Search

A mathematical formalism is presented that, based on the relative proportions of the 18O/16O-distinctive CO2 species C16O2, C16O18O, and C18O2 in a given CO2 pool, allows the calculation of 18O/16O ratios for the two different oxygen positions in the CO2 molecules of this particular pool. The quantities of the isotopically distinctive CO2 species can be determined as mass peak intensities 44I (for C16O2), 46I (for C16O18O), and 48I (for C18O2) by suitable adaptations of conventional mass spectrometric techniques. The δ18O(1,2) values obtained for each of the above positions furnish information as to either a probabilistic (“homogeneous”; with δ18O(1)=δ18O(2)) or a nonprobabilistic (“nonhomogeneous”) 18O/16O distribution (with δ18O(1)≠δ18O(2)) in the respective CO2 pool. Using δ18O(1,2), we may consequently obtain information pertaining to the oxygen sources of the CO2 molecules in the pool as well as to reservoir changes reflected by differential diffusion rates of the isotopically distinctive CO2 species that have left and entered the reservoir. This approach may become potentially important for an assessment of the sources and the reservoir dynamics of the atmospheric CO2 pool. Keywords: Carbon dioxide; oxygen isotopes; isotope variation measurements; mathematical modeling

A carbon and oxygen isotope survey based on 42 samples from the Amba Dongar carbonatite complex of Gujarat, India, indicates that the magmatic differentiation series sovite → alvikite → ankeritic carbonatite is beset with a distinct isotope trend characterized by a moderate rise in 13 C coupled with a sizeable increase in 18 O. From an average of −4.6 ± 0.4 ‰ [PDB] for the least differentiated (coarse) sovite member, δ 13 C values slowly increase in the alvikite (−3.7 ± 0.6 ‰) and ankeritic fractions (−3.0 ± 1.1 ‰), whereas δ 18 O rises from 10.3 ± 1.7 ‰ [SMOW] to 17.5 ± 5.8 ‰ over the same sequence, reaching extremes between 20 and 28 ‰ in the latest generation of ankeritic carbonatite. While an apparent correlation between δ 13 C and δ 18 O over the δ 18 O range of 7–13 ‰ conforms with similar findings from other carbonatite complexes and probably reflects a Rayleigh fractionation process, the observed upsurge of 18 O notably in the ankeritic member is demonstrably related to a late phase of low-temperature hydrothermal activity involving large-scale participation of 18 O-depleted groundwaters. As a whole, the Amba Dongar carbonatite province displays the characteristic 13 C/ 12 C label of deep-seated (primordial) carbon, reflecting the carbon isotope composition of the subcontinental upper mantle below the Narmada Rift Zone of the Indian subcontinent.

The Neoproterozoic–Cambrian transitional sequence in the Mussoorie and Garhwal Hills of the Lesser Himalaya is represented by dolomites of the Upper Krol Formation and chert-phosphorite assemblages of the lowermost Tal Formation. These rocks provide valuable information regarding the assumed existence of a stratified ocean during the terminal Neoproterozoic and beginning of earliest Cambrian (Nemakit-Daldynian). The isotopic compositions of carbonate and organic carbon in the transition profile can be best interpreted as reflecting an oceanic anoxic event (OAE) prior to the phase of early Cambrian phosphate formation. The chert-phosphorite beds record negative Ce and positive Eu anomalies. While Ce is an important paleooceanic redox indicator, Eu is rather immobile under normal diagenetic conditions. The observed Eu anomaly is linked to the high Ba content in these phosphorites and is indicative of an anoxic (or sulfate reducing) diagenetic environment. The suggested stratification of the ocean during the Late Neoproterozoic and Nemakit-Daldynian times would have been linked to the reduction of oxygen supply to the zone below the redoxcline, causing 12 C -and P-rich organic matter previously produced in the photic zone to be trapped in large quantities within the deeper anoxic zone. This would, in turn, increase the 13 C content of the water in the photic zone. Moreover, in the deeper anoxic waters, reduction of Ce4+ to Ce3+ would result in an enrichment of dissolved Ce (as Ce3+). Upwelling of this anoxic bottom stratum would bring the 12 C -and Ce-enriched waters back to the oxic marine realm. However, on entering the highly oxic productive zone, the excess Ce is expected to be removed from the water by oxidation to insoluble Ce4+ followed by precipitation or preferential scavenging. Due to these oxidation reactions, the waters of the shallow oxic zone would tend to become rapidly depleted in Ce. In other words, ocean mixing and upwelling during the early Cambrian were ultimately responsible for the transport of 12 C , P and Ce to the oxic shelves. Sharp drops in both δ 13 C carb and δ 13 C org support the existence of oceanic anoxia followed by oceanic mixing at this juncture of Earth history. The large spread of the δ 34 S values of early diagenetic pyrites within the phosphorite bearing sequence in contrast to the limited spread of δ 13 C carb values suggests that phosphatisation took place under exclusively suboxic diagenetic non-sulfate reducing condition and pyritisation began only when phosphate layers entered the zone of sulfate reduction.

