A case for late-Archaean continental emergence from thermal evolution models and hypsometry
The secular cooling of the Earth's mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans, in the oceanic bathymetry and in the area of emerged continental crust. The evolution of these variables is of fundamental importance to the geochemical coupling of mantle, continental crust, atmosphere and ocean. To explore this further, we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature, continental area and hypsometry. In this paper, we investigate the continental freeboard predicted using different models for the cooling of the Earth. We show that constancy of the continental freeboard (± 200 m) is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110-210 °C hotter than present. Such numbers imply either a very limited cooling of the planet or, most likely, a change in continental freeboard since the Archaean. During the Archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere, thus limiting crustal thickening due to mountain building processes and the maximum elevation in the Earth's topography [Rey, P. F., Coltice, N., Neoarchean strengthening of the lithosphere and the coupling of the Earth's geochemical reservoirs, Geology 36, 635-638 (2008)]. Taking this into account, we show that the continents were mostly flooded until the end of the Archaean and that only 2-3% of the Earth's area consisted of emerged continental crust by around 2.5 Ga. These results are consistent with widespread Archaean submarine continental flood basalts, and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from ∼ 2.5 Ga. The progressive emergence of the continents as shown by our models from the late-Archaean onward had major implications for the Earth's environment, particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the Earth's deep and surface reservoirs.