Carbon dioxide cycling and implications for climate on ancient Earth

The crustal Urey cycle of CO2 involving silicate weathering and metamorphism acts as a dynamic climate buffer. In this cycle, warmer temperatures speed silicate weathering and carbonate formation, reducing atmospheric CO2 and thereby inducing global cooling. Over long periods of time, cycling of CO2 into and out of the mantle also dynamically buffers CO2. In the mantle cycle, CO2 is outgassed at ridge axes and island arcs, while subduction of carbonatized oceanic basalt and pelagic sediments returns CO2 to the mantle. Negative feedback is provided because the amount of basalt carbonatization depends on CO2 in seawater and therefore on CO2 in the air. On the early Earth, processes involving tectonics were more vigorous than at present, and the dynamic mantle buffer dominated over the crustal one. The mantle cycle would have maintained atmospheric and oceanic CO2 reservoirs at levels where the climate was cold in the Archean unless another greenhouse gas was important. Reaction of CO2 with impact ejecta and its eventual subduction produce even lower levels of atmospheric CO2 and small crustal carbonate reservoirs in the Hadean. Despite its name, the Hadean climate would have been freezing unless tempered by other greenhouse gases.