Our modeling of biosignature gases in the mid-Proterozoic atmosphere is revealing intriguing implications for climate stability and ‘false negatives’ in remote life detection.Work between UCR and GT has focused on the development of 3D Earth system models designed for interrogating ocean-atmosphere chemistry on a reducing planet, with a focus on coupled O₂-O₃-CH₄cycling.A pair of complementary PNAS papers reveals exciting implications regarding the availability of all three of these gases during the mid-Proterozoic (Alternative Earth 3):

Earth’s oxygen cycle and the evolution of animal life. Christopher T. Reinhard, Noah J. Planavsky, Stephanie L. Olson, Timothy W. Lyons, Douglas H. Erwin, Proceedings of the National Academy of Sciences, USA . doi: 10.1073/pnas.1521544113

This paper describes challenging consequences of low oxygen to animal life. In the time leading up to the emergence of animals, from about 1.8 billion to 600 million years ago, the oxygen distribution in the oceans would have been extremely patchy, and even well-ventilated, shallow-water oases near the coasts would have been seasonally choked by anoxia or even toxic, sulfide-rich waters rising up from the deep. “So for part of the year they would be great places for animals to live, but for part of the year they would have been really inhospitable," Reinhard explains. These results are now a centerpiece in the team’s assertion that low oxygen in the ocean and atmosphere may have challenged the rise of complex life for hundreds of millions of years. Co-author Erwin is a member of the MIT team. Chris worked with the GT press office to issue a release that led to international coverage:

Before Animals, Evolution Waited Eons to Inhale, by Ben Brumfeld, Georgia Tech News Center, 25 July 2016.

How Oxygen May Have Jumpstarted the Rise of Animals, by Eva Botkin-Kowack, Christian Science Monitor.

Did Animals Splutter into Existence Rather Than Explode? by Richard Gray, Daily Mail Online, 25 July, 2016.

Limited Role for Methane in the Mid-Proterozoic Greenhouse. Stephanie L. Olson, Christopher T. Reinhard, Timothy W. Lyons. Proceedings of the National Academy of Sciences, USA . doi: 10.1073/pnas.1608549113

This paper describes unexpected and severe limits to the preservation of methane (CH₄) in the early ocean and its accumulation in the atmosphere between about 1.8 billion to 800 million years ago. These findings present us with a serious new challenge to explain what mix of greenhouse gases kept climate warm during the middle chapter of Earth’s history. Specifically, our calculations demonstrate that anaerobic oxidation of CH₄ coupled to SO₄^2- reduction is a highly effective obstacle to CH₄ accumulation in the atmosphere, possibly limiting atmospheric pCH₄ to less than 10 ppmv for the second half of Earth history regardless of atmospheric pO₂. Based on our recent pO₂ constraints from Cr isotopes, we predict that reduced UV shielding by O₃ would have further limited pCH₄ to very low levels similar to those seen today. Our model results, if correct, limit potential climate warming by methane for a large portion of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time.Another important implication is that these CH₄ concentrations would have been below detection capabilities if viewed remotely via present and foreseeable technology. Thus, analogous absences of measurable methane could be a false negative in the search for biosignatures on exoplanets. The UC Riverside team issued a press release that was also picked up as the lead story in a recent issue of Geochemical News:

Methane Muted: How Did Early Earth Stay Warm? by Sean Nealon, UCR Today, Sept. 26, 2016

A highlight in our ongoing proxy development is the most complete record to date of very low oxygen—and its direct impact on the biosphere—during the mid-Proterozoic (Alternative Earth 3) »

A shale-hosted Cr isotope record of low atmospheric oxygen during the Proterozoic.Devon B. Cole, Christopher T. Reinhard, Xiangli Wang, Bleuenn Gueguen, Galen P. Halverson, Timothy Gibson, Malcolm S.W. Hodgskiss, N. Ryan McKenzie, Timothy W. Lyons and Noah J. Planavsky. Geology. June 2016.doi: 10.1130/G37787.1

