Impact of the Last Glacial Cycle on Common-Era temperature and energy reconstructions from terrestrial borehole temperatures in North America

Abstract:

Reconstructions of past climatic changes from borehole temperature profiles are
important independent estimates of temperature histories over the last
millennium. There remain, however, multiple uncertainties in the
interpretation of these data as climatic indicators and as estimates of the
changes in heat content of the continental subsurface due to long-term
climatic change. One of these uncertainties is associated with the often
ignored impact of the last glacial cycle on the subsurface energy content, and
the estimate of the background quasi steady-state signal associated with
the diffusion of accretionary energy from the Earth's interior. Here we provide
the first quantification of the impact of the Laurentide ice sheet
on energy and surface temperature reconstructions during the Common Era from measurements of
terrestrial borehole temperatures in North America. We use basal temperature
values from the data-calibrated Memorial University of Newfoundland Glacial
Systems Model to quantify the extent of the perturbation to
steady-state temperature profiles and to derive spatial maps of the expected
impacts on measured profiles over North America. Furthermore, we present
quantitative estimates of the potential effects of temperature changes during
the last glacial cycle on Common-Era surface temperature reconstructions for North America. The range of these possible impacts are
estimated using synthetic basal temperatures for a period covering 120 ka to
the present day that include the basal temperature history uncertainties from
an ensemble of results from the calibrated numerical model. For all the
locations, we find that within the depth ranges that are typical for available
boreholes used to estimate surface temperature changes during the Common era (~600 m), the induced perturbations to the steady-state temperature profile are on the order of 10 mW/m², decreasing
with greater depths. Results indicate that site-specific heat content estimates
over North America can differ by as much as 50%, if the energy contribution
of the last glacial cycle in those areas of North America that experienced
glaciation is not taken into account when estimating recent subsurface energy
change from borehole temperature data.