Archean Atmosphere and Climate

The Earths rock record really begins at ~3.8 Ga. The period prior to that, which we have been discussing so far, is called the Hadean Era. The period between 3.8 Ga and 2.5 Ga is called the Archean Era. It is still poorly understood compared to more recent time periods; however, there are rocks of Archean age on all 7 continents and there is a vast literature describing what those rocks tell us about the Archean atmosphere and climate. We shall not, in this briefchapter, attempt to summarize all of this evidence. Instead, we focus on a few topics that may help the reader to understand what we do know about the Archean Earth, as this may guide our thinking about the earlier period for which the rock record is absent.

(i) Archean Climate

The basic climate issue regarding the Archean and the Hadean, eras is often called the faint young Sun problem. The Sun is thought to have been about 30 percent less luminous at the time when the Solar System formed and to have brightened more or less linearly since that time (Fig. 8.1). This is a theoretical prediction from stellar evolution models, but it is nonetheless considered to be a robust result. (This claim is supported by the fact that the prediction has not changed much in the last 25 years.) The consequences for Earth's climate are clear: Ifatmospheric composition had not changed over the Earth's history, the mean global surface temperature would have been below freezing prior to about 2 Ga. This can be seen from the upper dashed curve in Figure 8.1. The lower dashed curve in Figure 8.1 represents the effective radiating temperature of the Earth, which can be thought of as the temperature that Earth's surface would have in the absence of an atmosphere. The shaded area between the two dashed curves represents the greenhouse effect of the model atmosphere, which in this case has been assumed to consist of present-day levels of N2 and CO2, along with variable amounts of H2O. The greenhouse effect increases with time in this calculation because the atmosphere holds more water vapor as the surface temperature rises.

Possible solutions to the faint young Sun problem have been reviewed elsewhere.3 The answer almost certainly involves higher concentrations of greenhouse gases in the past. Greenhouse gases are those that absorb and emit thermal infrared radiation between about 5 ^m and 100 ^m. The most likely candidates are CO2, CH4 and possibly C2H6 (ethane). The latter gas has only recently been suggested, but it is a powerful infrared absorber that may have provided 10 degrees or more of greenhouse warming. H2O is also a powerful greenhouse gas, but its concentration is limited by its saturation vapor pressure and so it is a feedback on climate, as opposed to being a climate forcer.

The argument for higher CO2 levels in the past is as follows: on long time scales, atmospheric CO2 concentrations are controlled primarily by the inorganic carbon cycle, or carbonate-silicate cycle. CO2 is injected into the atmosphere by volcanoes and it is removed by the weathering of silicate rocks on land, followed by deposition ofcarbonate sediments in the ocean (Fig. 8. 2). The term weathering refers to the physical and chemical alteration of rocks through erosion and interaction with dissolved species in rainwater. The process ofsilicate weathering requires liquid water in order to proceed at an appreciable rate. Hence, if Earth's mean surface temperature were to fall below the freezing point ofwater, as suggested in Figure 8.1, silicate weathering should slow, or stop entirely and volcanic CO2 should accumulate in the atmosphere. The accumulation would continue until the greenhouse effect ofthe added CO2 became high enough to melt the ice and allow silicate weathering to proceed once again. Calculations4 show that a CO2 partial pressure of ~0.3 bars would have been sufficient to compensate for a solar flux reduction of 30 percent. The present atmospheric CO2 concentration, by comparison, is approximately 300 ppmv, or 3 x 10-4 bars. Hence, the amount of CO2 required to compensate for the faint young Sun is roughly 1000 times the present atmospheric level (PAL). Although this may sound high, it is only a small fraction of the available CO2. Earth has the equivalent of60-80 bars of CO2 tied up in carbonate rocks on the continents. Only a tiny fraction ofthis inventory would have been needed to compensate for the faint young Sun.

