Last week, the third (and most exciting in my book) episode of the haze saga was published.
It all started with some hard work and a great idea by Aubrey Zerkle, which she had while puzzling over data showing correlations between indicators of changes in the carbon cycle with changing atmospheric chemistry. “Methane-producing microbes formed an organic haze on Early Earth” screamed the headlines, while tabloids ran with the more prosaic “Farts of haze before the Great Oxidation Event” (I wish) The correlations in the initial data were admittedly a little on the weak side, but they were there and the story was definitely cool enough to warrant a follow-up. Aubrey and I were very fortunate to obtain funding from NERC to dive into this.
With their support we hired Gaz and, after a while, episode two came out in 2015. There, we confirmed that these weird correlations between sulfur and carbon occurred in pulses at multiple locations across the globe – up to 5 times in the 200 million years preceding the GOE. Cool beans – but what the entire idea was lacking was some sort of driver. What could possibly have caused the Earth to flip-flop between clear skies and a Titan-like organic haze? And why did this only seem to occur in the run up to the GOE?
Episode three involved the highest resolution study ever performed on rocks of this age, but was enabled by a severe annoyance. A broken fork-lift at the coreshed limited our sampling during out trip to Pretoria, which forced us to return to South Africa mid-way through the project. It was on this unplanned return trip that we collected a high-resolution record across one of the carbon/sulfur “wiggles” as we had come to call them. While the initial study had about 5 points across each wiggle, we collected samples every 50 cm over about 100 meters of stratigraphy. After a substantial amount of work (every multiple sulfur isotope measurement requires about 5 hours of time per sample), Gaz (and Ken) produced an exquisite data set, with spectacular correlations between the organic carbon isotopes and multiple sulfur isotopes, occurring over a 20 meter interval. These data points show an abrupt onset of the event, with a smooth continuous transition back to baseline conditions. Gaz was able to estimate this event lasted for well over 100,000 years, and possibly as a long as one million years.
Taken together, the best way we found to explain our data is biological. Oxygenic photosynthesis was evidently producing O2 locally at this time, but it was all consumed in the water column and had not yet leaked into the atmosphere. We surmise that a large and sustained pulse of nutrients from some ancient tectonic event, dramatically enhanced primary productivity in the basin. In sediments, a fair amount of decaying organic matter is always converted to methane, but this is normally eaten by other microbes performing methanotrophy or other methane-oxidation processes coupled to sulfate-reduction. However, the historically low sulfate concentrations in the Neoarchean, enabled substantially higher methane fluxes than ever before, eventually tipping the atmosphere into a hazy state. Feedbacks within the coupled chemistry/climate/biological systems maintained this hazy state for a substantial amount of time. To top it off – Gaz made some interesting ties into the evolution of atmospheric chemistry. These periods of enhanced methane concentration would have promoted substantial hydrogen escape to space – a key mechanism for planetary oxidation. Somewhat paradoxically, these burps of biogenic methane leading to haze may have played a role in accelerating the timing of the great oxidation event!
After almost 4 years in review, Aubrey’s paper on nitrogen cycling during the GOE was finally published in Nature last week. Although the 7-planet system in Trappist-1 caught most of the headlines in that particular news cycle, we did have one pretty spectacular write up from blogger Diana Crow. For those of you that know Aubrey, Diana has done pretty remarkable job of channeling Dr. Zerkle:
The (Mostly) Untold Story of the Oxygen Revolution
Here is a open source link to the paper. Nature is debuting a new format in which the full source of the paper is available to everyone for reading online, but it is unable to be saved. The official paper is here.
All I really want to add right now is a word of thanks to the reviewers and to congratulate Aubrey for sticking with it. This paper really is a story of persistence. In the review process, we learned of a non-standard contamination process that partially affected some of the original data. Over three years, multiple re-extractions and measurements in three different laboratories were needed to settle on the final interpretation, which was different than the original. For all of those included in the process, thanks for your guidance in getting this awesome work out there. And to anyone reading, a “parental-type” remark in closing – Stick with it! Persistence really does pay off!
Carl Sagan was an inspiration to so many of us. As a modern-day scientist and educator, I am amazed at both the strength of his research career coupled with his gift for communication, which he so selfless shared with the world, and would be a happy person if I could contribute only a small fraction of what he did to either of these endeavors. In that vein, this ones for you, Carl. Carl famously referred to how Earth would be seen from a far away solar system as a Pale Blue Dot. While this has likely been true for most of the 4.5 billion year history of Earth, there are times when the atmospheric chemistry may have changed. In particular, my colleagues and I have been involved in an interesting recent discovery that for time periods in Earth history, the atmosphere may have been enriched in organic haze, similar to Saturn’s moon Titan (see Zerkle et al. 2012 and Izon et al. 2015 below, and (hopefully coming soon!) Izon et al. 2017 above). Giada took this work to the next level, exploring both the climatic consequences of hazy early earth, and studying the observability of hazy exoplanets around stars of different spectral types. Giada also calculated the color of the sky you might have seen if you happened to be hanging around 2.7 billion years ago. Check out the paper for details, but spoiler alert – and I only wish Carl was around to appreciate this and/or to help me craft more beautiful prose – Earth also been a Pale Orange Dot.
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 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, and compared these to estimates made from orbit using the Hyperion satellite and quantified against 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. Really cool stuff, with relevance to both future work in the Atacama and other deserts, as well as Mars.
The paper is available here: http://dx.doi.org/10.1117/12.2241520
I was recently part of an effort to document the various fundamental measurements that are needed to push forward our ability to constrain exoplanet atmospheres.
Additional details on the publications page…
The Official Announcement