Radiation in the oceans
These 2 images are cross sections of the Atlantic ocean, running from the equator to the North Pole. They show the formation of North Atlantic Deep Water - cold water in the north Atlantic is denser than the warm water at the equator and gradually sinks, forming water that moves along the bottom of the ocean. Scientists have managed to track the formation of deep water in recent decades due to an act of humans. In the 1950s and 1960s, the United States and the Soviet Union set off hundreds of nuclear weapons in the Earth’s atmosphere. These tests scattered the unstable, radioactive debris reactions over planet at small amounts, and that radioactive debris allowed tracking of this Deep Water. One part of that debris is tritium, a hydrogen atom with 2 neutrons. Its half-life is 12 years, so over time it decays into Helium-3, but before it decays its chemical properties are the same as hydrogen. It can bond with oxygen and form water, so the pulse of tritium released in these airborne nuclear tests largely went into the oceans.
The water molecules containing that tritium mixed rapidly in the upper layers of the ocean, but took much longer to sink to depth. The contours in this plot show how the tritium released in the late 1950s gradually moved deeper as cold waters from the North Atlantic sank towards the Atlantic Ocean bottom.
This tritium allowed us a signal that we could track as the oceans mixed. Shallow waters are mixed around the world in a matter of years to decades by the winds, but the bulk of the oceans, the deep water, takes much longer, centuries to millennia, to overturn completely. Once water sinks to the ocean bottom, it can take thousands of years before it fully comes back to the surface. The presence of this tritium pulse allowed scientists to watch waters as they moved from the surface to the deeper parts of the Atlantic.
-JBB
Image credit: Toggweiler, 1994 https://www.gfdl.noaa.gov/bibliography/related_files/jrt9401.pdf
