The Archaean Eon (3.85 - 2.50 Ga)

The Archaean Eon began after the Late Heavy Bombardment. It ended shortly before the Great Oxidation Event. One of the primary research interests in the Archaean Eon is the Faint Young Sun paradox. The sun was 20% cooler, but there was liquid water on Earth. Scientists are investigating whether greenhouse gases might have warmed the earth sufficiently to allow for liquid water. After a short period with high methane, the atmosphere was primarily carbon dioxide and other gases from volcanic emissions. Because of absorption of oxygen by rock formations, the atmosphere remained anoxic even with generation of oxygen by algae. It might have had an orange haze atmosphere (Figure 7-3).

Unlike the lack of geologic data from the Hadean Eon, geologic formations remain from the Archaean Eon and reveal the composition of the atmosphere, oceans, and crust. Sulphur isotopes in ancient rocks indicate that an organic tholin haze probably covered the earth (Figure 7‑3) during the entire Archaean Eon (3.85 Ga to 2.1 Ga). The earth’s early haze might have been like the “tholin haze” on Saturn’s moon Titan (Figure 7‑4).[1] Although the orange haze might have only allowed 10% of the Sun’s light to reach the ground surface, cyanobacteria and probably other photosynthesizing organisms could have functioned at this low light level. An atmosphere with high levels of carbon dioxide would have been optimal for algae since algae consume carbon dioxide in the photosynthesis process.

The sun (Figure 7‑6) was 25% cooler in the Hadean and 20% cooler at the beginning of the Archaean than the present sun. Because of lower solar radiation, Earth’s cold climate should have frozen all ice on earth’s surface. Not only was there liquid water, but the types of rocks that formed in the Archaean seas normally form in water temperatures ranging 26 ⁰C to 85 ⁰C.[2] [3] These temperatures range from room temperature to almost boiling. This dichotomy between the apparent and theoretical states of water on the early Earth is the Faint Young Sun Paradox, which is the “most fundamental problem in paleoclimatology.” [4]

Carl Sagan discovered the Faint Young Sun paradox. He thought that elevated greenhouse gases levels might have kept the Earth’s climate warm enough for liquid water. Figure 7-7 and Figure 7-8 apply to the modern atmosphere during the Phanerozoic Eon, but the principles apply to the Faint Young Sun paradox. The primary incoming component of the energy balance is short wave (visible) radiation from the sun. Of the component that reaches the earth's surface, part of it is reflected. The amount of reflection depends on the type of surface. The land surface radiates long wave radiation back to the atmosphere. The atmosphere captures most of this energy but some passes through to space. The atmosphere radiates long wave (infrared) radiation back to the earth and into space.

Greenhouse gases in the present atmosphere (carbon dioxide, methane, ammonia, etc...) keep the atmosphere much warmer than it would be if the atmosphere were just nitrogen and oxygen. Even though greenhouse gases are at very low concentration in the atmosphere, parts per million (ppm) range, they intercept wavelengths of infrared radiation that other gases do not intercept and thus would otherwise exit into space. For example, there is a gap in absorbed radiation at 9 to 15 microns (2nd graph in Figure 7-8); however, carbon dioxide, partially fills this gap (4th figure in Figure 7-8). Thus, carbon dioxide holds radiation in the atmosphere that would have escaped into space. The present carbon dioxide level of 410 ppm is much higher than the 1960 concentration of 320 ppm.

The greenhouse effect is named after greenhouses and acts like a solar cooker (Figure 7-9). The glass allows short wave solar radiation to enter a solar cooker, but it prevents infrared radiation from moving out of the solar cooker; thus, the temperature rises. Greenhouse gases such as carbon dioxide allow solar radiation to reach the earth’s surface but prevent infrared radiation from escaping.

The Earth's natural carbon cycle has several inputs and outputs. The industrial revolution altered the carbon cycle because it added carbon in fossil fuels (Figure 7-10). In the Archaean, carbon dioxide was absorbed by algae and rocks and emitted by volcanoes. Carbon dioxide also diffuses into the ocean, and either became part of the photosynthesis cycle or reacted with calcium or other positive ions and formed calcium carbonate or other sediments. In the modern world, algae photosynthesis in the ocean accounts 40% of the carbon removal each year.