A mathematical formalism is presented that reflects the interrelationship between the gross oxygen isotopic composition of a given carbon dioxide reservoir and its composition in terms of the three most abundant 18 O/ 16 O-distinctive CO 2 species, i.e., 12 C 16 O 2 , 12 C 16 O 18 O and 12 C 18 O 2 (representing mass numbers 44, 46 and 48, respectively). Utilizing this formalism, a determination of the relative proportions of the masses 44, 46 and 48 of a CO 2 pool would permit us to calculate the average 18 O/ 16 O ratio of the CO 2 -bound oxygen of such a pool with similar precision as obtained by currently used “conventional” methods of 18 O/ 16 O analysis. Moreover, if the oxygen component of the CO 2 molecules of such a reservoir were derived from two different sources, the quantitative distribution of the above isotopically different CO 2 variants would allow a determination of the gross oxygen isotopic compositions of the two sources. Application of the functional relationships pertaining to the relative abundances of the three principal 18 O/ 16 O-distinctive CO 2 molecules also permits quantitative estimates of the efflux of CO 2 from a carbon dioxide pool or, alternatively, the influx into such a pool of CO 2 from an extraneous source. Pending the development of satisfactory mass spectrometric techniques for the determination of mass 48, the proposed approach might prove ultimately useful in refined model parameterizations aimed at a better understanding of environmentally relevant CO 2 fluxes.

Lower plants (algae, lichens, mosses) from the Antarctic continent have been analysed for their stable carbon isotope composition. In contrast to lichens and mosses which exhibit quite normal δ13C-values the studied microbial benthos is characterized by an extremely low carbon isotope fractionation (δ13C-values up to −1.4‰. vs. PDB). Limited CO2 availability and bicarbonate uptake are probably the main factors responsible for this phenomenon.

The bias in favour of the light carbon isotope (12C) inherent in the principal carbon-fixing enzymatic reaction of the photosynthetic pathway can be traced back to sedimentary organic matter over 3.8 Gyr of recorded Earth history, indicating that microbial (prokaryotic) photoautotrophs were prolific on this planet not long after its formation. As a consequence, biological modulation of the terrestrial carbon cycle was established very early and was in full operation by the time of formation of the oldest sediments.

The fossil sulfide and carbonate breccia of Decaturville, Missouri, is described and the structural position of the breccia mass discussed. The large and the small carbonate rock fragments show remnant textures typical of marine carbonates. The isotopic compositions of the carbonate fragments and the “flow” carbonates of the breccia compare well with those of sedimentary carbonates from the host sequence of Mississippi Valley-type stratiform lead-zinc deposits of southeastern Missouri. Therefore, an involvement of deep-seated (mantle) carbon in the process of breccia formation is not indicated.

With the currently available geological record at hand, there is no doubt that microbial (prokaryotic) ecosystems have been prolific on the Archaean Earth since 3.5, if not 3.8 Gyr ago. While the information encoded in the oldest record (>3.5 Gyr) is blurred by a metamorphic overprint, the paleontological and biogeochemical evidence pertinent to the existence of life at times < 3.5 Gyr is so firmly established as to be virtually unassailable. In spite of the marked impairment of the oldest sedimentary record, the residual evidence preserved is more than adequate to build a cogent case for the initiation of life processes as early as 3.8 Gyr ago, with the concomitant establishment of a biogeochemical carbon cycle based on the operation of (photo)autotrophic carbon fixation.

