Atmospheric Traces of Oxygenic Photosynthesis
OVERVIEW | 3.2 to 2.4 billion years ago (mid-late Archean)
The precise timing of the onset of oxygenic photosynthesis is a matter of intense debate. Current estimates span over a billion years of Earth history, ranging from prior to 3.7 billion years ago (Ga), the age of the oldest sedimentary rocks, to 2.4–2.3 Ga, coincident with the first permanent rise of atmospheric oxygen at the so-called “Great Oxidation Event” (GOE). Even without consensus on when biological oxygen production emerged, pinpointing the evolution of this process is essential for understanding Earth’s planetary evolution. If oxygenic photosynthesis evolved early, well before the permanent rise of atmospheric oxygen, the transition to a more oxidizing world in the Proterozoic is likely to be a reflection of Earth’s tectonic history, such as the emergence and stabilization of continents and related shifts in the temporal patterns of volcanism and associated fluxes of reduced gas. Alternatively, biological evolution (specifically, the emergence of oxygenic photosynthesizers) may have directly triggered this switch in Earth states. We are exploring these alternative models and their implications for the systematics of planetary oxygenation on Earth from a combined experimental, empirical and theoretical perspective.
Experimentally constraining the kinetics of terrestrial sulfide oxidation
The oxidation of reduced crustal minerals in terrestrial environments represents a crucial component of the Earth surface redox system from a number of perspectives. Poorly understood, however, is how these processes respond to very low levels of O2. (A pO2 value of 0.001% of the present atmospheric level is thought to characterize much of the Archean.) How biological catalysis modulates the kinetics of sulfide oxidation is another significant gap in our understanding of the quantitative atmospheric information embedded in these proxies. To improve our ability to accurately represent the dynamics of the O2 cycle and atmospheric chemistry more broadly, we are attempting to fill these gaps. Our results will provide crucial information for use in (1) the quantitative interpretation of proxy records (e.g., trace metal enrichment patterns, S-isotopes, and the presence/absence of reduced detrital minerals) and (2) more accurate mechanistic representations of processes and feedbacks regulating atmospheric O2 and CO2 levels throughout Earth’s history.
Our experiments, led by graduate student Aleisha Johnson (ASU), are demonstrating that even the trace levels of O2 we expect in the Archean are enough to oxidize sulfide minerals, possibly producing some of the geochemical signals we see in Archean rocks. Only 30 chemical sensors in the world are capable of making these measurements, and they all live in a single drawer, in a single lab, at Aarhus University in Denmark. Johnson, Chris Reinhard (GT), and Steve Romaniello (ASU) traveled to Aarhus for two weeks in June to conduct the first set of successful experiments, and Johnson presented this work at the AGU Fall Meeting in December. The measurements are the result of collaboration among Johnson, Reinhard, Tim Lyons (UCR), and two of our team’s international collaborators, Niels Peter Revsbech (Aarhus University) and Don Canfield (U. Southern Denmark).
In parallel, Reinhard, Chris Huber (GT), Ed Bolton (Yale), and Noah Planavsky (Yale) are developing reaction-transport models of terrestrial weathering systems designed to incorporate the new kinetic information obtained by the experimental work. This coupled experiment-model framework will allow the first rigorous demonstration of the pO2 levels at which sulfide oxidation at Earth’s surface becomes ‘transport limited’—that is, reaction progress limited by the replenishment of O2 at the mineral surface rather than the kinetics (rate) of the oxidation reaction. This threshold is a critical stability point in Earth’s oxygen cycle that is still poorly constrained. This framework will also allow for more accurate quantitative estimates of trace metal fluxes from Earth’s exposed crust into the oceans at very low levels of atmospheric O2.
An Archean uranium isotope record
The evolution of oxygenic photosynthesis fundamentally changed all major elemental cycles and ecosystem structure on Earth, yet the timing of this biological innovation is still poorly constrained. It is now generally agreed that organic biomarkers from Archean rocks, formerly the most commonly invoked evidence for the early rise of biological oxygen production, do not provide robust evidence for oxygenic photosynthesis. This failing lies mostly with the high thermal alteration states of the host rocks and the high frequency of post-depositional contamination. Given these challenges, emphasis has turned to inorganic geochemical proxies for tracking the waste product of oxygenic photosynthesis: molecular oxygen (O2). One of the most promising of these inorganic geochemical proxies involves isotopes of uranium (U). Significant uranium isotope (238U/235U) fractionation is contingent on the formation of a mobile U(VI) pool generated by oxidation of U(IV) in continental rocks. The oxidation of U(IV) appears to require O2 levels orders of magnitude higher than those obtainable on a prebiotic planet, making U isotopes an attractive new tool to track early oxygen.
