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Chairman's Summary Remarks: Discussion Proceedings of the NATIONAL ACADEMY OF SCIENCES Volume 53 * Number 6 * June 15, 1965 SYMPOSIUM ON THE EVOLUTION OF THE EARTH'S ATMOSPHERE BY INVITATION OF THE COMMITTEE ON ARRANGEMENTS FOR THE ANNUAL MEETING Presented before the Academy April 26, 1966 CONTENTS CHAIRMAN'S SUMMARY REMARKS. Preston E. Cloud, Jr. 1169 THE HISTORY OF OCEAN WATER AND ITS EFFECT ON THE CHEMISTRY OF THE ATMOSPHERE. Heinrich D. Holland 1173 BIOCHEMICAL, BIOLOGICAL, AND ATMOSPHERIC EVOLUTION. .Barry Commoner 1183 GEOCHEMICAL ASPECTS OF ATMOSPHERIC EVOLUTION. Charles F. Davidson 1194 FOSSILS, EARLY LIFE, AND ATMOSPHERIC HISTORY. Alfred G. Fischer 1205 HISTORY OF MAJOR ATMOSPHERIC COMPONENTS. .L. V. Berkner and L. C. Marshall 1215 CHAIRMAN'S SUMMARY REMARKS* BY PRESTON E. CLOUD, JR. UNIVERSITY OF MINNESOTA The papers introduced by this note are themselves summaries of fields and view- points related to the question of atmospheric history, now being attacked from different quarters. They comprise a symposium on the subject, held at the spring meeting of the Academy on April 26, 1965. This summary cf summaries is at- tempted primarily in the hope that it may bring out for those unable to be present for the oral presentation the high degree of interrelatedness between papers from different disciplines which was such a striking feature of this symposium. It seems desirable also at least to state the gist of H. C. Urey's introductory paper on the "Nature and Source of the Primary Atmosphere," which set some boundary condi- tions on the problem, but which, according to Dr. Urey's preference, is not here published, because it was primarily a condensation of a previous paper by him.1 In order to discuss the problem of the atmosphere, Urey points out, we must have some idea as to how the earth originated. The presently favored model calls upon the cold aggregation of discrete particles. Volatiles, including H20, and solubles would be trapped within the accumulating rubble. Subsequent degassing would give rise to H, CH4, C02, N, NH3, etc. The early oceans and atmosphere were es- sentially devoid of oxygen. When and how did oxygen first appear and subse- 1169 Downloaded by guest on October 1, 2021 1170 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. quently build up in the atmosphere? Small quantities of oxygen have probably been available from the beginning as a result of photolytic dissociations in the outer atmosphere, but this is a self-limiting process. Anaerobic life, however, could evolve in the absence of 2. This provides the foundation from which organisms capable of the photosynthetic release of oxygen arose. Photosynthesis eventually produced an oxygenous atmosphere, building up to present levels. In the early stages, the opacity of water to UV radiation would offer sufficient shielding for aquatic forms of life to develop within the photic zone. H. D. Holland's contribution on "The History of Ocean Water and Its Effect on the Chemistry of the Atmosphere" deals with recent progress toward understanding the factors that control the composition of the seawater-atmosphere system. Rubey's paper of 19512 is the point of departure. Some of Rubey's conclusions, however, have to be modified in the light of Sill6n's paper of 1961,3 in which that author showed that the pH of sea water is probably buffered more effectively by silicate mineral systems than by carbonate mineral systems. The oceans today are reservoirs through which elements are in transit. For all cations except sodium, residence time appears to be less than 20 million years, and even sodium probably has a residence time that is less than one tenth the age of the earth. Dr. Holland recognizes three classes of effects that control the major element composition of the ocean-atmosphere system: (1) Degassing of the earth-e.g., the concentration of chloride ion in the sea and the nitrogen and rare gas content of the atmosphere. (2) Effects of mineral equilibria-e.g., major cation ratios, pH of sea water, and CO2 content of the atmosphere. (3) Biological reactions-e.g., the 02 content of the atmosphere and the sulfate content of sea water. The apparent equilibration of ocean water with sediments, and the large size of the chloride reservoir in the oceans, seem to eliminate more than minor fluctuations in the major element composition of ocean water and the CO2 pressure of the atmosphere during intervals of the order of 100 million years. Perhaps the greatest contribu- tion of this stability to atmospheric evolution has been to provide a base for bio- chemical reactions believed responsible for the build-up of oxygen to present atmospheric levels. The oxygen content of the atmosphere at any time is a prod- uct of the balance of 02 production via photosynthesis and its consumption in organic decay, oxidation of volcanic gases, and oxidation of surface rocks. Barry Commoner ("Biochemical, Biological, and Atmospheric Evolution") em- phasizes the principle that features of modern organisms which are both basic and very widespread