Geothermal heat flux into deep caldera lakes Shikotsu, Kuttara, Tazawa and Towada

Abstract

key words: physical limnology, heat flux, stratification, density, deep water running header: geothermal heat flux into deep caldera lakes for submission to "Limnology" -2 - AbstractGeothermal heat fluxes into the deepest waters of four caldera lakes were measured.Temperature profiles within the stratification period between July and November 2007 allowed a quantification of the acquired heat. Due to their enormous depth, heat input from 5 the lake bed was locally separated from heat fluxes at the surface. In conclusion, a direct measurement of geothermal heat input could be accomplished. Although enhanced geothermal activity could be suspected in all cases, two lakes showed a geothermal heat flux of 0.29 or 0.27 W/m² (Lake Shikotsu and Lake Tazawa), as found in other regions not affected by volcanism, while both other lakes (Lake Kuttara and Lake Towada) showed a 10 much enhanced heat input of 1 or 18.6 W/m², respectively. In conclusion within our investigated set, all lakes acquired more heat from the underground than the continental heat flux average. Hence, the heat flux into the lakes from the ground was not dominated by the temperature gradient implied by the inner heat of the Earth. Other effects like the general temperature difference of deep lake water and the groundwater or local sources of heat in the 15 underground deliver more important contributions. Obviously the flow of water in the underground can play a decisive role in the heat transport into the deep waters of lakes.-3 -

Thermal Regime of A Deep Temperate Lake and Its Response to Climate Change: Lake Kuttara, Japan

“…When the lake is ice-free throughout the year, such an increase in bottom temperature is not seen in December-March. It is because the vertical circulation in the lake, which prevails during the weak stratification or approximately isothermal conditions in December-March, tends to dissipate the leaked water [18,21]. At the other points, such an increase in bottom temperature was not observed.…”

“…This suggests that the vertical circulation in spring occurred enough at these depths. At depths of 140 m, however, geothermal-water leakage after the circulation seems to slightly increase the temperature, thus being a little far from the T md value [18].…”

“…It is seen that, at depths of more than 90 m, the temporal change of water temperature is less than 1 • C. During the intense stratification of June-August, the bottom temperature at 3.86 • C (6 June 2015) increased up to 4.07 • C (29 August 2015). Since the heat flux toward the bottom is little in hypolimnion under the strong stratification, the increase in bottom temperature probably depicts the leakage of relatively warm water with some dissolved solids from below the bottom [18,19]. In fact, a geophysical survey by the magnetotelluric (MT) method in the Kuttara caldera suggests that there is a reservoir of geothermal water below the bottom at the deepest point [20].…”

“…The heat flux H Rin by river inflow is less than 1 W/m 2 at any time and thus is neglected. The heat flux H s at bottom was evaluated at the order 1 W/m 2 at the deepest point [18,19]. The magnitude of H s could be restricted to the deepest point or around, since the increase in bottom temperature was not observed at the other points (Figure 1).…”

Significance of Electromagnetic Surveys at Active Volcanoes: Toward Evaluating the Imminence of Wet Eruptions

“…On the other hand, Lake Kuttara is covered with ice in winters. Though it is not an active crater lake, slight advective heat transfer by relatively warm water is suggested, based on the limnological study on the temperature profile of the lake [28].…”