In research led by Xiangli Wang (Yale), we have analyzed 238U/235U of more than 250 samples of banded iron formation and black shale from 26 globally distributed units. The data show a marked increase in U isotope variability roughly 3.1 Ga, suggesting that oxygenic photosynthesis evolved around this time. The lack of significant fractionations prior to 3.1 Ga in multiple units suggests that abiotic processes (UV oxidation) are unlikely to lead to large U isotope fractionations in the sedimentary record. We propose that these results, if they hold up as we suspect they will, provide a critical temporal tie point—the emergence of oxygenesis— in the history of life on Earth. This 3.1 Ga timing is generally consistent with other recent assertions about the earliest records of oxygenesis, including recent studies within the UCR and University of Wisconsin teams.
Fingerprinting ‘cryptic oxygenesis’ during the Archean
Theoretical models predict that it is possible for large-scale oceanic plumes of dissolved O2 (‘oxygen oases’) to exist despite reducing atmospheric conditions—long after the evolution of oxygenic photosynthesis. The existence of such environments is critically important for two reasons: (1) they allow for the potential to geochemically fingerprint ‘cryptic oxygenesis’ (that is, localized oxygen accumulation on a pervasively reducing Earth surface), and thus provide the most temporally precise tie-points between geochemical signatures and microbial evolutionary history, and (2) the spatial extent and long-term stability of such environments may place important constraints on the timing of Earth surface oxygenation by regulating surface redox fluxes between Earth’s oceans and atmosphere. These settings also carry intriguing ecological implications for Archean life, including aerobic microbial pathways such as nitrogen cycling and associated geochemical signatures.
Building on previous work by other members of the Alternative Earths team, graduate student Joshua Stanford (GT) is leading attempts to apply a unique, coupled molybdenum (Mo) and chromium (Cr) isotope approach toward rigorously fingerprinting oxygen oasis environments in the Archean rock record. Initial work centers on manganese (Mn)-rich strata from South Africa’s Transvaal Basin, deposited at ~2.9 Ga. This work results from a broader collaboration across the Alternative Earths team, including Chris Reinhard (GT), Noah Planavsky (Yale), and Tim Lyons (UCR), and will build on further ground-truthing of our coupled Mo-Cr isotopic technique in younger strata deposited subsequent to the rise of atmospheric oxygen. Similar studies are underway with international collaborator Hari Tsikos (Rhodes University) with a focus on younger units proximal to the Great Oxidation Event.
In particular, we are searching for distinctive Mo isotope records that point to primary Mn oxides that can be taken as indicators of free O2 in the surface ocean. In the absence of convincing organic biomarker data, such approaches are our best windows to the earliest production and accumulation of oxygen in the ocean and atmosphere. In parallel, graduate student Stephanie Olson (UCR), collaborating with Reinhard and Lyons, has led the charge toward developing a systematic ‘rubric’ for fingerprinting different styles of biospheric oxygen release as manifest in the geochemical rock record. In particular, the approach systematizes the series of distinct geochemical signals that would be predicted from: (1) marine oxygen oases, (2) localized oxidation by short-range O2 fluxes from microbial mats/crusts, and (3) large-scale atmospheric oxygenation events.
UV photo-oxidation of Mn(II) and Fe(II) during the Archean
Numerous processes other than oxidation with O2 can cause significant redox cycling at Earth’s surface. For example, UV oxidation and anoxygenic photosynthesis can lead to the formation of ferric iron-rich sediments under anoxic conditions, and the former has been completely unexplored for its potential impact in terrestrial weathering environments. Biological processes provide viable alternative explanations that may have dominated the deposition of banded iron formations, yet the capacity of UV photochemistry to modify major geochemical cycles is clear. That this pathway could account for at least subtle proxy redox signals (specifically false positives for O2) cannot be dismissed a priori, yet ours is the first thorough exploration of this possibility.
