Palaeoclimates: the first two billion years

Abstract

Earth's climate during the Archaean remains highly uncertain, as the relevant geologic evidence is sparse and occasionally contradictory. Oxygen isotopes in cherts suggest that between 3.5 and 3.2 Gyr ago (Ga) the Archaean climate was hot (55-85 degrees C); however, the fact that these cherts have experienced only a modest amount of weathering suggests that the climate was temperate, as today. The presence of diamictites in the Pongola Supergroup and the Witwatersrand Basin of South Africa suggests that by 2.9 Ga the climate was glacial. The Late Archaean was relatively warm; then glaciation (possibly of global extent) reappeared in the Early Palaeoproterozoic, around 2.3-2.4 Ga. Fitting these climatic constraints with a model requires high concentrations of atmospheric CO2 or CH4, or both. Solar luminosity was 20-25% lower than today, so elevated greenhouse gas concentrations were needed just to keep the mean surface temperature above freezing. A rise in O2 at approximately 2.4 Ga, and a concomitant decrease in CH4, provides a natural explanation for the Palaeoproterozoic glaciations. The Mid-Archaean glaciations may have been caused by a drawdown in H2 and CH4 caused by the origin of bacterial sulphate reduction. More work is needed to test this latter hypothesis.

Figure 1

Geologic time-scale showing major climatic and evolutionary events during the Precambrian Era.

Figure 2

Diagram illustrating the faint young Sun problem. Solid curve represents solar luminosity relative to today. The dashed curves represent Earth's effective radiating temperature (T_e), and its mean surface temperature (T_s), as calculated using a one-dimensional climate model. Fixed atmospheric CO₂ and a fixed relative humidity profile were assumed in the calculation (from Kasting et al. 1988).

Figure 3

Biogenic methane flux, $\Phi ({\text{CH}}_4)$, and net primary productivity (NPP) for two different anaerobic ecosystems on the anoxic Archaean Earth. In one ecosystem, autotrophic methanogens dominate primary productivity; in the other, H₂-based anoxygenic photosynthesizers dominate. The two systems have different NPP but nearly the same methane flux. The modern biogenic methane flux and marine NPP (÷1000) are shown for comparison (from Kharecha et al. 2005).

Figure 4

Mean global surface temperature at 2.8 Ga for different CO₂ partial pressures and CH₄ mixing ratios. The assumed solar flux is 80% of today's value. The two dashed curves represent the freezing point of water and an upper limit on P_CO2 derived from palaeosols (Rye et al. 1995) (from Pavlov et al. 2000).

Figure 5

Stratigraphic sequence of the Huronian Supergroup in Southern Canada, showing the three glacial diamictites. Atmospheric O₂ levels increased dramatically during this time-interval (see text) (from Young 1991).

Figure 6

Compilation of Δ³³S versus time from rocks of all ages. Most of the data are from Ono et al. (submitted) and references therein. The error bars at 2.8 and 3.0 Gyr bracket the data from the Hardey and Mosquito Creek formations in Western Australia (Watanabe et al. 2005).

Figure 7

(a) ‘Case 1’ atmosphere from Kasting & Walker (1981). Volcanic outgassing of H₂ is balanced by escape to space. (b) ‘Case 2’ atmosphere from the same paper. No volcanic outgassing. Escape of H₂ to space is balanced by flow of O₂ from the atmosphere into the ocean.