Oxygen Stasis and the Rise of Eukaryotes and Metazoans
The importance of a full understanding of the controls on ocean-atmosphere O2 levels during the mid-Proterozoic is difficult to overstate. The evolution of O2 levels in the mid-Proterozoic ocean-atmosphere system forms the backdrop for the initial emergence and subsequent evolutionary stasis of eukaryotic life. Furthermore, it provides the possibility of a remarkably long period of Earth’s history during which many of the links between tectonics, climate, and life may have been short-circuited and/or amplified in unusual ways. Finally, it provides the preface that is essential reading for any story about the proximate causes of the subsequent emergence of complex life in the late Neoproterozoic. The central question in this regard is whether ocean-atmosphere O2 levels were low enough to inhibit the evolution and ecological emergence of complex multicellular life, or must we seek mechanisms strictly associated with internal biology to explain this event—or both? Our developing framework for very low oxygen levels during the mid-Proterozoic in the deep ocean, shallow ocean, and atmosphere are the baseline against which the dramatic environmental, climatic, and biotic events and triggers of the later Proterozoic should be assessed.
Evidence for extremely low atmospheric oxygen through the mid-Proterozoic
Efforts to constrain the level of post-GOE atmospheric pO2 have historically been sparse and loose, ranging from ~1% of present atmospheric levels (PAL) to <~40% PAL. This is obviously a wide range of values that may nonetheless all exceed those required to support animals. We proposed to use the chromium (Cr) isotope system to track the initial oxidation of Earth’s terrestrial surface. This Cr effort is, by implication, a search for the presence/absence and biogeochemical significance (or lack thereof) of a particular metabolic pathway (oxygenic photosynthesis), because the oxygen concentrations needed to drive oxidation of manganese Mn(II) are orders of magnitude higher than prebiotic levels. Here, we exploit the ‘inverse’ of this O2 sensitivity—as in looking for a fingerprint that may verify very low post-GOE oxygen levels.
Work led by graduate student Devon Cole (Yale) and Noah Planavsky (Yale), working together with team members at GT and UCR, has produced a new and transformative Cr isotope record through Earth history—corroborating our past preliminary results and focusing on the mid-Proterozoic/Neoproterozoic transition. This work comprises more than 300 new Cr isotope measurements from 20 different formations and shows a first-order shift in oxidative cycling of Cr around 800 million years ago (Ma). Although we are working to refine the exact oxygen levels needed to induce oxidative Cr cycling in typical continental environments, these data indicate that prior to 800 Ma, atmospheric pO2 was < 1% PAL—and close to theoretical estimates of the oxygen needs of the earliest metazoans. If correct, this is strong evidence that environmental oxygen levels were low enough to have directly impacted the diversification of complex life (Cole et al., submitted). Another intriguing possibility is that oxygen levels were lower for periods of the Proterozoic than they were during Archean oxygen highs, given suggestions of Archean oxidative Cr cycling. If correct, this conclusion marks one of the most fundamental shifts in our view of Earth’s oxygenation in decades—and provides strong impetus to continue our reassessment of basic aspects of Earth’s oxygen cycle.
In work at UCR, graduate student Dalton Hardisty and Tim Lyons have been leading the development and application of the iodine (I) proxy for reconstructing surface ocean oxygen levels using I/(Ca+Mg) ratios in carbonate sedimentary rocks. Application of this technique to a large suite of carbonate samples spanning Earth’s history suggests significant spatiotemporal variability in surface ocean oxygen levels for much of Precambrian time, including frequent upward mixing of O2-poor deep waters during the mid-Proterozoic (Hardisty et al., in review). These results provide independent empirical support for the theoretical models of the evolutionary and ecological landscape faced by early complex life now under development through collaboration among GT, Yale, UCR, and the Smithsonian Institution (see below).