Graduate student Parker Castleberry (ASU) is leading an experiment studying the abiotic UV photo-oxidation of Mn(II) and Fe(II) in aqueous solution. The goal is to determine whether the high UV fluxes expected for the Archean (which lacked ozone) were sufficient alone to oxidize dissolved Mn(II) and Fe(II) and produce Fe and Mn oxide deposits, which are classically interpreted to involve O2 or at least photoferrotrophic bacteria. Preliminary results suggest that many previous experiments were hampered by either trace O2 in the laboratory system or the use of inappropriate UV light sources (e.g., mercury vapor lamps), which do not accurately represent Archean solar conditions.
The role of microbial metabolism in controlling the atmospheric sulfur cycle during the Archean
The ‘flagship’ proxy for ancient atmospheric chemistry is the rare sulfur isotope systematics of Archean and Proterozoic sedimentary rocks. In particular, the non-mass-dependent sulfur isotope (NMD-S) anomalies preserved during the Archean suggest exceptionally low atmospheric pO2, likely 0.001% of the present atmospheric level. However, an important component of the NMD-S paleobarometer for quantifying ancient atmospheric chemistry is the requirement for atmospheric abundance of some reduced gas species sufficient to catalyze the production and deposition of these anomalies at the surface of sulfur aerosols under a variety of redox states. Thus, the NMD-S system potentially contains quantitative information on the atmospheric abundances of methane (CH4) and molecular hydrogen (H2), which have been little explored.
Building from ongoing work led by graduate student Stephanie Olson (UCR), we are exploring the role of anaerobic methane oxidation (AOM), a consortial microbial metabolism catalyzing the consumption of CH4 with dissolved sulfate (SO42-), in modulating the levels of reduced gases in Earth’s Archean atmosphere and, by extension, NMD-S signals exported into the rock record. Working closely with Chris Reinhard (GT) and Tim Lyons (UCR), Olson has performed a large suite of experiments with an Earth system model of intermediate complexity (GENIE) modified during previous work (described elsewhere in this report) to include parameterized O2-O3-CH4 photochemistry and microbial AOM in the oceans. This work has yielded a somewhat startling result: even very low oceanic SO42- levels are sufficient to throttle the oceans’ net CH4 production capacity, driving atmospheric CH4 to very low values; however, at SO42- levels recently estimated for Archean oceans, our model framework predicts the possibility of extremely high atmospheric CH4, with implications for climate, atmospheric redox, and the production/stability of photochemical atmospheric hazes.
An ecophysiological throttle on Earth system oxygenation
The evolution of oxygenic photosynthesis during the Archean would have represented a potentially dramatic shift in the net energy flow through Earth’s biosphere. However, more primitive forms of photosynthesis, involving reduced species such as Fe2+ and H2S, would have competed with Earth’s nascent oxygenic biosphere for essential nutrients in surface aqueous environments. Indeed, it is possible that this competition may have stifled the ultimate oxygenation of Earth’s ocean-atmosphere system for long periods of Archean time subsequent to the evolution of cyanobacteria.
Chris Reinhard (GT), Sean Crowe (UBC), and Kazumi Ozaki (U. Tokyo) are working together to empirically and theoretically understand the dynamics of this potential ‘ecophysiological throttle’ on Earth system oxygenation during the Archean. Crowe is leading experimental work with a newly isolated pelagic photoferrotroph (a bacterium that utilizes Fe2+ as its electron donor during photosynthesis), which reveals significantly lower light requirements compared to cyanobacteria. Reinhard, in collaboration with Crowe and Ozaki, has incorporated the new physiological data into a 1-D competitive photosynthesis model for the surface ocean. Results suggest that the growth parameters of photoferrotrophic bacteria are such that they should very effectively ‘choke off’ nutrient supply to cyanobacterial ecosystems if the ocean interior is reducing enough to accumulate dissolved Fe2+, which are indeed the conditions thought to characterize the Archean. Ozaki, in collaboration with Reinhard and Crowe, has been working to parameterize these relationships for implementation in an Earth system model of the global oxygen cycle.