The surface waters of the seas, down to depths of 200 meters, are areas
of rapid mixing in which temperature, density, and salinity are almost
uniform. Below the surface water is a zone in which temperature
decreases and density and salinity increase with depth. This zone, known
as the thermocline, may reach a depth of 1000 meters. Because density is
increasing here, vertical motion is reduced, and exchanges between the
surface and the deep waters are impeded. Knowledge of temperature,
density, and salinity is important to understanding what happens to
radionuclides in the ocean. Physical conditions affect the rates of
physical movement of radioactivity in the mixed (surface) layer, the
degree to which radionuclides are held at the thermocline, and the
processes by which radionuclides pass the thermocline and enter the
deep-water cycles and upwellings.
[Illustration: _Men aboard the research vessel_ Shimada _pulling in
plankton nets during sampling operations at sea_.]
The surface currents of the ocean are largely wind driven and their
patterns generally well known. New concepts of the vertical and
horizontal diffusion of substances introduced into the ocean were
developed, however, in studies of ocean-borne fallout during and after
nuclear tests in the Pacific.
The first of these surveys was conducted near Eniwetok and Bikini.
Scientists aboard a Navy vessel sampled water and plankton to depths of
300 meters at some 90 points spread over an area of 78,000 square miles
to determine the disposition of early fallout from the nuclear
detonations. Some weeks later another expedition voyaged from Eniwetok
to Guam and returned, covering an area of 375,000 square miles to follow
(by sampling) the mass of water-borne radioactivity resulting from the
test and to note the intervening effects of diffusion, dilution,
biological uptake, and decay. In 1958 two more surveys were conducted,
the first to ascertain the spread and depth—with samplings below the
thermocline—of a radioactively tagged water mass immediately following
an underwater detonation, and the second to follow the westward drift of
the tagged water mass.
Significantly, it was found that plankton immediately take up large
amounts of radioactivity. Planktonic forms, in fact, proved to be the
most sensitive indicators of the presence of radioactivity in the marine
environment. Further, the daily vertical migrations of plankton—down in
response to sunlight and up at night—seemed a part of the process by
which radionuclides move from the upper waters to the deeps.
The expedition scientists noted that the masses of low-level
radioactivity moved in the ocean significantly slower than the surface
currents, a circumstance attributable in large measure to biological
factors. The distribution of residual radioactivity in the sea a month
after the close of a nuclear testing program could be determined by
counting radioactivity in plankton samples.
It was established that strontium-90 and cesium-137, important in
fallout on land, enter the marine cycles only in minute amounts.
Practically no fission products are found in fish. Since strontium-90 is
not concentrated strongly by marine organisms, the question of what
happens to it in the ocean remains unanswered. Studies have suggested,
however, that strontium moves in solution and thus indicates the
movement of water. If this is true, strontium-90 may be contained in the
deep currents and eventually will be brought again to the surface. Some
observers believe this process has begun.