  Changes that occur on our planet can be tracked back to one of two energy
sources: the sun and the Earth's internal energy. The motion of tectonic
plates, volcanism, mountain building and the reshaping of our planet's surface
over geologic time depend on the Earth's internal energy. Tectonic activity is
driven by internal energy and affects the rate at which energy is tapped, i.e.,
the cooling rate of our planet. Petrologic data indicate that cooling did not
occur at a constant rate over geologic history. Interior cooling was mild until
~2.5 billion years ago and then increased (Figure 1). As the Earth cools, it
cycles water between its rocky interior (crust and mantle) and its surface.
Water affects the viscosity of mantle rock, which affects the pace of tectonics
and, by association, Earth cooling. We present suites of thermal-tectonic
history models, coupled to deep water cycling, to show that the petrologically
constrained change in the Earth's cooling rate can be accounted for by
variations in deep water cycling over geologic time. The change in cooling rate
does not require a change in the global tectonic mode of the Earth. It can be
accounted for by a change in the balance of water cycling between the Earth's
interior and its surface envelopes. The nature and timing of that water cycling
change can be correlated to a change in the nature of continental crust and an
associated rise of atmospheric oxygen. The prediction that the rise of oxygen
should then be correlated, in time, to the change in the Earth's cooling rate
is consistent with data constraints.
