Jökull - 31.12.2001, Síða 64

Guðmundsson et al.
The close correlation between past extent of
glaciers during the Holocene and seismic veloci-
ties in glaciofluvial sediments on Breiðamerkursandur
(Bogadóttir et al, 1987; Boulton and Dobbie, 1993)
and Skeiðarársandur may provide a tool to study the
past extent of glaciers in areas like Skeiðarársandur
where sedimentation rates are high. Moraines may
quickly get buried in sediments or washed away in
jökulhlaups. Seismic soundings can then be used to
reveal the existence or absence of compacted and/or
coarse-grained sediments in areas suspected of having
been ice covered at some stage during the Holocene.
The unconsolidated glaciofluvial sediments that
make up the bulk of Skeiðarársandur must have been
formed since the end of the Weichselian glaciation
~ 10,000 years ago. The compacted sedimentary
rocks inferred at 100-150 m depth in the central and
southern reaches of the sandur are in all likelihood
older than the Holocene. They may be sediments
deposited at the end of the Weichselian glaciation
that were subsequently covered and compacted un-
der glaciers during re-advances. Alternatively, these
may be older sediment accumulations that survived
the Weichselian glaciation.
If ~100 km3 of sediments have accumulated
on Skeiðarársandur during the Holocene (104 years)
as suggested by the seismic soundings, the aver-
age sedimentation rate has been ~10-2 km3/yr or
~1 km3/century. The sediment transport of the rivers
on Skeiðarársandur, excluding large jökulhlaups, has
been estimated as 9.5 million tonnes per year (Tóm-
asson, 1990). If it is assumed that the dry density of
the uncompacted sediments is 1500-1700 kg m-3, the
accumulated volume amounts to 0.6 km3/century. A
part of these sediments are deposited offshore, sug-
gesting a maximum deposition rate on the sandur of
0.5 km3/century. The most likely source for the re-
maining sediments is large jökulhlaups. Recent his-
tory shows several high-magnitude jökulhlaups per
century (Þórarinsson, 1974). In the 1996 jökulhlaup
a minimum of 180 million tonnes of sediments were
carried with the floodwater and most of it was de-
posited on the sandur (Snorrason et al., 2002; Maria
et al., 2000). Assuming the same density as before
these sediments have a volume of ~0.1 km3. Thus,
~5 events of similar magnitude per century over the
last 10,000 years seem to be required to account for
the formation of Skeiðarársandur. Since some mate-
rial is probably removed by coastal erosion this esti-
mate should be taken as a minimum value.
It is unlikely that the rate of sediment accumu-
lation on the sandur has been constant during the
Holocene. Firstly, sedimentation was probably very
rapid while the Weichselian glacier was melting. Sec-
ondly, some jökulhlaups may have deposited consid-
erably greater volume of sediments than did the 1996
jökulhlaup. Thirdly, increased sediment concentra-
tion due to surges of Skeiðarárjökull may have been
a contributing factor, as suggested by Knudsen and
Marren (2002) for the upper Jökuldalur valley in early
Holocene. However, despite the contributions from
surges and normal river flow, it seems that on average
a few high magnitude jökulhlaups per century over
the last 10,000 years are required to explain the ac-
cumulation of sediments on Skeiðarársandur. If large
glaciers did not exist during the Holocene thermal op-
timum (Eyþórsson, 1951), the rate of sedimentation
during relatively ice-free periods may have been con-
siderably less than at present. However, this need not
have been the case. The Grímsvötn area may have had
a local ice cap during most of the Holocene and activ-
ity in Grímsvötn may have caused jökulhlaups sim-
ilar to those from Katla in historical times. Volcanic
eruptions in Grímsvötn may therefore have been a rich
source of sediments even at times when Vatnajökull,
in its present form, did not exist. Further work on the
stratigraphy of Skeiðarársandur and the origins of the
sediments underlying the sandur is required to resolve
this question.
Acknowledgments
The measurements in 1997 were carried out by Gunn-
ar Þór Gunnarsson, Heiðrún Guðmundsdóttir, Kristín
Jónsdóttir, Laura Sandri and Jóna Finndís Jónsdóttir.
The 1999 survey was done by Eyjólfur Magnússon,
Hallgrímur Indriðason, Hálfdán Agústsson, Kristín
Martha Hákonardóttir, Mattias Lindman, Rita Ma-
losti, Sigurður Karl Lúðvíksson and Sigurlaug Hjalta-
dóttir. Anna María Ragnarsdóttir and Jón Benedikts-
son at Hótel Skaftafell provided sleeping accommo-
62 JÖKULL No. 51