Serpentinization and the Great Oxidation Event

Jim Kasting

Dept. of Geosciences

Penn State University

Dick Holland (1927-2012)

• Dick worked on many things during his career, but elucidating the details about the rise of O2 was perhaps his greatest love, as well as his greatest scientific contribution

• When we got together, talk would invariably turn to the question: What caused the Great Oxidation Event (GOE) at 2.5 Ga?

• Dick’s passing spawned a special volume of Chemical Geology in 2013. Much of this talk is based on my own contribution to that volume

‘Conventional’ geologic O2 indicators

• Blue boxes indicate low O2

• Red boxes indicate high O2

• Dates have been revised; the initial rise of O2 is now placed at ~2.45 Ga

H. D. Holland (1994)Colorized by Y. Watanabe

(Detrital)

Farquhar et al., Science (2000)

73 Phanerozoic samples

High O2 Low O2

• This story was strongly supported by sulfur isotope evidence published by James Farquhar and colleagues in 2000• So-called “mass independent fractionation” of S isotopes is seen before ~2.45 Ga, but not afterwards

Updated sulfur MIF record• 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

• Alternative explanations cannot explain the corresponding MIF signal in 36S

Reinhard and Planavsky, Nature (2013)

Grey circles—SIMSOpen circles—bulk rock

Resolved and unresolved questions

• Thus, for many of us, the question of when atmospheric O2 first rose (and stayed high) has been largely resolved– The answer is 2.45 Ga, and the event is often termed

the Great Oxidation Event, or GOE

• But the question of why it rose at that time continues to be debated

What caused the GOE (Great Oxidation Event) at

~2.4 Ga?• 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

• The best evidence for this comes from measurements of Mb in shales

http://www.primalscience.com/?p=424

Science (2007)

• Mb is forms an insoluble sulfide in reduced environments• A Mb enhancement in shales requires oxidative weathering of sulfides on land, followed by transport of soluble molybdate ion to sediments• The best way to do this, in my view (and that of Reinhard and Planavsky), is for the entire atmosphere to become O2-rich for short time periods

• Additional evidence from Cr isotopes suggests that O2 was being produced as far back as 3.0 Ga (Crowe et al., Nature, 2014)

• If one accepts these arguments for O2 production during the Archean, the real question becomes: What delayed the rise of atmospheric O2?– Obviously, something was holding O2 concentrations

down, but what exactly was doing so?

Published hypotheses for the cause of the GOE*

1. Progressive mantle oxidation (Kasting et al., 1993)2. Holland’s 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 H2O originally)

• Volcanic gases therefore become more oxidized with time

• 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)

• 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

2. Holland’s tectonic evolution/volcanic outgassing

model • During the last

decade of his life, Dick Holland published two papers outlining his hypothesis for the cause of the rise in O2 (Holland, GCA, 2002, 2009)

• In his view, the composition of volcanic gases was the key…

Holland’s f-value analysis (GCA, 2002)

• Holland analyzed the redox state of volcanic gases by calculating their f-values• Volcanic gases with f >1 lead to a reduced environment• A key assumption is that 20% of outgassed CO2 is buried as organic C

The carbon isotope record

• 13Ccarb = 0 corresponds to 20% organic carbon burial

• Except during times of transition, this is about what we see

Holland’s 2009 model• In his 2009 GCA paper, Dick

proposed an explicit model for determining the timing of the GOE

• Gradual growth of the continents resulted in increased recycling of C and S relative to H2O, and hence in lower f-values– So, volcanic gases do not

become more oxidized, but their C/H and S/H ratios change

• If one picks parameters carefully, the GOE occurs at ~2.5 Ga

Holland, GCA (2009)

• Big question with this analysis: What controls the organic carbon burial fraction in ancient sediments?– If the 20% organic C burial fraction is controlled by

redox balance, as seems likely, then invoking this as a constraint on O2 evolution involves circular reasoning

– An alternative hypothesis is that the organic C burial fraction is controlled by the C:P ratio in igneous rocks (Junge et al., JGR, 1975), but this also seems unlikely

3. Submarine/subaerial outgassing and the GOE

• Two different papers have argued that the rise of atmospheric O2 at ~2.4 Ga was caused by a switch from predominantly submarine to predominantly subaerial volcanism• Kump and Barley outlined the geologic evidence for this switch and analyzed data from submarine and subaerial volcanic gases

Kump and Barley, Nature (2007)

• Hypothesis: The rise in atmospheric O2 was caused by a switch from submarine to subaerial volcanism

• f was >1 in the Archean and

<1 afterwards, in their view• Much of the change in f,

however, is driven by the implicit assumption of 20% organic C burial– Hydrothermal fluids are rich in H2S

compared to CO2

More reduced

Nature (2007)

Submarine/subaerial outgassing and the GOE

• Gaillard et al. (Nature, 2011) performed detailed modeling of the outgassing process and emphasized the switch from submarine outgassing of H2S to subaerial outgassing of SO2

Gaillard et al., Nature (2011)

• In none of Gaillard’s cases did Holland’s f value ever exceed 1 for submarine outgassing (primarily because CO2 outgassing increases at high pressures, leading to more organic carbon burial)Þ This mechanism doesn’t

seem to work• It does somewhat better if one

uses an alternative approach to analyzing the global redox budget (see below), but still is not a sufficient answer

Nature (2011)

Subaerialoutgassing

Submarineoutgassing

• Before discussing the remaining hypotheses for triggering the GOE, we need to step back and talk about how best to quantify the principle of redox balance– Most of this discussion follows J.F. Kasting,

Chem. Geol. (2013)

The principle of redox balance• It’s not just a good idea, it should be a law…• Two ways to think about it

1. Conservation of free electrons

2. When one thing is oxidized, something else must be reduced

• Although this methodology for analyzing the GOE differs from Holland’s f-value analysis, all of Holland’s papers emphasized global redox balance, so this way of thinking is definitely not new

Two important redox budgets

• The atmospheric redox budget (at left) must be balanced for low-O2

atmospheres. This is how we formulate our Archean photochemical models• The global redox budget (at right) must be balanced for all atmospheres. This is the budget of the combined atmosphere-ocean system

Combined atmosphere-ocean system

Redox budget formulation• Define “neutral” oxidation state gases: H2O, CO2, N2, and

SO2

• Other gases are either oxidized or reduced compared to these. Express the differences in terms of H2 equivalents, e.g.

