Anthropogenic tritium in South Atlantic bottom water

Abstract

By observation of the invasion of man-made substances into the oceans, we learn about the nature and rates of mixing and flow. One of the more useful of these tracers is tritium (³H), which was produced by the atmospheric thermonuclear weapons tests of the 1950s and 1960s and which entered the oceans in a pulse-like fashion, totally dominating the natural background. The production and entry of this radioisotope (t_1/2 ∼ 12.5 yr)¹ into the ocean took place largely in the Northern Hemisphere, so that corresponding concentrations of tritium in the Southern Hemisphere are 5–10-fold smaller. Unfortunately, this means that the dynamic range (the ratio of maximum observable to minimum detectable concentrations) of tritium is limited for southern hemispheric work. If the minimum detectable quantity (that is, the detection limit) of tritium could be reduced, the evolution of this transient tracer would still provide valuable insight into mixing and advection. By increasing sample size and altering sample handling techniques, we have now extended the detection limit by more than an order of magnitude, and have detected what we believe to be man-made tritium in Antarctic bottom water at about 40° S in the western South Atlantic.