Publications

Biological Regulation of Atmospheric Chemistry En Route to Planetary Oxygenation PNAS, doi 10.1073/pnas.1618798114
Links to Paper: PDF (restricted access until 2018) ; PURE ; Article available upon request
Media Coverage: StA Press release ; Sci-News ; Futura Science (in French) ; ABC (in Spanish) ; My blog post
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This paper looks at the correlation between d13Corg and D36S/D33S in a high resolution section of core GKF-01, sediments laid down roughly 2.5 billion year old ago. Following our earlier work, we interpret these data as enhanced biogenic methane fluxes leading to organic haze in the atmosphere. We propose a mechanism for these events and tie them into the evolution of the atmosphere, arguing that the amount of hydrogen escape during these events may have accelerated the timing of the great oxidation event.

Permanent onset of the aerobic nitrogen cycle during the Great Oxidation Event Nature, 542, 465-467 (2017)
Links to Paper: PDF (restricted access) ; PURE ; Read-Only full version of paper
Media Coverage: PhysOrg ; Diana Crow’s blog ; My blog post

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In this paper, Aubrey Zerkle has measured nitrogen isotope compositions of rocks that document the disappearance of Sulfur MIF – thus defining the timing of Earth’s great oxidation event. Amazingly, within a few centimers of stratigraphy of where MIF-S disappears (implying O2/O3 in the atmosphere), the nitrogen cycle shifts to a modern-like form. This implies that biology instantaneously responded to the new niches available in an oxygen rich world, setting the stage for the (eventual) evolution of eukaryotic life.

The Pale Orange Dot: The spectrum and Habitability of Hazy Archean Earth Astrobiology, 16(11), 873-899 (2016)
Links to Paper: PDF (full open access) ; PURE
Media Coverage: Nature Highlight ; SkyMania ; My Ode to Carl Sagan

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Giada Arney wrote this really cool paper on the habitability and astronomical detectability of a hazy Earth. This builds on some of the really cool work that Aubrey and Gaz have done (see Zerkle et al. 2012 and Izon et al. 2015 below, and (coming soon!) Izon et al. 2017 above). documenting episodes in the Neoarchean when Earth’s atmosphere oscillated between hazy and clear. Giada’s work resolves a long standing conundrum surrounding the hazy Early earth – namely, that the albedo of a haze enshrouded planet should be high enough to cause a catastrophic global cooling, thus affecting the microbes that are creating the methane in the first place (see Haqq-Misra et al. 2008). Using the newest estimates of the scattering of “fractal” organic haze molecules, Giada shows that a hazy planet would be cold, but not deadly, and in fact, haze might provide a UV shield more significant than that provided by ozone in the modern day. Not satisfied with this supremely cool result, Giada continues to explore the astronomical detectability of this hazy Archean Earth, and discuss some interesting nuances of how this phase in our planets history could be observed on exoplanets.

Spectral identification and quantification of salts in the Atacama Desert Proc. SPIE 10005, Earth Resources and Environmental Remote Sensing/GIS Applications VII, 100050I
Links to Paper: PDF (restricted access) ; PURE (open-access pre-print)
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I have a long-standing interest in salts in the Atacama Desert, and some day promise to unleash a flurry of publications onto the world on such awesome topics as quantitative rainfall paleoproxies, geochemical and biological heterogeneity across the rainfall gradient, and on finding the driest place on Earth. In the mean time, this super-cool study will have to suffice. Jen Harris (link) is a whiz at remote sensing, and this project was aimed at seeing if the amazingly high (atmospheric) salt contents seen in the hyper-arid core of the Atacama could be detected from orbit. She made XRD measurements of soils I had sampled in 2012 (link to fieldwork pictures), and compared these to estimates made from orbit using the Hyperion satellite and mineral spectral databases. Jen was able to quantitatively determine sulfate concentrations from orbit, but had a harder time with nitrate and (much lower abundance) perchlorates.

The Need for Laboratory Work to Aid our Understanding of Exoplanetary Atmospheres Community White paper
Links to Paper: PDF (full open access) ; PURE
Media Coverage: NExSS announcement
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With a number of colleagues I helped co-author this white paper by the newly formed NExSS collaboration. We document the various fundamental measurements that are needed to push forward our ability to constrain exoplanet atmospheres. Students or anyone looking for a research question – Have a look!

