ABSTRACT
Unlike some of the other subjects in this book, my subject is of recent origin, and I cannot enlist in its cause any of the scientists who took part in the original Challenger Expedition. That is, not if the word ‘tracer5 is defined in the sense in which it is usually used today. If I were to use the term in its wider and more literal definition, it would be appropriate to almost any measurement of a property of sea water, for most such properties have been used to trace (i.e. study the movement of) the oceans at some time in the past. Temperature, salinity, density, potential vorticity, nutrients, plankton abundance and speciation have all been used as tracers, and I could take virtually the entire field of marine science as my subject. However, today oceanographers use the word for a more restricted range of vari- ables, the main defining features apparendy being that the measurements should require esoteric chemical skills incomprehensible to all but a select few, and that it should take hours to produce a single measurement.
