Effects of Long-Term History on Borehole Paleoclimatology

Temperature changes at the surface of the earth propagate downward into the earth and are stored as perturbations to the subsurface equilibrium thermal regime associated with heat flow from the interior of the earth. Within the context of Borehole climatology, a methodology that has been widely used to reconstruct past ground surface temperature histories (GSTH), we examine the effect of the reconstructed GSTH arising from the changes that took place to 120 kyrs before the present. The changes include the presence of an extended ice sheet, the duration of this cover, and the basal conditions of the ice sheet to account for the climatological interpretation of borehole temperature profiles. We have extracted 120 kyrs to present ground surface temperature time-series for a range of ice-cover histories from a data-calibrated glacial systems model for the last North American ice complex. These time-series provide a transient upper boundary condition for a 1-D subsurface heat transport model. This forcing introduces a thermal anomaly on a synthetic steady-state geothermal regime with equilibrium surface temperature of 8.0 C and geothermal gradient of 20~K~km^-1 under the assumption of an homogeneous subsurface. The corresponding results are sampled for depths ranging from 600~m down to 6000~m and are used to reconstruct the climate-induced subsurface anomalies. We use a series of Monte-Carlo experiments (typically between 700 and 1000), where Gaussian noise with zero mean and standard deviations between ±0.1 K and ±0.5 K, were added to the forcing time series in order to give an estimate of the range of variability of the subsurface anomalies.