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Nine downhole in situ temperature estimates were obtained from five holes on DSDP Leg 26. The heat-flow determinations were reliable at four sites. Closely spaced temperature measurements in two holes provided a good confirmation of the reproducibility of the measured temperatures using the downhole recorder. A number of temperature-versus-time records clearly shows the decay of penetration heating of the probe, permitting theoretical extrapolation of in situ temperature to equilibrium. Heat flux is nearly constant with depth at the two sites, each with four temperature points (including sea-bottom-water temperature). There is close general agreement between the Leg 26 sites and the results of the nearest shallow ocean probe. The Leg 26 sites show no systematic variation with age.

Heat flow measurements have been made in five deep crustal holes drilled into the mid‐Atlantic ridge by the Deep‐Sea Drilling Project ship Glomar Challenger. The data provide new information on the hydrothermal circulation that probably controls the crustal temperature distribution near ocean ridge spreading centers. Nearly constant very low heat flow of 0.6±0.1 μcal cm−2 s−1 (25±4 mW m−2) was found for three deep holes and three standard ocean probe measurements across one 3‐km‐wide sediment pond on 3.5‐m.y.‐old sea floor and to a depth of 400 m into the underlying basaltic basement. The heat flow predicted by conductive cooling models of plate accretion is much higher, about 6.4 μcal cm−2 s−1 (270 mW m−2). The result suggests that heat is carried to the surface by extensive hydrothermal circulation that extends at least to the lower part of layer 2, such that the heat flux based on conductive estimates represents only a fraction of the total flux. A borehole into 16‐m.y.‐old sea floor appears to have penetrated a convective system. Temperature measurements in the basement part indicate that seawater was flowing rapidly down the hole into a permeable horizon near the bottom at 324‐m depth. The heat flow of 0.5 μcal cm−2 s−1 (21 mW m−2) measured in the overlying sediments suggests that before the hole was drilled, water at about 10°C may have been flowing horizontally in this horizon.

A method of measuring the in situ sediment temperatures in deep bore holes drilled to depths of several hundred meters or more beneath the sea floor has been developed. The technique, as presently used aboard the Deep‐Sea Drilling Project (DSDP) drilling vessel Glomar Challenger, involves the emplacement of a temperature sensor, located below a self‐contained digital temperature recorder package, a short distance into the undrilled, thermally undisturbed sediment at the bottom of the drill hole. By measuring the in situ temperature at various depths in a single drill hole it is possible to calculate the thermal gradient for various intervals in the hole. This information, in conjunction with thermal conductivity data measured aboard ship on the sediment cores recovered from the drill hole, permits computation of the heat flow through the oceanic crust. Heat flow values measured in deep drill holes in the Indian and Pacific oceans and in the Bering and Red seas are in generally good agreement with the regional geothermal flux as determined by conventional near‐surface heat flow measurements, suggesting that the thousands of existent shallow heat flow values are representative of the earth's heat flux. Where multiple downhole temperature measurements made at one site permit calculation of interval heat flow values, there is no consistent indication of a significant vertical increase or decrease in heat flux, such as might be caused by long‐term changes in bottom water temperature or the upward migration of interstitial fluids. We note, however, that a more detailed set of temperature measurements in a single hole is required to verify this conclusion. Downhole heat flow values made within a specific physiographic region, such as the Red Sea or the Ninety East ridge, appear to be less variable than, but equal to, the heat flow values calculated using thermal gradient measurements made at shallower depths beneath the sea floor. This observation is in accordance with theoretical considerations which indicate that temperature measurements in deep drill holes are less susceptible than conventional heat flow measurements to the disturbing thermal effects of small‐scale surface topography, short‐term variations in bottom water temperatures, and local sedimentary processes (slumping, erosion).

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