Work led by graduate student Devon Cole (Yale) and Noah Planavsky (Yale), working together with team members at Georgia Tech and UC Riverside, has produced a new chromium isotope record through Earth’s early history. Comprising more than 300 new chromium isotope measurements from 20 rock formations around the world, this work shows a first-order shift in oxidative cycling of chromium around 800 million years ago—suggesting an environmental trigger for the singularly dramatic shift in the structure and complexity of the biosphere at that time. These data further suggest that that prior to 800 million years ago, atmospheric O₂ levels were <<1% of present atmospheric level (PAL)—contrasting sharply with previous estimates that O₂ concentrations were as high as 40% PAL. If correct, oxygen was clearly low enough to have directly hindered the diversification of complex life. This conclusion marks an essential shift in our view of Earth’s oxygenation—and provides strong impetus to continue our reassessment of basic aspects of Earth’s oxygen cycle.

We continue to calibrate the utility of the chromium and uranium isotope proxies through studies of modern systems; analyses of metamorphosed, weathered, and hydrothermally altered rocks; and careful comment on the work of others »

Integrated geochemical-petrographic insights from component-selective δ²³⁸U of Cryogenian marine carbonates. Ashleigh v.S. Hood, Noah J. Planavsky, Malcolm W. Wallace, Xiangli Wang, Eric J. Bellefroid, Bleuenn Gueguen, and Devon B. Cole, Geology, November 2016. doi:10.1130/G38533.1

Here we present the first texture specific stable uranium isotopes (an emerging redox proxy) from different components of a ~650 million year old reef in South Australia. We found high variability in U isotopes between marine carbonates (e.g. microbial frameworks and marine cements) and late-stage phases formed during the burial history. Large variability between poorly- and well-preserved phases within a single hand sample of rock may span almost the entire range of uranium isotope values seen in the modern Earth system. This work call for a change in carbonate trace metal work methodology  to make sure we thoroughly check samples for preservation before chemical analysis. In addition, the uranium isotope values of the best-preserved phases in these reefs reinforce the presence of anoxic and iron-rich oceans in the lead-up to the rise of animal life. This paper has been promoted with a cover image for the November issue of Geology.

The chromium isotope composition of reducing and oxic marine sediments. Bleuenn Gueguen, Christopher T. Reinhard, Thomas J. Algeo, Larry C. Peterson, Sune G. Nielsen, Xiangli Wang, Harry Rowe, Noah J. Planavsky, Geochimica et Cosmochimica Acta. 1 July 2016. doi:10.1016/j.gca.2016.04.004

From our analyses of the Cr isotope composition of reducing sediments from the upwelling zone of the Peru Margin and the deep Cariaco Basin, we found that authigenic Cr in anoxic sediments may reliably reflect the first-order Cr isotope composition of deep waters and may be suited to tracking geologically short-term changes in ocean oxygenation.

Chromium isotope fractionation during subduction-related metamorphism, black shale weathering, and hydrothermal alteration. Xiangli Wang, Noah J. Planavsky, Christopher T. Reinhard, Huijuan Zou, Jay J. Ague, Yuanbao Wu, Benjamin C. Gill, Esther M. Schwarzenbach, Bernhard Peucker-Ehrenbrink. Chemical Geology. 20 March 2016. doi:10.1016/j.gca.2016.04.004

This work suggests, among other things, that primary Cr isotope signatures may be preserved even in instances of intense metamorphic alteration at relatively high fluid–rock ratios. 

No evidence for high atmospheric oxygen levels 1,400 million years ago. Noah J. Planavsky, Devon B. Cole, Christopher T. Reinhard, Charles Diamond, Gordon D. Love, Genming Luo, Shuang Zhang, Kurt O Konhauser, Timothy W. Lyons. Proceedings of the National Academy of Sciences 113:19, 10 May 2016. doi: 10.1073/pnas.1601925113

Here we point out fundamental problems with the inorganic and organic geochemical work presented in Zhang et al. (2016, PNAS), which proposed oxygen levels of ∼ 4% present atmospheric levels (PAL) during the mid-Proterozoic.

