Methane and Climate During the Precambrian Era James F

Precambrian Research 137 (2005) 119–129

Methane and climate during the Precambrian era James F. Kasting ∗

Department of Geosciences, Penn State University, University Park, PA 16802, USA Accepted 1 March 2005

Abstract

The Sun was substantially less bright in the distant past, yet Earth’s surface temperature remained above freezing. Higher concentrations of the greenhouse gases CO2 and CH4 were likely responsible for keeping the early climate warm. CH4 concentrations of 1000 ppm or higher are predicted for the Late Archean/Paleoproterozoic atmosphere prior to the rise of O2. Photolysis of this CH4 may have created an optically thin organic haze during much of this time. The rise of O2 at 2.3 Ga eliminated most of the methane and probably triggered the Paleoproterozoic glaciations. CH4 concentrations could have remained elevated throughout much of the Proterozoic, however, as a consequence of low concentrations of dissolved O2 and sulfate in the deep oceans and a corresponding increase in organic matter recycling by fermentation and methanogenesis. © 2005 Elsevier B.V. All rights reserved.

Keywords: Methane; Precambrian era; Paleoproterozoic glaciation

1. Introduction in CO2 in determining past surface temperatures and in regulating climate stability. The present manuscript Manfred Schidlowski has been more than just an summarizes our recent work on this topic. isotopic geochemist during his illustrious career. He has also been one of the most original thinkers on the topic of biogeochemical cycles of various elements, 2. The faint young Sun problem especially carbon, during the Precambrian. His early papers inspired me to learn about the inorganic and The fundamental problem of climate during the Pre- organic carbon cycles and to investigate the role that cambrian is often termed the “faint young Sun (FYS) CO2 may have played in paleoclimate during the very problem.” Standard models of solar luminosity predict distant past. In the last few years, my students and I that the Sun was ∼30% less luminous at the time when have broadened our interests to include CH4 as well. it ﬁrst formed, ∼4.6 Ga, and that it has increased in lu- We now believe that variations in atmospheric CH4 minosity monotonically since that time. A convenient concentrations were equally as important as variations formula, from Gough (1981),is

S0 ∗ Fax: +1 814 863 7823. S = . (1) E-mail address: [email protected]. 1 + 0.4t/t0