CH4 + 2 H2O CO2 + 4 H2

H2O2 + H2 2 H2O

• Thus, the total outgassing rate of reductants can be written as

Global redox balance equationSetting H2 sources equal to H2 sinks yields the following equation:

Here out(Red) = total outgassed flux of reduced gases OW = oxidative weathering of the continents and seafloor burial(CaSO4) = burial of gypsum or anhydrite burial(Fe3O4) = oxidation of ferrous iron without using O2

(includes BIFs and serpentinization) burial(CH2O) = burial of organic matter burial(FeS2) = burial of pyrite

Catling & Claire’s* Koxy parameter

• Let Koxy represent the ratio of H2 sinks to sources, notcounting oxidative weathering and burial of sulfate (which are only important at high O2) and escape of hydrogen to space (which is only important at low O2)

• The atmosphere switches from reduced to oxidized when Koxy becomes >1

*Catling and Claire, EPSL (2005) Claire et al., Geobiology (2006)

Differences between my analysis and Catling &

Claire’s approach1. Budgeting done in terms of H2 rather

than O2

2. I take SO2 as the neutral oxidation state for sulfur, whereas most authors, including Holland, used sulfate

– S was neglected by Claire et al. (2006)

3. I lump metamorphic and volcanic fluxes together

4. I include anaerobic iron oxidation (e.g., BIF deposition and serpentinization) in my model

Magnitudes of terms

Relative magnitudes today (mostly from Dick Holland’s work):

Term Rate (1012 mol/yr) 2 burial(CH2O) 20 ± 6.6

5 burial(FeS2) 7* ± 4

out(red) 4.8 ± 3.6

burial(Fe3O4) 0.4 ± 0.2

Koxy 5.2*Corrected because of mistake in Holland (2002), Tables A1 and A2

• This is a big problem, because we need Koxy < 1 during the Archean to maintain a reduced atmosphere– It helps if one uses Berner’s organic carbon burial

rate, which is a factor of 2 smaller, but then one needs to rebalance all of the fluxes to remain consistent with the carbon isotope record

4. Catling & Claire’s continental oxidation model

• In the models of Catling et al. (2001), Catling and Claire (2005), and Claire et al. (2006), loss of H to space oxidized the continents, resulting in a smaller flux of reduced metamorphic gases

• There is indeed evidence that much of the oxygen stored in continental rocks was emplaced before the GOE (see diagram at right)

Catling et al., Science (2001)

Catling and Claire model• Modern metamorphic reduced

gas fluxes were scaled up by factors of 20-50 using thermodynamic equilibrium arguments

• But– The continents may have been

much smaller back then– Thermodynamic equilibrium does

not apply at metamorphic temperatures

– Most of the modern metamorphic reduced gas flux is CH4. And most of that CH4 is thermogenic, i.e., it is produced from heating of buried organic matter. Only about 20% of it is abiotic, produced by serpentinization

Þ Catling and Claire may have overestimated the metamorphic flux of hydrogen during the Archean

Serpentinization• One needs a kinetic

mechanism for oxidizing rocks without using O2 or sulfate

• Serpentinization is one such mechanism that operates today

• Ultramafic rocks interact with warm water to form serpentine minerals, producing hydrogen in the process

Serpentine cabochon from China. This is approximately 39 millimeters by 23 millimeters (From Geology.com)

Serpentinization• Iron is excluded from

the serpentine minerals, so it goes into magnetite

3 FeO + H2O Fe3O4 + H2

• This mechanism is aided by the fact that Archean continental rocks were observably more ultramafic (greenstone belts, komatiites)

Serpentine cabochon from China. This is approximately 39 millimeters by 23 millimeters (From Geology.com)

5. Serpentinization of seafloor

• But, if the continents contained a lot of ultramafic rock that was prone to serpentinization, wouldn’t the seafloor have been undergoing serpentinization, as well?

• Today, most water-rock interactions occur within the midocean ridge hydrothermal vents

• This was probably even more true during the Archean, especially if the continents were smaller

EPSL, 2010

• The Archean mantle would have been hotter, leading to a higher degree of partial melting at the midocean spreading ridges

• Definition: Convective Urey ratio = heat generated by radioactive decay within the mantle/mantle convective heat flow

Blue curves represent thermal evolutionmodels for different present-day convectiveUrey ratios (from Korenaga, 2008)

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% MgOArchean 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• None of the published mechanisms provides a fully

satisfactory explanation for the GOE– However, elements of several mechanisms are probably part of

the story. In particular, Earth’s tectonic evolution is important to consider

• The f-value analysis is circular because the assumption of 20% organic C burial is hard-wired into it. The flux balance approach of Catling and Claire (2006) and Kasting (2013) avoids this circularity

• Changes in the composition and thickness of the oceanic crust may well have been the most important factor in triggering the GOE. How can we better quantify this?