Evolution Of Atmospheric Oxygen Research Articles

Weathering of Granite Under an O2- and CO2-rich Atmosphere 2.45 Ga at Pronto, Ontario, Canada

The palaeosols at Pronto, Ontario, Canada, developed at ~2.45 Ga on an Archaean granite. Although they have been studied by several investigators (e.g. Gay and Grandstaff, 1980; Mossman and Farrow, 1992), debates have continued as to whether or not a major loss of Fe occurred during the soil formation. Loss or retention of Fe during weathering is crucial in models for the evolution of atmospheric oxygen. The ambiguity of the data on the Pronto palaeosols was partly due to a possibility that some rock samples collected by previous investigators from the upper parts of the palaeosol section might represent foreign soils. Therefore, we have collected a suite of rock samples, including the parental granite, palaeosols, and overlying sedimentary rocks, at Pronto, and conducted a detailed mineralogical and geochemical investigation on these samples to elucidate the O2 and CO2 contents of the 2.45 Ga atmosphere. We have examined the changes in rock textures, mineral textures, mineral abundances, analysed the contents of major elements and 40 trace elements (including REE) of the bulk rock samples, and also analysed the major and REE contents of selected minerals using an electron microprobe analyser (EPMA).

Evidence in Trace Elements and Fe3+/Fe2+ Ratios of Archaean and Early Proterozoic Shales for the Early Development of Oxic Atmosphere

A currently popular model for the evolution of atmospheric oxygen, termed the Cloud-WalkerKasting-Holland (C-W-K-H) model, postulates essential absence of free O2 in the atmosphere prior to 2.2 Ga, then a dramatic rise of p% from ~ 1% PAL (present atmospheric level) during the period between 2.2 and 1.9 Ga. Another major rise in pO2 to nearly the present level at -600 Ma has been suggested by some researchers. An implication of this theory is that episodic rises in O2 were major causes for the evolution of organisms, such as the appearance of eukaryotes at ~2.0 Ga and of metazoa and H2S-oxidizing bacteria at ~600 Ma. An alternative model for atmospheric evolution, termed the Dimroth-Ohmoto (D-O) model, postulates an essentially constant atmospheric pO2 level (within _+50% of PAL) since ~4 Ga. According to this model, the 02 requirement for all aerobic organisms (>10% PAL?) has been satisfied since the early stage of Earth's history, and the evolution of aerobic organisms was not caused by changes in the atmospheric pO2 level. Ohmoto (1997) has recently summarized some geochemical data and concepts that support the D-O model. They include: (1) isotopic (Pb, Sr, Nd, and HI) and trace element (REE, Nb/U) data on igneous and metamorphic rocks of Archaean ages that support the Armstrong's (1981) crustal growth model postulating an essentially constant volume for the continental crust since ~4.0 Ga; (2) isotopic (O, Nd and Sr) and elemental data on Banded Iron Formations (BIFs), suggesting that major BIFs were formed by submarine hydrothermal fluids locally discharged within anoxic basins surrounded by oxic oceans; (3) micro-scale variations in the sulphur isotopic compositions of pyrite crystals in 3.4-2.2 Ga shales, suggesting that they were formed by sulphate-reducing bacteria in sulphate-rich oceans; (4) textural and paragenetic data on detrital grains of uraninite and pyrite in quartz-pebble conglomerates of >2.2 Ga age, suggesting that these minerals were mostly diagenetic or hydrothermal, rather than detrital, in origin; (5) Fe3+/Ti and FeZ+/Ti ratios of pre-2.2 Ga palaeosols, suggesting soil formation under an oxic atmosphere; (6) carbon isotopic compositions of organic matter in 3.8-2.0 Ga sedimentary rocks, suggesting that temporal and spatial fluctuations in redox state of sedimentary basins, much like those occurring in modem Earth, were common during this period; and (7) ~a3C data on marine carbonates and TOC data on shales of various geologic ages, suggesting essentially constant rates of production and consumption of atmospheric oxygen since at least 3.5 Ga. Here I present other lines of supportive evidence for the D-O model, which come from our own chemical analyses of more than 300 samples of shales of 3.8-0.5 Ga in age.