Here, S0 represents the present solar luminosity, most improbable occurrence regardless of changes in 2 ∼1370 W/m , t0 = 4.6, and t = time in Ga prior to the CCN abundance. Greenhouse warming is required in present epoch. This increase in luminosity is a con- any tenable solution to the FYS problem. sequence of the gradual conversion of hydrogen into helium inside the Sun’s core. This increases the core density, causing it to contract and heat up, thereby mak- 3. Solutions involving atmospheric CO2 ing the nuclear fusion reactions proceed faster. If our physical theories of stellar evolution are cor- More than 20 years ago, Walker et al. (1981) rect, the only strictly astrophysical way out of the FYS proposed that increased levels of atmospheric CO2 problem would be if the early Sun was more massive were the most likely solution to the FYS problem. Our than today. At the Sun’s position in the stellar main reasoning was based on the following observation: On sequence, a star’s luminosity increases as roughly the long (million year or greater) time scales, the primary 4th power of its mass, M. Planetary orbital radii are regulator of atmospheric CO2 is the carbonate–silicate proportional to 1/M, and stellar fluxes fall off as the in- cycle. CO2 is removed from the atmosphere–ocean verse square of distance, so the intensity of sunlight re- system by weathering of silicate rocks on land, ceived by the Earth scales as ∼M6. Variousastronomers followed by deposition of carbonate sediments in have suggested that the Sun could have been more mas- the ocean. CO2 is returned to the atmosphere by sive when it first formed and that it lost mass by way volcanism (see Broecker and Sanyal, 1998 for a recent of a greatly enhanced solar wind. However, this idea discussion of this cycle). On a colder Earth, silicate has recently been laid almost to rest by Wood et al. weathering would be inhibited, whereas volcanism (2002), who estimated mass loss rates from several would continue; hence, CO2 should accumulate in the nearby young stars based on their “astrospheric” Ly atmosphere. The resulting increase in the greenhouse ␣ absorption (radiation absorbed by hydrogen at the effect would offset the initial temperature decrease—a shock where the stellar wind collides with the surround- negative feedback loop. In the extreme case of global ing interstellar medium). If their analysis is correct, the glaciation, silicate weathering would cease almost Sun could indeed have been up to 20% brighter initially. entirely and CO2 would accumulate in the atmosphere However, the period of rapid mass loss was short, 200 until the greenhouse effect became large enough to million years or less, so that solar luminosity would melt the ice. This sequence of events is thought to have have dropped to its standard, subluminous value by actually occurred during “Snowball Earth” events in 4.4 Ga. This is probably too early to have had any in- the Neoproterozoic, at 600 and 750 Ma (Hoffman et fluence on the origin and subsequent evolution of life al., 1998), and possibly in the Paleoproterozoic as or to have left any possible trace in the rock record. well, at 2.3 Ga (Evans et al., 1997; Pavlov et al., 2000). One other, partly astrophysical solution to the FYS In the past several years, two objections have been problem has been suggested recently. Shaviv (2003) raised regarding the high-CO2 solution to the FYS has argued that the stronger solar wind from the young problem. The first is empirical. Rye et al. (1995) stud- Sun would have led to a decrease in the intensity of cos- ied paleosols (ancient soils) dated at 2.2–2.8 Ga and mic rays hitting Earth’s atmosphere. This could have used these to place an upper limit on atmospheric pCO2 led to a decrease in cloud condensation nuclei (CCN) during this time (see Fig. 1). This limit is temperature- and a corresponding decrease in cloudiness. The result- dependent and is of the order of 10−2 bar for surface ing lowering of Earth’s albedo could have warmed the temperatures near the modern global average value, climate to some extent. I will not comment extensively 288 K. This is a factor of 20 or more lower than the CO2 on this mechanism here except to point out that the partial pressure required to offset the 25% reduced so- connection between cosmic ray flux and cloudiness is lar luminosity at 2.8 Ga. Their argument was based on highly speculative and, furthermore, the mechanism is the absence of the mineral siderite, FeCO3, from these unlikely to work as well as the proposer thinks it will. If samples. Siderite is a reduced mineral which would be the atmospheric greenhouse effect remained constant, quickly oxidized in modern soils but which ought to offsetting a 30% decrease in solar luminosity would re- have been present in ancient soils if CO2 levels were quire a decrease in planetary albedo from 0.3 to ∼0—a sufficiently high. This suggests that during the Late J.F. Kasting / Precambrian Research 137 (2005) 119–129 121

warming was required throughout the time (since ∼3.0 Ga) for which a reliable geological climate record exists.