A close look at cycling of key nutrients—nitrogen and phosphorous—reveals an intriguing influence on early life and the evolution of atmospheric biosignatures »

Cyanobacterial Diazotrophy and Earth’s Delayed Oxygenation, Stephanie L. Olson, Christopher T. Reinhard, and Timothy W. Lyons, Frontiers of Microbiology, 23 September 2016.doi:10.3389/fmicb.2016.01526

Evolution of the global phosphorus cycle, Christopher T. Reinhard, Noah J. Planavsky, Benjamin C. Gill, Kazumi Ozaki, Leslie J. Robbins, Timothy W. Lyons, Woodward W. Fischer, Chunjian Wang, Devon B. Cole, Kurt O. Konhauser, Nature, Accepted.

A provocative result from our Tectonics Working Group suggests a planet’s starting temperature is much more important to its habitability than previously thought »

Can mantle convection be self-regulated?Jun Korenaga. Science Advances. 19 Aug 2016. doi: 10.1126/sciadv.1601168

Alternative Earths Co-I Jun Korenaga suggests that simply being in the habitable zone isn’t sufficient to support life. For decades, it was thought that planets are able to self-regulate their internal temperature via mantle convection. A planet might start out too cold or too hot, but it would eventually settle into the right temperature. Now, Korenaga presents a general theoretical framework that says a planet also must start with an internal temperature that is just right. He explains the degree of self-regulation expected for mantle convection and suggests that self-regulation is unlikely for Earth-like planets, implying that chance factors in planetary formation are even more important for the evolution of planets that are more massive than Earth. Korenaga also organized a press release:

A New Goldilocks for Habitable Planets, by Jim Shelton, Yale News, 19 August 2016.

The Tectonics Working Group also made steps towards estimating the extent of continental area exposed above the oceans through Earth’s history—a key parameter in global biogeochemical (phosphorous-carbon-oxygen) models »

Global water cycle and the coevolution of Earth's interior and surface environment. Jun Korenaga, Noah J. Planavsky, Dave Evans, Philosophical Transactions A, In press.

We are delineating a causal link between tectonics and climate during the Precambrian »

Continental arc volcanism as the principal driver of icehouse–greenhouse variability. Ryan N. McKenzie, Brian K. Horton, Shannon E. Loomis, Daniel F. Stockli, Noah J. Planavsky, Cyn-Ty A. Lee, Science, 22 April 2016. doi: 10.1126/science.aad5787

Led by NASA MIRS Visiting Scholar Erik Melchiorre, analysis of the mineral stichtite provides a window into the conditions associated with a potentially habitable environment on early Earth and other bodies of the solar system  »

Conditions of Stichtite (Mg₆Cr₂(OH)₁₆[CO₃] · 4H₂O) formation and its Geochemical and Isotope Record of Early Phanerozoic Serpentinizing Environments, Erik B. Melchiorre, Ralph Bottrill, Gary R. Huss, Amanda Lopez, Geochimica et Cosmochimica Acta, 22 October 2016. doi: 10.1016/j.gca.2016.10.020 

We are exploring the redox evolution of the Archean ocean and the origins of oxygenic photosynthesis, life in the ocean, and the Great Oxidation Event »

Transient episodes of environmental oxygenation and oxidative continental weathering during the late Archean. Brian Kendall, Robert A. Creaser, Christopher T. Reinhard, Timothy W. Lyons, Ariel D. Anbar. Science Advances, 2015. doi: 10.1126/sciadv.1500777

Oxygen isotope perspective on crustal evolution on early Earth: A record of Precambrian shales with emphasis on Paleoproterozoic glaciations and Great Oxygenation Event. Ilya N. Bindeman, Andrey Bekker, and O. Zakharov, Earth and Planetary Science Letters, 2016.

http://dx.doi.org/10.1016/j.epsl.2015.12.029

PI Timothy Lyons (UCR) and Co-I Ariel Anbar (ASU) discuss the complexity of rising atmospheric oxygen on Earth more than two billion years ago in a popular science feature published in the February 2016 issue of BioScience, a wide-reaching, peer-reviewed journal that has served as a forum for integrating the life sciences since 1964:

