Jökull - 01.12.1985, Blaðsíða 76

A and B remains, however, indeterminate. Area B, for
example, appears to have been subjected to extensive
soil erosion relatively recently, resulting in small lichen
sizes representing the period since initial soil stripping,
but it may also have been affected by substantial rock
weathering, large proportions of palagonite breccias
(Zone X especially), and/or successive generations of
lichens, all acting to produce relatively small thallus
dimensions. In addition, the extent to which these
growth-inhibiting factors may have been, or are still,
operating even in areas C and D, remains uncertain;
even in these lower, younger areas, growth may have
been discontinuous, interrupted or disrupted (e.g. by
progressive wind blasting and rock disintegration).
EVIDENCE OF ABSOLUTE AGE OF SAN-
DUR DEPOSITS
(1) Lichenometríc evidence
The absolute age of any sandur deposit, i.e. its period
of formation and/or of abandonment by meltwaters,
may be determined only where a reliable lichen growth
curve may be established, or where the sandur can be
shown to be morphologically related to a well-dated
moraine system. Jaksch (1970, 1975) has correlated
maximum lichen size on the terminal moraines with
three known positions of the Sólheimajökull since 1890.
These points plot as a straight line (Curve 1, Fig. 2), and
the line has been extrapolated to represent a maximum
thallus diameter of 110 mm. Lichen size data collected
on the same moraines in August 1983 by Dugmore and
Williams, however, indicate several differences with
Jaksch’s data. Jaksch, for example, appears to have
found larger lichens on the younger surfaces, but small-
er lichens on the older surfaces, compared with those
found by Dugmore and Williams. The average line
plotted for the latter data, as shown in Curve 2 (Fig. 2),
indicates a more rapid growth rate, averaging c. 85 mm/
100 years (for the period 1860-1947) compared with
Jaksch’s curve which represents an average growth rate
of c. 73 mm/100 years (for the period 1890—1945). In
addition, Dugmore and Williams’ curve suggests that
the initial colonization period may be as much as 20-25
years at Sólheimajökull, compared with Jaksch’s esti-
mate of about 15 years (Fig. 2). Nevertheless, the
discrepancy in the dates predicted by the two curves
reaches a maximum of only 9 years in 1840, and hence
both curves seem to provide a reasonably close
correspondence in dates based on lichen growth over
the past 140 years. Although both plots appear as a
straight line, it is likely that the growth rate decelerates
with increasing thallus age, such that the lines plotted
must represent the minimum age of a given deposit.
The use of these lichen growth curves for dating the
proglacial sandur deposits is based on the assumption
that the rates of lichen growth on the moraines are not
significantly different from those on the sandur
deposits. Studies elsewhere (e.g. Maizels and Petch,
1985) seem to support this general assumption. The
maximum thallus diameter found within each mor-
phological zone (Fig. 1) has been plotted on Fig. 2,
indicating the likely ages of formation and/or abandon-
ment of deposits in areas C and D, and the largest sizes
attained in areas A and B. In areas C and D, the Zone
XI deposit thus appears to date from c. 1818 AD (curve
1) or c. 1829 (curve 2); the T2 terrace deposit appears to
date from c. 1840 (or c. 1848 curve 2). The T3 deposit
appears to date from c. 1883 (or c. 1882), and T4 from
about 1935 (or c. 1930). The T5 deposits are believed to
have formed since 1969, and indeed, these deposits are
not apparent on the 1960 aerial photographs. The pré-
dicted dates of the intermoraine outwash deposits
appear to correspond closely to the dates proposed by
Jaksch (op. cit.) and Carswell et al. (op. cit.) for the
adjacent moraine deposits. The distal outwash deposits
appear to date from c. 1925 (or 1923) possibly associ-
ated with a 1918 jökulhlaup; the intermediate deposits
from c. 1937-1940 (or 1933—1936), corresponding
almost exactly with the known ice limit for 1937; and the
proximal to c. 1949—1951 (or 1945—1947), also closely
matching the known ice-marginal position for 1947.
The apparent absolute ages of deposits in area A are
highly variable, with the terrace surfaces being last
colonized by lichen thalli between c. 1843 and 1898 (or
1852 and 1898, curve 2). Within the flood channels
lichen thalli appear to date from between 1868 and 1909
(1873—1868). These high sandur deposits, however, are
clearly of much greater age, and, according to historical
evidence (Landnámabók) appear to date from before
the tenth century (see below). The deep flood channel
that truncates the high sandur on its northern margin
and which extends into the settlement of Skógar in the
west, has clearly been settled for at least several hund-
red years. Hence it appears that the observed lichen
sizes on the high sandur terraces in the west do not
represent the absolute age of the deposit, but may
reflect severe environmental conditions, interrupted or
cyclic thallus development in an area that may only have
been stripped of soil and vegetation cover towards the
mid-nineteenth century. Evidence presented by Bjarna-
son (1974) suggests that sheep numbers in Iceland
increased rapidly from the early eighteenth century
onwards, resulting in increasingly high grazing press-
ures. This extensive heavy grazing may in turn have
resulted in extensive erosion of sandur soils, such that
74 JÖKULL 35. ÁR