Tritium was produced by the atmospheric nuclear weapons tests of the late 1950s and early 1960s. As the heaviest isotope of hydrogen, it was oxidized to water and rained out to the surface of the oceans and continents over the years after the tests. Since the tests were largely in the Norther Hemisphere, most of the tritium (about four fifths) has been deposited north of the equator, predominantly at higher latitudes.
This tritium "dye" entered the surface ocean in the mid-1960s, and is
working its way down into the abyss. Also, tritium has been leaking out
of the Arctic Ocean, where the tritium spike has been delayed and is only
now bleeding out. We see this as a stain of high tritium (red in the
images below) concentrated along the boundary currents around the
southern edge of Greenland.
Below is a map of tritium on the 26.8 kg/m3 isopycnal surface in 1981.
Click on it to download a postscript version of the map.
That we don't see evidence further north on Greenland's eastern flank is a result of the fact that we couldn't get close enough to sample in 1981 because of ice.
And below is an approximate north-south section (viewed from Europe) of
tritium vs depth. Click on it to download a postscript version.
The map below shows the station locations for our cruises in 1981 (click for a perspective movie, or here if it looks dark):
To do the visualizations, we regridded the data onto an approximately 30 X 30 X 30 rectilinear grid (compressed vertically near the ocean surface) using a modified (quadratic least squares with distance-squared weighting) Shepard's algorithm with spatially anisotropic corellation scales.
To see a movie of the tritium distributions in the North Atlantic in 1981, click here , or here for a gamma corrected version (if the first one looks too dark on your workstation). What you see at first is a map of tritium color coded blue to red for 0 to 10 TU, on an isopycnal surface (27.1). It is at about 1000m depth in the subtropical gyre. We then examine vertical sections (meridional, then vertical), and then isosurfaces of tritium starting with the 10 TU isosurface and ending with the 1 TU surface. Note the sharp deliniation between the subpolar and subtropical gyres.
Information from the tritium may be combined with its daughter isotope helium-3
(a stable, inert gas) to compute the
tritium helium age , which is roughly the time elapsed since a fluid
parcel was last in contact with the amosphere. To see a map of tritium-helium
age on an isopycnal (density surface) in the North Atlantic, click
here , or
here for a gamma-corrected version.
What you see is a color coded map (blue is 0 years, red is 12 years)
of age on the 26.8 isopycnal, which is at about 300-400 m depth in the
subtropical gyre. Superimposed on this is a simulation of float dispersal
using the one degree resolution CME velocity fields at approximately 250m
depth (data courtesy Frank Bryan, National Center for Atmospheric Research).
Some of the floats disappear beneath the isopycnal surface during the simulation,
especially in the subpolar gyre.
Notice the tendency in the subtropics for age to increase along float
trajectories.
In some respects, we can think of tritium as a dye that was injected into the ocean in the early 1960s, but it is also radioactive, decaying with a half life of 12.45 years. By summing tritium with its daughter product, helium-3, we can construct a true dye tracer which we call "Zeta". 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.
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