4. Methane greenhouses and anti-greenhouses during the Archean

All of this is not very worrisome from a climatol- ogist’s standpoint because there is every reason to believe that other greenhouse gases, methane in particular, were present in abundance in the low-O2 Archean atmosphere. Note that the evidence for an essen- tially anoxic Archean atmosphere has recently grown Fig. 1. Global mean surface temperature as a function of pCO2 and much stronger, based on the observation of mass- CH4 mixing ratio. Calculations are for a solar luminosity of 80% of independently fractionated sulfur isotopes in Archean the present value, assumed to be valid at ∼2.8 Ga. The dashed curves rocks (Farquhar et al., 2000, 2001; Kasting, 2001; show the freezing point of water and the upper limit on pCO2 derived from paleosols (from Pavlov et al., 2000). Pavlov and Kasting, 2002; Mojzsis et al., 2003; Farquhar and Wing, 2003; Ono et al., 2003). The photochemical lifetime of methane in such a low-O2 atmo- Archean/Paleoproterozoic Eras atmospheric CO2 levels were not high enough to keep the climate warm. sphere would have been of the order of 1000–10,000 Note that there is no firm evidence for glaciation until years, as compared to 10–12 years today (Kasting et al., ∼2.3 Ga, implying that the Late Archean climate was, 1983; Zahnle, 1986; Kasting and Brown, 1998; Pavlov if anything, warmer than today. et al., 2001). Thus, if methane had a substantial biological source during the Archean, as seems likely (see A second objection to the high-CO2 solution has been raised on theoretical grounds (Sleep and Zahnle, below), then relatively high atmospheric CH4 concen- 2001; Zahnle and Sleep, 2002). Sleep and Zahnle trations are predicted. This is illustrated in Fig. 2, which shows the results have argued that CO2 was removed from the Archean atmosphere—ocean system primarily by carbonitiza- of photochemical calculations performed by Pavlov et tion of seafloor rather than by continental weathering, as it is today. If so, the negative feedback loop between atmospheric CO2 levels and climate may not have op- erated effectively at that time, and the Archean climate could have been quite cold. Sleep and Zahnle overes- timate this effect, I would argue, because their model does not account for the fact that once the oceans froze over CO2 would no longer be transferred effectively between the atmosphere and oceans. (The ice on a “hard Snowball Earth” should have been at least a kilometer thick at all latitudes.) Hence, if the surface temperature had gotten low enough for this to happen, volcanic CO2 should have accumulated in the atmosphere, as suggested originally by Walker et al. (1981). However, it may well be that surface temperatures would have Fig. 2. Methane flux required to sustain different CH4 mixing ratios in an anoxic, “Archean” atmosphere. Calculations are from Pavlov et hovered just above the global glaciation point, which is −5 al. (2001) and have been extrapolated linearly below f(CH4)=10 . not consistent with the lack of evidence for glaciation The dashed lines show estimated production rates from methanogens at that time. So, Sleep and Zahnle’s basic point is and off-axis midocean ridges (both present-day values) and from well taken: it may well be that additional greenhouse impacts at 3.8 Ga. Impact flux includes serpentinization of ejecta. 122 J.F. Kasting / Precambrian Research 137 (2005) 119–129 al. (2001). In these calculations the surface CH4 mixing Biologists have long speculated that methanogens ratio was held fixed at different values, and the photo- are evolutionarily ancient (cf. Woese and Fox, 1977), chemical model was used to find the net rate of CH4 based partly on the fact that they occupy low-lying destruction (mostly by photolysis at Ly ␣, 121.6 nm). branches of the Archaeal domain and partly on the sup- Because CH4 is not re-formed at an appreciable rate, position, supported here, that CO2 and H2 should have the column destruction rate must be balanced by an up- been abundant on the early Earth. This does not imply ward flux of CH4 from the surface, shown by the solid that methanogens are the most ancient organisms. In- curve. The uppermost dashed line shows the present deed, the fact that they are confined to one branch of biological CH4 production rate, 535 Tg CH4/year, or the Archaea, the Euryarchaeota, strongly suggests that 1.2 × 1011 molec cm−2 s−1 (Houghton et al., 1994). they are not. House et al. (2003) suggest that elemental (Photochemists typically omit the term ‘molec’ when sulfur metabolism may be more ancient. However, it is using these units.) Today, that biological flux supports still a reasonable supposition that methanogens origi- −6 an atmospheric CH4 mixing ratio of 1.8 × 10 by vol- nated early, perhaps as early as the isotopic evidence ume, or 1.8 ppmv. In the anoxic Archean atmosphere, for life begins, ∼3.9 Ga (Rosing, 1999). (I will not take such a flux would have supported a CH4 mixing ratio a position here as to whether this isotopic evidence for of ∼400 ppmv. This value is slightly lower than found life is definitive. A skeptic might postulate that carbon in the previous study of Kasting and Brown (1998) be- isotopes might be fractionated abiotically.) cause the total hydrogen mixing ratio In the past few years my students and I have given considerable thought to the question of how much CH f = f + f , 4 tot(H2) (H2) 2 (CH4) (2) could have been produced by an anaerobic Archean − ecosystem. One approach to this question was sug- was held constant at 2 × 10 3 in these simulations, gested by Kral et al. (1998) (see also Lovley, 1985; whereas Kasting and Brown assumed a constant H2 − Conrad, 1996), who pointed out that methanogens (and, mixing ratio of 10 3. The hydrogen budget will be dis- indeed, anaerobes in general) grown under laboratory cussed in more detail below. conditions are capable of pulling H levels down until Let us return now to the climate calculations shown 2 the free energy yield from reaction R1 is of the order of in Fig. 1. The solid curves in this figure show calculated 35 kJ/mol, about the energy needed to synthesize one global average surface temperatures at different CO 2 mole of ATP. This behavior occurs under conditions in partial pressures and CH mixing ratios. The short- 4 which other nutrients (e.g., phosphorus and nitrogen) dashed curve shows the upper limit on pCO from the 2 are not limiting. Based on this approach, Kral et al. paleosol data; the long-dashed line shows the freezing (also Kasting et al., 2001) predicted that a methanogen- point of water. The calculations show that atmospheres based ecosystem could produce a global CH :H ratio with CH mixing ratios of 100–1000 ppmv could have 4 2 4 of between 10:1 and 20:1. Given a total hydrogen mix- maintained above-freezing global temperatures even −3 ing ratio, ftot ≈ (1–2) × 10 , this implies that CH4 if CO2 was at today’s concentration (370 ppmv, or − mixing ratios of 500–1000 ppmv are plausible. 