Deep Water Cycling and Delayed Onset Cooling of the Earth

Abstract

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.