Numerical studies on the Impact of the Last Glacial Cycle on recent borehole temperature profiles: implications for terrestrial energy balance

Abstract

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 the 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 (LGC) on the subsurface energy content, and on the estimate of the background quasi steadystate signal associated with the diffusion of accretionary energy from the Earth's interior. Here, we provide the first estimate of the impact of the development of the Laurentide ice sheet on the estimates of energy and temperature reconstructions 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 (MUN-GSM) to quantify the extent of the perturbation to estimated 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 the borehole reconstructions over the last millennium for North America. The range of these possible impacts is 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 (≈ 600 m), the induced perturbations to the steady-state temperature profile are on the order of 10 mW m −2 , 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 changes from borehole temperature data.

Impact of borehole depths on reconstructed estimates of ground surface temperature histories and energy storage

“…We confirm that the origin of the uncertainties due to the maximum depth of BTPs arise because downward propagation of past surface temperature changes (>1000 years before present (B.P.)) continue to have a nonnegligible contribution to the thermal state of the subsurface at the depth range where the determination of the equilibrium surface temperature, T 0 , and geothermal gradient, Γ 0 , are carried out [ Rath et al , ; Beltrami et al , ]. This quasi steady state acts as the reference and initial conditions that allows retrieval of the historically driven energy balance changes at the surface.…”

“…Some of these older events have contributed negative subsurface anomalies (i.e., colder events; see Table ) as is the case for most areas of Canada and northern Europe where the existence of the Little Ice Age (LIA) has been documented from boreholes ranging from 400 to 600 m in depth [e.g., Beltrami et al , ; Lewis , ; Luterbacher et al , ]. These subsurface heat content differences are expected to also vary spatially and with depth depending on the previous specific climatic history at the surface for each BTP site, e.g., the areas affected by the last glacial cycle in high northern latitudes [e.g., Rath et al , ; Beltrami et al , ].…”

Ground surface temperature and continental heat gain: uncertainties from underground

“…An important source of uncertainty is related to thermal noise from lithological variability, that is, high-frequency variation of the thermal properties of the heterogeneous subsurface. Also, previous climate events such as warming from the Last Glacial Cycle obscure the semi-equilibrium geothermal regime, and physically add a band of noise to the geothermal regime at all depths (Rath et al 2012, Beltrami et al 2014. Thus, it is crucial to obtain intervals of confidence for the models of GST histories inferred from borehole temperature profiles (BTPs), as these data have the potential to yield, among other things, information on the land component of the global energy balance of the climate system (Beltrami 2002a, Hansen et al 2005.…”

Geological controls on the present temperature field of the western Sverdrup Basin, Canadian Arctic Archipelago

“…() and would not affect general pattern of the temperature variation below 1000 m from the surface. Numerical studies of the impact of the Last Glaciation Cycle on recent borehole temperature profiles show that the disturbances to the geothermal gradient are no large at the depth from surface to 600 m (Beltrami et al ., ).…”