3.7 × 10 4 bar) or only slightly higher—well within Note that the total hydrogen mixing ratio is deter- the limit imposed by the paleosol data. By contrast, at- mined largely by the balance between volcanic mospheres with 10 ppmv of CH or less are not able 4 outgassing of reduced species and the rate of hy- to satisfy this constraint. Thus, the FYS problem can drogen escape to space (Walker, 1977; Kasting be resolved if methane production rates were similar to and Brown, 1998). Holland (2002) estimates a those today. modern H outgassing rate of 5 × 1012 mol/year, or Should this have been the case, though? Methane 2 1.9 × 1010 cm−2 s−1. Assuming that hydrogen escapes is produced today by methanogenic bacteria, most of at the diffusion limit (Hunten, 1973; Walker, 1977) whom metabolize either hydrogen or acetate (repre- sented here as acetic acid): ∼ 13 Φesc(H2) = 2.5 × 10 ftot(H2), (3) CO2 + 4H2 → CH4 + 2H2O, (R1) then balancing outgassing with escape yields → + . ∼ −3 CH3COOH CH4 CO2 (R2) ftot(H2) = 2 × 10 . If hydrogen escaped more slowly J.F. Kasting / Precambrian Research 137 (2005) 119–129 123 than this due to energy limitations higher up, as seems likely, then even higher H2 and CH4 mixing ratios are possible. We have recently constructed a more elaborate model of such a methanogen-based ecosystem. This model is not yet published, so it is not appropriate to present detailed results here. However, we have addressed several of the shortcomings of the Kasting et al. (2001) model. In particular, we argue that phosphorus should not have been limiting in such a system, even if it were removed efficiently during BIF formation (Bjerrum and Canfield, 2002). Nor should fixed nitrogen have been the limiting factor, as it appears that biological N-fixation originated early (Falkowski, 1997; Fig. 3. Negative feedback loop involving hydrocarbon haze par- Kasting and Siefert, 2001), and biological N-fixation ticles that may have stabilized surface temperatures in the Late tends to keep up with the supply of other nutrients Archean/Paleoproterozoic, prior to the rise of atmospheric O2 (from Pavlov et al., 2003). (Tyrell, 1999). Productivity would have been limited, however, by the relatively slow rate of diffusion through the atmosphere–ocean interface. This diffusion rate absorbs sunlight in the stratosphere and re-radiates it can be estimated by using the piston velocity approach to space from there, creating an anti-greenhouse effect described by Broecker and Peng (1982). When this that cools the surface (Pavlov et al., 2001). A similar kinetic limitation is imposed, and a thermodynamically phenomenon is thought to occur on Saturn’s moon, limited marine ecosystem is assumed, the methanogens Titan, which is where the term “anti-greenhouse are not able to draw down H2 as effectively as in the effect” was coined (McKay et al., 1991). Hence, the old model. However, CH4:H2 ratios of 3:1 are still CH4–climate system also contains a negative feedback likely. Thus, our basic conclusion still appears to hold: loop that could have stabilized methane concentrations Once methanogens appeared, they should have con- and surface temperatures at the point where an verted most of the available atmospheric hydrogen into optically thin organic haze existed (Fig. 3)(Pavlov methane, sustaining CH4 mixing ratios of ∼1000 ppmv et al., 2001). This is, if you will, a “Gaian” control or higher. Walker (1977) arrived at this same conclu- mechanism that could conceivably have regulated sion based on a more heuristic model of the early Earth. climate during the Late Archean/Paleoproterozoic There is an additional wrinkle to the climate calcu- eras. lations. Methanogens grow fastest at high temperatures The final episode of the Archean climate story or, more accurately, thermophilic methanogens have would have come in the early Paleoproterozoic, shorter doubling times than mesophilic ones (Cooney, ∼2.3 Ga, when atmospheric O2 levels first rose. I 1975). This creates a positive feedback loop in the argued above that the nature of this event is now es- climate system: higher temperatures lead to faster tablished conclusively by data on mass-independently methanogen growth, which leads to additional methane fractionated sulfur isotopes. It has also been known for production and still higher temperatures. There is some time (Roscoe, 1969, 1973; Walker et al., 1983) a limit, though, to how far this cycle can proceed. that the initial rise in O2 is correlated with the Paleo- Photochemical models (e.g., Zahnle, 1986; Pavlov proterozoic glaciation(s). Fig. 4, redrawn from Young et al., 2001) predict that methane should begin to (1991), shows this correlation explicitly for the Huro- polymerize and form hydrocarbon haze particles when nian Supergroup in southern Canada. This sequence is the atmospheric CH4:CO2 ratio exceeds unity. This bounded by radiometric age dates of 2.2 Ga at the top criterion is realized in the upper left-hand corner of and 2.45 Ga at the bottom. The three layers labelled Fig. 1. Note that high temperatures promote CO2 loss G1 (Ramsey Lake), G2 (Bruce), and G3 (Gowganda) along with CH4 production, thus driving the system represent glacial deposits. Below the lowermost glacial towards generation of haze. The haze, once it forms, layer, the Matinenda Formation contains abundant 124 J.F. Kasting / Precambrian Research 137 (2005) 119–129

