Geochemiical Aspects of Atmospheric Evolution by Charles F

1194 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. DR. COMMONER: Stereospecificity must have originated very early; various possible mechanisms which might either select or generate organic compounds with right-left specificity have been proposed, but in the absence of an established mechanism it is difficult to be at all precise about the time. DR. BRITTON CHANCE (University of Pennsylvania): I understand that the key to your hypothesis is that the modern catalyst developed last. I would like to ask what type of catalyst you imagine to operate in the primitive anaerobic metabolism which is considered as the first step. It seems to me that the development of catalysts at this point would be essential. DR. COMMONER: Perhaps I might restate the point made in my paper as follows: In the modern catalyst the active site represents an arrangement of only a few amino acid residues. M\Iy proposal was that, nonspecific protein synthesis being already present, there might appear, with some reasonable probability, among the numerous proteins produced, a few with the surface configuration of amino acids required for a particular catalytic effect. It is also possible that the probability of achieving the necessary amino acid arrangement might be enhanced by coordination with a metal. Hence, catalytically active proteins might appear although a mechanism for ensuring a highly specific amino acid sequence was absent. In effect, I suggest that catalytic activity of a protein is not necessarily dependent on its total amino acid sequence, and that catalytic activity is an earlier attribute of proteins than precise amino acid sequence. DR. CLOUD: I am afraid there is not time for further discussion of this paper. GEOCHEMIICAL ASPECTS OF ATMOSPHERIC EVOLUTION BY CHARLES F. DAVIDSON UNIVERSITY OF ST. ANDREWS, SCOTLAND One of the major problems of geochemistry relates to the composition of ancient sedimentary rocks. It can be summarized in a single simple question. Are these early strata isochemical with the sediments originally deposited, or have they sometimes been profoundly modified by additions and abstractions of chemical elements during their postdepositional history? In the field of economic geology this issue is the basis of the long-continued controversies between syngeneticists and epigeneticists on the origin of strata-bound sulfide ore-bodies. The same arguments arise about the genesis of most deposits of dolomite, many kinds of iron ore, and stratiform occurrences of barites, celestine, and other minerals. Lately, a new aspect of this old dispute has become prominent with, on one side, the isochemical school which seeks to interpret the abnormal composition of some ancient sediments in terms of their deposition under an oxygen-free atmosphere, and on the other, the actualistic school which finds a uniformitarian explanation of com- positional abnormalities in the chemical effects of intrastratal waters, proof of extraordinary atmospheres being denied. Recent discoveries of unusual potassium-rich shales in Scotland and in Arizona present fresh grounds for discussions of this kind. In the Lower Cambrian fucoid Downloaded by guest on October 2, 2021 VOL. 53, 1965 N. A. S. SYMPOSIUM: C. F. DAVIDSON 1195 beds of the North-West Highlands of Scotland, a radiometric survey by Dawson of the Geological Survey of Great Britain has recently demonstrated the existence of a potassic shale horizon 10-15-ft thick carrying 10-12 per cent K20, underlain by a dolomitic shale 25-30-ft thick with 8.5 per cent K20, these strata having a strike- length of over 100 miles. The potassium is present as minute (<0.005 mm) crystals of adularia feldspar, almost free from sodium.' Simultaneously, Granger and Raup2 of the U.S. Geological Survey have shown that a 120-ft-thick siltstone horizon of the Upper Pre-Cambrian Dripping Spring quartzite of Arizona carries con- sistently, at widely separated localities, from 10.4 to 14.6 per cent K20, between 60 and 90 per cent of the rock being formed of sodium-poor adularia (Na2O = 0.18%). Other Proterozoic shales of similar chemistry have been described by Vaes3 from the copper mines of Katanga; and the Lower Cambrian siltstones of Georgia, at one time quarried near Cartersville as a source of potassium, are ap- parently of a comparable character.4 Less spectacular "authigenic" concentra- tions of adularia are on record from the Beltian dolomites of Montana and from Ordovician strata in Minnesota.5 There is no volcanicity associated with these adularia shales, and in most developments the strata would be classified by a hard-rock geologist as unmetamor- phosed. It is beyond belief that such highly feldspathic sediments (120 ft thick in Arizona) could have accumulated by alluvial processes-a mode of origin also ruled out by their soda content and their very fine grain size. Equally