Developing the catalytic Mn-Cr-O2 cycle as a proxy for atmospheric chemistry
To quantitatively link Cr isotope signatures to environmental oxygen levels, Chris Reinhard (GT), Tim Lyons (UCR), and Noah Planavsky (Yale) have continued to develop plans for experiments with Yuanzhi Tang (GT) and Brad Tebo (OHSU) for the first wave of microbial Mn-Cr oxidation work. We are studying the biogeochemistry of enzymatic manganese oxide formation and the fractionation of Cr stable isotopes under different environmental conditions. Graduate student Shiliang Zhao (GT) is working with Tang and Tebo on the reactivity of Mn oxides under variable environmental conditions, and visiting scholar Sahib Zada (GT) is working on isolation and characterization of Mn oxidizers from different environmental settings. These two students are working on the preparation of cell-free extracts for initial experiments assessing oxidation of Cr by biogenic Mn oxides (without cell presence) under varied environmental conditions. These experiments include both fungi and bacteria; variables include pH and Zn presence during the formation.
Exploring the causes and consequences of very low pO2 during mid-Proterozoic time
Very low pO2 during mid-Proterozoic time (discussed above) would have limited appreciable O2 to “oxygen oases” within the surface ocean for much of mid-Proterozoic time. These oases—local-to-regional settings dynamic on biologically relevant timescales—may have been inhospitable environments for the origin of animal life but could also have catalyzed evolutionary innovation through their intrinsic insolation and instability once certain ecological conditions were met. Understanding of the ultimate mechanistic causes and ecological consequences of very low pO2 during the mid-Proterozoic remains an exciting research frontier.
Work led by Chris Reinhard (GT) and Noah Planavsky (Yale), together with a large collaborative group composed of researchers within and beyond the Alternative Earths Team, has resulted in the development of a new model for understanding the secular evolution of nutrient-limited energy flow through Earth’s biosphere (Reinhard and Planavsky et al., in review). By combining an exceptionally large geochemical database of phosphorus (P) distributions in siliciclastic marginal marine sediments with a biogeochemical Earth system model, we propose that energy flow through the biosphere was stifled for much of Precambrian time, with P-limited conditions under both very low and very high atmospheric pO2 states. At the same time, N-metal co-limited conditions prevented transitions between these states (a ‘nutrient bistability’). Our model provides a coherent mechanism for maintaining the stability of a low-O2 world after the evolution of cyanobacteria and implies a ready mechanism for ‘arresting’ oxygenation on ocean-bearing planets, perhaps permanently, despite the presence of oxygenic photosynthesis at the surface for billions of years.
Work led by graduate student Stephanie Olson (UCR), together with Tim Lyons (UCR) and Chris Reinhard (GT), has focused on the development of 3-D Earth system models designed to interrogate ocean-atmosphere chemistry on a reducing planet. In particular, we have incorporated parameterized O2-O3-CH4 photochemistry and an explicit representation of a consortial microbial metabolism that oxidizes CH4 anaerobically with dissolved SO42-. Results of this new model suggest that CH4 would have been an ineffective climate stabilizer even on a pervasively reducing Earth surface. Further, non-steady-state dynamics in the O2-O3-CH4 system may have played an important role in climate destabilization during late Proterozoic ‘Snowball Earth’ glaciations (Olson et al., in review).
We are also exploring the potential consequences of low atmospheric pO2 on the ecological landscape faced by early eukaryotic and metazoan life. Work led by Reinhard, Planavsky, Olson, Lyons, and Doug Erwin (Smithsonian) is attempting to define a path forward in efforts to understand how spatial and temporal variability in levels of surface ocean and benthic O2 may have constrained the evolutionary ecology of early eukaryotic and metazoan life (Reinhard et al., in review). Results for Earth system modeling and simple dynamic models of local surface ocean O2 levels indicate a ‘patchy’ oxygen environment for much of mid- and late-Proterozoic time, even at pO2 levels well above those reconstructed with the Cr isotope proxy. These results are consistent with independent proxy data derived from the I/Ca system, led by graduate student Dalton Hardisty (UCR; see above).