The Great Oxidation Event: Evolving understandings of how oxygenic life on Earth began, by Richard Blaustein. BioScience(2016). doi: 10.1093/biosci/biv193

We are improving geochemical constraints on the chemical evolution of Earth’s oceans and atmosphere:

Limited Zn and Ni mobility during simulated Iron Formation diagenesis. Robbins, L.J., Swanner, E.D., Lalonde, S.V., Eickhoff, M., Paranich, M.L., Reinhard, C.T., Peacock, C.L., Kappler, A., Konhauser, K.O. Chemical Geology, 2015. doi:10.1016/j.chemgeo.2015.02.037

As we continue to constrain the Neoproterozoic rise in oxygen (Alternative Earth 4), we are discovering greater variability and heterogeneity in marine oxygen levels than previously assumed, which implies intriguing influences on biological innovation »

Oceanic oxygenation events in the anoxic Ediacaran ocean. Swapan K. Sahoo, Noah J. Planavsky, G Jiang, Brain Kendall, Jeremy D. Owens, Xiangli Wang, X Shi, Ariel D. Anbar, Timothy W. Lyons in Geobiology, 1 March 2016. doi: 10.1111/gbi.12182

Enrichments of redox sensitive elements and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran-early Cambrian ocean. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well-oxygenated Phanerozoic oceans, in which anoxic events caused mass extinctions followed by fast recovery. In contrast, OOEs in ecologically limited, anoxic oceans of the Ediacaran-early Cambrian may have stimulated biotic innovations followed by prolonged evolutionary stasis.

Exceptional Silicified Preservation of Earth’s Earliest Animal Ecosystems. Lidya Tarhan, Ashleigh v.S. Hood, Mary L. Droser,James G. Gehling and Derek E.G. Briggs, Geology, November 2016.doi:10.1130/G38542.1 

An Alternative Earths and Foundation of Complexity inter-NAI team effort found evidence that some Earth’s earliest animals, the Ediacaran biota, were preserved in silica cements due to silica-rich oceans of the late Precambrian. This work may help solve an ongoing debate about how these fossils have been preserved, and relates their fossilization to Earth's unusual ocean conditions ~555 million years ago. The early precipitation of silica cements the sediment surrounding the fossils enabled exception preservation of these enigmatic, soft-bodied animals. Geochemical and petrographic analysis of the silica cements reinforces this hypothesis. Further the operation of a this time window through Earth’s early history supports that the emergence of  Ediacaran biota reflects biotic innovation instead of opening of a new preservation window. The article was featured in news section of Science:

How Earth’s Oldest Animals Were Fossilized, by Lucas Joel, Oct. 19, 2016

Thick sulfate evaporite accumulations marking a mid-Neoproterozoic oxygenation event (Ten Stone Formation, Northwest Territories, Canada). E.C. Turner and Andrey Bekker, GSA Bulletin, 2016. 
doi: 10.1130/B31268.1

With new Alternative Earths team member, William Gilhooly (Indiana University), we are exploring carbon and sulfur cycling and novel microbial ecologies in a modern lake analog for the anoxic-sulfidic conditions common to oceans on the Early Earth »

Carbon and sulfur cycling below the chemocline in a meromictic lake and the identification of a novel taxonomic lineage in the FCB superphylum, Candidatus Aegiribacteria. Trinity L. Hamilton, Roderick J. Bovee, Sarah R. Sattin, Wiebke Mohr, William P. Gilhooly III, Timothy W. Lyons, Ann Pearson, Jennifer L. Macalady. Frontiers in Microbiology 7, 27 April 2016. doi: 10.3389/fmicb.2016.00598

Mahoney Lake in British Columbia is an extreme meromictic system with unusually high levels of sulfate and sulfide present in the water column. At times throughout Earth's history the deep oceans have been oxygen depleted. During several of these events, regional accumulation of sulfide created areas of the ocean that were euxinic. As is common in strongly stratified lakes, Mahoney Lake hosts a dense, sulfide-oxidizing phototrophic microbial community where light reaches the chemocline. Below this “plate,” the euxinic hypolimnion is anoxic, eutrophic, saline, and rich in sulfide, polysulfides, elemental sulfur, and other sulfur intermediates.