incredible is the concept that they were formed by diagenesis under a potassic sea. In each known occurrence, however, they either underlie or are interbedded with a succession of dolomites; and this distribution pattern lends weight to the hypothesis that they originated by reaction between some common sediment and potassium-rich fractionates of intrastratal brines, possibly derived from the lixiviation of overlying evaporites which have not survived. The dolomitization may be attributed to the magnesian content of the same ground waters. In the context of the present paper, the adularia shales are relevant in that they give support to the supposition that widespread horizons of abnormal "sedimentary" rocks, unparalleled among modern sediments, may originate from postdepositional metasomatic changes involving ground waters. The anactualistic facies found ini old sedimentary sequences which have been claimed as evidence of former atmospheres-in particular, uraninite-bearing pyritic conglomerates, taconite-type iron ores, and extensive developments of dolomite-all seem capable of explanation in much the same manner. The ancient stratigraphical successions within which these rocks occur include those of southern Africa, possibly dating from as early as 3,400 million years B.P.; the Krivoi Rog series of the Ukraine, ranging upwards from 2,600 million years; the Huronian formation of Ontario, deposited more than 2,100 million years ago; and the Proterozoic strata of southern Karelia, from 2,000 million years onward. In the following pages the evidence relating to each no- tional anactualistic facies is reviewed, and some observations are added on red-beds and phosphorites which are also relevant in considering atmospheric evolution. The Evidence of Banket Conglomerates.-The main geochemical argument sup- porting the concept of an anoxygenic Pre-Cambrian atmosphere rests on an in- terpretation of the manner of mineralization of the uraninite-bearing pyritic quartz- pebble conglomerates known as bankets, which are found in each of the ancient Downloaded by guest on October 2, 2021 1196 EVOLUTION OF THE EARTH'S ATMOSPHERE PROC. N. A. S. stratigraphical sequences mentioned above and in the undated Lower Proterozoic strata of Brazil. As is well known, these bankets form the gold-uranium ores of the Witwatersrand and of Serra de Jacobina, and the uranium ores (free from re- coverable gold) of Blind River and of southern Karelia. In most instances, the mineralization is found in or close to basal pebble-beds, overlying major unconform- ities and bottoming thick sequences of molasse-type sediments. According to syngeneticists, the bankets are isochemical with the pebble-beds as originally deposited, and much of the pyrite and uraninite still exhibits a pristine detritat form, though all the gold has been recrystallized. Since detrital uraninite does not survive in modern alluvials, the mineralogy of the conglomerate reefs has been explained on the grounds that they were deposited before an oxygenated atmosphere came into being. Nowhere, however, has it been possible to point to a primary source of uraninite whence the detritus could have been derived. There is no need to repeat the many arguments advanced elsewhere6 to illustrate the subjective nature of the observations on which these conclusions are based; but one line of evidence does merit emphasis. A reducing atmosphere could not affect the manner in which the processes of sedimentation are controlled by the force of gravity, and in a reducing environment the commonest refractory heavy minerals of Archaean shields, such as monazite, zircon, ilmenite, and rutile, must be as readily subject to alluvial concentration as in an oxidizing one. Anactualistic conditions, therefore, do not explain why, if the commercial banket ores are heavy- mineral alluvial fractionates, the tenor of uranium is almost invariably much in excess of that of thorium, this being the antithesis of what is found not only in all modern placers but also in the bedrock of all Archaean shields which have been mass-sampled. From the analyses listed in Table 1 it will be seen that the content of zirconium and titanium in uranium-rich bankets is no more than in an average sandstone. The absence of any significant concentration of zircon, ilmenite, and rutile demonstrates unequivocally that the mineralized reefs are not fossil placer deposits. They are not metamorphosed black sands. The heavy minerals present TABLE 1 COMPOSITION (%) OF BANKETS COMPARED WITH AVERAGE SANDSTONES TiO2 ZrO2 Fe U30s SiO2 I 0.13 0.04 1.69 0.157 89.96 II 0.25 0.05 2.51 0.05 86.56 III 0.02-0.33 n.d. 0.05-0.75 0.08 85-95 1 0.36 0.016 2.10 0.035 84.99 2 0.46 0.024 2.16 0.073 78.67 3 1.42 0.027 7.60 0.56 75.04 4 0.60 0.095 7.92 0.164 75.44 A 0.15 0.016 0.57 n.d. 87.22 B 0.50 0.025 3.1 0.0001 78.33 C 0.42 0.051 1.30 n.d.

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