2010 Annual Science Report
VPL at University of Washington Reporting | SEP 2009 – AUG 2010
Understanding Past Earth Environments
We study the chemical and climate evolution of the Earth as the best available proxy for what other inhabited planets might be like. A particular focus is on the “Early Earth” (formation through to the 1.6 billion years ago) which is poorly represented in the geological record but comprises half of Earth’s history. We have studied the total pressure of the Archean atmosphere (prior to 2.5 billion years ago), developed constraints on CO2 concentration, studied the oxygen and nitrogen cycles, the fractionation of sulfur isotopes and explored the effect of hazes on early Earth climate.
An important unknown in characterizing the Archean atmosphere is the total pressure, dominated today by oxygen (which was largely absent then) and Nitrogen (which has received little study). Goldblatt et al. (2009) showed that there is around twice as much nitrogen in Earth’s crust and mantle than in the atmosphere, and that mantle nitrogen is of subduction origin. This means that there might well have been more nitrogen in the Archean atmosphere than today, which would have caused a warming by pressure broadening.
In parallel, further progress has been made towards developing a paleobarometer for the Archean atmosphere at 2.7 Ga from gas-bubble size in sea-level basalt flows and raindrop impact-crater size, using computerized X-ray tomography for the former and 3D laser scanning for the latter. Software for visualizing and quantifying low-contrast amygdales and for measuring the diameter and volume of raindrop craters has been created, vesicle formation in cooling lava has been modelled and experiments on raindrop crater formation in different sedimentary media have been performed (work in progress by Som & Buick). Catling and Zahnle (2009) have studied atmospheric escape.
The oxygen cycle is dominant in determining the redox condition of the atmosphere. Catling (2010) has reviewed this. Whilst the Archean troposphere was reducing, there was an oxygen source in the stratosphere from photolysis. By implementing a passive tracer in a general circulation model (GCM) we have shown that, whilst downward transport would have contributed some surface O2 on early Earth prior to the major rise of oxygen, this would not have been sufficient for aerobic respiration(work in progress by Haqq-Misra).
Some of the most important constraints on redox are supfur isotopes. We are developing this constraint by examining the changes in isotopic compositions in various model atmospheres, and comparing with the geologic record. The atmospheric chemistry model has been enhanced to include isotopic sulfur species, and the wavelength grid has been enhanced to resolve sub-Angstrom changes in isotopologue spectra of SO2. The model predicts magnitudes of sulfur fractionation that are at odds with other recent papers which made many simplifying assumptions (Claire & Kasting, 2010). See the postdoctoral report by Mark Claire for a more detailed discussion.
Goldblatt & Zahnle (2010) have explored how clouds could affect the Faint Young Sun paradox. Rosing et al. (2010, with VPL member Sleep) have suggested a new constraint on pCO2 in the Archean, which would make this paradox harder to solve. Tian and Kasting (manuscript in review) are suggesting that NH3 may have been relevant as an Archean greenhouse gas, contrary to earlier work.
PROJECT INVESTIGATORS:Roger Buick
Project InvestigatorDavid Catling
Project InvestigatorJames Kasting
PROJECT MEMBERS:David Des Marais
RELATED OBJECTIVES:Objective 1.1
Formation and evolution of habitable planets.
Indirect and direct astronomical observations of extrasolar habitable planets.
Earth's early biosphere.
Production of complex life.
Environment-dependent, molecular evolution in microorganisms
Co-evolution of microbial communities
Effects of environmental changes on microbial ecosystems