Mechanisms of heat transport through the floor of the equatorial Pacific Ocean
LocationEquatorial Pacific Ocean
KeywordGeothermal resources; Heat budget; Ocean circulation; Ocean bottom; Marine geophysics; Pleides (Ship) Cruise 3; Knorr (Ship : 1970-) Cruise KN73-4
The equatorial Pacific heat flow low, a major oceanic geothermal anomaly centered on the equatorial sediment bulge, was investigated using deeply penetrating heat flow probes (6-11 meters penetration) within three detailed surveys (400 km2) and along over 10,000 km of continuous seismic profiles (CSP). Previous heat flow measurements in this region defined a broad region characterized by a heat flux well below 1 HFU. We report 98 new measurements collected during cruises PLEIADES 3 and KNORR 73-4 that verify the anomalous nature of the heat flux and also define non-linear temperature gradients (concave down). Temperature field disturbances due to perturbations of a purely conductive heat transport regime are incapable of suitably explaining either of these observations . A simple model incorporating heat transport by both conduction and fluid convection through the sediments fits the observations. A volume flux of (hydrothermal) fluid in the range of 10-6 to 10-5 cm3/sec/cm2 (0.1 liter/yr/cm2) is required. The sense of the flow for all measurements exhibiting non-linear gradients is upward out of the sediment column; no evidence for the recharging of the system was observed. Investigation of a well-defined boundary of the low zone at 4°N and 114°W showed a transition from low and variable heat flow to values compatible with thermal models that correlated with a change in the nature of the basement from rough to smooth. A few outcrops occur in the area of rough basement, but otherwise the region is well-sedimented (greater than 200 meters). Measurements within a detailed survey centered at this transition showed a dramatic increase in heat flow from 1.21 HFU to values greater than 3 HFU over a horizontal distance of 10km. A similar transition from non-linear to linear temperature gradients was not observed as nearly every measurement was non-linear. Heat flow measurements located in well-sedimented, outcrop-free areas (A environments) were associated with linear gradients and a heat flux greater than 1 HFU, however, several of these values were well below the theoretical heat flow for the appropriate age crust. Values measured in environments other than A exhibited variable heat flow and non-linear gradients. The average value of measurements located in A environments within the equatorial Pacific heat flow low was 1.37±0.27 HFU. The previously reported average was 0.92±0.48 HFU based on several measurements from L-DGO cruise VEMA 24-3. The average heat flow measured at a survey located outside the low heat flow zone on crust of 55 ±5 m.a. was 1.76 ±0.30 HFU which is in good agreement with the theoretical value of 1.60. The measurements in this survey were not located in A environments suggesting that crustal convection has ceased or is greatly attenuated within crust of this age. Error analysis of the geothermal data reduction using the convective/conductive heat transport model suggests that the volume flux parameter is sensitive to temperature measurement errors greater than a few millidegrees. Volume fluxes less than 10-7 cm/sec are difficult to distinguish from the purely conductive case assuming instrumental accuracies of 0.001°C. Resolution of the volume flux deteriorates as heat flow decreases and is poor for values less than 0.5 HFU. A detailed survey located within the low zone confirmed previous measurements of low heat flow, however, due to the low value of heat flow (about 0.5 HFU) the small-scale variability could not be clearly defined.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1981
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