Controls on Atmospheric O 2 : The Anoxic Archean and the Suboxic Proterozoic James Kasting Dept. of Geosciences Penn State University Conventional geologic O 2 indicators Blue boxes indicate low O 2 Red boxes indicate high O 2 Dates have been revised; the initial rise of O 2 is now placed at ~2.45 Ga H. D. Holland (1994) Colorized by Y. Watanabe (Detrital) The GOE based on sulfur MIF Definitive evidence for a Great Oxidation Event, or GOE, comes from sulfur mass-independent fractionation (S-MIF) recorded in ancient sediments As S-MIF data have accumulated, the cliff at 2.45 Ga has become even more pronounced Small, but finite, 33 S values immediately after this may be caused by reworking of older sediments Reinhard and Planavsky, Nature (2013) Grey circlesSIMS Open circlesbulk rock What caused the GOE? In one sense, the answer to this question is easy: The rise of O 2 was caused by cyanobacteria, the only true bacteria capable of performing oxygenic photosynthesis In another sense, though, the rise of O 2 is a mystery, as both cyanobacteria and oxygenic photosynthesis appear to predate the GOE by several hundred million years Thus, the real question seems to be: What delayed the GOE? Obvious, but overly simplistic explanations for the GOE The first two explanations that occur to just about everybody are: 1.Organic carbon burial increased at 2.4 Ga because of some biological innovation (e.g., the invention of heterocysts for N fixation in cyanobacteria) 2.Volcanic outgassing rates decreased with time, causing the supply of reduced gases (mostly H 2 ) to fall below the O 2 production rate --Unfortunately, neither of these explanations is consistent with the carbon isotope record The carbon isotope record 13 C carb = 0 corresponds to 20% organic carbon burial Except during times of transition, this is about what we see. Thus, there is no evidence for a secular increase in organic carbon burial with time [Figure from Catling and Kasting, in prep.] The carbon isotope record Increasing the overall volcanic outgassing rate also doesnt work, because it implies greater organic carbon burial in the past [Figure from Catling and Kasting, in prep.] Published hypotheses for the cause of the GOE * 1.Progressive mantle oxidation (Kasting et al., 1993) 2.Hollands tectonic evolution/volcanic outgassing model (Holland, 2002, 2009) 3.Submarine versus subaerial outgassing mechanisms (Kump and Barley, 2007; Gaillard et al., 2011) 4.Continental oxidation and hydrogen escape (Catling et al., 2001; Catling and Claire, 2005; Claire et al., 2006) 5.Serpentinization of seafloor (Kasting and Canfield, 2012) 6.Banded iron-formation triggers (Isley and Abbott, 1999; Barley et al., 2005; Goldblatt et al., 2006; Bekker et al., 2010) 7.Various biological triggers Ni famine for methanogens (Konhauser et al., 2009) Nitrogenase protection mechanisms; Mo/V availability (Anbar and Knoll, 2002; Grula, 2005; Zerkle et al., 2006; Scott et al., 2008, 2011; Kasting and Canfield, 2012) * See J. F. Kasting, Chem. Geol. (2013) 1. Progressive mantle oxidation The idea here was that H escape to space oxidizes the upper mantle (because the H came from H 2 O originally) Volcanic gases therefore become more oxidized with time, lowering the sink for O 2 Some support for this hypothesis was provided by sulfide barometry in Ga peridotitic diamonds, which suggested that the upper mantle was more reduced at that time Kasting et al., J. Geol. (1993) Volcanic O 2 sink The H 2 :H 2 O ratio in volcanic gases is determined by the equilibrium reaction H 2 O H 2 + O 2 pH 2 /pH 2 O = K eq /fO 2 Mantle fO 2 is near QFM (~10 8.5 atm at 1450 K), so pH 2 /pH 2 O Collectively, H 2 and other reduced gases account for % of the total O 2 sink Decreasing fO 2 by 1-2 log units in the Archean would have a major effect on the O 2 budget Unfortunately, studies of Cr (J.W. Delano, 2001) and V (D. Canil, 1997, 2002; Li and Lee (2004) concentrations in ancient basalts and peridotites appear to have ruled out this hypothesis These elements partition differently into the melt as a function of their redox state But the idea that one needs to get more H 2 out of the early Earth to delay the rise of O 2 remains valid 5. Serpentinization of seafloor It may be possible to get more H 2 out of the solid Earth without any change in mantle redox state Certain types of rocks (ultramafic rocks) can be oxidized by warm water, releasing hydrogen during the process The alteration process is referred to as serpentinization Serpentinization is a minor sink for O 2 today (~1% of the total, according to Norm Sleep) Serpentinization Serpentinization happens when ultramafic rocks are exposed to warm water, either on the continents or on the seafloor Iron is excluded from the serpentine minerals, so it goes into magnetite 3 FeO + H 2 O Fe 3 O 4 + H 2 Archean continental rocks do appear to have been more ultramafic Think greenstone belts and komatiites) Serpentine cabochon from China. This is approximately 39 millimeters by 23 millimeters (From Geology.com) But, theory predicts that the Archean seafloor should also have been more ultramafic Serpentinization of seafloor is potentially a bigger sink for O 2 than continental serpentinization, particularly since the continents may have been much smaller at that time EPSL, 2010 The Archean mantle would have been hotter, leading to a higher degree of partial melting at the midocean spreading ridges More melting makes the resulting igneous rock more like the mantle, which is rich in Fe and Mg Such models also predict very thick oceanic crust, which would cool slowly, possibly giving rise to widespread hydrothermal circulation Modern seafloor: wt% MgO Archean seafloor: wt% MgO A recent paper based on a statistical analysis of ~70,000 major and trace element measurements of various continental rocks supports the idea that the early crust was ultramafic According to these authors, the percentage of fractional melting during volcanism has decreased from ~35% in the Archean to ~10% today A sharp decrease in fractional melting occurred right near the Archean-Proterozoic boundary This supports the idea that more serpentinization, and hence more H 2 production, was occurring during the Archean Keller and Schoene, Nature (2012) Conclusions Free O 2 was evidently being produced well before the GOE at ~2.4 Ga The key question then becomes: What delayed the GOE? Most mechanisms for delaying the GOE require larger O 2 sinks during the Archean. H 2 is the most likely culprit A more reduced Archean mantle could, in principle, have provided more H 2, but this explanation has been ruled out by studies of V and Cr concentrations in ancient rocks Serpentinization of ultramafic seafloor is another potential source for H 2 during the Archean The question of what determined the timing of the GOE remains unresolved Also unresolved (but not discussed today) is why Proterozoic O 2 levels stabilized for a billion years or more at levels well below that of today