Multiple oscillations in Neoarchean atmospheric chemistry EPSL 431. 264-273 (2015)
Links to Paper: PDF (full open access) ; PURE
Media Coverage: Phys.Org
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This paper follows up on the hypothesis of Zerkle et al., (2012) that Earth’s atmosphere may have occasionally contained an organic haze. The original evidence was from a single drill core, so this study was designed to see if the coupled carbon/sulfur isotope trends occurred elsewhere. Gaz showed that the signature occurs in 5 other drill cores as well, and may potentially be a way to cross-correlate different cores. These cores were taken from modern day Australia and South Africa. Although we don’t know exactly where these two landmasses were relative to eachother 2.5 billion years ago, it seems clear that coupled carbon/sulfur anomalies are a widespread feature of the late Neoarchean.

Modeling the Signature of Sulfur Mass-Independent Fractionation produced in the Archean Atmosphere Geochimica et Cosmochimica Acta 141, 365-380 (2014)
Links to Paper: PDF (full open access); PURE
Media Coverage: Astrobiology Magazine
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Earth’s ancient rock record contains bizarre patterns in sulfur isotopes which could have only been created if the atmosphere at that time did not have any oxygen in it. With colleagues, I developed a new model of atmospheric chemistry including sulfur isotopes and showed that current experimental data are not of sufficient quality to accurately interpret the rock record. Stay tuned though – it’s just a matter of time before we will be able to constrain the presence of gases other than oxygen in Earth’s atmosphere!

Abiotic ozone and oxygen in atmospheres similar to prebiotic Earth Astrophysical Journal Letters 792(90), 1-15 (2014)
Links to Paper: PDF (full open access); PURE
Media Coverage: SciTechDaily
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It has long been assumed that the huge abundance of oxygen in Earth’s atmosphere is diagnostic of a living planet. Indeed, the notion that this O2 (and its derivative O3) is observable over interstellar distances that motivates the search for life on other planets. Carl Sagan once famously said “Extraordinary claims require extraordinary evidence” – This paper, co-led by Shawn and Anti, subjected the claim in the first sentence to this high standard. We found that in certain extreme cases, O2 can be created in atmospheres without a biologic source, raising the serious possibility of a “false positive” for life detection. Luckily there are other observable features in those sorts of atmospheres which could potentially fully discriminate between these cases!

Using dimers to measure biosignatures and atmospheric pressure for terrestrial exoplanets Astrobiology, 14, pp 67-86, (2014)
Links to Paper: PDF (full open access); PURE
Media Coverage: UW Today; Huffington Post; Science
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Atmospheric pressure is one of the many factors affecting planetary habitability, but is difficult to measure on extrasolar planets. There is no current (or planned) space telescope that can utilize the standard technique used to measure pressure, so Amit developed a novel methodology which can be used by the JWST, scheduled for launch in 2018. This method, involves observing O2 molecules bound as dimers O2-O2, works best on planets with Earth’s atmospheric pressure or higher and for modern or higher oxygen concentration. The O2-O2 dimer also forms an easily observable biosignature, so this also is a new method to detect life on other planets!

Evidence of martian perchlorate, chlorate, and nitrate in Mars meteorite EETA79001: Implications for oxidants and organics Icarus, 229, pp 206-213, (2014)
Links to Paper: PDF (restricted access); Article available upon request; PURE
Media Coverage: Astrobiology Magazine
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After being part of studies which explored perchlorate formation in the atmospheres of Earth and Mars, I was privileged to be part of the second ever direct detection of perchlorate in a piece of Mars. We analyzed a carbonate clast from martian meteorite EETA790001, which had previously been shown to be of Martian origin. We measured material dissolved from this clast by Ion Chromotagrpahy and showed that it contained perchlorate,chlorate, and nitrate. We also presented nitrate isotopic values which further confirmed that the clast was not contaminated during the time it sat in/on the Antarctic ice. When combined with other indirect measurements of perchlorate inferred from Viking and Mars Science Laboratory missions, we think that perchlorate may be ubiquitous on Mars, which has implications for the survivability of organic materials.

