Jökull - 01.01.2005, Blaðsíða 82

Kate T. Smith and Hreinn Haraldsson
DISCUSSION
Trees in the Markarfljót sandur area
These mature buried birch trees at Drumbabót are un-
common in the Markarfljót area and Iceland in gen-
eral, both in terms of their size in relation to their
age and also in the nature of their burial. Compari-
son of the radiocarbon date for the sampled Drumba-
bót tree which was probably around 60–80 years old
when it was killed (1230±35 14C yr BP) with Har-
aldsson’s (1981) date for the layer with tree remains
at Teigsaurar (1485±65 14C yr BP) and bearing in
mind that gradual vegetational succession means that
the underlying surface may have been stable for some
100–200 years before the trees really took hold, sug-
gests that trees were probably growing across the
Markarfljót sandur for some centuries prior to burial
of the woodland. The biggest tree trunks and the
most widespread birch woods in the lower part of
the sandur field, Landeyjar, are also of similar age,
dating from the mid first millennium AD (Haralds-
son 1981). This indicates that burial events of this
scale did not happen during this time on decadal or
even centurial timescales, but were large-scale, low
frequency events.
As yet, no similarly large areas of mature trees
have been discovered elsewhere in the valley or other
tree remains buried by very thick tephra-rich deposits.
Woody fragments and cavities in tephra layers and
soils where woody plants once grew are common
in low, valley-side prehistoric sediment exposures all
along the Markarfljótsaurar margins and in Þórsmörk.
Most of these fragments are less than one to five cen-
timetres in diameter but on occasion may reach simi-
lar sizes of around 20 cm across. This evidence for
woodland extends in most profiles on lower slopes
away from the sandur plain right up to the soils di-
rectly beneath the Landnám tephra layer. Much work
in Iceland indicates that the impact of settlement ac-
tivities and later climatic deterioration significantly
reduced tree cover (as discussed by Dugmore et al.
2005), and extensive evidence has been collected for
this in the Markarfljót-Eyjafjöll area (e.g. Påhlsson
1981, Buckland et al. 1991, Simpson et al. 2001).
The Drumbabót site is unusual in that mature wood-
land grew here over a wide area but was extinguished
perhaps some centuries prior to the settlement period
and that there is no evidence here, or in other riparian
zones in Landeyjar, of regrowth of similar woods after
the burial event.
Drumbabót flood
The deposits at Drumbabót are most similar to jökul-
hlaup deposits which make up the sandur surfaces
along the south coast of Iceland (e.g. Skógasandur,
Sólheimasandur, and Mýrdalssandur; e.g. Jónsson
1982, Maizels 1989, 1991, 1993, Tómasson 1996).
Bedding of these sands suggests transport by a water-
dominated flow. The lack of discrete bands of lithics
or extensive low-flow facies with fine ripples or lam-
inations, combined with the extremely good preser-
vation of the trees suggests rapid emplacement of the
sands within one flow event as opposed to normal out-
wash conditions. Cross bedding and ripple lamina-
tions in the deposit at Aurasel suggest low flow water
transport perhaps into standing water forming a delta.
These two different situations may indicate variation
in the type of flow across the area or perhaps represent
different time slices during the duration of the event.
Perhaps Drumbabót lies closer to the main Þverá flood
channel than the deposits north of Aurasel, explaining
the difference in thickness of the deposits as well as
the different flow regimes represented.
If these deposits at Drumbabót, Aurasel and along
the Markarfljót upvalley are indeed the products of a
single event they are best explained by a flood orig-
inating from close to or perhaps even beneath Mýr-
dalsjökull passing down the Markarfljót. Haraldsson
(1993) interpreted the Drumbabót deposits as the re-
sult of a devastating flow caused by a subglacial erup-
tion, a thesis supported by the more recent work pre-
sented here. Tephrochronological dating of upstream
events places the flood before the deposition of the
Landnám tephra (871±2 AD, Grönvold et al. 1995)
but after the deposition of Layer H (1540±50 14C yr
BP, Smith 2004). No other evidence has been found
for this event along the northwest or southern channel
margins at Langanes or on the higher slopes north of
Þórsmörk giving upper limits to the flow. However, it
most probably did overtop the gorge as it entered the
wider middle valley flowing across the relatively flat
lava surfaces to Einhyrningsflatir, along previously
82 JÖKULL No. 55