Lecture 12: Global Redox Balance/Causes of the rise of O2

Abiol 574

Global redox balanceCauses of the Great Oxidation Event

Dick Holland (1927-2012)

• Dick Holland passed away just over a year ago and will be missed by many of us

• His passing, though, has spawned a special issue of Chemical Geology and a special session of this conference this past Monday

• I used this opportunity to work on quantifying the global redox budget and evaluating various hypotheses for causing the GOE at 2.4 Ga

Published hypotheses for the cause of the GOE*

1. Holland’s tectonic evolution/volcanic outgassing model (Holland, 2002, 2009)

2. Submarine versus subaerial outgassing mechanisms (Kump and Barley, 2007; Gaillard et al., 2011)

3. Continental oxidation and hydrogen escape (Catling et al., 2001; Catling and Claire, 2005; Claire et al., 2006)

4. Serpentinization of seafloor (Kasting and Canfield, 2012)

5. Banded iron-formation triggers (Isley and Abbott, 1999; Barley et al., 2005; Goldblatt et al., 2006; Bekker et al., 2010)

6. 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., in press

What caused the GOE (Great Oxidation Event) at

~2.45 Ga?• In one sense, this question is easily answered: 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

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

A “whiff” of oxygen before the GOE

Anbar et al., Science (2007)• Anbar and colleagues measured enhanced Mo concentration in the Mt. McRae shale, dated at 2.50 Ga (50 m.y. before the GOE)

The carbon isotope record• The carbon isotope

record also shows no sign of a secular change in organic carbon burial

• The fraction of carbon leaving the system as CH2O is given by

• So, for 13Ccarb = 0, forg 0.2

Hayes and Waldbauer, Phil. Tran. Roy.Soc. B (2006)

Lomagundi event

• So, if O2 was being produced prior to 2.45 Ga, the real question has become: What delayed the rise of atmospheric O2?

• To answer this question, we need to think about the redox budget in more detail…

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, 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

J.F. Kasting, Chem. Geology, submitted

• Let’s look at some specific hypotheses for what caused the rise of atmospheric O2…

3. Catling & Claire’s continental oxidation model

• The amount of O2 stored as Fe+3 in the continental crust exceeds the amount of organic carbon

• The extra O2 was likely produced by loss of hydrogen to space

• Much of the Fe+3 was evidently emplaced prior to the GOE need anaerobic mechanisms for oxidizing ferrous iron

Catling et al., Science (2001)

H2 from continental metamorphism• In the Catling & Claire model,

reduced gases produced from continental metamorphism were what slowed the O2 rise

• The continents were less oxidized during the Archean

• 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

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

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)

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) 10 ± 5

out(red) 4.8 ± 3.6

burial(Fe3O4) 0.4 ± 0.2

Koxy 5.8

• I’ll come back to the Koxy method of redox budget analysis

But first

• Dick Holland had his own methodology for evaluating redox budgets…

1. 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 argument for 20% organic C burial

Hayes and Waldbauer (2006)

• Throughout most of Earth’s history, carbonates have remained near 0 per mil, apart from short-lived, but occasionally spectacular, excursions• Holland hard-wired the assumption of 20% organic C burial into his model

f-values for modern subaerial volcanoes

• Modern subaerial volcanoes have f-values that are generally <1

• That, in Holland’s view, is why the modern atmosphere is rich in O2

Holland, GCA (2002)

f-values vs. C/S/H2O ratios

Holland, GCA (2002)

• f-values become lower as C/H2O and S/H2O ratios become higher• Hence, one way to keep the Archean atmosphere reduced is to outgas more H2O, and hence more H2, relative to CO2 and H2S

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

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

Holland, GCA (2009)

2. Submarine/subaerial outgassing and the GOE

Kump and Barley, Nature (2007)

Gaillard et al., Nature (2011)

• Two different recent 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• Is this really true?

• 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

• This analysis, however, leaves out some important terms in the redox budget; furthermore, it assumes that 20% of outgassed CO2 is buried as organic matter, which need not always have been true

More reduced

Nature (2007)

• Gaillard et al. pursued the submarine-to-subaerial outgassing hypothesis, arguing that the key factor was an increase in the SO2:H2S ratio as the outgassing moved to lower pressure

• They didn’t do a redox budget analysis, though

Nature (2011)

• So, we did…• In none of the cases of

Gaillard et al. did Holland’s f value ever exceed 1Þ This mechanism doesn’t

really seem to work

• Nonetheless, it’s a nice quantitative model that is susceptible to analysis

Nature (2011)

Subaerialoutgassing

Submarineoutgassing

• Alternatively, one can analyze the mechanism of Gaillard et al. using the Catling/Claire Koxy parameter

• For the present day, this yielded

• In Gaillard et al.’s model, submarine outgassing roughly doubles the outgassing flux of reductants, so

Þ So, this mechanism helps, but it does not produce a reduced Archean by itself (need Koxy < 1)

(Units are 1012 mol/yr)

ConclusionsIt is possible to explain the reduced Archean, even if oxygenic photosynthesis evolved well before 2.4 Ga1. Need to have slower burial of organic carbon and pyrite during the Archean (as emphasized by Dick Holland)

– Sulfur cycle was slower in the past because sulfur was less mobile– Organic carbon burial could have been slower if less inorganic carbon

entered the system (smaller continents, less weathering of carbonates, no oxidative weathering)

2. Greater anaerobic oxidation of ferrous iron (e.g., BIF deposition and serpentinization of seafloor) also helps3. A switch from submarine towards subaerial outgassing could also be part of the story4. Biological innovations that increase productivity could have helped. However, one has to maintain consistency with the carbon isotope record, which appears at face value to indicate that organic C burial rates have not changed