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Normally in the deep ocean, the bottom water temperature
(BWT) is stable over time, and varies only slightly with depth
as the adiabatic gradient below the level at which water reaches
its maximum density. Shallow waters, in contrast, do not generally
have such stable temperatures. The bottom water temperature
(BWT) controls the boundary condition for conduction of heat
upward through the sediments. Temperature transients in shallow
BWTs are expected; to measure heat flow in shallow sediment
in the presence of such transients is extremely difficult
(Lewis et al., 1991). The transients are not generally the
same in magnitude or timing from year to year.
As an example, a plot of the bottom water temperature (BWT)
vs. water depth for various Gulf of Mexico stations (Figure
6) shows that the depth of minimum water temperature is between
1,900 and 2,300 m. This represents the maximum density of
water, near 4oC. Below this depth, the water column in the
Gulf of Mexico is more variable from place to place. At water
depths of less than 1,500 m, larger temperature gradients
appear. Eastern Gulf Stations at water depths of 902 and 604
m, respectively, have large variations in heat flow near the
sediment-water interface, consistent with and controlled by
past transients in the BWT.
Figure 6. Water column
temperatures as a function of depth.
Eastern Gulf Stations at water depths of 1,373 and 1,412
m, respectively, have very constant heat flow in the upper
3 m of sediment, and show no sign of past transients in the
BWT. However at one Station (1,472 m water depth), a small
change in heat flow might be due to recent transients. Therefore,
BWTs must be considered when evaluating the heat flow measurements
in water depths less than approximately 1,000 m in this region
of the Gulf of Mexico.
Figure 7. Small but
significant differences in water temperatures measured down
to a depth of 1,900 m at four Stations. Upper station has
had a recent change in heat flux in the sediments, lower station
illustrates three adjacent stations which had no changes in
heat flux in sediments.
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