2002). Clearly, methane is not a reliable bioindicator if large abiotic sources of CH4 can be identified. Fig. 2 shows my best estimates for the present abiotic methane source from off-axis midocean ridges and for the possible methane source associated with impacts at 3.8 Ga. The off-axis ridge flux is thought to originate from serpentinization of ultramafic rock (peridotite) deep beneath the ocean floor in slow-spreading ridges (Kelley et al., 2001). During hydrothermal alteration of ultramafic (Fe- and Mg- rich) rocks, serpentine minerals are created. These tend to exclude ferrous iron; thus, the iron must find another stable phase. For thermodynamic reasons, it forms magnetite, Fe3O4, a mineral in which two of the three iron atoms have been oxidized to the ferric state (Berndt et al., 1996). Hence, some other substance must be reduced during the serpentinization process. Fig. 4. Stratigraphy of the Huronian Supergroup in southern Canada. When H2O is the oxidant, hydrogen is released The three layers labelled “diamictite” are assumed to be glacial in origin (from Young, 1991, redrafted by Y. Watanabe). 3FeO + H2O → Fe3O4 + H2. (R3)

When dissolved CO2 is present, methane is formed detrital pyrite and uraninite—both evidence of low-O2 conditions. Above the uppermost glacial layer, the instead Lorrain redbed formation gives evidence of high-O2 CO2 + 12FeO + 2H2O → CH4 + 4Fe3O4. (R4) conditions. Evidently, the glaciations occurred right at the time when O2 levels first rose. (Ongoing sulfur The estimated CH4 flux shown in Fig. 2 was derived isotope work by A. Bekker and co-workers at Harvard as follows. Kelley et al. (2001) measured dissolved University may lead to a more detailed chronology of CH4 concentrations of 0.2 mmol/kg in fluids emanating events within the glacial sequence.) The occurrence from the Lost City ventfield on the flank of the (slow- of widespread, perhaps global (Evans et al., 1997) spreading) Mid-Atlantic ridge. This water exits the glaciation at this time fits in well with the methane vents at 50–60 ◦C. The flux of water passing through greenhouse story just related. An increase in atmo- the ridges can be estimated from the total heat flow spheric O2 concentration could have wiped out most anomaly, that is, the amount of geothermal heat given of the CH4, thereby reducing the greenhouse effect off at the ridges in excess of what would be expected and triggering glaciation. This story is internally self- in the absence of hydrothermal circulation. Mottl and consistent, along with being quantitatively believable. Wheat (1994) estimate a total heat flow anomaly of 65 × 1018 cal/year, of which (45–50) × 1018 cal/year occurs off-axis. This number includes both fast- and 5. Hadean (abiotic) methane levels slow-spreading ridges. For a vent-fluid temperature of 50 ◦C, this corresponds to a circulation rate of Although most of our interest has been in the ∼1 × 1015 kg/year. If half the world’s ridges are post-biotic Archean, the question of abiotic methane slow-spreading, and if all water emanating from them levels is regularly brought up. In part this is because has the same dissolved CH4 concentration as at Lost methane could have been involved in HCN synthesis City, then the global CH4 source from this process is (Zahnle, 1986; Kasting and Brown, 1998) and the 1 × 1011 mol/year, or 3.7 × 108 cm−2 s−1. According origin of life. More recently, it is because of the interest to Fig. 2, this would produce an atmospheric CH4 in methane as a bioindicator on extrasolar planets mixing ratio of ∼0.5 ppmv. (The actual calculations (Schindler and Kasting, 2000; Des Marais et al., shown in Fig. 2 extended down to only 10 ppmv J.F. Kasting / Precambrian Research 137 (2005) 119–129 125

