Alexander Anatolevich Pavlov

Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia

Simple calculations show that if deep-water H2S concentrations increased beyond a critical threshold during oceanic anoxic intervals of Earth history, the chemocline separating sulfidic deep waters from oxygenated surface waters could have risen abruptly to the ocean surface (a chemocline upward excursion). Atmospheric photochemical modeling indicates that resulting fluxes of H 2S to the atmosphere (.2000 times the small modern flux from volcanoes) would likely have led to toxic levels of H 2S in the atmosphere. Moreover, the ozone shield would have been destroyed, and methane levels would have risen to .100 ppm. We thus propose (1) chemocline upward excursion as a kill mechanism during the end-Permian, Late Devonian, and Cenomanian‐Turonian extinctions, and (2) persistently high atmospheric H2S levels as a factor that impeded evolution of eukaryotic life on land during the Proterozoic.

Greenhouse warming by CH4 in the atmosphere of early Earth

Earth appears to have been warm during its early history despite the faintness of the young Sun. Greenhouse warming by gaseous CO2 and H2O by itself is in conflict with constraints on atmospheric CO2 levels derived from paleosols for early Earth. Here we explore whether greenhouse warming by methane could have been important. We find that a CH4 mixing ratio of 10(-4) (100 ppmv) or more in Earths early atmosphere would provide agreement with the paleosol data from 2.8 Ga. Such a CH4 concentration could have been readily maintained by methanogenic bacteria, which are thought to have been an important component of the biota at that time. Elimination of the methane component of the greenhouse by oxidation of the atmosphere at about 2.3-2.4 Ga could have triggered the Earths first widespread glaciation.

New insights into Archean sulfur cycle from mass-independent sulfur isotope records from the Hamersley Basin, Australia

We have measured multiple sulfur isotope ratios ( 34 S/ 33 S/ 32 S) for sulfide sulfur in shale and carbonate lithofacies from the Hamersley Basin, Western Australia. The v 33 S values (v 33 SWN 33 S30.515UN 34 S) shift from 31.9 to +6.9x over a 22-m core section of the lower Mount McRae Shale (V2.5 Ga). Likewise, sulfide sulfur analyses of the Jeerinah Formation (V2.7 Ga) yield v 33 S values of 30.1 to +8.1x over a 50-m section of core. Despite wide variations in v 33 S and N 34 S, these two shale units yield a similar positive correlation between v 33 S and N 34 S. In contrast, pyrite sulfur analyses of the Carawine Dolomite (V2.6 Ga) yield a broad range in N 34 S (+3.2 to +16.2x) but a relatively small variation and negative values in v 33 S( 32.5 to 31.1x). The stratigraphic distribution of N 33 S, N 34 S, and v 33 S in Western Australia allows us to speculate on the sulfur isotopic composition of Archean sulfur reservoirs and to trace pathways in the Archean sulfur cycle. Our data are explained by a combination of massindependent fractionation (MIF) in the atmosphere and biological mass-dependent fractionation in the ocean. In the Archean, volcanic, sulfur-bearing gas species were photolysed by solar ultraviolet (UV) radiation in an oxygen-free atmosphere, resulting in MIF of sulfur isotopes. Aerosols of S8 (with v 33 Ss 0) and sulfuric acid (with v 33 S6 0) formed from the products of UV photolysis and carried mass-independently fractionated sulfur into the hydrosphere. The signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments. Pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass-dependent fractionation on mass-independent atmospheric effects. Multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur-metabolizing organisms. A 2003 Elsevier Science B.V. All rights reserved.

Methane-rich Proterozoic atmosphere?

Methane mixing ratios of 100–300 ppm in the Proterozoic atmosphere (0.75–2.3 Ga) would have been sufficient to offset the climatic effects of the faint early sun and maintain the warm climate during those ∼1.5 b.y. The major argument against this type of the atmosphere is the short atmospheric oxidation time of methane after the first oxygenation event ca. 2.3 Ga. Here we argue that the net methane flux from the oxygen-poor Pro tero zo ic ocean could have been 10–20 times higher than the present total biological methane flux. We demonstrate that increased methane production would have been sufficient to maintain methane concentrations at 100–300 ppm, which would keep the surface warm throughout the Proterozoic without invoking high CO 2 levels (although the CO 2 abundance could have been higher as well). A second oxygenation event at the end of the Proterozoic would have resulted in a decrease of methane flux and could have caused the first Neoproterozoic “snowball” glaciation.

TRANSONIC HYDRODYNAMIC ESCAPE OF HYDROGEN FROM EXTRASOLAR PLANETARY ATMOSPHERES

Hydrodynamic escape is an important process in the formation and evolution of planetary atmospheres. Transonic steady state solutions of the time-independent hydrodynamic equations are difficult to find because of the existence of a singularity point. A numerical model is developed to study the hydrodynamic escape of neutral gas from planetary atmospheres by solving the time-dependent hydrodynamic equations. The model is validated against an analytical solution of the escape from an isothermal atmosphere. The model uses a two-dimensional energy deposition calculation instead of the single-layer heating assumption, which is not sufficiently accurate for hydrodynamic escapefrom ahydrogen-richplanetaryatmosphere.Whenapplied totheatmospheresofextrasolar planets, themodel results are in good agreement with observations of the transiting extrasolar planet HD 209458b. The model predicts that hydrogen is escaping from HD 209458b at a maximum rate of 6 ; 10 10 gs � 1 . The extrasolar planet is stable under the hydrodynamic escape of hydrogen. The rate of hydrogen hydrodynamic escape from other possible extrasolar planets is investigated using the model. The importance of hydrogen hydrodynamic escape for the long-term evolution of extrasolar planets is discussed. Simulation shows that through hydrodynamic escape of hydrogen, a planet at the orbit of Mercury (0.4 AU) and with 0.5 Uranus mass can lose about 10% of its mass within 850 million yr if the solar EUV radiation is 10 times the present level. This calculation provides an indication of how Mercury may have evolved during the early days of the solar system. Subject heading gs: planetary systems — planets and satellites: general

