Controls on Atmospheric O2: The
Anoxic Archean and the Suboxic
Proterozoic
James Kasting
Dept. of Geosciences
Penn State University
‘Conventional’ geologic O2 indicators
(Detrital)
H. D. Holland (1994)
• Blue boxes indicate low O2
Colorized by Y. Watanabe
• Red boxes indicate high O2
• Dates have been revised; the initial rise of O2 is now placed at
~2.45 Ga
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, 33S values
immediately after this may
be caused by reworking of
older sediments
Grey circles—SIMS
Open circles—bulk rock
Reinhard and Planavsky, Nature (2013)
What caused the GOE?
• In one sense, the answer to
this question is easy: The rise
of O2 was caused by
cyanobacteria, the only true
bacteria capable of performing
oxygenic photosynthesis
• In another sense, though, the
rise of O2 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?
http://www.primalscience.com/?p=424
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 H2) to
fall below the O2 production rate
--Unfortunately, neither of these explanations is
consistent with the carbon isotope record 
The carbon isotope record
• 13Ccarb = 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 doesn’t
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.
2.
Progressive mantle oxidation (Kasting et al., 1993)
Holland’s tectonic evolution/volcanic outgassing model
3.
Submarine versus subaerial outgassing mechanisms (Kump
4.
Continental oxidation and hydrogen escape (Catling et al., 2001;
5.
6.
Serpentinization of seafloor (Kasting and Canfield, 2012)
Banded iron-formation triggers (Isley and Abbott, 1999; Barley et
7.
Various biological triggers
(Holland, 2002, 2009)
and Barley, 2007; Gaillard et al., 2011)
Catling and Claire, 2005; Claire et al., 2006)
al., 2005; Goldblatt et al., 2006; Bekker et al., 2010)
–
–
*See
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)
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 H2O originally)
• Volcanic gases therefore
become more oxidized with
time, lowering the sink for O2
• Some support for this
hypothesis was provided by
sulfide barometry in 3.3-3.5
Ga peridotitic diamonds,
which suggested that the
upper mantle was more
reduced at that time
Kasting et al., J. Geol. (1993)
Volcanic O2 sink
• The H2:H2O ratio in volcanic
gases is determined by the
equilibrium reaction
H2O  H2 + ½O2
pH2/pH2O = Keq/fO2½
– Mantle fO2 is near QFM (~108.5
atm at 1450 K), so pH2/pH2O 
0.024
• Collectively, H2 and other
reduced gases account for 1020% of the total O2 sink
– Decreasing fO2 by 1-2 log units in
the Archean would have a major
effect on the O2 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 H2 out of the
early Earth to delay the rise of O2 remains valid
5. Serpentinization of seafloor
• It may be possible to get more
H2 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 O2 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 + H2O  Fe3O4 + H2
• 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 O2 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: 10-13 wt% MgO
Archean seafloor: 18-24 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 H2 production, was occurring
during the Archean
Keller and Schoene, Nature (2012)
Conclusions
• Free O2 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 O2 sinks
during the Archean. H2 is the most likely culprit
– A more reduced Archean mantle could, in principle, have provided more
H2, 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 H2
during the Archean
• The question of what determined the timing of the GOE remains
unresolved
• Also unresolved (but not discussed today) is why Proterozoic O2
levels stabilized for a billion years or more at levels well below that
of today