CH4. The loss rate has been extrapolated linearly an organic-rich impactor, the overall reaction would be to lower CH4 mixing ratios to show the abiotic limits.) 2C + 2H2O → CH4 + CO2. (R5) Whether or not this estimate can be extended to the Hadean is debatable. In one sense, I have maximized Thus, if all of the carbon in carbonaceous impactors the estimate for the present abiotic CH4 flux by followed this pathway, exactly half of it would be con- blithely extrapolating from measurements at a single verted to CH4. ventfield. Even so, I may have greatly underestimated Kress and McKay have made estimates of the the Hadean CH4 flux. Depending on how plate amount of CH4 produced from impacts, based on im- tectonics was operating at that time, the amount pact fluxes from Chyba (1990). They obtained CH4 of ultramafic rock exposed at the seafloor could production rates in excess of 500 Tg CH4/year, i.e., conceivably have been much greater than today. (N. higher than the present biological CH4 flux, prior to Sleep, private communication, has speculated that this 3.5 Ga. I consider these estimates to be too high. My was not the case, and that most of the seafloor would own estimate follows. Zahnle and Sleep estimate a to- have been covered with normal, mafic basalt, as it is tal mass influx rate of 2 × 1014 g/year at 3.8 Ga. For today, but this conclusion is far from certain.) Today, impactors of C1 composition containing 3 wt.% car- approximately 21 × 1015 kg of ferrous iron is oxidized bon, this corresponds to an influx of 6 × 1012 g C/year, to the ferric state during aqueous alteration of seafloor or 5 × 1011 mol C/year. Thus, for the stoichiometry (Lecuyer and Ricard, 1999). The main oxidant today is shown in reaction R5, the CH4 production rate would dissolved sulfate. If the Hadean seafloor was ultramafic have been 2.5 × 1011 mol/year, or 9.3 × 108 cm−2 s−1. in composition, and this same amount of iron was This by itself would generate a CH4 mixing ratio of oxidized by serpentinization, the resulting CH4 source ∼1 ppmv, according to Fig. 2. 12 9 −2 −1 would be 1.5 × 10 mol/year, or 5.6 × 10 cm s . Somewhat more CH4 could have been generated by Faster creation of seafloor on a younger, hotter Earth serpentinization of finely divided impact ejecta, assum- could boost this number by a factor of 5–10, so ing a crust of ultramafic composition. Zahnle and Sleep an abiotic source of order 1010–1011 cm−2 s−1 is estimate that the mass of the ejecta is ∼50 times that of within the realm of possibility. This is close to the the impactors themselves. If we assume a mantle-like modern biological CH4 flux and suggests that we iron concentration, ∼8 wt.%, and allow this to serpen- should be cautious about using methane as a bioindi- tinize by reaction R4, then the resulting CH4 flux would cator. have been 1 × 1012 mol/year, or 3.7 × 109 cm−2 s−1. A second likely abiotic source for methane during The dashed line in Fig. 2 represents the sum of this in- the Hadean would have been from impacts. In a pre- direct CH4 production and the direct production in the vious analysis of the effects of impacts on early atmo- plume, which is 4.6 × 109 cm−2 s−1. This is sufficient spheric composition (Kasting, 1990), I concluded that to produce a CH4 mixing ratio of ∼7 ppmv. impacts should have produced mainly CO, not CH4. It should be noted that all of these estimates should The CO could have come from oxidation of organic be time-dependent. The impact flux may have increased carbon in carbonaceous asteroids or comets or from exponentially backwards in time with an e-folding reaction of metallic iron with ambient CO2 in iron-rich time of ∼200 million years (Kasting, 1990). This as- asteroids. While coming to this conclusion, I assumed sumes that the heavy bombardment period was con- that thermodynamic equilibrium would prevail at high tinuous, which may or may not have been the case temperatures within the impact plume and that chem- (Ryder, 2003). The methane photolysis rate should also ical reactions would be kinetically inhibited below a have increased backwards in time as a consequence “freeze-out” temperature of 3500 K. More recently, of enhanced UV emission from the rapidly rotating Kress and McKay (2004) have argued that, because young Sun. The estimated loss rate shown in Fig. 1 of the presence of catalytic dust particles in impact includes an enhancement of ∼2.5 in the solar Ly ␣ plumes, the effective freeze-out temperature would be flux, which is appropriate for a time of ∼4.0 Ga (Walter much lower, ∼500 K. In this case, CH4 becomes the and Barry, 1991). More detailed analysis of both bom- thermodynamically favored carbon-containing gas. For bardment rates and solar UV flux history would be 126 J.F. Kasting / Precambrian Research 137 (2005) 119–129 needed to improve significantly on the numbers derived here.