UV shielding of NH3 and O2 by organic hazes in the Archean atmosphere

The late Archean atmosphere was probably rich in biologically generated CH4 and may well have contained a hydrocarbon haze layer similar to that observed today on Saturns moon, Titan. Here we present a detailed model of the photochemistry of haze formation in the early atmosphere, and we examine the effects of such a haze layer on climate and ultraviolet radiation. We show that the thickness of the haze layer was limited by a negative feedback loop: A haze optical depth of more than ∼0.5 in the visible would have produced a strong “antigreenhouse effect,” thereby cooling the surface and slowing the rate at which CH4 was produced. Given this climatic constraint on its visible optical depth, the amount of UV shielding provided by the haze can be estimated from knowledge of the optical properties and size distribution of the haze particles. Contrary to previous studies [Sagan and Chyba, 1997], we find that when the finite size of the particles is taken into account, the amount of UV shielding provided by the haze is small. Thus NH3 should have been rapidly photolyzed and should not have been sufficiently abundant to augment the atmospheric greenhouse effect. We also examine the question of whether photosynthetically generated O2 could have accumulated beneath the haze layer. For the model parameters considered here, the answer is “no”: The upper limit on ground level O2 concentrations is ∼10−6 atm, and a more realistic estimate for pO2 during the late Archean is 10−8 atm. The stability of both O2 and NH3 is sensitive to the size distribution and optical properties of the haze particles, neither of which is well known. Further theoretical and laboratory work is needed to address these uncertainties.

Organic haze on Titan and the early Earth

Recent exploration by the Cassini/Huygens mission has stimulated a great deal of interest in Saturns moon, Titan. One of Titans most captivating features is the thick organic haze layer surrounding the moon, believed to be formed from photochemistry high in the CH4/N2 atmosphere. It has been suggested that a similar haze layer may have formed on the early Earth. Here we report laboratory experiments that demonstrate the properties of haze likely to form through photochemistry on Titan and early Earth. We have used a deuterium lamp to initiate particle production in these simulated atmospheres from UV photolysis. Using a unique analysis technique, the aerosol mass spectrometer, we have studied the chemical composition, size, and shape of the particles produced as a function of initial trace gas composition. Our results show that the aerosols produced in the laboratory can serve as analogs for the observed haze in Titans atmosphere. Experiments performed under possible conditions for early Earth suggest a significant optical depth of haze may have dominated the early Earths atmosphere. Aerosol size measurements are presented, and implications for the haze layer properties are discussed. We estimate that aerosol production on the early Earth may have been on the order of 1014 g·year−1 and thus could have served as a primary source of organic material to the surface.

Organic haze in Earth's early atmosphere: Source of low-13C Late Archean kerogens?

High concentrations of greenhouse gases would have been required to offset low solar luminosity early in Earth9s history. Enhanced CO 2 levels are probably at least part of the solution, but CH 4 may have played a significant role as well, particularly during the Late Archean era, 2.5–3.0 Ga, when methanogenic bacteria were almost certainly present. Indeed, biological CH 4 production should have led to CO 2 drawdown by way of a negative feedback loop involving the carbonate-silicate geochemical cycle. We suggest here that the atmospheric CH 4 /CO 2 ratio approached the value of ∼1 needed to trigger formation of Titan-like organic haze. This haze was strongly depleted in 13 C relative to 12 C and was produced at a rate comparable to the modern rate of organic carbon burial in marine sediments. Therefore, it could provide a novel explanation for the presence of extremely low- 13 C kerogens in Late Archean sediments.

A coupled ecosystem-climate model for predicting the methane concentration in the Archean atmosphere.

A simple coupled ecosystem-climate model is described that canpredict levels of atmospheric CH4, CO2, and H2during the Late Archean, given observed constraints on Earthssurface temperature. We find that methanogenic bacteria shouldhave converted most of the available atmospheric H2 intoCH4, and that CH4 may have been equal in importance to CO2 as a greenhouse gas. Photolysis of this CH4 may have produced a hydrocarbon smog layer that would have shielded the surface from solar UV radiation. Methanotrophic bacteria would have consumed some of the atmospheric CH4,but they would have been incapable of reducing CH4 to modern levels. The rise of O2 around 2.3 Ga would have drastically reduced the atmospheric CH4 concentrationand may thereby have triggered the Huronian glaciation.

Passing through a giant molecular cloud: “Snowball” glaciations produced by interstellar dust

[1]xa0In its motion through the Milky Way galaxy, the solar system encounters an average -density (≥330 H atoms cm−3) giant molecular cloud (GMC) approximately every 108 years, a dense (∼2 × 103 H atoms cm−3) GMC every ∼109 years and will inevitably encounter them in the future [Talbot and Newman, 1977]. However, there have been no studies linking such events with severe (snowball) glaciations in Earth history. Here we show that dramatic climate change can be caused by interstellar dust accumulating in Earths atmosphere during the solar systems immersion into a dense (∼2 × 103 H atoms cm−3) GMC. The stratospheric dust layer from such interstellar particles could provide enough radiative forcing to trigger the runaway ice-albedo feedback that results in global snowball glaciations. We also demonstrate that more frequent collisions with less dense GMCs could cause moderate ice ages.