Decrease of seawater CO₂ concentration in the Late Archean: An implication from 2.6Ga seafloor hydrothermal alteration

Before continents attained a critical aerial dimension on the early Earth, hydrothermal carbonation of subseafloor crust is considered to have played the dominant role in fixing CO₂ from the CO₂-rich ocean. However, it is uncertain how and when the seawater CO₂ level decreased and the strong carbonation of oceanic crust ceased. Here we report the depth profiles of the volume concentration and the carbon isotopes of calcites in the Late Archean/Paleoproterozoic volcanic rocks (Fortescue and Hamersley groups), exposed in the southwestern Pilbara Craton, Western Australia. The depth profiles indicate that 2.6Ga seafloor hydrothermal carbonation is well preserved in the study area and that the CO₂ content of subseafloor crust per seafloor unit area is estimated to be clearly lower than those in the Early and Middle Archean and similar to the Phanerozoic equivalents. This suggests that the CO₂ concentration in seawater decreased from the Middle Archean to the Late Archean. This period broadly corresponds to the time of the first appearance of supercontinent on Earth. The amalgamation of continents has the potential to decrease seawater CO₂ concentration due to the removal of platform carbonate to continental interior. Subsequent fragmentation of supercontinent likely cause the carbonate deposition around newly created continental shelves. It is therefore implied that seawater CO₂ concentration in the early Earth was lowered by not only the hydrothermal carbonation of subseafloor crust but also through the formation and breakup of supercontinent in the Late Archean.