One of the real workhorse paleo-tracers,
and one of the very first to be developed, was based on
what are called isotopes, which are
the same element
but they have different masses.
Oxygen 18 has two more neutrons in the nucleus than oxygen 16.
A neutron doesn't have any charge.
It doesn't mean that you have to have more electrons. The oxygen 18 and
16 have the same number of electrons so it means they have the same chemistry.
They are both oxygen, but they have different masses.
Different processes in a hydrological cycle tend to separate
these two isotopes.
And so we can measure the relative abundances
of the two isotopes either in calcium carbonate (CaCO3)
on the seafloor, or in ice (H2O) to get
clues about how the earth worked at earlier times.
The planet acts like a giant still,
distilling the light isotope, to form the great ice sheets.
When water evaporates it tends to favor the light isotope because
the lighter one can go into the vapor phase more easily.
And then as that water vapor gets
selectively rained out, the rainfall tends to choose for the heavy isotope.
And so that means that the water vapor that's left
in the atmosphere, as you go further and further north where
it's colder and colder, tends to get lighter and lighter and lighter in its
proportion of oxygen 16 to 18.
Until finally, when it forms an ice sheet, the ice sheet is very light.
By very light I mean something like it has 5% less
oxygen 18 relative to 16 of what the ocean has,
which doesn't sound like a huge change.
But it actually is a very large change, very easy to measure.
and the way geochemists describe this is to do it in "per mil" instead of percent.
5% is the same as 50 per mil. And the fact that it's
lighter means put a minus sign here. The ice sheets have an isotopic
signature of minus 50 per mil relative to
the ocean which starts out at zero per mil.
And then when you take enough water out of the oceans to make this isotopically
light ice sheet, the water that's left behind is a little bit enriched.
It turns out that the ocean can be
about one or two per mil heavier during a time
when there's a lot of ice on the Earth.
We can measure the isotopic composition of the ice just
by putting it in a mass spectrometer and counting the different kinds of atoms.
But we can also figure out what the isotopic composition of the ocean
was in the past from sediment cores by looking at the oxygen in
calcium carbonate.
Here is an equilibrium reaction where we're changing this heavy
oxygen for the two light oxygens in this calcium carbonate here.
And it turns out that which chemical the oxygen 18 wants
to be bound to depends on the temperature
at which the calcium carbonate forms.
Its actually a bit of a trick because, measuring the proportion of oxygen 18 to 16
in calcium carbonate in sediments, there could be
a component of the signal that comes from the global ice volume, that can
tell us about the shrinking and growing of ice sheets through the ice age,
But it can also be a function of the local temperature, because when
you change the temperature it changes how the oxygen wants to be distributed.
No paleo-proxy is ever used in isolation.
There are other proxies that can be used to help
constrain and pin down the local temperature, for example, and then
from suite of many of these tracers we can begin
to put together a picture of the world in the past.