Jökull - 01.12.1985, Blaðsíða 71

Lichenometric Dating and Tephrochronology of Sandur
Deposits, Sólheimajökull Area, southern Iceland
JUDITH K. MAIZELS and
ANDREW J. DUGMORE
Department of Geography
University of Aberdeen
St. Mary’s, High Street
Old Aberdeen AB9 2UF
Scotland U.K.
abstract
A series of proglacial sandur deposits infront of
Sólheimajökull has been dated using a combination of
morphological, lichenometric, tephrochronologic and
historical evidence. The lichenometric data, based on
measurements of Rhizocarpon geographicum thalli, sug-
gest that a reliable growth curve may be established for
about the last 140 years. Mean growth rates appear to
have ranged between 73 and 85 mm per 100 years, while
the maximum diameter recorded reached 110 mm. The
dates predicted from the lichen sizes for the period of
outwash formation and/or abandonment for this period
conform closely to the morphologic and historical evi-
dence, with deposits formed particularly in about 1840,
1883, 1935 and 1969. The outwash deposits which, accor-
ding to morphologic and historical evidence, are much
older, tend to exhibit smaller and more variable lichen
sizes than those found on the younger deposits. While the
lichenometric data suggest dates of formation between c.
1818 AD and 1910 AD, the tephrochronologic evidence
from soil sections on the sandar and within the lateral
moraine sequence, indicate dates of accumulation associ-
ated with glacier expansion at some time between c. 600
AD (Skógasandur) and c. 1400 AD (Sólheimasandur).
Hence the lichenometric data cannot be used to date
deposits older than about 140 years in age, for the lichen
sizes observed on these older deposits appear to reflect
the period of exposure by soil erosion, or the harsh
environmental conditions and rapid rates of rock weath-
ering that hinder lichen colonization and subsequent
growth.
INTRODUCTION
The proglacial area of Sólheimajökull in southern
Iceland comprises a complex series of moraine and
sandur deposits many of which are believed to date
from early Neoglacial times. The younger moraine
ridges in the proximal zone of the valley have been
dated by Jaksch (1970, 1975) using lichenometric
methods supported by known ice-limits since 1890 AD.
More recently, Jaksch’s data have been supplemented
by field measurements of lichen diameters (Dugmore
and Williams, pers. comm.) and of tephrochronologic
evidence from the lateral moraine sequence on Eystri-
heiði, east of the Jökulsá valley (Fig. 1) (Dugmore, in
prep.). The latter evidence in particular has allowed the
dating of older moraines in the Sólheimajökull valley.
However, only minor attempts have been made to try to
determine the relative and absolute age of the sandur
deposits, most of which are clearly much older than
those studied by Jaksch. The major objective of this
study is therefore to attempt to determine the relative
age of the proglacial sandur deposits using topographic
and morphologic evidence, and their absolute age using
a combination of lichenometric methods and the
tephrochronological evidence of Dugmore. Jaksch also
concluded that lichen diameters never exceeded a maxi-
mum of 70 mm within the proglacial area, such that 50
mm marked the 1890 ice-limit, and 50-70 mm dimen-
sions represented all earlier deposits. He accounted for
the absence of larger thalli (the individual lichens) on
these earlier deposits in terms of rapid rates of rock
weathering in this cool, maritime environment. The
second objective of this study is therefore to test
whether this size limit extends to other proglacial
deposits and, if so, to confirm that lichenometric dating
is not feasible for deposits older than c. 1900 AD (1983
data) that is, deposits formed 80-85 years ago.
STUDY AREA
Sólheimajökull forms an 8 km long outlet glacier on
the southern side of the Mýrdalsjökull ice cap in south-
ern Iceland (Fig. 1, inset). The glacier terminus Iies at
an elevation of about 100 m, and is located on the
JÖKULL 35. ÁR 69