A comprehensive sulfur and oxygen isotope study of sulfur cycling in a shallow, hyper-euxinic meromictic lake. William P. Gilhooly III, Christopher T. Reinhard, TW Lyons. Geochimica et Cosmochimica Acta, 2016. doi:10.1016/j.gca.2016.05.044 

With another new Alternative Earths team member, Jeremy Owens (Florida State), we are tracking geochemical evolution of the ocean and impacts on marine ecology during periods of oxygen deficiency »

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle. Jeremy D. Owens, Christopher T. Reinhard, Megan Rohrssen, Gordon D. Love, Timothy W. Lyons. Earth and Planetary Science Letters. 16 June 2016. doi: 10.1016/j.epsl.2016.05.046

Understanding the global redox state of the oceans and its cause-and-effect relationship with periods of widespread organic-carbon deposition is vital to interpretations of Earth's climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency. Based on this compilation of new and published data from an organic-rich locality within the proto-North Atlantic Ocean during the Cenomanian–Turonian boundary event, we found: (1) temporal trace metal drawdown suggests a global expansion of oxygen deficiency prior to euxinia, (2) modeling Mo drawdown suggest the marine inventory reduction could be biolimiting with <7% euxinia, (3) eukaryotic biomarkers contribution decreases during the metal-depleted OAE, and (4) progressive global deoxygenation and nutrient limitations coincide with known turnover pattern. 

What Sparked the Cambrian Explosion? »

PI Timothy Lyons (UCR) and International Collaborator Donald Canfield (U. of Southern Denmark) discuss the relationship of atmospheric oxygen and early animal evolution in a Nature News Feature:

"What Sparked the Cambrian Explosion? An evolutionary burst 540 million years ago filled the seas with an astonishing diversity of animals. The trigger behind that revolution is finally coming into focus,” Douglas Fox. Nature 530, 268–270 (18 February 2016) doi:10.1038/530268a 

Alternative Earths team members contributed to the latest Astrobiology primer»

The Astrobiology Primer v2.0, various authors including Alternative Earths team members Jennifer Glass and Eva Stüeken, Astrobiology, 1 August 2016. doi:10.1089/ast.2015.1460

Alternative Earth 3

Chemostratigraphy of the Shaler Supergroup, Victoria Island, NW Canada: a record of ocean composition prior to the Cryogenian glaciations. Thomson, D., Rainbird, R.H., Planavsky, N., Lyons, T.W., Bekker, A., Precambrian Research 263, p. 232-245 (2015). doi:10.1016/j.precamres.2015.02.007

Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution. C.A. Partin, A. Bekker, N.J. Planavsky, T.W. Lyons, Chemical Geology 417, p. 148-162 (2015). doi:10.1016/j.chemgeo.2015.09.004

Decline in oceanic sulfate levels during the early Mesoproterozoic. G. Luo, S. Ono, J. Huang, T.J. Algeo, C. Li, L. Zhou, A. Robinson, T.W. Lyons, S. Xie, Precambrian Research 258, p. 36-47 (2015). doi:10.1016/j.precamres.2014.12.014

Marine redox conditions in the middle Proterozoic ocean and isotopic constraints on authigenic carbonate formation: Insights from the Chuanlinggou Formation, Yanshan Basin, North China. C. Li, N.J. Planavsky, G.D. Love, C.T. Reinhard, D. Hardisty, L. Feng, S.M. Bates, J. Haug, Q. Zhang, X. Chu, T.W. Lyons, Geochimica et Cosmochimica Acta 150, p. 90-105 (2015). doi:10.1016/j.gca.2014.12.005

Current perspectives on microbial strategies for survival under extreme nutrient starvation: evolution and ecophysiology (Chapter 7). Glass, J.B, C.B. Kretz, M.J. Warren, C.S. Ting, Microbial Evolution under Extreme Conditions, editor: C. Bakermans. De Gruyter (2015).

