2013 Annual Science Report
VPL at University of Washington Reporting | SEP 2012 – AUG 2013
Understanding Past Earth Environments
For much of the history Earth, life on the planet existed in an environment very different than that of modern-day Earth. Thus, the ancient Earth represents a planet with a biosphere that is both dramatically different than the one in which we live, but that is also accessible to detailed study. As such, it serves as a model for what types of biospheres we may find on other planets. A particular focus of our work was on the “Early Earth” (formation through to about 500 million years ago), a timeframe poorly represented in the geological and fossil records but comprises the majority of Earth’s history. We have studied the composition, pressure and climate of the ancient atmosphere; the delivery of biologically available phosphorus; studied the sulfur, oxygen and nitrogen cycles; and explored atmospheric formation of molecules that were likely important to the origins of life on Earth.
In this task we study the Earth’s early environments as proxies for habitable environments unlike the modern Earth. This year we made progress in the areas of Earth’s geochemical history and its implications for life, Earth’s early climate evolution, and the evolution of atmospheric oxygen levels.
Studying the Earth’s geochemistry, Buick and colleagues discovered a soluble and reactive phosphorus species (phosphite) in early Archean carbonates (Pasek et al., 2013) that was likely of meteoritic origin. Phosphite’s delivery during the Late Heavy Bombardment may have driven chemistry to form cell membranes and make nucleotides, a precursor to the RNA World hypothesis for the origin of life. Stüeken, Catling and Buick synthesized data to provide a model of sulfur cycling on the early Earth (Stüeken et al., 2012) which shows that biological weathering of sulfide minerals on land started in the late Archean, providing the oldest evidence of non-marine life. Buick and students Stüeken and Som, with colleagues, also published work on new techniques for selenium isotope analysis (Stüeken et al., 2013) and the measurement of vesicle size distribution, a possible probe of atmospheric pressure, in ancient lava flows (Som et al., 2013). Claire and colleagues showed that mass-independent fractionation of sulfur is conserved between bulk rock and grain scale, and can also constrain mechanisms of pyrite formation, providing an additional probe into the environments of the early Earth (Farquhar et al., 2013)
We also made progress in understanding the long-term evolution of Earth’s climate through a paper by new team members (Wordsworth and Pierrehumbert), suggesting H2 might have been an effective greenhouse gas on early Earth. The commentary on that paper also came from team members (Kasting, 2013c). The commentary explained why H2 is a good greenhouse gas, but also explained why H2 may not have been necessary to keep the early Earth warm.
In understanding the history of O2 in the Earth’s atmosphere we outlined possible causes for the first rise of oxygen at ~2.5 Ga. (Kasting, 2013a, 2013b), and a framework for understanding both the first and second rise of oxygen was provided (Catling, 2013a, 2013b). Claire and colleagues argued for the evolution of oxygenic photosynthesis 2.7 Ga based on sulfur isotope measurements (Kurzweil et al., 2013). .One specific mechanism for oxidizing the atmosphere, the escape of hydrogen to space, received specific attention in the form of a review by Zahnle, Catling and Claire (Zahnle et al., 2013) and a discussion in the context of other controls on redox budgets (Catling, 2014). Catling and Krissansen-Totton are currently re-examining the multi-billion year record of marine carbon and sulfur isotopes to derive optimal statistical estimates of their implications for oxygen fluxes into the atmosphere and ocean.
This year we also advanced our modeling repertoire. Most significantly, Domagal-Goldman and Robinson have worked to generalize our 1-D atmospheric chemistry and climate models and automate changes in boundary conditions. This will allow us to rapidly run the models for a variety of environments to quantify the effects of specific metabolism on the atmospheric evolution of the Earth, and eventually on the detectability of specific biosignatures. We have also upgraded our particle treatment to make the photochemical and climate codes more consistent with the spectral model.
PROJECT INVESTIGATORS:Roger Buick
Project InvestigatorDavid Catling
Project InvestigatorJames Kasting
PROJECT MEMBERS:Shawn Domagal-Goldman
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