The Roy Hill shale of the Jeerinah Formation, Pilbara Craton, Western Australia contains enigmatic trace metal enrichments that suggest Archean oxygen accumulation. But do they reflect an atmospheric 'whiff' of oxygen, a marine oxygen oasis, or the activities of a microbial soil crust?
Archean oxygen oases
Oxygen oases beneath an anoxic atmosphere. From Olson et al. 2013
Earth's well oxygenated ocean-atmosphere system, upon which all complex life depends, is the consequence of cyanobacterial oxygenic photosynthesis. But the relationship between biological oxygen production and the accumulation of atmospheric oxygen is complex. Several lines of evidence now suggest that the origin of oxygenic photosynthesis substantially predates the first geochemical hints of atmospheric oxygen during the Paleoproterozoic Great Oxidation Event (GOE)--potentially by as much as half a billion years. Using an Earth system model, I have shown that biologically significant oxygen accumulation likely occured in surface 'oxygen oases' beneath an anoxic atmosphere and within an otherwise anoxic ocean. Ongoing work involves identifying these environments in the geologic record and distinguishing the geochemical signatures of marine oxygen oases from so-called 'whiffs' of atmospheric oxygen and oxidative weathering processes involving microbial crusts. In addition to having implications for the evolution of complexity on Earth, these environments provide an important warning for remote life detection studies: non-detection of atmospheric oxygen does not preclude biological oxygen production, surface oxygen accumulation, or biological oxygen consumption on an exoplanet.
Proterozoic oxygen regulation
Possible oxygen stabilizing feedback. From Olson et al. 2016a
Despite significant progress towards constraining the timing and magnitude of Earth's oxygenation events, we still lack a mechanistic model for explaining why atmospheric oxygen remained low for so long or why it eventually rose in the Neoproterozoic. One possibility is that oxygenation negatively impacts the bioavailability of critical nutrients, such as nitrogen. In this scenario, a negative feedback disfavors oxygenation because increases in oxygen tend to reduce the oxygen generating potential of the biosphere. If correct, pervasive oxygenation may have required a yet unidentified external perturbation--and it is not obvious that today's oxygen-rich atmosphere was an inevitable consequence of oxygenic photosynthesis. Understanding Proterozoic oxygen regulating mechanisms is therefore integral to understanding the likelihood of complexity elsewhere in the Universe.
Oxygen & the rise of animals
Trace fossils tracking the movement of an early animal along a microbial mat in the Ediacara Member, Flinders Ranges, South Australia. Photo credit: S. Olson
The realization that oxygen oases can persist beneath an anoxic atmosphere, coupled with experimental results suggesting that the earliest animals required very little oxygen, has invited challenges to the widely held belief that Earth's oxygenation trajectory controlled the timing and tempo of the rise of animals. But, existing biological experiments do not distinguish between surviving and thriving, and they do not directly speak to issues relating to origination, reproductive success, and diversification. Meanwhile, geochemical proxies and Earth system models generally lack the spatial and temporal resolution to constrain the oxygen landscape experienced by benthic animals during their individual lifetimes. An alternative approach involves examining the Ediacaran trace fossil record, which reflects the behaviors of the earliest bilaterians. Within the framework of a model for benthic oxygen heterogeneity, I have recognized an oxygen seeking behavior--suggesting that the first bilaterians had a high metabolic oxygen demand compared to Ediacaran seawater. Although questions remain regarding the role of oxygen in triggering the origination of animals, these results are compatible with a limiting role for oxygen in their subsequent diversification and the emergence of increasingly complex ecosystems.
This work was presented at AbSciCon 2017 (Olson et al).
This work was presented at AbSciCon 2017 (Olson et al).
Oxidation, glaciation, & Earth's enduring habitability
Atmospheric methane levels (colored contours) as dependent on atmospheric oxygen and marine sulfate levels. From Olson et al. 2016b
The Earth has been persistently habitable for at least the 3.8 billion years, despite continuously increasing solar luminosity and episodes of extreme low-latitude glaciation. Earth's enduring habitability is the result of negative feedbacks that modulate the composition of Earth's greenhouse. For example, warming arising from increases in carbon dioxide stimulates higher weathering rates and greater carbon dioxide drawdown, which tends to stabilize surface temperatures and allows Earth system recovery from climatic perturbation. But estimates of the carbon dioxide levels necessary to warm the early Earth in the face of a faint young sun are incompatible with the geologic record, likely requiring significant greenhouse contributions for other gases. Of particular interest is methane because the oxidative collapse of a methane greenhouse could explain the association of oxygenation and glaciation events in the Proterozoic. But, I have modeled the methane cycle on the early Earth, and I found that, although low levels of oxygen allow substantial methane production, methane escape from the ocean is severely limited by sulfate--precluding high levels of atmospheric methane for most of Earth history. This work raises two important questions: How did the mid-Proterozoic Earth avoid glaciation at any latitude? And what are the cause and effect relationships between oxidation, glaciation, and biological innovation in the Neoproterozoic?
Spectral fingerprints of life elsewhere
Synthetic reflectance spectra centered on oxygen, ozone, and methane bands for each of Earth's major geologic eons. Black lines correspond to no absorption from the target gas. The blue lines represent an optimistic scenario and the red lines represent a conservative scenario. Modified from Reinhard et al. 2017
Reconstructing the co-evolution of life and environment throughout Earth's >3.8 billion years of inhabitation illuminates the diversity of 'Earth-like' exoplanets and informs our search for life elsewhere in the Universe. Ongoing efforts involve coupling biogeochemical, photochemical, and spectral models to identify remotely detectable signs of life on the early Earth. I am particularly interested in identifying potential biosignatures during the Boring Billion Biosignature Blindspot (BBBB) of the mid-Proterozoic, during which conventional biosignature gases like oxygen and methane may not have been remotely detectable.