Global Temperature Trends Since 20 Ma Driven by Tectonic Degassing of Carbon Dioxide

The climatic journey from global warmth of the Mesozoic and Early Cenozoic to the period of great Pleistocene ice ages was far from monotonic. Episodic advances and retreats of this southern hemisphere ice cap occurred for most of Oligocene and Miocene time. However, an important reversal of the climatic trend occurred for a period between ~17-13.9 Ma, (Miocene Climatic Optimum = MCO), when the East Antarctic ice sheet may have largely disappeared. The forcing underlying the MCO remains enigmatic.

The MCO was originally recognized by a substantial depletion in the benthic oxygen isotope record, which reflects both deep sea temperatures and global ice volume. Recent marine paleotemperature measurements indicate that the magnitude of mid-Miocene ocean warming significantly exceeded a widely cited figure of + 3-5^oC and was bi-hemispheric. We estimate that peak MCO ocean warming was on the order of 6-7^oC, globally integrated.

The magnitude and global nature of the MCO warmth and its concurrent reduction in ice volume demand a potent and persistent driver. Here, we investigate the possibility that global temperature and ice volume evolution through the MCO was paced by changes in a slow, but powerful, deep earth process: the rate of degassing of CO₂ and other volatiles controlled by changes in plate tectonics. We take advantage of high precision astronomical dating of magnetic polarity durations that allow us to monitor the spreading rate history of mid-ocean ridges over time, independent of the assumption that any ridge system exhibited constant spreading rate. The resulting globally averaged crustal production curve shows highly dynamic changes over the past 20 Ma. Oceanic plate generation was significantly higher than present before ~ 5 Ma, and reached a local maximum from ~19-14 Ma at ~30% greater than the late Pleistocene average.

We constructed a simple carbon cycle mass balance forced by the new sea-floor spreading curve, which we consider as a first-order proxy for tectonic degassing by all processes. The MCO provides a temperature calibration point to optimize model parameters, which are then held fixed over time. We find that the tectonic CO₂ forcing was sufficient to explain both the amplitude and timing of Neogene temperature change, and likely resulted in high levels of atmospheric CO₂ during the MCO (900-1200 ppm).