The formation of sulfate, nitrate and perchlorate salts in the martian atmosphere Icarus, 231, pp 51-64, (2014)
Links to Paper: PDF (full open access); PURE
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In 2008, NASA’s Phoenix lander performed the first ever wet chemistry experiment on another planet. Phoenix scooped some Martian soil into a box, added water that was brought from Earth, stirred, and measured the concentrations of the salts that dissolved into the water. In one of the more surprising findings in the last decade of Planetary Science research, Phoenix found that the dominant soluble salt in the Martian soil was perchlorate (ClO4). This salt is extremely rare on Earth (at least the naturally occurring variety- it is produced industrially for fireworks and rocket fuel), and only occurs naturally in desert environments. In 2010, I was part of a paper that described for the first time how the atmospheric production of perchlorate occurs over Chile’s Atacama desert, using a model of atmospheric chemistry. Here, Meg took the exact same mechanism and applied it in a model of Martian atmospheric chemistry – and showed that it didn’t work at all. Our model predicts sulfate and nitrate concentrations that fit with observations and theory on Mars, but we predict way too little perchlorate. We conclude by arguing that there must be another mechanism (currently unknown to science) that can produce substantial quantities of perchlorate seen on Mars.

The rise of oxygen and the hydrogen hourglass Chemical Geology, (362) 26-34 (2013)
Links to Paper: PDF (full open access); PURE
Media Coverage: Extended Synopsis at BMSIS
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The most profound change in Earth’s climate and chemistry over it’s 4.5 billion year history occurred 2.4 billion years ago when the surface oceans and atmosphere became flooded with O2 gas. The scientific community has been considering the question “What caused the rise of Oxygen?” for 100 years, and the fundamental answer still remains unclear. This article was written for a commemorative journal issue celebrating the life of Dick Holland, who dedicated his scientific career to this question, as well as mentoring multiple generations (including myself) on the careful use of geochemistry to understand past environments. The paper – lead by Kevin Zahnle – argues for the primary importance of hydrogen escape in thinking about the evolution of the Earth, which for some reason is generally ignored by Earth scientists.

Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars Astrobiology 13 (9), 833-849, (2013)
Links to Paper: PDF (restricted access); PURE (open access version);
Media Coverage: Nature; First Author Andrew Rushby’s Excellent Blog post; The Onion (!)
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Andrew updated the concept of the continuous habitable zone here, by focusing on both stellar evolution and stellar age. Fits to L(t) for multiple spectral types are included as well as estimates of when and how long a planet in a given location will remain in the habitable zone. This paper got mentioned by the Onion, which makes me feel like my life’s work is complete

Atmospheric sulfur rearrangement 2.7 billion years ago: evidence for oxygenic photosynthesis Earth and Planetary Science Letters 366, 17-26 (2013)
Links to Paper: PDF (restricted access); PURE; Article available upon request
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In this paper, undergraduate student (!) Florian Kurzweil presented data from 2.71 billion year old outcrop samples from the Kidd Creek area in Ontario, Canada. The minor sulfur isotope data, in particular the D36S/D33S slope, was substantially different from those previously published 2.73 billion year old samples from Western Australia. I took the opportunity to publish some updated modelling, as I wasn’t entirely happy with the O2/CH4 runs I had published in Zerkle et al. 2012. I re-did these models using flux boundary conditions, which hopefully made them a bit easier to interpret. We used the updated models to argue that the evolution of oxygenic photosynthesis may have caused the change in the sulfur isotope D36S/D33S slope from -1.5 to -0.9 in the Neoarchean.

Pathways for Neoarchean pyrite formation constrained by mass-independent sulfur isotope fractionation Proceedings of the National Academy of Sciences, 110 (44), p. 17638–17643 (2012)
Links to Paper: PDF (full open access); PURE
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This paper by James Farquhar used Secondary Ion Mass Spectrometry on some of the samples as measured by Zerkle et al. 2012. In Aubrey’s (and many) sulfur isotope papers, the samples are ground up portions of multiple grams of rock chips. If the isotope signals are different in different parts of the rock chip, the signal that is measured from the ground powders will be an average of all those present in the rock, which sometimes can hide interesting behavior. This study was the first to show that the D36S/D33S slope was identical between the bulk scale and on the scale of individual grains in the rocks. This gives important credence to the concept that the D36S/D33S slope is set by atmospheric processes and is not altered much between generation and preservation in sediments. This paper also makes some cool insights into the way that pyrite forms.

