Lecture 12: Global Redox Balance/Causes of the rise of O 2 Abiol 574 Global redox balance Causes of the Great Oxidation Event

Dick Holland ( ) 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 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 

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 CH 2 O is given by So, for  13 C carb = 0, f org  0.2 Hayes and Waldbauer, Phil. Tran. Roy. Soc. B (2006) Lomagundi event

So, if O 2 was being produced prior to 2.45 Ga, the real question has become: What delayed the rise of atmospheric O 2 ? 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-O 2 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: H 2 O, CO 2, N 2, SO 2 Other gases are either oxidized or reduced compared to these. Express the differences in terms of H 2 equivalents, e.g. CH H 2 O  CO H 2 H 2 O 2 + H 2  2 H 2 O Thus, the total outgassing rate of reductants can be written as

Global redox balance equation Setting H 2 sources equal to H 2 sinks yields the following equation: Here  out (Red) = total outgassed flux of reduced gases  OW = oxidative weathering of the continents and seafloor  burial (CaSO 4 ) = burial of gypsum or anhydrite  burial (Fe 3 O 4 ) = oxidation of ferrous iron without using O 2 (includes BIFs and serpentinization)  burial (CH 2 O) = burial of organic matter  burial (FeS 2 ) = burial of pyrite J.F. Kasting, Chem. Geology, submitted

Let’s look at some specific hypotheses for what caused the rise of atmospheric O 2 …

3. Catling & Claire’s continental oxidation model The amount of O 2 stored as Fe +3 in the continental crust exceeds the amount of organic carbon The extra O 2 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)

H 2 from continental metamorphism In the Catling & Claire model, reduced gases produced from continental metamorphism were what slowed the O 2 rise The continents were less oxidized during the Archean Modern metamorphic reduced gas fluxes were scaled up by factors of using thermodynamic equilibrium arguments But –The continents may have been much smaller back then –Thermodynamic equilibrium does not apply at metamorphic temperatures  Catling and Claire may have overestimated the metamorphic flux of hydrogen during the Archean

Catling & Claire’s * K oxy parameter Let K oxy represent the ratio of H 2 sinks to sources, not counting oxidative weathering and burial of sulfate (which are only important at high O 2 ) and escape of hydrogen to space (which is only important at low O 2 ) The atmosphere switches from reduced to oxidized when K oxy becomes >1 * Catling and Claire, EPSL (2005) Claire et al., Geobiology (2006)

Magnitudes of terms Relative magnitudes today (mostly from Dick Holland’s work): TermRate (10 12 mol/yr) 2  burial (CH 2 O) 20 ±  burial (FeS 2 ) 10 ± 5  out (red)4.8 ± 3.6  burial (Fe 3 O 4 )0.4 ± 0.2  K oxy  5.8

I’ll come back to the K oxy 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 CO 2 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 O 2 Holland, GCA (2002)

f-values vs. C/S/H 2 O ratios Holland, GCA (2002) f-values become lower as C/H 2 O and S/H 2 O ratios become higher Hence, one way to keep the Archean atmosphere reduced is to outgas more H 2 O, and hence more H 2, relative to CO 2 and H 2 S

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 H 2 O, 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 O 2 at ~2.4 Ga was caused by a switch from predominantly submarine to predominantly subaerial volcanism Is this really true?

The rise in atmospheric O 2 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 CO 2 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 SO 2 :H 2 S 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 1  This mechanism doesn’t really seem to work Nonetheless, it’s a nice quantitative model that is susceptible to analysis Nature (2011) Subaerial outgassing Submarine outgassing

Alternatively, one can analyze the mechanism of Gaillard et al. using the Catling/Claire K oxy parameter For the present day, this yielded In Gaillard et al.’s model, submarine outgassing roughly doubles the outgassing flux of reductants, so  So, this mechanism helps, but it does not produce a reduced Archean by itself (need K oxy < 1) (Units are mol/yr)

Conclusions It is possible to explain the reduced Archean, even if oxygenic photosynthesis evolved well before 2.4 Ga 1. 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 helps 3. A switch from submarine towards subaerial outgassing could also be part of the story 4. 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