Snowball Earth at low solar luminosity prevented by the ocean–atmosphere coupling

Abstract

The standard solar model proposes that the solar luminosity was 30% lower than the present level at 4.5 billion years ago (Ga). At low solar radiation, the climate model predicts that the Earth should have been completely covered by ice in the first 2 billion years, i.e. in the snowball Earth climate mode, when the atmospheric CO2 content was at the present level. However, snowball Earth condition is inconsistent with various sedimentological, paleontological, and geochemical evidence. Such controversy is collectively known as the ‘Faint Young Sun’ (FYS) paradox. Though various models have been proposed, the FYS paradox has not yet been resolved. In this study, we develop a model by considering the ocean–atmosphere coupling to show that high atmospheric CO2 level could be sustained at low seawater pH. The modeling result indicates that 0.1 bar atmospheric CO2 level that was required to prevent snowball Earth in early Archean could be sustained at seawater pH of 6.8–7.2. Although the absence of siderite in Archean paleosols has been used to argue against high atmospheric CO2 level, we suggest that siderite precipitation in paleosols was not controlled by the atmospheric CO2 level alone. Instead, siderite could precipitate in anoxic conditions with various amount of CO2 in the atmosphere, suggesting siderite cannot be used to reconstruct the atmospheric CO2 level. Therefore, the new model suggests that the snowball Earth condition could be prevented by the coupling of atmosphere and ocean systems, and thus the emergence of the ocean in the very beginning of Earth evolution might be the key to the subsequence evolution of habitability.