Explaining the Paleoproterozoic Rise of O2

Details Explaining the Paleoproterozoic Rise of O2 Explaining the Paleoproterozoic Rise of O2

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p21c..09a&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #P21C-09

Other

0315 Biosphere/Atmosphere Interactions, 0325 Evolution Of The Atmosphere, 0400 Biogeosciences, 5407 Atmospheres: Evolution

Scientific paper

The Paleoproterozoic rise of atmospheric O2 is not understood. Geochemical evidence indicates pO2 \leq 5 x 10-4 atm before ~ 2.2 Ga and >= 0.03 atm by 2.0 Ga [1]. Most models of this rapid oxygenation account for the timing in an ad hoc manner. In contrast, we suggest oxygenation was driven by rising solar luminosity, a hypothesis consistent with emerging geochemical evidence. Solar luminosity was { ~} 80 % of present value 2.75 Ga. Climate models indicate that pCO2 > 0.2 atm was required before 2.75 Ga if CO2 alone accounted for greenhouse enhancement that averted global glaciation [2]. Paleosols reveal pCO2 <= 10-1.4 atm between 2.75 and 2.2 Ga [3]. This constraint may extend earlier [4]. Given insufficient CO2, CH4 likely helped maintain surface temperature. pCH4 > 10-4 atm would have been adequate 2.75 Ga; Methanogens could sustain this level as long as pO2 was negligible [5]. Atmospheric CH4 and O2 are incompatible. If CH4 was a critical greenhouse gas, increases in pO2 caused global cooling. Temperature and photosynthetic O2 production are probably positively correlated between 273 and 300 K. Combined, these effects constituted a negative feedback on pO2: Excess O2 production cooled the surface, decreasing O2 production until pCH4 was restored. Methanogenesis was decoupled from this feedback as the bulk of Archean methanogens presumably lived in the deep ocean, insulated from surface temperature. Hence, the CH4 greenhouse acted as a cap on pO2 in the Archean. This cap would have eased as rising solar luminosity permitted "titration" of CH4 by increased O2 production; pO2 increased slowly as pCH4 decreased. This feedback suppressed pO2 until solar luminosity allowed the CO2-H2O greenhouse to maintain equable temperatures without appreciable CH4. This threshold was crossed { ~} 2.5 to 2.0 Ga [2], explaining Paleoproterozoic oxygenation. As Earth approached the threshold the CH4 buffer shrank and the likelihood of the feedback being temporarily overwhelmed by sudden O2 blooms increased. This hypothesis is consistent with observations of extreme climatic instability between 2.5 and 2.0 Ga, including evidence of at least two, possibly three, episodes of global glaciation 2.4 to 2.2 Ga [6]. We predict that Paleoproterozoic glacial episodes should be preceded by increased oxygen production- excess O2 production is predicted to cause pCH4 erosion and cooling. 13C-enriched carbonates underlying the glaciogenic Timeball Hill Fm, S. Africa [6] indicate enhanced carbon burial associated with such O2 production. Significantly, there is no evidence of glaciation following the largest Paleoproterozoic 13C excursion,{ ~} 2.2 Ga; solar luminosity at this time was sufficient to avoid glaciation despite pCH4 << 10-4 atm. Greenhouse considerations no longer limited the rise of pO2 after this time, consistent with paleosol and other evidence of oxygenation. Further study of 13C in Paleoproterozoic and late Archean carbonates are critical to test and extend this hypothesis. 1. Rye & Holland (1998), Am. J. Sci. 298, 621; 2. Kasting (1987), Precamb. Res. 34, 205; 3. Rye et al. (1995) Nature 378, 603; 4. Sleep & Zahnle (2001) JGR 106, 1373; 5. Pavlov et al. (2000) JGR 105, 11981; 6. Bekker et al. (2001), Am. J. Sci. 301, 601.

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