Meta-omic signatures of microbial metal and nitrogen cycling in marine oxygen minimum zones. Glass J.B., C.B. Kretz, S. Ganesh, P. Ranjan, S.L. Seston, K.N. Buck, W.M. Landing, P.L. Morton, J.W. Moffett, S.J. Giovannoni, K.L. Vergin, F.J. Stewart, Frontiers in Microbiology 6, 998 (2015). doi:10.3389/fmicb.2015.00998

Cryptic cross-linkages among biogeochemical cycles: Novel insights from reactive intermediates. C.M. Hansel, T. G. Ferdelman, and B. M. Tebo. Elements 11, p. 409-414 (December 2015). doi: 10.2113/gselements.11.6.409

Emerging biogeochemical views of Earth’s ancient microbial worlds. T.W. Lyons, D.A. Fike, and A. Zerkle. Elements 11, p. 415-421 (December 2015). doi: 10.2113/gselements.11.6.415

Reconstruction of secular variation in seawater sulfate concentrations. T.J. Algeo, G.M. Luo, H.Y. Song, T.W. Lyons, D.E. Canfield. Biogeosciences 12, p. 2131-2151 (2015). doi:10.5194/bg-12-2131-2015

The evolution of the global selenium cycle: Secular trends in Se isotopes and abundances. Stüeken, E.E., Buick, R., Bekker, A., Catling, D., Foriel, J., Guy, B.M., Kah, L.C., Machel, H.G., Montañez, I.P., Poulton, S.W., Geochimica et Cosmochimica Acta 162, p. 109-125 (2015). doi:10.1016/j.gca.2015.04.033

“Microbes that Meddle with Metals” — May 2015 edition of Microbe Magazine (the official magazine for the American Society of Microbiology). Jennifer Glass (GT) published this feature article with NASA Exobiology and the NASA Astrobiology Postdoc Fellowship program included in the acknowledgements.

Alternative Earth 4

Late Proterozoic transitions in climate, oxygen, and tectonics, and the rise of complex life. In: Earth-Life Transitions: Paleobiology in the Context of Earth System Evolution. N.J. Planavsky, L.G. Tarhan, E. Bellefroid, C.T. Reinhard, G. Love, T.W. Lyons, In P.D. Polly, J.J. Head, and D.L. Fox (eds.), Earth-Life Transitions: Paleobiology in the Context of Earth System Evolution. The Paleontological Society Papers 21. Yale Press, New Haven, CT.

Dynamic changes in sulfate sulfur isotopes preceding the Ediacaran Shuram Excursion. M.R. Osburn, J. Owens, K.D. Bergmann, T.W. Lyons, and J.P. Grotzinger, Geochimica et Cosmochimica Acta 170, p. 204-224 (2015). doi:10.1016/j.gca.2015.07.039

Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period. B. Kendall, T. Komiya, T.W. Lyons, S.M. Bates, G.W. Gordon, S.J. Romaniello, G. Jiang, R.A. Creaser, S. Xiao, K. McFadden, Y. Sawaki, M. Tahata, D. Shu, J. Han, Y. Li, X. Chu, A.D. Anbar, Geochimica et Cosmochimica Acta 156, p. 173–193 (2015). doi:10.1016/j.gca.2015.02.025

Ediacaran marine redox heterogeneity and early animal ecosystems. C. Li, N.J. Planavsky, W. Shi, Z. Zhang, C. Zhou, M. Cheng, L.G. Tarhan, G. Luo, and S. Xie, Scientific Reports 4, (2015). doi:10.1038/srep17097

The molecular record of Cryogenian sponges—A response to Antcliffe (2013). Love, G.D. and Summons, R.E. Paleontology58, p. 1131-1136 (2015). doi: 10.1111/pala.12196

Marine organic matter cycling during the Ediacaran Shuram excursion. Lee, C., Love, G.D., Fischer W.W., Grotzinger, J.P., Halverson, G.P., Geology. (2015). doi: 10.1130/G37236.1