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Early Earth: Not So Hot?

February 15th, 2007 by Hasenkopf · No Comments

Early Earth was a strange place. Take incident solar radiation, for example. The Sun had about 80% of its current intensity 2.5 billion years ago. Today, if you placed a giant screen over the sun and blocked out 20% of the light emitted, the Earthâ€™s average surface temperature would be below waterâ€™s freezing point. Yet, it is pretty well accepted from geological records that there was abundant liquid water flowing on Earthâ€™s surface at that time. This apparent contradiction is termed the Faint Young Sun paradox . The paradox goes away if you assume early Earthâ€™s atmosphere had a greenhouse effect that makes todayâ€™s greenhouse effect look puny in comparison. In fact, according to oxygen isotope records, early Earth surface temperatures could have been as high as 45 – 85 degrees C! Granted geological scales of time, temperature, etc., are hard for a modern-atmosphere dweller such as myself to comprehend, but those numbers seem crazily high. Two recent papers by Kasting, et al. have hypothesized that climate early on Earth was much more temperate than the inside of a sauna, and also physically reconcile such a climate with the oxygen isotope record (see Kasting et al., 2006a , Kasting et al., 2006b).

The high temperature climate told by the oxygen isotope record is based on the premise that 18O and 16O fractionate differently according to temperature. Low surface temperatures promote sediments enriched in 18O, while high temperatures do the opposite. Looking at the figure below (appears as Figure 2 in Kasting, et al., 2006a),which is compiled from average oxygen isotope ratios of marine carbonates over the past 3.4 billion years, you can see why paleoclimatologists deduce a much warmer early Earth climate than the present.


However, the authors point out that this hot early Earth scenario creates contradictions with other aspects of the geologic record (namely paleoweathering and glacial records). They propose a mechanism that would preferentially depress the fractionation of 18O into sedimentary rocks-which is what we actually measure to gauge isotopic oxygen levels of sea water in paleoclimates – and therefore make the record indicate a warmer climate than actually was present. The gist of their idea centers on the fact that isotopic oxygen composition of sedimentary rocks is a function of both the ambient temperature and a baseline isotopic oxygen composition of sea water that is controlled primarily by interactions with basalt rock. The hot Earth described earlier with temperatures in the 45-85 degrees C range results from assuming that the baseline isotopic oxygen composition is constant over time in sea water. Over short timescales, this is an alright assumption, however you run into problems for larger ones.

18O-incorporation of sea water occurs from interaction with basalt rocks at temperatures over 350 degrees C. Such high temperatures occur in the ocean only in ridges with deep geothermal vents. Kasting et al. hypothesize that because oceans were much shallower on early Earth, pressures would be lower within hydrothermal systems, limiting circulation and the exposure of hot enough water to 18O-rich basalt. Also, shallower oceans would allow heated water to rise and boil off before reaching the critical temperature, so no 18O would be exchanged between the basalt and sea water. Therefore, the baseline amount of heavy oxygen isotopes present in early oceans would be less than in todayâ€™s deeper oceans. Consequently, early Earth surface temperatures are assumed to be higher than they actually were when this ocean-depth dependent baseline is not taken into account.

Oscillating glacial periods-which are represented in the geological record- do not seem as viable with a hot planet. So assuming you have a temperate early Earth, how could you push it past its tipping point and into a glacial period? Kasting et al. suggest studying records of mass-independent fraction (MIF) of sulfur. Looking at the figure below (Figure 5 in Kasting et al., 2006a), the main concepts you need to know to interpret it are (1) high MIF (y-axis) values are believed to be caused by photolysis of SO2 and this signal is preserved by sulfur leaving the atmosphere in more than one form and not getting remixed in the ocean, and (2) MIF values near zero result from either SO2-shielding from UV light or oxidation of outgassed SO2. For instance, our current oxygenated atmosphere both shields SO2 from UV photons and oxidizes it into sulfate, so todayâ€™s MIF signal is low.

Notice the MIF dip in the figure between 2.8 and 3.2 billion years ago. Kasting et al. offers four hypotheses about why that dip might be there. To me, the most interesting of the four hypotheses is the following train of thought: the extra greenhouse effect needed to produce a warm (not hot) early Earth was supplied by higher levels of CO2 and H2O. Then, at some time before 3.2 billion years ago, methanogens began producing high enough levels of methane to bring the CH4/CO2 ratio to 0.6. At this point, an organic haze would form, similar to that found on Saturnâ€™s moon Titan, which would block out both UV and a portion of the visible solar spectrum. The haze would then cause the lower MIF signals seen in the figure above and an anti-greenhouse effect, possibly spurring an ice age.

The authors point out that their hypotheses of why there are low MIF signals during that time span are speculative and will rely on geologists making additional MIF analyses of sulfur in rocks from the same period. They also mention that if their hypothesis is valid, then burgeoning life must have had a profound effect on shaping early Earth climate and perhaps conditioned it for future oxygen-reliant life.

Tags: climate

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