tend to be primitive. On this and other grounds, he supposes that the first organisms were probably anaerobic heterotrophs, deriving energy from the partial oxidation of geochemically produced organic substrates. These gave rise, eventually, to organisms in which light-absorption enhanced anaerobic exergonic metabolism, and later to photosynthesizers, capable of carbon assimilation. Primi- tive, anaerobic photosynthesizers are the likely ancestors of organisms capable of the photolysis of water with release of molecular 02. These, in turn, were the major source of oxygen to the then essentially oxygen-free atmosphere, culminating in organisms capable of the aerobic oxidation of organic substrates. The primitive aerobes established the metabolic conditions for the terrestrial and aquatic organ- isms which dominate the modern biosphere. A sizable portion of Dr. Commoner's Downloaded by guest on October 1, 2021 VOL. 53, 1965 N.A.S. SYMPOSIUM: P. E. CLOUD, JR. 1171 paper is also devoted to the question of the origin of life, and in what sequence the capabilities leading to the first organism may have arisen. C. F. Davidson's contribution on "Geochemical Aspects of Atmospheric Evolu- tion" deals with the evidence of the rocks themselves-the Pre-Cambrian strati- graphic and geochemical record. He pays particular attention to the occurrence of alleged detrital uraninite and pyrite (readily oxidized) in certain basal conglomer- ates, and to iron ores, dolomites, red-beds, and sedimentary phosphorites. Profes- sor Davidson gives geochemical and sedimentary reasons for regarding the uraninite and pyrite in question as not detrital, but introduced from later hydrothermal waters by leaching and transport from overlying volcanic rocks and other sources. The dolomites are regarded as epigenetic and having no bearing on the question of the CO2 content of the contemporary atmosphere. Red-beds (continental), Davidson argues, appear to be confined mainly to the younger Pre-Cambrian because the older rocks are generally at a higher grade of metamorphism; in them original red pig- ments have disappeared, forming chlorite. Sedimentary phosphorites older than 2330-2500 million years (K-Ar date of metamorphism) are believed by Davidson to be related to a delicate CO2 balance at levels comparable to those of the present day. Such reasoning leads Dr. Davidson to warn that metasomatic changes subsequent to deposition may have biased the evidence of the rocks. M\Jany of the early strata on which geochemical interpretations have been based may not be isochemical with strata originally deposited, but may have been profoundly modified during post- depositional history. As far back as he looks, he sees no convincing evidence that the atmosphere ever differed greatly from that of the present. A. G. Fischer deals with "Fossils, Early Life, and Atmospheric History." Dr. Fischer proposes a model in which both Protozoa and M\etazoa evolved in complete respiratory dependence on host plants, far back into the Pre-Cambrian. Dependence on local sources of oxygen in the hydrosphere is visualized as an isolating mechanism which brought about an extremely diverse evolution of animal types of phylum grade-all small, naked, and soft. Oxygen is considered to be built up mainly within the atmosphere, oxygenation of the oceans-at-large being brought about from the atmosphere. When this happened, the previously isolated communities of organisms were allowed to spread, intermingle, and compete. Development of calcareous exoskeletons in the early Paleozoic is interpreted as probably related to protection from radiation and dessication. A corollary of this hypothesis, of course, is that we should eventually find records of these developments in the Pre-Cambrian sediments. The final paper, on "History of -Major Atmospheric Components," by L. V. Berkner and L. C. Marshall, was presented by Dr. Berkner. They postulate as an initial premise that the earth accumulated without an external primordial atmos- phere. Those atmospheric constituents which were not bound chemically in solids of the planetesimal particles were lost from their low gravitational fields before agglomeration of the earth. Following Rubey and others, the present atmosphere of the earth is considered to be of secondary origin. The significant juvenile source of atmospheric gases, excepting only oxygen, is volcanic. All free atmos- pheric 02 must come from H20 by photodissociation or photosynthesis. Computations by Berkner and Marshall of the "Urey effect" lead to the conclu- sion that 02 generated by photodissociation in the primitive atmosphere would be Downloaded by guest on October 1, 2021 1172 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. self-regulated at less than 0.1 per cent of present atmospheric level-insufficient for oxidative metabolism. Other factors related to the generation of atomic oxygen and ozone in the primitive and intermediate atmosphere, however, indicate that these substances will tend to be generated in a thin layer near the earth's surface.
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