| Abstract: | A coupled GCM-dynamical ice sheet model is used to investigate the
possibility of significant Antarctic glacial ice during the Cretaceous.
The model is used to test the sensitivity of the climate-cryosphere system
to changes in paleogeography, atmospheric CO2, orbital configuration, and
ocean heat transport. An asynchronous GCM-ice sheet coupling scheme allows
for long (10^6 year) integrations of ice sheet initiation and subsequent
variability though multiple orbital cycles.
Preliminary model results support the existence of isolated ice caps on
high plateaus and mountainous regions of the Antarctic continental
interior despite the overall warmth and high CO2 of the Cretaceous. The
model produces highly dynamic ice caps, growing and shrinking in response
to orbital (astronomical) forcing, with the greatest variability in ice
volume occurring during periods of high eccentricity. With high
eccentricity, south polar latitudes experience exceptionally cool summers
(and minimal summer ablation) when obliquity is low and precession places
aphelion during austral summer. Warm summers occur when obliquity is high
and precession aligns perihelion with austal summer. In GCM-ice sheet
simulations of the Paleogene, a specific range of atmospheric CO2 (2x-3x
present values) and high eccentricity orbits produce astronomically paced
changes in ice volume large enough to account for tens of meters of
eustatic sea level change, but without the ice caps reaching the coast.
Similar climate-cryosphere behavior is possible during the Cretaceous.
These results suggest significant glacial ice was possible during the
"greenhouse" climates of the Cretaceous and ice volume response to orbital
forcing may have been paced by the 400 kyr eccentricity cycle. While the
existence of highly dynamic glacial ice was possible during the overall
warmth and elevated CO2 of the Cretaceous and Eocene, simulations of
earliest Oligocene glacial inception suggest CO2 values would have had to
drop below 3x CO2 for the continental-scale glaciation of Antarctica.
During the Cretaceous, the proximity of Australia and India to Antarctic
increased southern hemisphere continentality. Thus, the CO2 threshold
value for glacial inception was likely different for the Cretaceous than
the earliest Oligocene. Additional GCM-ice sheet simulations are underway
to better constrain the "greenhouse" glacial history of the Cretaceous and
to explore ice sheet behavior under different values of CO2. |