Geothermal gradients in British lake sediments

Abstract

Temperatures have been measured in the water and sediments of Loch Ness and Windermere to investigate the possibility of determining the geothermal heat flow through the bottom of deep British lakes. The accuracy is limited by corrections for annual and long term variations in bottom water temperatures.In Loch Ness (max depth 232 m) the annual temperature variation of the deep water is normally <0.5°C. There is evidence of a benthic water layer overlying the sediments in the deepest parts of the loch in which the annual variations are probably not more than 0.2°C. When corrections are made, the mean of 13 measurements of heat flow in Loch Ness is 1.45 µcal cm−2 s−1. Values increase steadily from 1.03 µcal cm−2 s−1 in the northeast to 1.74 in the southwest, with local high values near the Foyers granite intrusion.Annual variations in bottom water temperature in Windermere (max depth 65 m) are >2.5°C, though variations in the benthic layer are less than this. Reliable geothermal heat flow measurements were only possible because an extended series of measurements was available. The mean of three acceptable results was 1.69 µcal cm−2 s−1.