Scientists have investigated Sagan’s concept that elevated greenhouse gases in the Archaean atmosphere kept the climate above the freezing point of water and provide a solution to the Faint Young Sun paradox. If carbon dioxide concentration in the atmosphere was 10% or greater (vs. 0.04% today), then the atmosphere would have held enough heat to keep water in the liquid phase during the Archaean. However, magnetite and siderite in ocean sediments from this period indicate that the maximum CO₂ concentration was no more than 10 times (0.4%) that of the present atmospheric concentration, which would be far too low to maintain the climate above the freezing point of water.[5]

Methane absorbs an enormous amount of heat, and it has been proposed as the solution to the Faint Young Sun paradox. There were several possible sources of methane. Early archaebacteria called methanogens added methane to the atmosphere. Methane flux from volcanoes was 5 to 10 times greater than the present. The combination of these sources could have increased methane concentration in the atmosphere to 0.1%.[6] The problem with the methane solution to the Faint Young Sun paradox is that methane would have reacted with carbon dioxide in the reduced atmosphere and formed an orange tholin haze (Figure 7-2 and Figure 7-3). The orange tholin haze would have reflected the Sun’s radiation back into space and resulted in global cooling rather than global warming. Even without the orange haze, atmospheric modelers tested the combined effect of estimated methane plus observed carbon dioxide levels in the Archaean and found that the global average temperature would lead to glaciation.[7] Another strike against methane as the solution to the Faint Young Sun paradox is that geologic evidence indicates that methane levels were not extremely high.

Ammonia (NH₃) is another potential global warming enhancer that scientists proposed as a solution to the Faint Young Sun paradox. However, solar radiation would have quickly ionized ammonia, and it would rain out of the atmosphere and into the oceans as ammonium ion (NH₄⁺).[8] It is unlikely that there was excessive ammonia in the Archaean atmosphere because the atmosphere was not strongly reducing. In summary, none of the greenhouse gas scenarios explains the Faint Young Sun paradox.

If Earth’s axis was angled toward the sun (high obliquity axis) during the Hadean, Archaean, and Proterozoic eons (4.5 Ga – 2.4 Ga), then the temperature at the poles would have been extremely hot during the periods when the pole pointed toward the Sun. There would have been liquid water, even hot water, at the poles. [9] [10] This concept is controversial. Some scientists argue that geologic evidence indicates a high obliquity axis. Scientists have attempted to find a physical cause of a shift in obliquity at the end of the Proterozoic Eon, such as luni-solar torque; however, these efforts have been unsuccessful. [11] [12]

[1] Trainer, Melissa G., Alexander A. Pavlov, H. Langley DeWitt, Jose L. Jimenez, Christopher P. McKay, Owen B. Toon, and Margaret A. Tolbert. "Organic haze on Titan and the early Earth." Proceedings of the National Academy of Sciences 103, no. 48 (2006): 18035-18042.

[2] Angela Hessler, Earth’s earliest climate, ed. Figen Mekik, The Nature Education Knowledge Project. Accessed on July 6 2013 at <http://www.nature.com/scitable/knowledge/library/earth-s-earliest-climate-24206248>

[3] James Kasting and M. Tazewell Howard, Atmospheric Composition and Climate on the Early Earth. Philos Trans R Soc Lond B Biol Sci. 361 (2006) no. 1474: 1733-1742.

[4] Georg Feulner, The faint young Sun problem, Reviews of Geophysics (2012) 50: RG2006, doi:10/1029/2011RG000375.

[5] Hessler, Earth’s earliest climate[6]

[6] Fuelner, Faint young sun.

[7] Jacob Haqq-Misra, Shawn Domagal-Goldman, Patrick Kasting, and James Kasting, A revised, hazy methane greenhouse for the Archaean Earth, Astrobiology (2008) 8:1127-1137.

[8] Feulner, Faint young sun.

[9] Yannick Donnadieu, Gilles Ramstein, Frederic Fluteau, et al., Is high obliquity a plausible cause for Neoproterozoic glaciations, Geophysical Research Letters (2002) 29(23): 42-1 to 42-4.

[10] R. Oglesby and J. Ogg, The effect of large fluctuations in obliquity on climates of the late Proterozoic, Paleoclimates (1999) 2: 293-316.

[11] Fuelner, Faint Young Sun.

[12] Gregory Jenkins, High-obliquity simulations for the Archaean Earth: Implications for climatic conditions on early Mars, Journal of Geophysical Research (2001) 106(E12): 32903-32913.