INTERGLACIAL CYCLE
7.2.1. Ice sheet evolution
Figure 7.1 displays the Antarctic ice sheet at several Stages during the last glacial cycle. The first picture is of the ice sheet during the Eem interglacial at 120 ky BP. Clearly, the ice sheet is significantly smaller than today, in particular in West Antarctica. However, as pointed out below, this model behaviour should not necessarily be interpreted as proof for a West Antarctic ice sheet collapse at that time, and is probably also caused by the specific startup conditions used.
During the subsequent glacial buildup (second picture), a moderate lowering of sea level by some 35-40 m is enough to initiate grounding in the Weddell Sea, and the ice shelf runs aground on a number of high sea-banks near the present ice shelf front. This obstructs the ice flow landinwards and results in a relatively rapid thickening, so that almost instantaneous grounding occurs over a large area. The situation in the ROSS Sea, on the other hand, is quite different. Here, the free water depth below the ice shelf generally increases towards the sea and consequently, grounding is of a more gradual nature.
Also the threshold for grounding appears to be larger and widespread
-
2500 2000- -
3000 25001
1500-
20001 1000 - 1500 500
-
1000200 - 500 BELOW 200
fig.7.1: Stages in the modeiled evolution of the Antarctic ice sheet during the last glacial-interglacial cycle
.
Hsl: worid-wide sea level stand; ATsl: temperature change. Times as indicated; contours are in m above present sea-level.grounding is only produced in the later stages of a glacial period, in particular when the sea level depression exceeds 100 m (third picture).
Comparing the simulation at 16 ky BP with the stationary 'glacial' geornetry in fig. 6.2 demonstrates that the Antarctic ice sheet has not fully reached its maximum extent, especially in the ROSS Sea. This was anticipated because of the long response time scale for grounding-line advance
(9
6.2.2.3). At this Stage, the Antarctic ice sheet volurne has grown to 31.0 X 106 krn3, corresponding to a global sea level lowering of 12.3 rn relative to the interglacial reference run. This is even less than the 16rn
found in the previous chapter.The concomitant deglaciation of the Antarctic ice sheet is essentially a partial desintegration of the West Antarctic ice sheet. Grounding-line retreat is triggered by a rise of world-wide sea-level, but lags behind. In the rnodel, it begins around 8000 years BP and is nearly completed by the present time.
This delay may be related to increased accumulation rates, which during the early stages of the Holocene thicken the ice and offset the retreating effect of rising sea level. The time lag between the onset of the recession and the beginning of the climatic warming is also the reason why the ice volurne reaches a maximum during the early stages of the glacial-interglacial transition: at that time accumulation rates have already increased, while the ice sheet domain has not yet started to shrink and the surface warrning signal has not reached basal shear layers.
Once the thresholds for grounding-line retreat are passed, overall retreat appears to take around 6000 years to complete. An important feature is that this disintegration seems to develop its own dynamics: environrnental boundary conditions do not change after 6 ky BP, and the process of grounding-line retreat, once it has been initiated, then becomes entirely internally controlled. It only stops when a new equilibriurn is achieved between ice thickness and sea depth. This happens when the ice shelf is able to stabilize the ice sheet and prevent excessive outflow. Stress conditions in the ice shelves then keep the grounded portions in place, and consequently, there is no collapse.
Fig. 7.1 also shows that the ice sheet retreats earlier in the ROSS Sea than in the Weddell Sea. Like the glacial buildup, this is caused by the existente of a eustatic threshold for grounding-line migration, which is larger for the ROSS Sea. A crucial role is also played by the bed adjustrnent process: it allows the grounding line to retreat into depressed inland areas, but subsequent isostatic rebound slows the retreat down and eventually results in a small advance. It is interesting to note that during this evolution, ice rises in the Ronne-Filchner ice shelf (Korff Ice Rise and Berkner Island) never completely disappear. In the East Antarctic ice sheet, on the other hand, changes in ice thickness are less pronounced and reflect the combined effects of accumulation and temperature changes much like the fixed-domain studies discussed earlier, onto which a thinning wave caused by postglacial grounding line retreat is superimposed.
The thresholds for grounding-line recession are also different for each outlet system. Retreat occurs first in the Totten Glacier area at about 9 ky BP and ends at 6 ky BP. In the NinnisIMertz glacier System, retreat Starts at 5 ky BP and is finally completed only by 3 ky BP. When the model is run for another 50000 years into the future, it produces an ice sheet virtually identical to the present interglacial reference state.
The overall response of the ice sheet in this experiment is summarized in fig.
7.2. Displayed are the model forcing functions and the evolution of ice volume and the equivalent sea level changes. Clearly, the dominant forcing is eustatic sea level, and the response is modulated by fluctuations in accumulation rate and ice temperature. Changes in surface altitude are of considerable interest in the interpretation of Information from deep ice cores. They are shown in the lower panel for two locations. Typical elevation changes at Vostok station (East Antarctic ice sheet) are generally within 150 rn, whereas the modelled fluctuations at Byrd (West Antarctic ice sheet) are substantially larger. These values should not be taken too literally, in particular during the first 50000 years or so which are needed for the model to forget its initial conditions.
However, they suggest that the climatic signal recovered at Vostok is probably not biased too much by level fluctuations. The situation for Byrd, on the other hand, is likely to be a lot more complicated because of important changes in the grounded ice domain.
kyears BP
fig.7.2: Forcing (upper panels) and evolution of some large-scale model variables (lower panels) in the 'standard' model experiment on the last glacial cycle. The vertical bar in the l0weSt panel refers to the glacial (G) and interglacial (I) steady states at Byrd (80°S 120°W) The corresponding bars for Vostok (78.5OS, 106.8OE) should be at -9 and -1rn, respectively. They are not shown because they almost coincide, which is because of the counteracting effects of changes in accumulation rate and ternperature, and to a lesser extent grounding-line movements. The conspicious 'low' during the Eem should not be taken too literally and may be a model artefact for reasons discussed in the text.