Geoscience Reference
In-Depth Information
The temperature of ice varies with depth. The temperature of subterranean ice
is typically fairly constant for a significant depth (down to 1,500m depth), but it
becomes warmer as depth increases and may reach melting point at the base
(Oard, 2005).
Past variations in accumulation rate, ice thickness, ice temperature, etc. are
typically disregarded in some models (steady state models). This is done not just
to simplify the calculations, but also because our knowledge about past changes of
ice sheet climate and dynamics is seldom good enough to justify application of
hypothetical complicated time-dependent models for dating the ice.
To compute the age of an ice layer, one needs to estimate the rate of
accumulation at the time it formed and the thinning function (i.e., the ratio of the
current thickness of a layer to its initial thickness). The age of the ice at a given
depth z is then calculated from:
þ
Depth
dz
Accumulation ð z Þ Thinning ð z Þ
Age(depth) ¼
0
The accumulation function has been modeled for various localities. However,
it depends on past temperatures, which in turn depend upon chronology—so,
some circular reasoning is involved. The thinning function has also been modeled,
but a number of assumptions are made that are dicult to appraise (Parrenin et
al., 2001). One proceeds from such initial data as the snow accumulation rate, the
temperature and viscosity of the ice, the velocity of ice movement, and bed relief
features. Models typically assume steady state ice flow and the absence of major
gaps in columns (Kotlyakov, 1996). Overall, the ice flow modeling process appears
to this writer as some sort of black magic.
Parrenin et al. (2007a) modeled ice flow at Dome C and Dome Fuji with a
1-D mechanical model and an analytical velocity profile, taking into account
variations in ice thickness and deducing the accumulation rate from the isotopic
content of the ice. The poorly constrained parameters of these models were
adjusted by utilizing independent age markers. They reconstructed changes in
surface and bedrock elevation and found a surface elevation 120m lower at the
Last Glacial Maximum and an LGM ice thickness 160m smaller than at present.
They inferred a value of 0.56 0.19mm/yr for basal melting at Dome C. Annual
layer thickness as a function of depth was the primary result of this model. It was
found that layer thickness varied considerably with depth, ranging from about
3 cm for the uppermost 500m to about 1.2 cm for depths of about 500 to about
1,800m, and dropping down toward zero as the depth approached 3,000m. The
profiles of vertical velocity are highly nonlinear at both sites, which suggests
complex ice flow effects, a consequence of the anisotropic behavior of the ice and
the bedrock relief. It was concluded that the accumulation reconstructions based
on the isotopic content of the drilled ice have reached their limits and accurate
estimates of past accumulation rates are required for an accurate age scale. New
independent proxies are needed to improve ice core chronologies and interpreta-
tions at low accumulation sites in Antarctica. Proposals for improved ice flow
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