A warm or a cold early Earth? New insights from a 3-D climate-carbon model

Abstract

Oxygen isotopes in marine cherts have been used to infer hot oceans during the Archean with temperatures between 60• C (333 K) and 80• C (353 K). Such climates are challenging for the early Earth warmed by the faint young Sun. The interpretation of the data has therefore been controversial. 1D climate modeling inferred that such hot climates would require very high levels of CO 2 (2-6 bars). Previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the Hadean and the Archean. Here, we revisit the climate and carbon cycle of the early Earth at 3.8 Ga using a 3D climate-carbon model. We find that CO 2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects. However, such high CO 2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts, producing an efficient stabilizing feedback. Moreover, the weathering of impact ejecta during the Late Heavy Bombardment (LHB) would have strongly reduced the CO 2 partial pressure leading to cold climates and potentially snowball Earth events after large impacts. Our results therefore favor cold or temperate climates with global mean temperatures between around 8• C (281 K) and 30• C (303 K) and with 0.1-0.36 bar of CO 2 for the late Hadean and early Archean. Finally, our model suggests that the carbon cycle was efficient for preserving clement conditions on the early Earth without necessarily requiring any other greenhouse gas or warming process.

Probable Cold and Alkaline Surface Environment of the Hadean Earth Caused by Impact Ejecta Weathering

“…For , we assume the same model as that of Charnay et al (), which is based on the results of Collins et al (). In this model, the amount of ejecta can be calculated using mass, velocity, density, and impact angle of each impact (see equations (S21) and (S22)).…”

“…According to Collins et al (), we assumed the initial size distribution of ejecta particles as follows: where is a cumulative number of ejecta particles whose mass is larger than . The coefficient, , is assumed to be between 0.87 and 0.95 (Charnay et al, ; Collins et al, ; Zahnle & Sleep, ). The coefficient, , is chosen so that it reproduces the total ejecta mass (see equation (A.20) in the supporting information).…”

“…On geological timescales of years or more, comparable to that on which the carbonate‐silicate cycle operates, the overall effect of impacts in the Hadean would be to cool the Earth. Impacts produce considerable ejecta, which is easily weathered by carbonic acid (Charnay et al, ; Sleep & Zahnle, ; Zahnle & Sleep, ). Such acid weathering of the impact ejecta is a source of carbonate alkalinity (i.e., dissolved cations), so it results in a decrease in atmospheric when those cations sequester carbon into carbonates.…”

Nucleic Acid Catalysis under Potential Prebiotic Conditions

“…Indeed, more recent 3D climate‐carbon models by Charnay et al. predict global mean temperatures between around 8 °C (281 K) and 30 °C (303 K) 3.8 billion years ago, suggesting that cold and even frozen environments may have been present on early Earth . Hydrothermal vent temperatures are highly variable, with gradients from the hot interior (>350 °C) to much colder seawater (or surrounding surface freshwater) .…”

“…[58] Indeed, more recent 3D climate-carbon models by Charnay et al predict globalm ean temperatures between around 8 8C( 281 K) and 30 8C( 303 K) 3.8 billion years ago, suggestingt hat cold and even frozen environments may have been present on early Earth. [151] Hydrothermal vent temperatures are highly variable, with gradients from the hot interior (> 350 8C) to much colder seawater (or surrounding surface freshwater). [51] This precludes the occurrence of biochemical processes on or near to the surface of the vent, particularly given that the function of typical mesophilic nucleic acid enzymes is lost above % 70 8C, but conditions in the immediate surroundings may have been rather more amenable.…”