Paradigm shift in determining Neoproterozoic atmospheric oxygen

We present a new and innovative way of determining the oxygen level of Earth’s past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 ± 15–m.y.–old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the midto late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured pO2 puts oxygenation of Earth’s paleoatmosphere ~100–200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life. INTRODUCTION Deciphering the oxygenation history of the atmosphere and oceans is critical to understanding weathering processes, sedimentary environments, climate change, mass extinctions, tectonic events, and the evolution of Earth’s biota. Earth’s atmosphere was not always plentiful in free oxygen, and finding a paleobarometer to measure its partial pressure over geologic time remains “a famously difficult challenge” (Lyons et al., 2014, p. 307). Accurately defining Archean, Proterozoic, as well as Phanerozoic atmospheric conditions remains problematic, despite the multitude of proxies from marine and non-marine archives (metals and isotopes of U, Cr, Mo, S, Zn, Fe, Se, C) for modeling atmospheric conditions (Lyons et al., 2014; Kunzmann et al., 2015; Liu et al., 2015; Sperling et al., 2015). Indeed, according to Canfield (2005, p. 1), the “sparse geologic record combined with [geochemical] uncertainties...yields only a fragmentary and imprecise reading of atmospheric oxygen evolution.” Major steps in unraveling the evolution of atmospheric oxygen and its close link to the evolution of life on Earth have produced some revolutionary hypotheses and concepts. In 1998, Canfield suggested that the deep ocean of the mid-Proterozoic was anoxic and ferruginous and atmospheric oxygen was <0.1% (Canfield, 1998). However, much remains to be quantified about the redox state of the Proterozoic ocean and atmosphere (Lasaga and Ohmoto, 2002; Canfield, 2005) and especially the emergence of marine animal life during the Neoproterozoic (Canfield et al., 2007; Lyons et al., 2014). The increasing number of elemental and isotopic proxies give us a better understanding of the “relative” redox state of the atmosphere and ocean during Earth’s early history, which shows that most of it was marked by low oxygen levels with two exceptional perturbations termed the Great Oxidation Event (GOE) and Neoproterozoic Oxidation Event (NOE). The models of earliest oxygen evolution document the GOE but are quite uncertain about the NOE (magnitude, onset, and trend) and its relationship to the diversification of plant life and the evolution of marine animal life. Several models suggest persistence of extremely low oxygen levels for this time period (Lyons et al., 2014), whereas others advocate more moderate and increasing but uncertain levels (Canfield, 2005, his figure 6). Most of these models leave unresolved the question of whether oxygenation of the atmosphere-ocean drove animal evolution or animal evolution drove oxygenation (Lyons et al., 2014). Halite is well established as a paleoenvironmental archive through analysis of fluid trapped in inclusions (Benison and Goldstein, 1999). This archive with primary fluid in inclusions is now accepted to stretch back to the Neoproterozoic (Spear et al., 2014). Inclusions in halite may contain two phases consisting of primary evaporative brine and a gas bubble. The gas may reflect the ambient atmosphere, gas trapped in soil/sediment columns, or gas incorporated during diagenesis (Lowenstein and Hardie, 1985; Hovorka, 1987; Schreiber and El Tabakh, 2000). It is our intent to demonstrate that trapped gas in halite is primary and then measure the inclusion gas to document the actual oxygen level of the Neoproterozoic atmosphere (Brand et al., 2015; Blamey et al., 2015). PRESERVATION CRITERIA Geologic archives, irrespective of material, whether used for proxy investigation or direct measurements need to be intensively screened for their preservation state (Brand et al., 2011), including our material of choice in this study, halite. We present a number of features and parameters that halite must possess to be considered a robust archive for retaining primary gas contents. Halite from highly concentrated brine (~10.6 seawater enrichment; McCaffrey et al., 1987) starts by forming cumulate crystals at the brine-air interface as floating individuals or rafts (Hovorka, 1987). These crystals will include bands of inclusions acquired during rapid growth, but they eventually settle out to the bottom of salinas, playas, or sabkhas of shallow water depth. Intermittently wet, syntaxial precipitation of halite initiates the formation of chevron and/or cornet crystals (Lowenstein and Hardie, 1985). Chevron halite forms rapidly on the cumulate substrate at the air-brine interface as vertically oriented and elongated crystals with fluid inclusion–rich and milky chevron bands aligned parallel to the crystal faces. With faster GEOLOGY, August 2016; v. 44; no. 8; p. 651–654 | Data Repository item 2016211 | doi:10.1130/G37937.1 | Published online XX Month 2016 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. 652 www.gsapubs.org | Volume 44 | Number 8 | GEOLOGY growth, more square to rectangular inclusions may form along the crystal faces, and upwardpointed cubes in continuous and extensive beds form (Schreiber and El Tabakh, 2000). Another type of halite that forms on the cumulate substrate at the air-brine interface are cornet crystals with increasingly widening upward-oriented inclusion bands (Hovorka, 1987). The inclusion bands tend to be parallel to the crystal face and more abundant during rapid daytime growth. In addition, patches of clear halite may be associated with chevron and cornet halite during primary growth but with the exclusion of inclusions (Lowenstein and Hardie, 1985). Halite may also form at subaqueous depth producing few to no inclusions from stratified dysoxic-anoxic bottom water (Schreiber and El Tabakh, 2000). Overall, the original halite crystal fabric must be devoid of any depositional and/or post-depositional tectonic and/or halotectonic folding, faulting, and fracturing to allow the preservation of bedding and crystal fabrics and textures (Spear et al., 2014). Recrystallized or diagenetically formed halite may be identified by unusually large, distorted, sporadically distributed interlocking mosaics of clear crystals with large and abundant inclusions (Schreiber and El Tabakh, 2000). Halite cement forms during early burial, and the entire process is complete by ~45 m depth (Casas and Lowenstein, 1989), and afterward halite is no longer susceptible to dissolution and alteration except under hightemperature, fluid burial, and tectonic conditions. Geochemistry may be an additional screening tool to identify the primary state of inclusions in halite. Bromine and d34S are two such tools to ascertain the primary and marine nature of halite. The sulfur isotope composition is largely controlled by the sulfur content of the ambient but geologically variable seawater, whereas Br content will range from 65 to 75 ppm at the onset of halite crystallization, depending on the partition coefficient, and reach ~270 ppm at the offset (McCaffrey et al., 1986). Also, the major ion chemistry of the inclusion fluids reflects the preservation potential of the halite (Spear et al., 2014). Maturation of halite deposits is generally complete within 45 m of burial manifested by the occlusion of all intercrystalline porosity by clear halite cement (Schléder et al., 2008). Halite fluid inclusions may contain oxygenic photoautotrophs such as Dunaliella and halophilic Archaea (Schubert et al., 2010), with the former potentially increasing the local oxygen concentration. Halite forms in brines with >325 gL–1 salinity that may be replete with halophilic bacteria imparting a red color on the water but lack live oxygenic photoautotrophs, thus local post-depositional oxygen production within inclusions is not an issue. In summary, if depositional conditions and fabric/textural and geochemical parameters support a primary state, then gases trapped in inclusions of cumulate, chevron, and cornet halite may contain gas reflective of the ambient atmosphere at the time of crystallization.