Glaciation and regional ground-water flow in the Fennoscandian Shield: Site 94

Results from a regional-scale ground-water flow model of the Fennoscandian shield
suggest that ground-water flow is strongly affected by surface conditions associated
with climatic change and glaciation. The model was used to run a series of numerical
simulations of variable-density ground-water flow in a 1500-km-long and approximately
10-km-deep cross-section that passes through southern Sweden. Ground-water flow and
shield brine transport in the cross-sectional model are controlled by an assumed time
evolution of surface conditions over the next 140 ka.

Simulations show that, under periglacial conditions, permafrost may locally or
extensively impede the free recharge or discharge of ground water. Below cold-based
glacial ice, no recharge or discharge of ground water occurs. Both of these conditions
result in the settling of shield brine and consequent freshening of near-surface water in
areas of natural discharge blocked by permafrost. The presence of warm-based ice with
basal melting creates a potential for ground-water recharge rates much larger than
under present, ice-free conditions. Recharging basal meltwater can reach depths of a
few kilometers in a few thousand years. The vast majority of recharged water is
accommodated through storage in the volume of bedrock below the local area of
recharge; regional (lateral) redistribution of recharged water by subsurface flow is minor
over the duration of a glacial advance (~10 ka). During glacial retreat, the weight of the
ice overlying a given surface location decreases, and significant upward flow of ground
water may occur below the ice sheet due to pressure release, despite the continued
potential for recharge of basal meltwater. Excess meltwater must exit from below the
glacier through subglacial cavities and channels. Subsurface penetration of meltwater
during glacial advance and up-flow during glacial retreat are greatest if the loading
efficiency of the shield rock is low. The maximum rate of ground-water discharge occurs
at the receding ice margin, and some discharge occurs below incursive post-glacial seas.

The simulation results suggest that vertical movement of deep shield brines induced by
the next few glacial cycles should not increase the concentration of dissolved solids
significantly above present-day levels. However, the concentration of dissolved solids
should decrease significantly at depths of up to several kilometers during periods of
glacial meltwater recharge. The meltwater may reside in the subsurface for periods
exceeding 10 ka and may bring oxygenated conditions to an otherwise reducing
chemical environment.