The Salt of the Earth - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 75

For the common seawater ions, residence times are of truly geo-
logical durations. For potassium it is about 7 million years; for mag-
nesium, 10 million years; for sodium, 70 million years—and for
chloride the residence time is a simply gigantic 100 million years.
The average chloride ion, therefore, will have been carried around the
world again and again on the ocean currents since the heyday of
the dinosaurs. These are average figures, we must recall. Some of
these chloride ions were washed into the sea last week. Others will
have been in the sea much longer. A single drop of seawater will con-
tain many chloride ions that have drifted there since the trilobites
roamed the sea floors, half a billion years ago.
These enormously long times mean that these common ions have
been well and truly mixed and homogenized in the oceans, the 'mix-
ing time' of which (the average time it takes for water particles to cir-
culate around the oceans) is about 1,000 years. That is not to say that
their concentrations are identical everywhere (that character will
become very important in the next chapter, where we discuss how
ocean currents work), but the relative proportions of these ions are the
same, as are the proportions of any stable chemical isotopes of them.
For the trace elements in seawater residence times are typically
much shorter, mainly because they quickly drop out of solution,
being crystallized biologically or chemically or by a combination of
the two. For iron, the residence time is a mere 200 years—so most of
the tiny amounts of dissolved iron in the oceans date back only to the
Industrial Revolution. The residence time is not much greater for such
elements as aluminium, lead, and zinc—and also less common (but
useful) ones such as the rare earth elements, including cerium, lantha-
num, and neodymium.
These very short residence times can be useful for geologists. Being
much shorter than the mixing time of the oceans, such elements can
act as oceanographic tracers, illuminating such processes as the