Dramatic Oxygen Fluctuations
OVERVIEW | ~2.3 to 2.0 billion years ago (mid-Paleoproterozoic)
Our studies of the middle portion of the Paleoproterozoic (2.2 to 2.0 Ga) are focused on whether Earth’s surface experienced a unidirectional oxygen rise or instead rose to high levels (potentially near-modern) and then crashed dramatically. More specifically, we are rigorously testing the idea that the middle Paleoproterozoic Earth was marked by high oxygen levels—in strong contrast to traditional arguments for far lower values. The resolution of this question is perhaps one of the most important issues in Earth history, as it points to the likelihood that the much later development of complex life was not solely contingent on high levels of oxygen at Earth’s surface. Work to date has focused on trying to place empirical constraints on ocean-atmosphere O2 levels during the Paleoproterozoic and developing quantitative theoretical tools for understanding the dynamics of large shifts in ocean-atmosphere oxygen levels.
Super-high Paleoproterozoic pO2 and Earth’s climate system
The initial hints of sustained high atmospheric pO2 levels during the Paleoproterozoic came from a protracted (ca. 200-million-year) interval of elevated δ13C values in the marine carbonate sedimentary rocks. These carbon isotope values were interpreted at face value to reflect elevated rates of organic carbon burial over an exceptionally long timescale; however, the implications of this non-uniformitarian perturbation to Earth’s crustal carbon inventory for the climate system have not been explored. Such an extended period of organic carbon burial would have been catastrophic for Earth’s atmospheric CO2 inventory, for example, without some unknown buffering mechanism. Together with inferred high pO2 (and thus low atmospheric pCH4), this scenario would place stringent constraints on possible greenhouse gas assemblages available to buffer Earth’s climate system.
Work led by Chris Reinhard (GT) has focused on developing a low-order Earth system model designed to explore the isotopic and climate consequences of potential crustal buffering scenarios for evolving atmospheric CO2 during a protracted period of high O2 levels. The model includes a ‘grey atmosphere’ climate system that responds to the radiative forcing effects of a range of greenhouse gas assemblages including CO2, CH4, and N2O. Initial model design and testing is well underway (Reinhard and Fischer, in review), and initial attempts have begun to assess plausible crustal buffering scenarios that can mechanistically accommodate ‘super-high’ Paleoproterozoic O2 levels. The long-term goal will be to use this simple model architecture as a framework for coupling with more complex models of ocean/atmosphere chemistry and secular mantle/crust evolution.
Secular crustal differentiation and the evolution of the sedimentary rock cycle
An important backdrop for studies of evolving ocean-atmosphere O2 levels based on redox-sensitive trace elements is the secular chemical evolution of Earth’s upper crust. Indeed, it is clear that a gradual shift from relatively mafic to more felsic chemical compositions in the bulk upper crust occurred during the late Archean and early Paleoproterozoic, with potentially significant ramifications for the baseline against which our most oft-used paleoproxy records are interpreted. Rigorously understanding the signal-to-noise ratio of trace element signals in Earth’s sedimentary rock record requires novel approaches toward backing out the ‘crustal baseline.’
Chris Reinhard (GT) and Noah Planavsky (Yale), in collaboration with postdoctoral researcher Rich Gaschnig (GT) and Roberta Rudnick (U. Maryland), have been developing a novel chromium (Cr) isotope and enrichment technique for fingerprinting the secular chemical evolution of Earth’s upper crust. Data from glacial tillites and marine siliciclastic sediments indicate a Paleoprotoerozoic transition between a ‘physical’ Earth surface Cr cycle that dominated Archean time to a ‘chemical’ Cr cycle driven by redox shifts that dominated the remainder of Earth’s history (Reinhard et al., in prep). This signal can be used further to quantify changes in source/sink terms in Earth’s oxygen cycle originating from weathering of the upper crust, with initial results suggesting that the secular evolution toward more felsic upper crust may have been critical for the long-term oxygenation of Earth’s ocean-atmosphere system.
Isotopic records of Paleoproterozoic atmospheric change
Assessments of the nature of the rise of oxygen in the Paleoproterozoic will require use of novel redox-sensitive proxies available in rock record through this interval. The loss of non-mass dependent sulfur just prior to this interval (~ 2.45 Ga) has provided a lower limit for atmospheric pO2, but an upper constraint has yet to be established. With this gap in mind, fractionations in the Cr isotope system have the potential to highlight a transition to oxygen levels above ~ 0.1% of present atmospheric levels. We have recently demonstrated the utility of black shale as a reliable Cr archive in the Precambrian (Cole et al., in review), and the ubiquity of shales through this interval may provide enough resolution to highlight a shift in the Cr isotope record.
Work at Yale led by graduate student Devon Cole and Noah Planavsky is examining the Cr isotope signal in mid-Paleoproterozoic black shales from the Francevillian Basin of Gabon and the Pretoria Group of Southern Africa (all collected prior to the start of the NAI award). These shale sequences record deposition through the Lomagundi carbon isotope excursion and a brief increase in redox-sensitive metal enrichments (such as U and Cr) that has been invoked by us to indicate a significant rise in pO2. Should atmospheric oxygen levels have risen to near modern levels, we would expect to see a fractionated Cr isotope signal preserved in the shale record indicative of oxidative terrestrial weathering through this period. Initial work has indicated Cr isotopes of pre-Lomagundi-age strata remain unfractionated, while a few preliminary samples coeval with the excursion appear to be fractionated above bulk silicate earth values. Future work will continue to examine the Cr isotope signal of shales through this interval, endeavoring to provide a detailed record of possible major fluctuations in atmospheric pO2 during this formative time in Earth history. This application of the Cr isotope proxy is an essential proof of concept.