Heat flow variations in a deep borehole near Sept‐Iles, Québec, Canada: Paleoclimatic interpretation and implications for regional heat flow estimates

Abstract

A deep (> 2000m) borehole in the Sept‐Iles intrusion, on the north shore of the Saint Lawrence River, in Québec, Canada, was repeatedly logged for temperature. Systematic variations of the temperature gradient with depth are not correlated with the thermal conductivity. We interpreted the temperature profile as follows:(1) During the last glacial maximum, the temperature at the base of the ice sheet was cold (≈−5°C); (2) When the region was below sea level, between 10 and 5ky B.P., the ground surface temperature was warm (≈ 6°C); (3) The average ground surface temperature dropped to ≈ 2°C at 5ky B.P. when the region rebounded above sea level; (4) The long time averaged ground surface temperature before the last glacial maximum was ≈0 ‐ 1°C; (5) The reference heat flow (36 – 37mW m−2) is 4–5mW m−2 higher than estimated from the upper 1000m of the heat flow profile.This interpretation can not be extrapolated to the entire region covered by the Laurentide ice sheet. Except for extremely deep (> 1500m) boreholes, the small uncertainty (< 15%) affecting heat flow estimates can not be eliminated.