The Penetration of Helium and Tritium into the Sargasso Sea
One of the most significant promises of transient tracers is
that by observing how they change in time, we can deduce the rates of
processes occurring in the ocean. Tritium is a radioactive
(half-life = 12.45 years) transient tracer, which
was produced in massive quantities by atmospheric nuclear weapons testing
in the late 1950s and early 1960s. The
transferral of tritium to the oceans
is a complex process, but the net effect was that there was a spike of
tritium added to the surface oceans, largely in the northern hemisphere.
The result is that surface water tritium values in the North Atlantic
climbed rapidly in the early 1960s, and reached a maximum around 1965.
Due to dilution with older waters below, and more tritium-impoverished
waters from the south, surface water tritium values decreased at a
rate exceeding the normal decay rate of tritium. Thus we have a period
of time (around 1965) which has been marked by this tracer.
We can see this marker moving down into the ocean! Using
our own tritium and helium-3 data,
we have a 15 year record for tritium and
helium-3 at a location near Bermuda in the Sargasso Sea.
The tritium mark is readily understood. The helium-3 mark must be
thought of in the following manner: near the ocean surface, any helium-3
produced by tritium decay is lost to the atmosphere, but deeper down,
it accumulates with increasing distance from the surface. The increase,
however, cannot go on indefinitely, since in the deep waters, there is
less tritium. Most notably, both maxima are moving down through the
thermocline at the same rate of 17 m/y. This is a direct measure of
vertical velocities in the thermocline. The vertical velocity in the
main thermocline is an important quantity in thermocline theories, and
numerical models of ocean circulation. How difficult would it be to
measure this with a "meter"? Keep in mind that this is .00005 cm/s.
One other thing that we can learn from tritium and helium-3 is how
mixing plays a role in distributing substances in the ocean.
Recognizing that helium-3 is just dead tritium (it is
the daughter product of tritium decay), we can reconstruct the
original tritium concentration by adding the two tracers together.
This, in effect, gives us a stable tritium tracer, which
we fondly refer to as Zeta . A perspective plot of Zeta
from the time series station
shows it to be decreasing steadily over
the years, with some filling up of the deeper waters. Over the course
of the time series, we see a two-fold reduction in the Zeta maximum
(yes, it also moves down into the thermocline at 17 m/y), and by
extrapolating back to 1965, an almost four-fold attenuation by 1989.
Looking at the three-dimensional distribution of Zeta in the North Atlantic
in 1981, we make the following observation: at no place in
the main thermocline of the subtropical gyre do we see any Zeta
more than 6.3 TU.
This is despite the surface maximum of 18 TU in 1965. No water exists
that has not been diluted less than three-fold since this time.
Note that the high Zeta values along the North American and Greenland
coasts north of Newfoundland are low salinity, high tritium water
flowing in from the Arctic.
For a brief movie (about 400-500 KBytes) showing the distribution of Zeta in
the North Atlantic, click here , but if
the image is a bit too dark, click here
for a gamma corrected version. (If you are on a SPARCstation, choose the
latter). The color scale is blue to red for 0 to 8 TU (greater than 8 TU is
red). The isosurface generated at the end of the movie starts at 10 TU and
decreases to 6 TU. Note the appearance of the subtropical subsurface Zeta
lens only when Zeta gets down to about 6 TU.
If you wish a postscript copy of a preprint discussing this time series
(about 750 KB) click here , and for
a color version of figure 1 for that manuscript click
here . This manuscript was submitted
to the Journal of Geophysical Research on March 21, 1994.
Return to WHOI-HIL Projects Page.
The tritium mark is readily understood. The helium-3 mark must be
thought of in the following manner: near the ocean surface, any helium-3
produced by tritium decay is lost to the atmosphere, but deeper down,
it accumulates with increasing distance from the surface. The increase,
however, cannot go on indefinitely, since in the deep waters, there is
less tritium. Most notably, both maxima are moving down through the
thermocline at the same rate of 17 m/y. This is a direct measure of
vertical velocities in the thermocline. The vertical velocity in the
main thermocline is an important quantity in thermocline theories, and
numerical models of ocean circulation. How difficult would it be to
measure this with a "meter"? Keep in mind that this is .00005 cm/s.