6. Methane in the Proterozoic atmosphere

Let us return now to the post-biotic atmosphere after what Holland (2002) has termed the Great Oxidation Event (GOE) at ∼2.3 Ga. We believe that atmospheric O2 concentrations had reached appreciable levels by this time, probably within a factor of ∼100 of the present atmospheric level (PAL; Pavlov et al., 2003). The mass-independent S isotope data only −5 require pO2 >10 PAL (Pavlov and Kasting, 2002); however, it is unlikely that O2 concentrations could have stabilized at such low levels. The loss process that limited O2 during this time was presumably oxidative weathering of the continents, as today (Holland, 1978), and it is unlikely that this would have been efficient below ∼10−2 PAL (Holland, 1984, 2003; Lasaga and Ohmoto, 2002; Ohmoto, 2003). Suppose, for the sake of concreteness, that pO2 was equal to ∼0.1 PAL during much of the Proterozoic. Canfield (1998) has argued that much of the deep ocean Fig. 5. Diagram illustrating the dependence of atmospheric CH should have been anoxic at O levels below ∼0.5 PAL 4 2 mixing ratio on surface CH4 flux (panel A) and the dependence of (see also Canfield et al., 2000 and Anbar and Knoll, surface temperature on CH4 mixing ratio (panel B) for “Proterozoic- −2 2002 in support of this view). H.D. Holland (private type” atmospheres containing between 10 and 1 PAL of O2. The communication) (also Logan et al., 1995) has pointed two curves in panel B are for solar fluxes appropriate for the Pa- out that this need not have been true, as the sinking leoproterozoic (S/S0 = 0.83) and Neoproterozoic (S/S0 = 0.94) (from Pavlov et al., 2003). of particulate organic matter could have been inhibited prior to the diversification of eukaryotes and the evolution of fecal pellets, but let us set aside this criticism for have been 10–20 times the present global methane flux, the moment and consider what such a “Canfield-type” which as we have seen earlier is ∼535 Tg CH4/year. Proterozoic ocean would have implied about rates of Pavlov et al. (2003) also explored the consequences methane production. This scenario has been recently of such high methane fluxes for the Proterozoic explored by Pavlov et al. (2003). There, we argued that atmosphere and climate. Their results are summarized marine primary production was probably comparable in Fig. 5. Fig. 5A shows atmospheric CH4 mixing to present values and that both O2 and sulfate were ratio as a function of the surface CH4 flux for O2 − scarce, or absent, in the deep oceans. (The argument for levels between 10 2 and 1 PAL. (The axes are inverted low sulfate was based on measurements of trace sulfate to facilitate comparison with Fig. 5B.) At all of abundances in Proterozoic carbonates.) If so, then or- these O2 levels, the CH4 mixing ratio increases ganic matter would not have been efficiently recycled nonlinearly (nearly quadratically) with CH4 flux. This by aerobic decay or by sulfate reduction, as it is today. nonlinear behavior has been understood for some time Hence, much of the organic matter produced by photo- (Thompson and Cicerone, 1986; Prather, 1996). It oc- synthesis may have been recycled by fermentation and curs because the main loss process for CH4 in an oxic methanogenesis. For reasonable estimates of the or- atmosphere is reaction with the hydroxyl radical, OH ganic carbon burial fraction in an anoxic marine ecosystem, the methane flux from marine sediments could CH4 + OH → CH3 + H2O. (R6) J.F. Kasting / Precambrian Research 137 (2005) 119–129 127

