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A COUPLED ATMOSPHERE-ECOSYSTEM MODEL OF THE EARLY ARCHEAN BIOSPHERE
ABSTRACT
A coupled atmosphere-ecosystem model has been developed to simulate the early Archean biosphere. The model incorporates kinetic and nutrient limits on biological productivity, along with constraints imposed by metabolic thermodynamics. We have used this model to predict the biogenic CH4 flux and net primary productivity (NPP) of the marine biosphere prior to the advent of oxygenic photosynthesis. Organisms considered include chemotrophic and organotrophic methanogens, H2-, H2S-, and Feusing anoxygenic phototrophs, S-reducing bacteria, CO-using acetogens, and fermentative bacteria.
CH4 production and NPP in our model are limited by the downward flux of H2, CO, S8, and H2S through the atmosphere-ocean interface and by the upwelling rate of Fe2+ from the deep oceans. For reasonable estimates of the supply of these compounds, we find that the biogenic CH4 flux should have ranged from ~⅓ to 2.5 times the modern CH4 flux. In the anoxic Archean atmosphere, this would have produced CH4 concentrations of 100 ppmv to as much as 35,000 ppmv (3.5%), depending on the rate at which hydrogen escaped to space. Recent calculations indicating that hydrogen escape was slow favor the higher CH4 concentrations. Calculated NPP is lower than in the modern oceans by a factor of at least 40. In our model, metabolism based on H2 and Fe is about equally productive, with S-based metabolism being considerably less productive. Internal recycling of sulfur within the surface ocean, neglected here, could conceivably raise rates of sulfur metabolism to much higher values.
Although explicit calculations of the methane greenhouse effect are not performed here, our model results are consistent with the idea that the early Archean mean surface temperature could have been very high, perhaps as high as the 55-85oC estimate obtained from oxygen isotopes in 3.3-Ga cherts. The climate could have been particularly warm if methanogens evolved before anoxygenic phototrophs, as this would have maximized the ratio of CH4 production to organic carbon burial. CH4 concentrations and surface temperatures should have declined once phototrophs evolved because increased primary productivity and organic carbon burial would have drawn down total atmospheric hydrogen mixing ratios. CH4 concentrations may have increased again (and the climate warmed) in the late Archean following the origin of oxygenic photosynthesis because primary productivity would no longer have been constrained by the supply of reductants.
A better understanding of the geologic record is needed to test these climate scenarios.
TABLE OF CONTENTS
List of Figures………………………………………………………………………………………………………. v
List of Tables………………………………………………………………………………………………………. vi
Preface……………………………………………………………………………………………………………….. vii
Acknowledgements…………………………………………………………………………………………….. viii
Dedication…………………………………………………………………………………………………………… ix
Chapter 1. INTRODUCTION………………………………………………………….. ..1
Chapter 2. NATURE OF THE ARCHEAN BIOSPHERE…………………………….. . 3
Limitations on primary productivity in the modern and Archean biospheres……… ..4 Anaerobic microbial ecosystems on the Archean Earth……………………………. ..6
H2– and CO-based metabolism…………………………………………………. ..7 Sulfur-based metabolism ………………………………………………………. 10
Iron-based metabolism ………………………………………………………… 11
Chapter 3. MODEL DESCRIPTION………………………………………………….. 13 Ecosystem model…………………………………………………………………… 13
Atmosphere model……………………………………………………………………. 15
The atmospheric hydrogen budget ……………………………………………… 16
Coupled atmosphere-ecosystem model…………………………………………….. 19
Chapter 4. RESULTS………………………………………………………………….. 23
Case 1: Methanogen-based ecosystem….………………………………………….. 23 Case 2: Methanogen-acetogen ecosystem …..……………………………………….. 26 Case 3: Anoxygenic phototroph-acetogen ecosystem……………………………… 29 Case 4: Sulfur-based ecosystem……………………………………………………. 35 Case 5: Iron-based ecosystem………………………………………………………. 37
Distribution of Archean marine primary producers………………………………… 38
Chapter 5. DISCUSSION……………………………………………………………… 39
H2 escape rates and implications for Archean climate…………………………….. 39 Effects of solar UV radiation on ecosystem productivity…………………………. 41 Constraints imposed by the carbon isotope record…………………………………. 43
Changes induced by the advent of oxygenic photosynthesis………………………. 47
Chapter 6. CONCLUSIONS……………………………………………………………. 48
Appendix A: Calculating dissolved H2 and CO using free energy constraints…………49
Appendix B: Abiotic uptake of atmospheric CO by the ocean………………………… 51
References……………………………………………………………………………… 55
Chapter 1. Introduction
Previous studies by our group (Kasting et al., 1983; Pavlov et al., 2000, 2001) and by others (Zahnle, 1986; Kiehl and Dickinson, 1987; Catling et al., 2001) have explored the photochemistry of methane in an anoxic early Earth atmosphere and have examined its effect on climate and on redox evolution of the crust. Two other studies (Kral et al., 1998; Kasting et al., 2001) have looked at the coupling between the Archean atmosphere and a hypothetical methanogenic ecosystem, but only in a pure thermodynamic sense and only in isolation from other likely components of an anaerobic Archean biosphere. Here, we present a more detailed analysis of the Archean ecosystem in which we estimate the relative productivity of H2-, S-, and Fe-based metabolism based on both kinetic and thermodynamic constraints. We are interested mostly in methane because of its importance to climate and its possible significance as a biomarker on extrasolar planets. However, we hope that our model will also elucidate the relative importance of different metabolisms, and in doing so shed light on the general pattern of biological/ecological evolution during the early stages of Earth history.