Tracking the ecological rise of eukaryotes
Graduate student Terry Tang (Yale) and Noah Planavsky (Yale) are leading a project that crosses Yale, UCR, GT, and J. Craig Venter Institute (JCVI) to track the rise of Earth’s first large-scale eukaryotic ecosystems. This ecosystem shift represents one of the most critical transitions in Earth history and has been linked to both Neoproterozoic oxygenation and the onset of low-latitude (‘Snowball Earth’) glaciation events. However, it is not known when this shift occurred. Although fossil and organic biomarker records provide crucial information on presence/absence of eukaryotic clades, they cannot speak directly and quantitatively to large-scale influence on carbon fluxes and ecological impact. A new proxy is thus required to determine when eukaryotic algae became an important part of Earth’s biosphere and to determine what role, if any, this innovation played in shifting Earth’s atmospheric composition.
Zinc (Zn) is the most abundant cofactor in eukaryotic enzymes. Our geochemical work in collaboration with biologist Christopher Dupont (JCVI) reveals a clear delineation between the zinc requirements of eukaryotes (algae) and prokaryotes (cyanobacteria). Tang has developed a Zn isotope approach to track the extent of Zn limitation and eukaryotic production throughout Earth history. Specifically, the isotope systematics of the pyrite and rock extracts (bitumen) and kerogen pyrolysate (extracted at UCR in Gordon Love’s lab and processed at Yale University) have been used to delineate regional- and global-scale shifts in the extent of primary productivity by algae. Further, recent work on modern and Cretaceous euxinic sediments have demonstrated that δ66Znpyr can capture a marine Zn isotope signature and that Δ66Znpyr-org can be used to track zinc bioavailability in the environment. We intend to apply this simple framework to track the evolution of the marine Zn cycle and the rise of eukaryotic algae to ecological dominance with unprecedented resolution.
Modeling the detectability of Earth’s mid-Proterozoic biosphere
Though important in its own right for understanding long-term climate sensitivity (see above), our modeling of the surface cycles of a suite of biogenic gases in the mid-Proterozoic atmosphere is revealing intriguing implications for ‘false negatives’ in remote life detection. In particular, recent isotopic constraints on evolving atmospheric pO2 and oceanic SO42- suggest the possibility that a range of conventional biosignature gases (O2, O3, CH4, N2O) would have been difficult to detect spectroscopically for much of mid-Proterozoic time, despite the great abundance of at least prokaryotic life in the ocean at that time.
Our modeling efforts, led by graduate student Stephanie Olson (UCR), Chris Reinhard (GT), and Tim Lyons (UCR) highlight the importance of long-term chemical disequilibrium between the surface ocean and the atmosphere. This disequilibrium allows for the recycling of biosignature gasses within the marine environment without detectable expression in the atmosphere. A critical implication is that extended intervals of Earth history might have appeared sterile through the filter of sea-air exchange—assuming searches that rely on the current suite of proposed biosignature gases for remote life detection and currently available and planned telescope technologies (Olson et al., in review). Future work will seek to identify novel biosignatures that are less likely to be muted by exchange with a liquid ocean, and these results might inform the design of future telescopes intended for remote (exoplanet) life detection.
Work led by undergraduate student Chloe Stanton (GT) and Jennifer Glass (GT) has been exploring the production kinetics of nitrous oxide (N2O) in iron-rich aqueous systems from an experimental perspective, with a focus on better understanding potential biotic/abiotic production pathways. Collaborating with Chris Reinhard (GT), the group is attempting to estimate local and global production rates of N2O in pervasively ferruginous ocean systems. This work ultimately feeds into a broader collaboration with Tim Lyons (UCR) and Jim Kasting (VPL), through which Stanton is learning and applying Kasting’s 1-D atmospheric photochemical code to estimate atmospheric N2O levels under a range of possible boundary conditions. Stanton has made one trip to Kasting’s lab, with plans for follow-up work in the coming year. Student and postdoc exchanges are a key strategy in our goal toward interdisciplinary research, with this project providing a key early example of the potential fruitfulness of this strategy.
In an ideal demonstration of synergism between NAI teams, Eddie Schwieterman (VPL) proposes to couple our growing, comprehensive library of proxy-constrained estimates of ocean-atmosphere chemistry from the Proterozoic with the photochemistry-climate and radiative transfer models of the VPL. The ultimate goals are the integrated tools necessary to extend these diverse biogeochemical states, or alternative Earths, toward a more complete understanding of the universe of possible exobiospheres that may be discovered in the coming decades—given the right technology.