But this reaction is also a major sink for OH. Hence, parameterization of CH4 absorption at all wavelengths the concentration of OH decreases as CH4 increases, is needed to resolve these uncertainties. and the photochemical lifetime of CH4 increases cor- respondingly. Fig. 5B shows the potential consequences for 7. Summary Proterozoic climate. The two curves show the effect Methane was arguably an important atmospheric of increased CH on global mean surface temper- 4 constituent and greenhouse gas during much of ature during the Neoproterozoic (S/S = 0.94) and 0 Earth’s Precambrian era. Small amounts of methane Paleoproterozoic (S/S = 0.83). In these calculations, 0 (1–10 ppmv) should have been formed abiotically CO was fixed at 320 ppmv, about 40 ppmv above its 2 on the Hadean Earth from impacts and from ser- preindustrial value. If these calculations are correct, pentinization of ultramafic rocks on the seafloor. then a CH mixing ratio of 300 ppmv (which could 4 Much larger CH concentrations (∼1000 ppmv conceivably result from an increase in CH flux by 4 4 or more) may have been present during the Late a factor of 20) should cause an increase in surface Archean/Paleoproterozoic, prior to the rise of O . This temperature by ∼15 K. Such an enhanced methane 2 methane would have been produced by methanogens greenhouse might explain the protracted warmth of that metabolized with CO and H from the atmosphere the middle Proterozoic. No evidence for glaciation 2 2 or acetate produced from fermentation of photosyn- is seen between the Paleoproterozoic glaciation at thetically produced organic matter. The rise of O at ∼2.3 Ga and the first of the Neoproterozoic glaciations 2 ∼2.3 Ga should have caused a drastic decrease in at- at 0.75 Ga. Pavlov et al. (2003) also suggested, without mospheric CH concentrations and may have triggered showing any geologic evidence, that a rise in either 4 the Paleoproterozoic glaciation. CH concentrations atmospheric O or dissolved sulfate near the end of 4 2 may have remained much higher than today, however the Proterozoic could have decreased the percentage (10–100 ppmv) throughout much of the Proterozoic of organic matter recycled through methane and pos- as a consequence of enhanced recycling of marine sibly triggered the Neoproterozoic “Snowball Earth” organic matter by fermentation and methanogenesis. glaciations. Additional work needs to be done to determine what This hypothesis is attractive in many respects. the radiative effect of CH would have been at this time. It explains the warm Mesoproterozoic and it could 4 Although this research on methane and climate provide an effective trigger for Snowball Earth. does not draw directly on the work of Manfred Schid- However, I should mention a caveat that has surfaced lowski, Manfred’s inspiration and encouragement over since the Pavlov et al. (2003) paper was published. A the years has meant a lot to this author. In particular, new version of our climate model in which a different I recall a very pleasant visit with the Schidlowski’s at algorithm was used in the thermal infrared (Mlawer et their house in Mainz prior to the 1983 ISSOL meeting. al., 1997; Segura et al., 2003) yields entirely different Then, as always, it was a great pleasure to talk science results for such “Proterozoic” atmospheres. Indeed, with Manfred and to share his enthusiasm for learning with the new model, we get an anti-greenhouse effect, about the early Earth and its biota. rather than a greenhouse effect, at CH4 concentrations of 100 ppmv (60 times present). This behavior is counter-intuitive but not impossible, as methane also Acknowledgements absorbs effectively in the visible and near-IR portions of the spectrum. Thus, gaseous methane can, in princi- Different parts of this work were supported by grants ple, produce the same time of anti-greenhouse cooling from NASA’s Exobiology program, the NASA Astro- that is generated by hydrocarbon haze. Interestingly, biology Institute, and NSF’s LExEn program. such anti-greenhouse behavior is not observed when the new model is used for “Archean” atmospheres References with no O2 or O3. Apparently, the presence of a warm, ozone-rich stratosphere makes a difference in these Anbar, A.D., Knoll, A.H., 2002. Proterozoic ocean chemistry and calculations for reasons that are obscure. An improved evolution: a bioinorganic bridge. Science 297, 1137–1142. 128 J.F. Kasting / Precambrian Research 137 (2005) 119–129

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