A TERRESTRIAL MESOARCHEAN-MESOPROTEROZOIC RECORD OF ATMOSPHERIC OXYGEN LEVELS

The history of atmospheric oxygen levels through Earth history has been the topic of intense interest since at least the 1960s (Cloud, 1968). By 2000, a two-step model had become strongly established in Precambrian research in which oxygen is hypothesized to have remained relatively constant except for large, geologically rapid, unidirectional increases in O₂ in the early Paleoproterozoic (the “Great Oxidation Event”) and the Neoproterozoic. Work in the last decade has begun to add detail to this simple view, discriminating between different O₂ levels rather than simply assessing its presence or absence (e.g., Lyons et al., 2014). While less common than marine records, terrestrial records such as paleosols form in direct contact with the atmosphere and thus are ideal records of atmospheric composition (e.g., Rye and Holland, 1998).

Uranium and chromium have redox-sensitive isotope systems that behave relatively simply in terrestrial soils. Uranium fractionation responds to O₂ levels above ~10^-5 times present atmospheric levels (PAL) (Partin et al., 2013), while chromium responds above ~10^-3 PAL (Crowe et al., 2013). Here we present paired U and Cr isotope measurements on a set of globally distributed paleosols that span the time interval from Mesoarchean (≥3.0 Ga) to Mesoproterozoic (1.1 Ga). These paleosols are all considered to have excellent preservation based on textural, mineralogical, and chemical examination. By pairing these two isotope systems, we construct a quantitatively constrained history of oxygen levels, rather than simply assessing “almost none” versus “more than almost none.” The record crosses several critical transitions in Earth history, including the traditional “Great Oxidation Event,” the Huronian glaciation, the emergence of the earliest fossil organisms, the expansion of oxidative photosynthesis, and the appearance of eukaryotes.