According to standard solar evolution models (e.g., Gough, 1981), the Sun was considerably dimmer in the past—a change that is best countered by an increased greenhouse effect in Earth’s atmosphere. Besides CO2 and H2O, the favored greenhouse gas is CH4 (Kiehl and Dickinson, 1987; Pavlov et al., 2000). On the present Earth, biotic sources for CH4 outweigh abiotic ones. The ratio of biotic to abiotic CH4 was estimated to be ~300 (Kasting and Catling, 2003), based on an extrapolation of measurements of dissolved CH4 in hydrothermal vent fluids emanating from the Lost City vent field (Kelley et al., 2001). New measurements (Kelley et al., 2005) indicate that dissolved CH4 concentrations at Lost City are higher than first thought by about a factor of 10; hence, the ratio of biotic to abiotic CH4 may only be ~30. Biotic production of CH4 probably outweighed abiotic production on the early Earth as well. Here, we estimate the global biotic production rate of CH4 during the early- to mid-Archean (~3.8-3.0 Ga), before the advent of oxygenic photosynthesis. We consider the identification of cyanobacterial and eukaryotic organic biomarkers in 2.7-Ga sediments by Brocks et al. (1999) as the earliest convincing evidence for oxygenic photosynthesis. Rosing and Frei (2003) have suggested that oxygenic photosynthesis evolved long before this, by 3.7 Ga or earlier, based on an analysis of lead isotopes, but we do not consider their conclusion to be definitive. Later on in the Archean, after the origin of oxygenic photosynthesis, CH4 production rates may have increased substantially as a consequence of increased production of organic matter (Catling et al., 2001). This only strengthens the conclusion reached here that methane was abundant enough to exert a major effect on Archean atmospheric chemistry and climate.
The primary goal of this study is thus to estimate the concentration of biogenic CH4 in the Archean atmosphere. A secondary goal is to assess the supply of nutrients to the global biota and to estimate global primary productivity. In the modern biosphere, primary productivity is limited mainly by the availability of fixed nitrogen (N), phosphate (P), and iron (see, e.g., Tyrrell, 1999). However, before the advent of oxygenic photosynthesis, the main limitation on productivity was probably the availability of electron donors such as H2, CO, H2S, and dissolved Fe2+ (Walker, 1977; DesMarais, 1998). In the weakly reducing Archean atmosphere, the three reduced gases would have had long atmospheric lifetimes and could have accumulated to substantial levels (Walker, 1977; Pavlov et al., 2001). Their transfer rates to the surface ocean, or to soils, would have been limited by diffusion and can thus be estimated quantitatively. Ferrous iron (Fe2+) was abundant in the deep ocean (Holland, 1984) and would have been supplied to the surface biosphere by upwelling at rates that can also be estimated quantitatively. These kinetic constraints are modeled explicitly in the present study.
Determining the concentrations of biogenic gases in the Archean atmosphere could provide useful information for the search for extraterrestrial life. Within the next 10-15 years, NASA’s two planned Terrestrial Planet Finder (TPF) missions will attempt to detect possible biosignatures in the atmospheres of Earth-like extrasolar planets. Methane is one of the potential biosignature gases in such atmospheres (Schindler and Kasting, 2000). An issue that is relevant for TPF is how much methane should be present on an inhabited planet compared to an uninhabited one. The present study helps shed light on this question.
A COUPLED ATMOSPHERE-ECOSYSTEM MODEL OF THE EARLY ARCHEAN BIOSPHERE
