D. Fabel, J. Harbor1, A. Stroeven2,
J. Kleman2, C. Hättestrand2, D. Elmore3 and D. Fink4
1 Department of Earth and Atmospheric Sciences, Purdue University,
U.S.A.
2 Department of Physical Geography and Quaternary Geology, Stockholm
University, Sweden
3 PRIME Lab, Purdue University, U.S.A.
4 Australian Nuclear Science and Technology Organisation.
Critical tests of global climate models
include their ability to reconstruct environmental conditions
during former periods of distinctly different or rapidly changing
climate, such as glacial maxima and periods of large-scale ice-sheet
growth and decay. Global climate signals and ice volume estimates
have been extracted from deep-sea, coral reef, and ice sheet
proxies. Over the past 20 years, since the CLIMAP reconstruction
of the Last Glacial Maximum (LGM), including LGM ice volumes,
there has been growing concern that ice sheet thicknesses and
volumes suggested by CLIMAP may have been overestimated, and
that new reconstructions are needed. However, CLIMAP estimate
of LGM ice volumes are broadly consistent with some recent new
evidence: (i) far-field evidence of sea level low-stands
imply more ice on earth than formerly thought, (ii) relative
sea level curves from glaciated regions do not normally cover
the late glacial period, and, hence, potentially underestimate
the volume of ice at LGM, (iii) the Arctic ocean may
have been covered by an ice shelf, and (iv) direct glacial-geological
observations of ice sheet extent have modified prevailing perceptions
of ice sheet volume (e.g. the Innuitian Ice Sheet is now
commonly accepted). However, key aspects such as the height and
evolution of individual paleo-ice sheets cannot be traced in
proxy sea level data, yet have a significant impact on climate
models. Potential records of the paleotopography of Northern
Hemisphere ice sheets are found in formerly glaciated areas in
northern Sweden, western Norway, Scotland, and eastern Canada.
In these locations, features such as the upper limit of glacially
eroded bedrock surfaces (trimlines) and differences in rock weathering
(weathering zones) have been interpreted as indicators of former
ice sheet height. This interpretation has been questioned by
others who have argued that such features may reflect internal
thermal boundaries between wet (warm-) based erosive ice and
dry (cold-) based ice that is effectively non-erosive. Resolving
this issue is critical because of differences in predicted ice
sheet configurations. For example, a large and thick late Weichselian
Fennoscandian ice sheet (FIS), as opposed to a much thinner,
and possibly multidomed, late Weichselian ice sheet. |
Over the past three years we have worked collaboratively to
examine deglaciation chronology and patterns of erosion and landscape
preservation in the northern Swedish mountains, the core area
of the Fennoscandian ice sheet using in situ produced cosmogenic
10Be and 26Al. The accumulation of in situ produced cosmogenic
10Be (half-life = 1.51 ± 0.05 x 106 yr) and 26Al (half-life
= 7.1 ± 0.2 x 105 yr) in quartz exposed to cosmic radiation
provides a means of determining the amount of time the rock has
been at or near the ground surface. Because nuclide production
decreases with depth, removal of two or more meters of irradiated
rock during one glacial event will create a zero age surface.
In this context, areas known to have been ice covered should
have exposure ages equivalent to deglaciation if they were significantly
eroded by ice and older exposure ages if they suffered limited
erosion or were completely protected. Ice cover of 10m or more
shields the underlying rock surface from most cosmic radiation,
so areas that undergo multiple cycles of ice sheet overriding
but no erosion should accumulate 10Be and 26Al cosmogenic nuclide
concentrations equivalent to the sum of the ice free periods,
minus decay during periods of ice shielding.
Patches of relict landform assemblages were identified in
the northern Swedish mountains through extensive field and air
photo mapping. Quartz-rich samples for cosmogenic radionuclide
analysis were collected from bedrock outcrops and erratics in
mapped relict patches. Erratics confirm that the sites were overridden
by ice and were dated to determine whether they were deposited
during the last glaciation. Bedrock in relict patches was sampled
to determine whether these sites were in fact eroded during the
last glaciation (to give a deglacial exposure age) or whether
they are relict (exposure ages reflect both post-glacial time
and inheritance from one or more previous ice-free period).
Our results from erratics provide deglaciation ages and indicate
that relict patches have been covered by ice during the last
glaciation. Bedrock samples from the same relict patches provide
much older cosmogenic ages suggesting insufficient glacial erosion
and hence preservation of these patches during multiple ice overriding
events. Relict patches therefore appear to represent areas of
frozen bed conditions at the base of the FIS supporting the current
swing in opinion toward the idea that, under given boundary conditions,
ice sheets are landscape 'preservers' rather than 'destroyers'. |