Permafrost

During a cold period the depth of permafrost will increase due to colder conditions;
however, permafrost growth is typically considered separate from the growth of ice sheets
during glacial periods. In reality, permafrost and ice sheets commonly coexist, generating a
single thermal regime. We produce thermal scenarios of a permafrost system with and without
an overlying ice cover. We compare the modeled vertical temperature profiles and permafrost
thickness estimates with available observations to provide constraints on the most likely
scenario of ice extent for the Qinghai-Tibet Plateau.
Our numerical representation of permafrost evolution treats the problem of 1-D advection
and diffusion with a logarithmic grid transformation that concentrates grid cells at boundary
interfaces. This more appropriately treats the thermal discontinuity between sediment,
permafrost, air, and/or ice. Geothermal heat flux into the base and overlying air or basal ice
temperatures serve as boundary conditions for the coupled domain. At steady state, the depth
of permafrost is determined by the heat flux at the permafrost-sediment interface, the surface
temperature, and the thermal conductivity of the permafrost. A temperature change at the
surface boundary decreases in amplitude as it propagates within the permafrost, and ultimately
affects the permafrost depth.
While permafrost will grow in response to cooler air temperatures, ice cover during a cold
period will insulate the permafrost and can lead to its degradation. The ice cover caps the
permafrost, shielding it from changing surface air temperature, and influencing the temperature
gradient through the permafrost. If enough time ensues, the temperature profile through
permafrost with an overlying ice cover and without an overlying ice cover will differ
significantly.We expect the current permafrost temperatures and resulting thickness
distributions to contain a memory of the bulk past surface conditions. The present existence of
permafrost is significant and allows for a comparison between our model results and scenarios
based on field observations, though we acknowledge that dry conditions and relatively shallow
permafrost in this area will limit our ability to decipher accurate details of surface temperature
conditions in the distant past. By comparing recent temperature profiles with generated
temperature profiles from different model scenarios we hope to add new constraints to the
extent of ice on the Qinghai-Tibet Plateau during the last glacial period.
In addition to this study of ice cover on the Qinghai-Tibet Plateau, these reconstructions
may shed light on the timing and extent of Last Glacial Maximum glaciation in Antarctica. One
region where past ice cover remains contentious is in the Dry Valleys; deciphering the
permafrost signatures there may support or rule out possible past surface temperature scenarios.
Key words: Permafrost, Qinghai-Tibet Plateau, Ice Sheet, Temperature
132