Ice core methodology - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 118

3.2.5 Ice flow modeling

Ice sheets and glaciers are sedimentary deposits consisting of sequences of layers,
deposited annually as snow accumulation. Snow layers sink into the ice mass and
are subjected to continuous thinning. This occurs initially as a result of densifica-
tion, by which the snow is transformed into ice, but later mainly due to flow-
induced vertical compressive strain. In this process the layers are stretched hori-
zontally until they are advected by the ice motion into an ablation zone where the
ice is removed by melting or calving of icebergs. The oldest ice is found near the
base and along the margin of the ice sheet. If the temperature reaches melting
point at the ice sheet base, the oldest layer sequences may also have been removed
by basal melting. Basal melting may drastically reduce the time range of the layer
sequences left behind in the ice mass.
The age of an ice particle at any given position in the core is estimated by first
estimating the path of the particle from the site of deposition on the surface, and
then estimating the time required for the particle to travel along this path from the
surface to its present position (see Figure 3.10 ). This requires modeling the
dynamic and thermal history of the ice mass during the entire period elapsed since
the ice particle was deposited at the surface. This depends upon past upstream
histories of accumulation rate/ablation rate, ice thickness, ice temperature, and
other ice flow parameters, and it becomes an increasingly dicult and uncertain
task to perform the further back in time that the dating is extended. In addition,
past changes in other boundary conditions (e.g., sea level in the case of an ice
sheet terminating in the sea) must be considered (Reeh, 1989).
On an ice sheet dome or a horizontal plateau there is little horizontal ice
motion and consequently no need to correct for advective transport for as long as
the dome or crest position has remained constant. This is the basis for choosing
drill sites on domes or slightly sloping crests, since at least the more recent parts
of ice core records from such locations should be simpler to date and interpret.
Nevertheless, the accumulation rate, ice thickness, internal ice temperature, and
ice flow parameters are likely to have changed with time, even if the dome position
has not. This causes temporal changes in the depth distribution of the vertical rate
of straining of the layers, an effect which must be considered when calculating the
time scale.

Figure 3.10. Ice particle flow paths.