Evidence in pre–2.2 Ga paleosols for the early evolution of atmospheric oxygen and terrestrial biota

The loss of Fe from some pre-2.2 Ga paleosols has been considered by previous investigators as the best evidence for a reduced atmosphere prior to 2.2 Ga. I have examined the behavior of Fe in both pre- and post-2.2 Ga paleosols from depth profiles of Fe3+/Ti, Fe2+/Ti, and sigma Fe/Ti ratios, and Fe3+/Ti vs. Fe2+/Ti plots. This new approach reveals a previously unrecognized history of paleosols. Essentially all paleosols, regardless of age, retain some characteristics of soils formed under an oxic atmosphere, such as increased Fe3+/Ti ratios from their parental rocks. The minimum oxygen pressure (PO2) for the 3.0-2.2 Ga atmosphere is calculated to be about 1.5% of the present atmospheric level, which is the same as that for the post-1.9 Ga atmosphere. The loss of sigma Fe, common in paleosol sections of all ages, was not due to a reducing atmosphere, but to reductive dissolution of ferric hydroxides formed under an oxic atmosphere. This reductive dissolution of ferric hydroxides occurred either (1) after soil formation by hydrothermal fluids or (2) during and/or after soil formation by organic acids generated from the decay of terrestrial organic matter. Terrestrial biomass on the early continents may have been more extensive than previously recognized.

Box models for the evolution of atmospheric oxygen: an update

A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O 2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disapperance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O 2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O 2 increase during the Middle and Late Proterozoic is identified as another outstanding question.

Box models for the evolution of atmospheric oxygen: an update

A simple 3-box model of the atmosphere/ocean system is used to describe the various stages in the evolution of atmospheric oxygen. In Stage I, which probably lasted until redbeds began to form about 2.0 Ga ago, the Earth's surface environment was generally devoid of free O 2, except possibly in localized regions of high productivity in the surface ocean. In Stage II, which may have lasted for less than 150 Ma, the atmosphere and surface ocean were oxidizing, while the deep ocean remained anoxic. In Stage III, which commenced with the disapperance of banded iron formations around 1.85 Ga ago and has lasted until the present, all three surface reservoirs contained appreciable amounts of free O 2. Recent and not-so-recent controversies regarding the abundance of oxygen in the Archean atmosphere are identified and discussed. The rate of O 2 increase during the Middle and Late Proterozoic is identified as another outstanding question.

Oxidant abundances in rainwater and the evolution of atmospheric oxygen.

A one-dimensional photochemical model has been used to estimate the flux of dissolved hydrogen peroxide (H2O2) and of other soluble species in rainwater as a function of atmospheric oxygen level. H2O2 should have replaced O2 as the dominant oxidant in rainwater at oxygen levels below 10(-3)-10(-2) times the present atmospheric level (PAL). The exact value of pO2 at which H2O2 becomes more important than O2 depends on the abundance of trace gases such as CO, CH4, and NO. H2O2 was probably an important oxidant even in an O2-free atmosphere, provided that CO2 levels were significant higher than today's. In model atmospheres containing free O2 the concentration of photochemically produced oxidants generally exceeds that of photochemically produced reductants. The oxidizing power of rainwater is therefore greater than that due to dissolved molecular O2 alone. The difference is small at present but becomes important at O2 levels less than 10(-3) PAL. At O2 levels between 10(-4) and 10(-5) PAL the oxidizing power of rainwater is almost independent of pO2. Precambrian soils in which a part or all of the Fe2+ in their source rocks has been oxidized to Fe3+ could therefore have developed in the presence of an atmosphere with very low values of pO2. On the other hand, the upper limit for pO2 during early and mid-Precambrian time suggested by the incomplete oxidation of FeO in soils developed on basaltic rocks is affected only slightly by the presence of photochemical products in rainwater.

A boundary condition on the evolution of atmospheric oxygen

The constancy of the 13C/12C ratio in marine carbonate over the last half-billion years provides evidence that an amount of organic carbon comparable to that presantly contained in the earth's sedimentary reservoir must have existed in early Paleozoic time. Since the amount of organic carbon is a direct measure of the net amount of oxygen produced by photosynthesis, this in turn implies no great change in available oxygen over that time period. Finally, unless the distribution of oxygen between the bound and molecular form has undergone a large change, the oxygen content of the atmosphere must have been comparable to its present value in early Paleozoic time.