The evolution of solar flux from 0.1 nm to 160 microns: quantitative estimates for planetary studies Nature Geosciences, 5, 359-363, 2012.
Links to Paper: PDF (restricted access) ; PURE (full open access PDF version of final article)
Open Source Model and Spectral files: VPL website
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Using observational data from the Sun and stars similar to the Sun but of different ages, we create a computer model of the Sun’s light and how it has changed over the age of the solar system. The predictions can be used at any place in the solar system, and at any time from when the Sun joined the main sequence until approximately 3.5 billion from now when it will turn into a Red Giant. The fluxes are “top of the atmosphere” fluxes (e.g., those seen in free space or at the surface of an airless body like an asteroid or the Moon). In order to predict the fluxes seen at the surface of a planetary body with an atmosphere, additional radiative transfer/photochemistry calculations need to be done. The model is open access. Feel free to use it, but please let me know if you modify it substantially.

A bistable organic-rich atmosphere on the Neoarchaean Earth Nature Geosciences, 5, 359-363, (2012)
Links to Paper: PDF (restricted access); PURE
Media Coverage: New Scientist; National Geographic
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This paper by Aubrey Zerkle examined a 2.65 – 2.5 billion year old drill core from South Africa. She documented a never-before-seen relationship between the carbon and sulfur isotope behaviors. Negative excursions in organic carbon isotopes (to d13Corg values < -3.7%) have long been interpreted to reflect that a substantial component of the organic matter was derived from carbon in the form of methane (CH4). Mass-independent fractionation of sulfur isotopes (in particular the D36S/D33S slope) are commonly interpreted as reflecting what was going on in the atmosphere at the time. Aubrey detailed that these two signature (organic carbon isotopes and multiple sulfur isotopes) co-vary, and suggested a provocative linkage between the two. She proposed that enhanced methane from the biosphere may have lead to temporary periods of atmospheric organic haze, similar to that seen on Saturn’s moon Titan. My contribution was providing some numerical models to show the feasibility that organic haze could change the photochemistry of sulfur in the atmosphere.

someday I will add blurbs for the rest of my papers…
Ensemble Properties of Comets in the SDSS, Icarus 218 (1):571-584, 2012.

Anode Effects on Microbial Fuel Cell Efficiency, Proceedings of the International Conference on Bioinformatics & Computational Biology, BIOCOMP 2011, July 18-21, 2011, Las Vegas Nevada, USA.

Astronomical biosignatures for sulfur-rich anoxic biospheres, Astrobiology, 11(5), 2011.

High rates of anaerobic methanotrophy at low sulfate concentration with implications for past and present methane levels, Geobiology, DOI: 10.1111/j.1472-4669.2010.00267.x, 2011.

Photochemical and climate consequences of sulfur outgassing on early Mars, Earth and Planetary Sciences Letters, 295 (3-4), 412-418, 2010.

Detecting Active Comets in the SDSS, Icarus 205 (2), 605-618, 2010.

Atmospheric origins of perchlorate on Mars and in the Atacama, J. Geophys. Res., 115,E00E11, 1-15, 2010

Modeling aqueous perchlorate chemistries with applications to Mars, Icarus, 207 (2) 675-685, 2010.

Nitrogen-enhanced greenhouse warming on the early Earth, Nature Geosciences, 2 (12), 891-896, 2009.

Identifying Transiting Planets with LSST, LSST Science Book, Version 2.0, arXiv:0912.0201, http://www.lsst.org/lsst/scibook, 2009 pp. 299-302

Quantitative Modeling of the Rise in Atmospheric Oxygen, PhD thesis, University of Washington, 2008.

Stellar SEDs from 0.3-2.5 microns: Tracing the Stellar Locus and Searching for Color Outliers in the SDSS and 2MASS, The Astronomical Journal, 134, 2398-2417, 2007.

Anaerobic methanotrophy and the rise of atmospheric oxygen, Philosophical Transactions of the Royal Society of London – A, 365, 1867-1888, 2007.

The Astrobiology Primer: An Outline of General Knowledge – Version 1, 2006, Astrobiology, 6 (5),735-813, 2006.

Biogeochemical modeling of the rise in atmospheric oxygen, Geobiology, 4, 239-269, 2006.

The loss of mass-independent fractionation in sulfur due to a Paleoproterozoic collapse of atmospheric methane, Geobiology, 4, 271-283, 2006.

The Ultraviolet, Optical, and Infrared Properties of Sloan Digital Sky Survey Sources Detected by GALEX, The Astronomical Journal, 130 (3):1022-1036, 2005

How Earth’s atmosphere evolved to an oxic state – A status report, Earth and Planetary Sciences Letters – Frontiers, 237, 1-20, 2005.