Jökull - 01.01.2013, Blaðsíða 66

Da̧bski and Tittenbrun
tests are performed on surfaces overgrown by Rhizo-
carpon lichen thalli. However, they obtained no clear
decrease of R-values on lichen-free surfaces.
Etienne (2002) argues that exfoliation of basaltic
surfaces deposited by Sólheimajökull (S Iceland)
commences c. 150 years after glacier release. It is
possible that exfoliation rejuvenates the surface of
basaltic boulders found on Fláajökull moraines de-
posited at the beginning of the 20th century and ear-
lier (sites II–Ia). Closer examination of micro-cracks
developed parallel to the rock surface (Figure 5) re-
veal that they could create relatively flat surface after
flake detachment from the rock. Therefore, we are
inclined to explain relative stabilization of the micro-
roughness parameters on old moraines by the process
of rock exfoliation. However, this issue calls for fur-
ther studies on micro-roughness of basaltic surfaces of
various age, preferably on moraines unambiguously
dated for 18th century and older.
The results of this study do not support results of
Chenet et al. (2010) that the historical maximum ex-
tent of Fláajökull dates to a period older than the clas-
sic LIA (i.e. late 19th century), as the indices of rel-
ative age for the oldest moraine do not differ signifi-
cantly from the second oldest moraine ridge (compare
test sites Ia and Ib). On the other hand, we did not ob-
tained unequivocal arguments that the LIA maximum
occurred at the end of the 19th century. Furthermore,
it is possible that the furthest moraine ridge does not
represent LIA glacier maximum, because of erosion
which could have erased older moraines (Kirkbridge
and Winkler, 2012).
CONCLUSIONS
This study presents the innovative use of the Handy-
surf E-35B electronic profilometer for distinguishing
between landforms created due to post-LIA deglacia-
tion. Initial, discrete increase in rock surface micro-
roughness following first decades after deglaciation is
registered. It can be inferred that micro-roughness of
basaltic surfaces, previously abraded and polished by
a glacier, increases for about 80 years and then sta-
bilises at an elevated level, probably due to exfolia-
tion. The development of micro-depressions on sur-
faces of basaltic boulders, as well as micro-cracks and
micro-cavities within weathering rinds, testifies to ad-
vancing surface deterioration, which, in the case of
the basalts studied here, seems dominated by physi-
cal weathering. At this stage, it is difficult to explain
the lack of visible chemical alternation of the rock.
There are statistically significant correlations between
the age-dependent study site range, selected param-
eters of micro-roughness (Ra, Rz, Rzmax), weath-
ering rind thickness, and Schmidt hammer R-values.
We inferred that there is a time-dependent increase in
weathering rind thickness which causes decrease in
R-values.
Given homogenous petrographic conditions and
rock with a fine-grained texture (e.g. basalts or lime-
stones), which is a very crucial condition due to lim-
itation of the instrument (limited range of measur-
able amplitude), the Handysurf E35-B electronic pro-
filometer can be successfully used in studies on the
development rate of initial weathering micro-relief,
and thus on the relative age of glacier landforms de-
veloped since the LIA maximum. However, this study
represents only a first attempt to use this instrument
and different methodologies (e.g. evaluation length,
sample size) should be exercised.
Results of this study do not support the notion
of an early 19th century age for the LIA maximum
extend of Fláajökull, as calculated by Chenet et al.
(2010). On the other hand, the age indices used in
this study seem to be robust only within first 80–110
years of weathering, given the type of rock and cli-
mate of the study area. Further comparative studies
within different glacial marginal zones are required.
Acknowledgements
We would like to thank Agnieszka Burakowska and
Piotr Dzierżanowski for microscopic analysis and
comments, as well as Marek Jastrzȩbski for assistance
in statistical treatment of the data. We are grateful
to Barbara Woronko for discussions on frost weath-
ering. Comments from Ívar Örn Benediktsson, an
anonymous reviewer and Bryndís Brandsdóttir sig-
nificantly improved the paper. Permissions from the
Icelandic Institute of Natural History and Vatnajökull
National Park are kindly acknowledged. This study
has been funded by the National Science Centre in
Poland, project N N306 034440.
66 JÖKULL No. 63, 2013