Publication Date:
2022-05-25
Description:
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
Description:
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.
Keywords:
Geothermal resources
;
Heat budget
;
Ocean circulation
;
Ocean bottom
;
Marine geophysics
;
Pleides (Ship) Cruise 3
;
Knorr (Ship : 1970-) Cruise KN73-4
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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