On the other hand, CO2 may not have been the only greenhouse gas whose concentration was high on the early Earth. We saw in the previous section that CH4 could also have been abundant. This is

Figure 8.1. Diagram illustrating the faint young Sun problem. The solid curve is solar luminosity relative to the present day value. The lower dashed curve is the effective radiating temperature, Te. The upper dashed curve is the mean global surface temperature, Ts. The shaded area in between the two dashed curves shows the magnitude of the greenhouse effect. (adapted from Scientific American, Vol 256, Kasting.30 Copyright (1988) with permission from Scientific American)

Figure 8.1. Diagram illustrating the faint young Sun problem. The solid curve is solar luminosity relative to the present day value. The lower dashed curve is the effective radiating temperature, Te. The upper dashed curve is the mean global surface temperature, Ts. The shaded area in between the two dashed curves shows the magnitude of the greenhouse effect. (adapted from Scientific American, Vol 256, Kasting.30 Copyright (1988) with permission from Scientific American)

particularly true after the origin of life, when organisms may have produced copious amounts of methane. The organisms that do so today are called methanogens. (Higher plants actually produce some methane, as well, but we shall ignore that because higher plants were not around during the Archean.) Methanogens are not true Bacteria; rather, they are all found on one branch ofthe the Archaeal domain on the rRNA tree (see Chapter 2). They are considered by most biologists to have evolved very early and so they were probably extant during the Archean era. Atmospheric O2 levels were low at that time (see below), causing the photochemical lifetime of CH4 to have been at least 1000 times longer than today (10,000 years instead of 10 years). The likely source of CH4 in the Archean was from methanogens living in the oceans and in marine sediments.22 Calculations suggest that the CH4 source was comparable to today's source (ibid.). Hence, the CH4 concentration, instead of being 1.6 ppmv, as today, may well have been over 1000 ppmv. At these concentrations, CH4 is an effective greenhouse gas, generating approximately 10 degrees of greenhouse warming.25 Furthermore, photolysis ofmethane in a low-O2 environment forms ethane, C2H6 and ethane is an even better greenhouse gas. Hence, a combination of CO2, CH4 and C2H6 appears to be a plausible mechanism for keeping the Archean Earth warm.

Other proposed constraints on Archean climate come from geochemistry and geology. Oxygen isotopes in cherts (SiO2) have been

Figure 8.2. Schematic diagram illustrating the carbonate-silicate cycle. This cycle controls the atmospheric CO2 concentration over long time scales.

used to argue that the Archean climate was hot (~70C).26 Indeed, the chert data and also O isotope data from carbonates, suggest that the Earth remained quite warm until as recently as 400 million years ago. On the other hand, geomorphic evidence indicates that the Earth experienced glaciations at 2.9 Ga, 2.4 Ga and 0.6-0.7 Ga.27 These data are not at all consistent with the O isotope data for a hot early Earth. The likely resolution to this problem, in our opinion, is that the oxygen isotopic composition of the oceans has changed with time (ibid.). This topic remains highly controversial and it may be that this story will change over time. However, we would argue that, while the Archean climate was warm, it was nowhere near as hot as the O isotope data suggest.

(ii) Archean O2 Concentrations

For many years, both geologists and biologists have been intrigued by the question ofhow much O2 was present in the Archean atmosphere and when atmospheric O2 first rose to appreciable concentrations. In the late 1960s, the geologist Preston Cloud used a variety of geologic evidence, including redbeds, detrital minerals and banded iron-formations (BIFs), to show that the atmosphere first became O2-rich sometime around 2.0 Ga. Cloud's hypothesis was further developed and elaborated by Heinrich Holland,5 whose name was mentioned earlier. The date of the Great Oxidation Event (GOE), as Holland termed it, was revised back to ~2.4 Ga, but Cloud's basic idea remained secure. Various geochemists have questioned this hypothesis, notably Hiroshi Ohmoto ofPenn State University, but their ideas have failed to gain traction.

About 7 years ago, a significant new development occurred that all but clinched the debate over the rise of atmospheric O2. James Farquhar and colleagues published a paper28 in which they used multiple sulfur isotopes in ancient rocks to test Cloud's hypothesis. Sulfur isotopes (32S, 33S, 34S and 36S) normally fractionate, or separate, along a predictable line that depends on their relative masses. (33S differs from 32S by 1 mass unit, whereas 34S differs from 32S by 2 mass units; hence, they normally fractionate along a line with a slope of In all rocks younger than about 2.0 Ga, the S isotopes plot along this mass fractionation line; however, in rocks older than 2.4 Ga, the S isotopes fall off it. (Rocks between 2.0 Ga and 2.4 Ga are close to the mass fractionation line, but not quite on it.) As part of their study, Farquhar et al photolyzed sulfur dioxide,

SO2, in the laboratory in the absence of O2 and they were able to show that the products of the reaction had mass independent S isotope signatures, i.e., they fell off the mass fractionation line. The fact that the S isotope data show essentially the same pattern as the conventional geologic data of Cloud and Holland provides strong support for their O2 evolution hypothesis.

