Synchronous debris-rich glacier advances in northeast Iceland triggered by an early Holocene climate fluctuation
Lindsay Sugden, Nick Hulton & Andrew Dugmore, School of Geosciences, University of Edinburgh, Drummond Street, Edinburgh, EH8 9XP
Figure 1 Geomorphology of Borgarfjorður showing margins of landforms
indicative of response to an Early Holocene climate fluctuation
Aim
To assess geomorphic evidence for climate-induced environmental changes in NE Iceland
during the early Holocene.
Figure 4 Detailed geomorphology of Vik-2 landform suite
Figure 5 Looking in direction of flow across Vikura2 landform suite.
A series of arcuit ridge crests are visible in the Vik-2 suite,
indicative of glacial activity, as shown in Figures 4 and 5.
Background
The evidence presented suggests there was an Icelandic geomorphic response to an extreme
early Holocene climate fluctuation, possibly the “8.2Ka cooling event” (Alley et. al., 1997). A
response to the 8.2ka event has not yet been recognised as such in Icelandic glacial records,
though it has been recorded on the North Icelandic shelf, surrounding North Atlantic and
Greenland. The existence of such a response represents a firm constraint on the timing,
magnitude and mechanisms of this event in the North Atlantic. The study has implications for
the use of periglacial and glacial landforms as climatic indicators. It also better defines the early
Holocene Icelandic tephrostratigraphy in the region.
Figure 3
Approach
Geomorphological and tephrochronological evidence from the Borgarfjorður area of north-east
Iceland are combined to produce a well-constrained Holocene chronology of geomorphic
events.
Results: Geomorphology
Thirteen landform suites indicative of debris-transport by ice with unique morphological features
are identified in the Borgarfjorður region.
• They originate from small accumulation areas usually surrounded by high cliffs providing
constant debris supply.
•They exhibit flow-like characteristics, maintaining a lobate form with a distinct terminus, and
containing a series of down-slope oriented ridges indicating flow direction. Most contain a
number of well-defined terminal ridges indicating a sequence of recessional stages or flow
events (see Figures 3, Brun-1, and 4, Vik-2).
Figures 4 & 5
• The landforms terminate at altitudes well below expected Little Ice Age limits (confined to high
corries), and above Younger Dryas limits (off the present coast-line).
Preservation Potential
The extent of topographic and lithological control on the preservation and genesis of landforms
is analysed through detailed assessment of drainage basin characteristics. Slope angle is
found to be the most important factor in landform preservation (Figure 2 below). This analysis
has enabled assessment of the extent to which the geomorphic record has been de-coupled
from the climate record through topographic and lithological control.
Figure 3 Detailed mapping of Borg-1 suite.
Several phases of flow events are signified by a sequence of prominent
transverse arcuit ridges. Patterns of downslope ridges indicate two main
flow-routes.
Figure 2 (right) Preservation
potential of drainage basins with
slope angle.
Figure 2 shows area and average
slope of each drainage basin in
the field area, highlighting the
basins in which geomorphic
evidence for debris/ice transport
has
been
observed
(this
‘evidence’ is the series of
landform suites shown in Figure
1). It is seen that most of the
basins containing geomorphic
evidence have average slope
angles of around 10°, while
steeper basins contain little
evidence.
N
1km
Results: Tephrochronology
Tephrochronology is used to date the landforms observed. By analysing tephra profiles from within and
outwith the landform limits it is possible to constrain ages for the landform suites and thus characterise
possible response mechanisms to the 8.2ka event (see Figure 6). Almost 150 tephra profiles have been
excavated, most containing tephra layers well-preserved in peat deposits. A minimum age of ~8,000
years is derived for the landform suites, based on tephrostratigraphy and soil accumulation rates.
Figure 6 (right) Cross-profile
through landform suite Vikura-2,
with associated tephra profiles
Part of the tephrostratigraphy of
landform suite “Vikura-2” is shown.
Tephras are inter-correlated based
on
appearance,
physical
characteristics and geochemical
analysis of key layers. Tephra
layers from profiles VK,J,E and H,
within the margins of the landform
suite, are correlated with tephras
from reference profile VD. The
position of these profiles is shown
in Figure 4. The well-documented
‘Hekla 4’ layer (4390+/-107 cal. yr.
BP as dated by Zillen et al. 2002),
is found half-way down profile VK.
Given constant soil accumulation
rates, a minimum age of ~8,000
years can be assigned for landform
genesis. Radiocarbon dates are
currently being processed to betterconstrain these results. Based on
stratigraphy of other landform
suites, genesis of all landforms
mapped in the field area is thought
to have occurred synchronously,
which is suggestive of a regional
response mechanism to (climatic)
forcing.
Conclusions
•Thirteen landform suites represent debris-rich ice advances from small accumulation
areas.
References
•Tephrostratigraphic dating suggests
synchronously ~8,000 years ago.
these
debris/ice
flow
events
occurred
Alley, R., Mayewski, P., Sowers, T., Stuiver, M., Tayloe, K. and Clark, P. (1997). "Holocene climatic instability: A
prominent, widespread event 8200 yr ago." Geology 25: 483-486.
Zillen, L. M., Wastegard, S. and Snowball, I. (2002). " Calendar year ages of three mid-Holocene tephra layers
identified in varved lake sediments in west central Sweden." Quaternary Science Reviews 21(14): 1583-1591.
•The landform-genesis events were triggered by the initiation of a cooler climate and
associated enhanced glacial and periglacial activity which exploited inherent slope
instabilities. This cooler climate may be related to the “8.2ka event”.