines (Fig. 3). Woodland cover at
Landnám is likely to have been dense, as the island lacked terrestrial herbivores. Erosion spots began fi rst in the highlands
(ca. AD 920) and appeared at lower elevations later (AD 1341), when thufur (frost hummocks) formed in the surface and
were mantled by subsequent tephra falls. The 3-D geometry of the tephra layers changes as eroding slopes develop because
the greatest sediment deposition occurs immediately downwind of the sediment sources.
12 Journal of the North Atlantic Volume 2
grazing at the time of the Norse settlement, and these
areas would have expanded with the subsequent
episodes of climatic cooling. Unsustainable grazing
would have decreased or eliminated species intolerant
to trampling and browsing, leading to creation of
many breaches in the vegetation cover (Gísladóttir
2001). The exposed soil would not have been able
to withstand erosion. These changes are likely to
have developed in heathlands where the micro-scale
pattern of plants form a heterogeneous pattern and
where thufur or turf hummocks characterize the land
surface (Fig. 11). Thufur are formed by a vegetated
mound of soil, the top surface of which is sensitive
to disruption by freeze-thaw cycles and grazing
pressure (Webb 1972). The plant communities that
characterize these environments consist of a mosaic
consisting of different species of mosses, dwarf
shrubs, grasses, and herbs, out of which mosses (e.g.,
Racomitrium lanuginosum [racomitrium moss])
and dwarf shrubs (e.g., Empetrum sp.) are the least
tolerant to trampling and grazing (Gísladóttir 1998).
of soil erosion by adding a conceptual model of
changed vegetation pattern, and linked maps and
representations of soil thickness and sediment flux
across the landscape (Fig. 13). The patterns and
progress of vegetation change and soil erosion and
the resulting sediment flux may be governed by
three key factors: contrasting sensitivities to the
breaching of vegetation cover, contrasting depths
of sediment in uplands and lowlands, and the rate
of development of rofabards.
Discussion
Upland heaths are likely to have been the most
sensitive areas to grazing impacts in the early historical
period in Iceland (AD 920 and AD 1341; Fig. 11).
Here the growing season is shorter than the lowlands,
and grazing may easily extend beyond the start and
after the end of the summer biomass production season
(Simpson et al. 2001). Some upland areas would
not have been able to support sustainable year-round
Figure 12. Kroshóll in South Iceland (Fig. 5) viewed from the South. The rocky knoll on the skyline is the location of the
207-m spot height (Figs. 5, 10). Deep soils that once covered the whole ridge have been reduced by rofabard erosion, the
extent of which is shown by the steep semi-vegetated slopes that surround the remaining upstanding areas of deep soils.
Vegetation cover has regenerated on the lower slopes stripped of their deep soils. The heavily managed home fi elds of the
Seljaland farms can be seen in the foreground. Short, steeper slopes mark the boundary between sandar (river fl ood plain)
in the foreground and truncated low angled fans at the foot of the main escarpment (site of the small house for a water
turbine). Photograph © Andrew J. Dugmore.
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 13
Figure 13. Models of landscape change: (A) pre-Landnám ca. A.D. 400, (B) ca. A.D. 920–1510, and (C) 1510–1947 A.D.
x-y is a cross section illustrating soil thickness from the upland ice margin to the valley fl oor. In (A), shallow soils are generally
associated with upland heaths (although there may be deeper sediment accumulations in sheltered areas [cf Fig. 6]).
At lower altitudes, soils will tend to be thicker as they will be older and will have benefi ted from sediments moving down
slope. The stepwise changes in soil cover on the fl oodplain have been created by episodic movements of the river channel.
In (B) (AD 920–1510), change is illustrated by the two idealized cross sections of soil thickness (a dashed line for Stage A,
and a solid line for Stage B). Areas that have maintained a vegetation cover will have also experienced profi le aggradation.
Anomalously great sediment accumulations are proposed to have taken place in stable areas close to sources of eroding soil.
In (C) (AD 1510–1947), two sets of cross sections are shown, one for soil thickness and one for sediment fl ux, a representation
of the amount of sediment transported across the landscape. Dashed lines represent Stage B, and solid lines represent
Stage C. The rate of movement of sediment is proposed to peak close to eroding soils and to have been at its greatest where
the soil cover was at its deepest.
14 Journal of the North Atlantic Volume 2
Those sensitive plant groups are frequent on top of
the thufur. The combination of this species pattern
and thufur formation in the heathland makes it an
extremely sensitive environment (Gísladóttir 2001).
The presence of thufur in past landscapes is shown
by the form of tephra layers within the soil profi le
(Dugmore and Buckland 1991), and the dimensions
of the fossil structures are similar to those of today,
ranging in size from 20–50 cm in height and 50–200
cm in diameter (Thoroddsen 1914). With vegetation
breaks occurring on thufur, a dense, meter-scale
patchwork of spot erosion could form, which then
Figure 14. An eroding soil showing the form of the slope characterised by the 21st-century profi le of Figure 15. Erosion of
the exposed sediment undercuts the turf causing collapse. Once they have formed, comparatively low levels of grazing can
help to maintain erosion on these slopes through browsing and trampling, particularly as sheep seek out the overhangs of
these eroding slopes for shelter. Photograph © Guðrún Gísladóttir.
Figure 15. Lowland rofabard development 14th–21st century AD Once rofabards have developed, small-scale lateral
movements of the eroding face can result in large-scale sediment mobilization, a process enhanced by the thickening of the
surviving soil profi le (Fig. 14).