A general theory of oxygen toxicity in man.

A GENERAL THEORY OF OXYGEN TOXICITY IN MAN* D. R. SHANKLIN, M.D.f Implicit in current biomedical thinking is the assumption that oxygen exchange occurs in a steady state ofthe external environment. While this maybe truefor practical purposes for the comparatively brief life span of any individual animal or person, it is demonstrably untrue for the panorama of evolution and particularly for those species recendy emergent. This hypothecationwillattempta fusion ofpresently available information on the evolution of atmospheric oxygen, human embryogenesis, and certain seeming paradoxes in a number ofanimal species, as viewed in the light ofour own researches [i]. The relation ofthese matters to adaptation to extrauterinelife in general, and to hyaline membrane disease in particular , is emphasized. In a series ofrecent papers, the late L. V. Berkner and Professor L. C. Marshall of the Southwest Center for Advanced Studies, Dallas, Texas, have formulated a definitive model ofthe oxygenic evolution of the atmosphere ofthis planet [2-6]. Since, as they ably point out, there is much evidence from the evolution of living organisms with implications for oxygenic evolution, and vice versa, it is seemly that we here abstract the Berkner-Marshall model with reference to some of the major sources of evidence and hypothecation. The solid primordial earth was probably without significant atmosphere . Principal among the evidences for this is the marked discordance between the low terrestrial concentrations and the much higher cosmic abundance ofthe rare gases [6-9]. These workers have concluded that the * This is the second part of a two-part paper, "On the Pulmonary Toxicity of Oxygen." Part r, "The relationship oftotal pressure and oxygen content to the effect ofoxygen on the lung," will bepublished in Laboratory Investigation [i], Supported by grants from the John A. Hartford Foundation , New York. t Professor ofobstetrics and gynecology and ofpathology, Chicago Lying-in Hospital, Pritzker School ofMedicine, University ofChicago. 80 D. R. Shanklin · Oxygen Toxicity in Man Perspectives in Biology and Medicine · Autumn 1969 earth lost the greater share ofthe gases during the agglomeration ofcomponent planetesimals [io]. Thus, Berkner and Marshall [3] agree with Fesenkov [11] that thepresent-day atmosphere isa secondary or evolutionary phenomenon. A confirmation ofthis exists in part in the calculation that the present-day supply ofAr*0 can be accounted for by its origin from K40 through radioactive decay over a period of approximately 5 X 10» years, a figure consistent with the age ofthe earth [12]. Examination ofthe physical chemistry ofthe complex compounds that make up a major share ofthe surface ofthe earth has led to the conclusion that the crust was not substantially molten at any one time [12-14]. Accordingly , large quantities ofgases are chemically bound in various ways. Growth ofthe initial atmosphere and oceans from volcanic gas discharge probably occurred over a very long period of time [8, 13-17]. Volcanic effluvient is predominandy water vapor, carbon dioxide, nitrogen, and sulfur dioxide, with lesser amounts of SO3, H3, Cl3, H3S, CO, HF, HBr, CH4, HCl, and NH3 [16]. Thus, the primitive atmosphere was without significant oxygen. This conclusion is confirmed by studies of the molecular evolution of life [18-27]. Since it is probable that photoexcitation played a major role in the synthesis ofearly compounds [28], a detailed consideration ofthe relation ofatmospheric components, ultraviolet radiation, and early cellular components was undertaken by Berkner and Marshall [2, 3]. Indeed, the absence ofoxygen in a free state is probably a necessity for the evolution ofliving things asweknowthem. Abelsonhas shown thatamino acids are stable at moderate temperature in the absence ofoxygen [29]; further, oxygen is a poison to nuclei ofpresent-day cells [27]. Since there is much oxygen in the atmosphere today, the general principle of uniformitarianism as applied to the atmosphere must be abandoned in favor ofan evolutionary view ofthe problem. This is supplied by the Berkner-Marshall model. The details ofultraviolet absorption in relation to oxygen accumulation and the evolution ofliving things are too intricate to relate here comprehensively . The conclusions ofBerkner and Marshall with reference to electromagnetic absorption are important to an understanding of oxygenic evolution. The fact thatonly oxygen and ozone significandy absorb radiation with wavelengths above 2,000 Â is ofgreat importance. Photochemical dissociation of water occurs principally in the...