Jökull - 01.12.1985, Page 75

(2) Lichenometric evidence
In areas C and D the maximum lichen thalli generally
conform to the expected relative ages suggested by the
morphological evidence. The maximum thallus dia-
meter in this central valley area reaches 110 mm on the
„proximal outwash deposit" of the Holar deposit. Thalli
on the T2 terrace, which is believed to be associated
with ice retreat following the Holar limit, average c.
90-95 mm, while north of the Holar deposit lichen
thallus diameters tend to become progressively smaller.
On T3 the lichen thalli range from 60 to 65 mm, while
on the more proximal intermoraine outwash deposits
themselves maximum thallus diameters range from 30-
35 mm between distal moraine ridges (Zones XXII,
XXIII), to 20—23 mm nearer the ice margin (Zones
XXIV, XXV), to only 9—14 mm on the proximal inter-
moraine outwash deposits (XXVI, XXVII). Similarly,
maximum thallus diameters become progressively
smaller through the terrace sequence of T2 to T5 as one
approaches the present Jökulsá. While T3 thalli reach
60—65 mm in diameter, those on T4 average only 20—
30 mm, and are totally absent on the T5 deposits (Table
1). Although this area appears to have had a complex
history of moraine deposition followed by meltwater
dissection and deposition, local variations in maximum
thallus diameter appear to be reasonably consistent. For
example, the morphological evidence suggests that
meltwater channels XXIV and XX (Fig. 1) are contem-
poraneous, and indeed the similarity of thallus sizes
(especially of the mean of the 5 largest thalli) for these
two zones supports this morphological interpretation.
Area A, by contrast, exhibits apparently anomalous
thallus sizes, for maximum diameters are not only
highly variable, both between and within individual
morphological zones, but also are relatively small when
compared with the largest sizes found in area C, an area
that is clearly much younger. Hence, the largest thalli
found on the high terrace surfaces range between only
51 and 91 mm, and those in the flood channels between
43 and 73 mm. In addition, many sites exhibited very
small lichens (10 mm) or were devoid of all thalli.
Similarly, area B exhibits relatively small thallus dia-
meters compared with area C, which clearly postdates
area B. Maximum thallus diameters range between only
65 and 86 mm, and local variability is again extremely
high, particularly on the lowest terrace (Zone X) where
over 70 per cent of search areas were devoid of lichens.
(3) Interpretation of relative age evidence
Two clear patterns of lichen size distribution in re-
lation to the relative ages of sandur deposits have
emerged. Firstly, the maximum thallus diameters on the
more recently deposited outwash sediments within vall-
ey fill areas C and D, conform fairly consistently to the
apparent morphological history of the proximal pro-
glacial zone. By contrast, however, the maximum thall-
us diameters on the adjacent higher terrace surfaces of
areas A and B, are anomalously small and locally highly
variable, suggesting that these sizes do not reflect the
age of the deposit and/or its period of abandonment by
meltwaters. A number of possible explanations may be
proposed for the small thallus diameters found on the
older, higher deposits:
(I) The observed lichen thalli may not represent the
first generation of thallus development in the area, but
an unknown generation in the longterm colonization of
these deposits, resulting from the natural decay and
subsequent replacement of older lichens.
(II) The lichens observed may indeed represent first
generation lichens, but have colonized a deposit that
was formerly soil-covered. Hence the „relative age“ in
fact may represent the relative age of more recent soil
stripping and erosion, rather than of original sandur
deposition. This possibility seems particularly likely in
area B, where evidence of extensive soil erosion exists
in the form of the few relict soil patches still remaining.
(III) Owing to relatively harsh environmental con-
ditions (see below) for lichen growth on the high ter-
races, thallus growth may not have been continuous
since initial colonization; hence, although the lichens
may be of first generation, their growth may have been
severely and repeatedly interrupted, and growth rates
hence very irregular.
(IV) The observed maximum thallus sizes may rep-
resent the largest dimension that can be attained before
rock weathering occurs. High rates of rock weathering
are readily apparent in areas A and B and include, in
particular, chemical disintegration, frost shattering and
exfoliation, frost heave and eolian abrasion. In many
sites thalli are only found on south-facing rock surfaces,
sheltered from strong, cold, sediment-laden winds blow-
ing off the ice cap.
(V) The absence of large thalli may reflect the ab-
sence of any suitable rock material for colonization.
Clasts that are too small, too exposed or of unsuitable
texture and composition would preclude thallus
development. Many sites in areas A and B are mantled
by large bouiders, cobbles and rock fragments providing
suitable substrates for lichen colonization, but a high
proportion of palagonite breccias in certain areas (e.g.
flood channels and Zone X in area B) precludes wide-
spread development of thalli, compared with areas
richer in fine grained basalts.
The relative contribution of each of these factors to
the presence of anomalously small lichen thalli in areas
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