2009 A.J. Dugmore, G. Gísladóttir, I.A. Simpson, and A. Newton 15
gradually developed into larger areas of active soil
erosion and resulted in denuded areas. Rofabards
could form at the edges of denuded areas and thicker
soils. Erosion of these exposures at a similar rate to
the historical development of rofabards at Krosshóll
(ca. 20 cm yr-1), or even the lower range of modern
rates measured elsewhere (1–10 cm yr-1), could then
result in the rapid stripping of large areas of soil. This
process could explain large-scale spatial change,
without requiring the rapid (>100 cm yr-1) movement
of individual eroding faces.
The denudation of large areas of sensitive upland
soils may have only had a comparatively modest
overall impact on regional sediment fl ux rates because
of the shallow soil profi les involved (Fig. 9).
This situation would have changed when large-scale
erosion developed within the deeper lowland soils
(Figs. 11, 12). Rofabards formed within the old
forest zones may only erode at a similar rate to soil
exposures in the uplands, but this process would
involve soils at least 2–5 m thick, and consequently
result in the movement of large volumes of soil (Figs.
14, 15). Initial breaches in the vegetation cover at
lower, ecologically less-marginal altitudes seem to
have developed later in historical time, and been
more widely spread apart, as the areas of surviving
vegetation cover tend to be substantial 10–100-m
scale patches (Fig. 9). Here the change in vegetation
pattern varied from the more-marginal upland areas
(Fig. 11), making the lowlands more resistant to
vegetation breaches. With similar rates of erosion at
individual soil exposures, a less-dense patchwork of
rofabards will produce a slower overall loss of soil
cover. As a result, there is no need to invoke radical
changes in the rate of erosion at individual soil exposures
to explain both changes in soil cover as well
as aeolian sediment fl ux in the south of Iceland.
Wider implications
Our refi ned model of soil erosion highlights two
particularly important processes: factors that trigger
the development of individual erosion faces,
and those that control the propagation of the erosion.
Triggering factors for soil erosion must be
closely related to the changing status of the vegetation
cover, and so to the net effect of grazing. The
impact of grazing will be primarily determined by
rangeland-management practices, such as livestock
type, stocking levels, and the management of day-today
grazing patterns, and its overall annual duration.
Rangeland management will also be infl uenced by
the availability of fodder collected elsewhere, such
as hay, and the potential to feed stock through the
winter and spring. In addition, the past will exert a
signifi cant infl uence, as the cumulative landscape
history will play a key role in determining biomass
productivity and sensitivity to change; for example,
past land-management decisions may have modifi ed
soil profi les and altered the composition and nature
of the vegetation. Breaches in the vegetation cover
will occur more easily in heterogeneous plant communities
where species are intolerant to stress and
where formation of thufur has occurred due to a
substantial mismatch between biomass production
and the grazing offtake. It may also be triggered by
catastrophic events such as freak weather, or volcanic
activity and tephra deposition.
Therefore, we would see land management,
through both its long-term impacts and response
to short-term environmental change, as playing the
critical role in determining the timing and location
of vegetation-cover disruption, and the triggering of
soil erosion. However, crucially, sensitivity to this
critical threshold may be altered by both long- and
short-term climatic changes.
Once soil erosion has been initiated, it may
be sustained and propagated as much by climatic
factors as land use, because a lower intensity of
grazing is required to maintain exposures of bare
soil than is needed to break an established vegetation
cover. In addition, key factors in determining
the erosion rates of bare soil slopes include the
purely climatic factors of needle ice formation,
rainfall, and wind. As differing combinations and
intensities of precipitation, wind, and temperature
can vary rates of erosion, it is possible that distinct
climate signals are embedded within the overall rate
of soil erosion and related sediment accumulation.
If the role of management practice can be clearly
defined, aeolian sediment accumulation in southern
Iceland over the last 1200 yr may be shown to contain
a proxy record of climate change.
Conclusions
There are three key factors in the development
and geomorphological impact of soil erosion in
southern Iceland: the density of breaks in vegetation
cover, the rate of soil erosion at these breaks,
and the depth of the eroding soil profi le. Rapid
denudation is associated with a high density of vegetation
breaches, which have tended to occur early
in historical time in upland heaths where soil profi les
were shallow at the time of settlement (generally
less than 0.5 m). Despite a widespread change in soil cover,
the overall impact on sediment fl uxes was not as
great as the later but less spatially extensive erosion
of deeper soils (generally greater than 2 m). Modern erosion is
characterized by a lower density of eroding slopes
and the exposure of deep soil profi les, so while the
overall rate of loss of soil cover is now well below
the historical average, the impact on sediment fl uxes
is greater because of the volumes of soil involved.
Land-management decisions played a primary role
in triggering soil erosion, but climate may substantially
determine the subsequent soil erosion rates.
16 Journal of the North Atlantic Volume 2
Acknowledgments
This work has been supported by grants from the
Leverhulme Trust, as part of the landscapes-circum-landnám
program, the UK Natural Environmental Research
Council, the University of Iceland Research Fund, and the
US National Science Foundation Offi ce of Polar Programs
Arctic Social Sciences (grant number 0732327 as part of
the International Polar Year Humans in the Polar Regions
project “IPY: Long Term Human Ecodynamics in the
Norse North Atlantic: Cases of sustainability, survival,
and collapse”). Paula Milburn and Jeanette Yates provided
fi eldwork assistance, and cartography was undertaken by
Gerry White, University of Edinburgh. We gratefully acknowledge
the support of the people of south Iceland, in
particular Kristján Ólafsson of Seljaland and Rósa Aðalsteinsdóttir
of Stóramörk
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