Jökull - 31.12.2001, Blaðsíða 22

Maciej Dgbski
who identifies 1901-1925 as one of the coldest pe-
riods in Iceland in the 2()th century. From compar-
ison with lichenometric dating of the neighbouring
Heinabergsjökull marginal zone (Evans et al., 1999)
Fláajökull glacier probably produced moraine ridges
close to the oldest Heinabergsjökull moraines in 1907
and 1913. Research on the Svínafellsjökull marginal
zone (Thompson and Jones, 1986), which also flows
south from Vatnajökull, dated moraines to 1870, 1904
and 1930, adding further support for advances of Fláa-
jökull between the end of I9th century and 1930.
On the basis of the above evidence it is suggested
that the following dates should be assigned to the old-
est Fláajökull moraine ridges: ridge 1-1870/94, ridge
11-1907, ridge III—1920/25 (Figure 3). There is one
hardly discernible ridge between ridges II and III
which probably formed in 1913 (Evans et al., 1999).
Supplementary counting of varves in clay sediments
of a dry lake between moraine ridges II and III gave
the mean of 87 varves in 1998, i.e. 2 additional years
from 1913.
Age of ridges IV, V, VI and VII.
Annual glacier variations have been monitored in Ice-
land since 1930 (Ahlmann and Thorarinsson, 1937;
Thorarinsson, 1943; Sigurdsson, 2000; National En-
ergy Authority www.os.is). These show that Fláa-
jökull retreated (along the research profile) 620 m be-
tween 1934 and 1960. This rapid recession was punc-
tuated only by short advances in 1932/33, 1937/38,
1941, a period of still-stand and advance between
1948-1952 and a short advance in 1958. From 1960
to 1996 the glacier front receded only 98 m, with slow
recession in the 1960s and 1970s, and with slow ad-
vance or still-stand in the 1980sand 1990s. Fromfield
observation in 1997, 1998 and 2000 it appears that the
ice front has not changed its position markedly.
Dating of ridges IV, V, VI and VII is based on
the glaciological measurements referred to the aerial
photography taken in 1989, and the map based on
aerial photography from 1946 (US Army Map Ser-
vice, 1949). Unfortunately there is no glaciologi-
cal data for the Hólmsárgarður profile in 1934-1939.
Lack of information on precise timing of the aerial
photography in 1946 when the glacier front receded
rapidly (100 m) makes dating of moraine ridge V im-
precise.
Some additional information was obtained from
conversation with farmers from the area, concerning
the date of the road construction that runs between
the VI and the VII moraine ridges. It was built in the
beginning of the 1960s and has since modified the hy-
drology of the Hólmsá river.
The following dates are therefore assigned to
moraine ridges: IV-1932/33, V-1934/35, VI-1943/-
47, VII—1965/2000. It is, however, obvious that Fláa-
jökull could not create high ridges in the 20í/l century.
Snorrason (1984) states that ridges V, VI and VII (Fig-
ure 3) are late Pleistocene in age. During the LIA
these ridges were therefore overridden by the glacier
but their cores remained unchanged. They emerged
again during 20lh century deglaciation with only a
thin new morainic cover.
LICHENOMETRIC ANALYSIS
In order to get additional information on the age of
the surface material of moraine ridges measurements
of Rhizocarpon agg. lichens (Innes, 1982) were con-
ducted in 2000. Ten areas about 300 m long and 20 m
wide were selected along the Hólmsárgarður profile,
situated on each of the moraine ridges and in depres-
sions between them. A 100 of the largest and ap-
proximately circular thalli were selected in each area.
Largest diameters (enveloping circle) were measured.
Figure 4 shows average diameters of five largest thalli
among the hundred from each field.
Haines-Young (1983) demonstrated the existence
of green-zone, a zone of relatively fast lichen growth
at the foot of a stoss slope of moraine ridges. This
was confirmed in the Fláajökull marginal zone (see
depressions IV/III and III/II in Figure 4). Therefore
only measurements done on the ridges were consid-
ered, those from the depressions were rejected. Rhi-
zocarpon agg. thalli appear on the youngest moraine
ridge only occasionally and are very small. For this
reason the VII moraine ridge was not included in the
lichenometric dating.
Lichenometric investigations within the Heina-
bergsjökull and Skálafellsjökull marginal zones, just
south of Fláajökull, conducted by Gordon and Sharp
20 JÖKULLNo. 51