Technical Comments

The Problematic Rise of Archean Oxygen

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Science  22 Feb 2002:
Vol. 295, Issue 5559, pp. 1419
DOI: 10.1126/science.295.5559.1419a

In their intricate study, Catling et al. (1) attempted to explain the rise of atmospheric oxygen on the early Earth. The model they presented relies on a unique anaerobic ecosystem in which necessarily complex microbial fermentations interact closely with methanogens to decompose cyanobacteria and control the global preservation rate of organic carbon. The model begins schematically with photosynthesis, under which 1 mol of organic carbon is produced and 1 mol of free oxygen is released for every mol of CO2 and H2O input [equation 2 of (1)]. Most of that net primary productivity is assumed in the model to convert rapidly to methane [equation 3 of (1); net CH2O yields 0.5CH4] to establish a greenhouse atmosphere of 100 to 1000 parts per million (ppm) CH4; the remainder is buried in sediments averaging ∼1% kerogen carbon. A steady amount of the methane is assumed to leak to the stratosphere to enhance hydrogen escape, which results in a net gain of ∼1013 mol O2 year−1 over ∼109 years [equations 4a and 6 of (1)].

The fate of the photosynthetic free oxygen produced each year, however, presents a serious problem for this proposal. Accepting the minimum biogenic-methane input of 3 × 1013 mol year−1 assumed by Catling et al. [notes 9 and 10 and equation 3 in (1)] requires that twice that amount of net primary productivity, or 6 × 1013 mol C, be recycled annually to CH4. Organic carbon escaping recycling and buried in sediments represents net production of another 1013 mol C year−1 [table 1 of (1)]. Thus, at least 7 × 1013 mol net cyanobacterial C are produced annually under the model, with 7 × 1013 mol free O2 being simultaneously released into the global anoxic environment. The oxidation of “graphite” settling into the troposphere following stratospheric photolysis of CH4 [equation 4b and table 1 of (1)] would leave 6.3 to 6.9 × 1013 mol free O2 to be consumed each year (2). Given any geologically and biologically plausible constraints for the Archean, no resources are available to scavenge this annual output of free oxygen. Oxidizing methane [note 9 of (1)] is obviously counterproductive; this would recycle what was produced and eliminate the “greenhouse,” the hydrocarbon smog itself, or both (3). It would also simultaneously add isotopically light CO2 to the atmosphere and diminish hydrogen escape. Highly reactive reduced gases (H2S, H2) were either insufficient to accomplish the necessary scavenging or are unsupported altogether by the rock and isotope records (4–8). Scavenging the oxygen by oxidizing the necessarily large amounts of dissolved Fe2+ also yields implausible rocks (6–9). The “back reaction of O2 and CH2O via respiration” [note 10 of (1)], essential to the model, would also use the same carbon twice by oxidizing the photosynthetic carbon used to produce the methane in the first place—and, in any event, is an aerobic process requiring pO2 greater than ∼0.002 atm (6–8, 10).

There is abundant evidence to support a low-O2 oxic atmosphere (∼0.003 atm) for the Archean (6–8,11, 12)—and it remains debatable whether other evidence or models to the contrary (1) can indeed distinguish with any certainty between pO2 values of either <0.0008 atm or ∼0.003 atm and a “transitional” value of ∼0.03 atm [reference 1 of (1)]. Both the amount of oxygen in early Earth's atmosphere and the rise of oxygen on the early Earth remain problematic.

*Senior Scientist, Emeritus, Smithsonian Institution

REFERENCES AND NOTES

Response: Towe suggests that much more O2 could accumulate in the early atmosphere than is consistent with the geologic record. We suggest, however, that kinetic losses of O2 operated at a greater rate than he allows, resolving the discrepancy—and thus, as is explained below, that early atmospheric CH4 was important (1).

Towe deduces a net annual oxygen flux of 6.3 to 6.9×1013 mol O2 (2–4). The dilemma that he poses, however, has its roots in the dismissal in his analysis of important O2 sinks and in his neglect of kinetics, or different rates of reactions between different species. The assumed CH4 flux of ∼3 × 1013 mol year−1 could be entirely consumed by 6 × 1013 mol O2 year−1 via CH4 + 2O2 = 2H2O + CO2. But not all CH4 would be oxidized in this way, because substantial O2 (of order 1013 mol year−1) would be scavenged by more reactive reductants supplied to the early environment by hydrothermal, volcanic, metamorphic and weathering fluxes. Such reductants include Fe2+, H2, CO, H2S, and SO2. Thus, on the more reducing early Earth, the annihilation of CH4 by O2 would have been incomplete. Excess CH4 would accumulate to a level at which its photolytic destruction promoted rapid escape of hydrogen to space. That the bulk of the CH4 reacts with O2 in this scenario is not “counterproductive” but is expected (5,6). Also, oxidation of CH4 to CO2would not significantly affect carbonate isotopes, as Towe implies, because, as with the present day, such CO2 would be mixed with larger cycling of CO2 from gross photosynthetic productivity.

In (1), we argued that the sink on O2 from reductants emanating from the crust was greater in the Archean to account for geological evidence of low atmospheric O2. Carbon isotopes show that roughly 20% of the CO2 flux into the biosphere has been fixed biologically and buried as organic carbon since at least ∼3.2 billion years ago (Ga) [note 8 of (1)]. The constant burial flux of photosynthetic carbon implies a constant O2 supply rate [via CO2 + H2O = CH2O (buried) + O2]. Given that unchanging O2 source, for O2 to rise at 2.4 to 2.2 Ga, the O2 sink must have decreased (1, 7). Today, reduced volcanic and metamorphic volatiles consume about a third of the O2 flux associated with organic carbon burial (8). The recycling of more reduced Archean crust via weathering and metamorphism would have provided a greater sink on O2 than the recycling of today's more oxidized crust. The flux of reductants need only have been around three to four times greater, relative to the CO2 flux, to consume the ancient O2 flux associated with organic burial plus some O2 associated with CH4 production (9). CH4 would then accumulate to 100 to 1000 times today's abundance (1,6, 7). Abundant CH4 explains why the O2 sink eventually diminished: The crust slowly lost reducing power to space via CH4-induced hydrogen escape (1). Once the O2 sink dropped below the O2 source, pO2 rose to a new equilibrium.

Thus, the early rock record is most plausibly reconciled by loss of O2 to excess reductants (7). Such conditions stabilize abundant CH4 (1). Abundant CH4 may help explain several major issues in Earth history—why the Earth was not frozen over when the Sun was 20 to 30% less luminous, why Earth irreversibly oxidized, and why low-latitude glaciation occurred in the Paleoproterozoic (given that greenhouse CH4 would be lost upon the Paleoproterozoic increase of O2).

REFERENCES AND NOTES

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