This does not mean that there was zero O2 in the Archean atmosphere, nor in the Hadean atmosphere. O2 should continually be formed in such atmospheres from reactions such as CO2 + hv ^ CO + O H2O + hv ^ H + OH O + OH ^ O2 + H

Here, 'hv' represents a UV photon capable of splitting CO2 or H2O. However, O2 should not have accumulated in the early atmosphere, as it would also have had photochemical sinks. In addition to CO2 and H2O, which are their major components, volcanic gases also contain smaller amounts of reduced gases, such as CO and H2. As pointed out in Section 8.1, James Walker showed that the H2 content ofthe early atmosphere would have been determined by the balance between outgassing from volcanoes and escape ofhydrogen to space. This concept of atmospheric redox balance has since been refined to include rainout of oxidized and reduced species that are soluble in water.3 A typical photochemical model calculation that includes these processes is shown in Figure 8.3. O2 is formed in the stratosphere by photolysis and it is consumed in the troposphere (the lower atmosphere) by reaction with H2, catalyzed by the byproducts of H2O photolysis. Such calculations predict that the O2 concentration near Earth's surface should have been of the order of 10-13 atm. Though finite, this is far too small to have any influence on biology or on prebiotic synthesis of organic compounds. Thus, both models and data suggest that atmospheric O2 concentrations were low throughout most of the Earth's early history.

At some time at or before 2.7 Ga, oxygen-pro ducing cyanobacte-ria evolved.3 The evidence for their appearance comes from complex organic compounds preserved in rocks: 2-alpha methyl hopanes, thought to come from the cell walls of cyanobacteria and steranes (derived from sterols) that are presumed to come from eukaryotes. Eukaryotes, which occupy the domain Eucharya on the rRNA tree, are organisms that have cell nuclei. The biosynthesis of sterols in eukaryotes requires free O2; hence, the presence ofsteranes in 2.7-Ga

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Volume Mixing Ratio

Figure 8.3. Vertical profiles of major atmospheric gases in a weakly reduced primitive atmosphere. (adapted from Science, Vol. 259, Kasting.4 Copyright (1993) with permission from Science).

rocks suggests that O2 was being produced at that time. Some early cyanobacteria probably lived together in microbial mats, later being preserved as laminated stromatolites in rocks. Other cyanobacteria may have lived in the surface ocean, especially in upwelling regions that were rich in nutrients like fixed nitrogen and phosphorus. Plumes of O2 from these local oxygen oases may have wafted into the otherwise anoxic Archean air, gradually being consumed, just as plumes of reduced pollutants are gradually consumed in today's oxidizing atmosphere. This "backwards" atmosphere persisted until about 2.4 Ga, at which time O2 became dominant and H2 and CH4 were relegated to less important roles.

(iii) Connection with the Glacial Record

One of the strongest arguments in support of the scenario described above is its success in explaining the glacial record. In the previous section, we mentioned that there were glaciations at 2.9 Ga and at ~2.4 Ga. The 2.4-Ga, or Paleoproterozoic, glaciation is the better documented of the two. Glacial diamictites (or tillites) from this period are found on at least 3 continents. In North America, they appear as part of the Huronian Formation in southern Canada, just north of Lake Huron. There are 3 diamictites in the Huronian (Fig. 8.4): the Ramsey Lake (lowermost), the Bruce (middle) and the Gowganda (uppermost). Below the Ramsey Lake diamictite in the underlying Matinenda Formation, one finds detrital uraninite (UO2) and pyrite (FeS2)—two reduced minerals that are evidence oflow oxygen. (The term "detrital" means that they were weathered out of their parent rock without dissolving. As these are reduced minerals, this could only have happened in the near-absence of O2.) Above the Gowganda diamictite, the Lorraine formation is a well-developed redbed. (A redbed contains oxidized iron in the form ofhematite, Fe2O3 and is therefore evidence of high O2.) So, it appears that this series ofglaciation occurred at the same time when

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