332
sliding law of fig. 11 is appropriate, and if the accumulation rate is large enough, cyclic
surging will occur. During a surge, the flow velocity increases dramatically (calculations
suggest a velocity of 2 centimetres a secondJ), and there results a massive iceberg flux into
the North Atlantic. On the lower branch of fig. 11, water production is virtually absent,
Qw is low in (3.22) since the flow is slow and the geothermal and viscous heat at the base
can be conducted away by the ice. The low value of Qw gives high N, consistent with low
11. On the upper branch, however, viscous heat dominates, and Qw is large, N is small,
also consistent with a high 11.
At the end of a surge, the rapid ice drawdown causes the water production to drop, and
the rapid velocities switch off. This mayor may not also be associated with re-freezing
of the basal mudstones.
When water saturated soils freeze, frost. heave occurs by sucking up water to the
freezing front via capillary action, and this excess water freezes (at least. for fine grained
clays and silts) in a sequence of discrete ice lenses. Heaving can occur at a typical rate
of perhaps a metre per year, though less for fine grained soils, and the rat.e of heave
is suppressed by large surface loads. Calculations suggest a surge period of perhaps a
hundred years, with a drawdown of a thousand metres, and a recovery period on the
order of 5,000-10,000 years. During the surge, the rapidly deforming basal muds will
dilate (in the deforming horizon, likely to be only a metre or so thick). At the termination
of a surge, this layer re-consolidates, and we can expect the total heave to be a certain
(small) fraction of the frost penetration depth. In effect, the ice lenses freeze the muds
into the ice stream, so that when the next surge phase is initiated, some of this frozen-in
basal sediment will be transported downstream, and thence rafted out into the North
Atlantic in iceberg discharge.
In fact, there is evidence that this rather glamorous sequence of events actually occurs. Heinrich events are layers of ice-rafted debris in deep-sea sediment cores from the
North Atlantic which indicate (or are consistent with) massive iceberg discharges every
7000 years or so. In addition, oxygen isotope concentrations in ice cores from Greenland
indicate that severe cooling cycles occurred during the last ice age. One theory has it that
such cooling events can be caused by a swit.ch-off of North Atlantic deep water (NADW)
circulation - effectively switching off the convective heat transport from equatorial latitudes and thus cooling the atmosphere. It seems that bunches of these cooling cycles
are terminated by Heinrich events, in the sense that following Heinrich events the climate
warms suddenly. There are two reasons why this should be so. On the one hand, the
sudden reduction in ice thickness should warm the air above, and also it can be expected
that a massive iceberg flux to the North Atlantic acts as a source of negat.ive thermal
buoyancy, which can re-initiate an otherwise stagnant circulation. Rather than being
lumbering beasts, glaciers and ice sheets show every sign of being dynamically active
agents in shaping the climate and the earth's topography.
sliding law of fig. 11 is appropriate, and if the accumulation rate is large enough, cyclic
surging will occur. During a surge, the flow velocity increases dramatically (calculations
suggest a velocity of 2 centimetres a secondJ), and there results a massive iceberg flux into
the North Atlantic. On the lower branch of fig. 11, water production is virtually absent,
Qw is low in (3.22) since the flow is slow and the geothermal and viscous heat at the base
can be conducted away by the ice. The low value of Qw gives high N, consistent with low
11. On the upper branch, however, viscous heat dominates, and Qw is large, N is small,
also consistent with a high 11.
At the end of a surge, the rapid ice drawdown causes the water production to drop, and
the rapid velocities switch off. This mayor may not also be associated with re-freezing
of the basal mudstones.
When water saturated soils freeze, frost. heave occurs by sucking up water to the
freezing front via capillary action, and this excess water freezes (at least. for fine grained
clays and silts) in a sequence of discrete ice lenses. Heaving can occur at a typical rate
of perhaps a metre per year, though less for fine grained soils, and the rat.e of heave
is suppressed by large surface loads. Calculations suggest a surge period of perhaps a
hundred years, with a drawdown of a thousand metres, and a recovery period on the
order of 5,000-10,000 years. During the surge, the rapidly deforming basal muds will
dilate (in the deforming horizon, likely to be only a metre or so thick). At the termination
of a surge, this layer re-consolidates, and we can expect the total heave to be a certain
(small) fraction of the frost penetration depth. In effect, the ice lenses freeze the muds
into the ice stream, so that when the next surge phase is initiated, some of this frozen-in
basal sediment will be transported downstream, and thence rafted out into the North
Atlantic in iceberg discharge.
In fact, there is evidence that this rather glamorous sequence of events actually occurs. Heinrich events are layers of ice-rafted debris in deep-sea sediment cores from the
North Atlantic which indicate (or are consistent with) massive iceberg discharges every
7000 years or so. In addition, oxygen isotope concentrations in ice cores from Greenland
indicate that severe cooling cycles occurred during the last ice age. One theory has it that
such cooling events can be caused by a swit.ch-off of North Atlantic deep water (NADW)
circulation - effectively switching off the convective heat transport from equatorial latitudes and thus cooling the atmosphere. It seems that bunches of these cooling cycles
are terminated by Heinrich events, in the sense that following Heinrich events the climate
warms suddenly. There are two reasons why this should be so. On the one hand, the
sudden reduction in ice thickness should warm the air above, and also it can be expected
that a massive iceberg flux to the North Atlantic acts as a source of negat.ive thermal
buoyancy, which can re-initiate an otherwise stagnant circulation. Rather than being
lumbering beasts, glaciers and ice sheets show every sign of being dynamically active
agents in shaping the climate and the earth's topography.