A TERRESTRIAL MESOARCHEAN-MESOPROTEROZOIC RECORD OF ATMOSPHERIC OXYGEN LEVELS
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.