My students and I construct numerical models of early atmospheric composition and climate. See, e.g., my review paper in Science (Kasting, 1993). We are particularly interested in the rise of atmospheric O2. There are strong reasons to believe that it first increased around 2.3 Ga, based on mass-independently fractionated sulfur isotopes in rocks (Pavlov and Kasting, 2002) and other geologic evidence. We don't understand, however, why O2 went up when it did. The cyanobacteria that produced the first O2 appear to have been around since at least 2.8 Ga (J. Brocks et al., Nature, 1999). Why, then, did it take another half billion years or more for atmospheric O2 to increase? One possibility is that the mantle oxidation state increased slightly as a consequence of loss of hydrogen to space (Kasting, 1993, Kump et al. 2001). This change could have made volcanic gases more oxidizing, thereby reducing the geologic sink for O2. However, this hypothesis is apparently contradicted by data on redox-sensitive metals (V and Cr) in ancient rocks, which suggest that the mantle oxidation state has remained constant for the last 3.5 Ga. Thus, this topic remains an area of research interest.

 

Fig. 1 Diagram showing geologic indicators of atmospheric oxygen. Red boxes indicate high O2; blue boxes indicate low O2. [From H. D. Holland, in Early Life on Earth, S. Bengtson, ed., 1994.]

 

            We are also interested in long-term climate evolution. One of the questions is:  which greenhouse gases were responsible for counteracting lower solar luminosity in the past? This question is often referred to as the "faint young Sun problem." High CO2 concentrations are one possible solution (Walker et al. 1981, Kasting, 1987, Kasting, 1993); however, high CH4 levels may have been important as well (Pavlov et al. 2000, Pavlov et al. 2001, Kasting et al. 2001, Pavlov et al. 2003). We are currently in the process of deriving new absorption coefficients for all of the important greenhouse gases (CO2, CH4, and H2O) so that the accuracy of our climate modeling can be improved.

 

Fig. 2 Diagram showing the greenhouse effect caused by different combinations of CO2 and CH4. The calculations were performed for a solar constant equal to 80% of the present value, which is appropriate for 2.8 Ga. The dashed blue line represents the freezing point of water. The solid red curve is the upper limit on atmospheric CO2 derived from paleosols, or ancient soils (R. Rye et al., Nature, 1995). Here, f(CH4) represents the CH4 volume mixing ratio. [From Pavlov et al. 2000]

 

            A third area of interest is Snowball Earth. Paleomagnetic evidence suggests that the oceans may have frozen over entirely at least 3 times during Earth's history: once at ~2.3 Ga (the same time that O2 levels rose) and twice or more in the Late Proterozoic, around 600 Ma and 750 Ma. The first of these Snowball Earth episodes can be naturally explained if rising O2 concentrations caused the collapse of a methane greenhouse that existed during the Archean/Paleoproterozoic Eras (Pavlov et al. 2000). The Late Proterozoic glaciations have been studied most extensively by Paul Hoffman at Harvard University. These glaciations are interesting both from the standpoints of both climatology and biology. We think that Earth escaped from such glaciations by building up volcanic CO2 in its atmosphere, thereby increasing the greenhouse effect (Caldeira and Kasting, 1992). How the photosynthetic algae and other light-dependent organisms made it through this catastrophe is still largely unexplained, however. We have been trying to show that Chris McKay's thin-ice model (C. P. McKay, Geophys. Res. Lett., 2000) is viable, and that significant sunlight penetrated through the ice in the tropics. A new paper on this topic has just been submitted for publication.

 

 

Fig. 3 Diagram illustrating how the Earth recovers from Snowball Earth episodes. Curves are solutions calculated by an energy-balance climate model (Caldeira and Kasting, 1992). Solid lines represent stable solutions; dashed lines represent unstable ones. The three different curves are for 3 different CO2 levels. The modern CO2 level (~300 ppmv) is on the right. For present solar flux, Earth escapes from the Snowball after buildup of 0.12 bars of CO2, or about 400 times the present atmospheric level.

 

            Our work on evolution of Earth's atmosphere and climate is carried out in conjunction with other researchers as part of Penn State's NASA Astrobiology group (http://psarc.geosc.psu.edu).

 

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