Project 157 of the International Geological Correlation Program is concerned with the timing of major events in biological history (such as the advent of bacterial sulfate reduction and the development of oxygen-releasing photosynthesis) and how these events may have been related to the formation of mineral and fossil fuel deposits. Both direct and indirect forms of microbial involvement are being investigated. Examples of direct involvement would be contributions of sulfate-reducing bacteria to the formation of certain stratiform sulfide mineral deposits and massive accumulation of microbial remains to form bog head coals and oil shales. An example of indirect involvement would be photosynthetic oxygenation of the Proterozoic atmosphere providing conditions amenable to the formation of red bed copper deposits. Two aspects of Project 157 are directly applicable to the search for petroleum reserves. One is the subproject thut deals with pre-Devonian crude oils; a major conclusion of our work in this area is that many Proterozoic sedimentary sequences have oil and gas potential and should not be neglected by explorationists. The other is the subproject that deals with organic diagenesis of modern microbial mat communities; among other things, work in this area should lead to the recognition of new biological tracer compounds that can be used for oil-to-source correlations.

The emergence and early evolution of life on Earth have long been intriguing subjects for scientific inquiry and a continuous source of intel­ lectual excitement. Current work at this frontier proceeds along a number of different avenues and comprises a variety of disciplinary approaches that together strongly suggest that the Earth was inhabited by life as early as almost 4 Gyr ago. The present state of the field as inferred from the results of a broad interdisciplinary approach has recently been summarized by J. W. Schopf and coworkers (cf. Schopf 1 983). It is with the advantage of such a synoptic view that the present review addresses a single facet of the theme-namely, the potential isotopic infer­ ences of the presence of CO2-fixing ("autotrophic") organisms for the carbon cycle of the early Earth. With biologically derived (organic) carbon as well as carbonates (limestone, dolomite, etc.) abundantly preserved in sediments, relevant isotopic evidence should have been encoded in the geologic record from its onset about 3.8 Gyr ago. It has been known since the pioneering studies by Nier & Gulbransen (1939), Murphey & Nier (1941), and Rankama (1948) that the trans­ formation of inorganic carbon into living matter entails a marked bias in favor of the light isotope C2C), with the heavy species (l3C) retained in the inorganic reservoir. Subsequent work performed by many investigators (e.g. Craig 1953, Park & Epstein 1960) has confirmed that all common

A model is presented in which the operation of the terrestrial carbon cycle has been simulated under a set of varying parameters, notably different flux ratios ϕ = Corg/Ccarb in the total rate of carbon transfer to sediments and different ratios κ = kCorg/kCcarb of the recycling constants of both sedimentary carbon species. It can be demonstrated that variations by ±50% of an assumed (and most plausible) value of ϕ = 0.125 (even over time spans of some 108 yr) have almost negligible effects on the isotopic composition of carbonate carbon and organic carbon in the sedimentary reservoir, provided the long-term average of this parameter (i.e., ultimately of the burial rate of organic carbon) remains approximately constant. Accordingly, the standard deviations displayed by the fossil δ13Ccarb record would allow for considerable variations of the assumed ϕ over the geologic past. With the Corg reservoir constituting the stoichiometric equivalent of photosynthetically produced oxygen, the evolution of the terrestrial O2 budget may be traced through geologic history within certain limits. Moreover, the model proves helpful in elucidating major isotopic anomalies in the sedimentary carbon record as well as in imposing limiting conditions on dating the beginnings of photosynthesis (about 3.7 × 109 yr ago according to our calculations).

On the basis of the sulfur isotope age curve of marine sulfate evaporites and currently available numerical data for the sulfur cycle the stationary terrestrial sulfate reservoir has been calculated as a function of time for the last 600×106 yr, this also giving information on the partial inventory of photosynthetic oxygen locked up in sedimentary and marine SO42−. The results obtained confirm previous concepts (arrived at by geochemical flux models) of a sizable interchange of material between both sedimentary sulfur reservoirs and, accordingly, varying amounts of oxygen stored as sedimentary sulfate (this amount fluctuating between some 0.5 and 1.5×1022 g during the Phanerozoic). With the constancy of the sedimentary δ13Ccarb record indicating that the crustal Corg reservoir and its stoichiometric oxygen equivalent (i.e., total photosynthetic oxygen) did not change very much during the last quarter of the earth's history, fluctuations in sulfate oxygen must have been buffered by the two other O2 inventories, namely, oxygen fixed as Fe(III) oxide and free oxygen of the atmosphere. Although the main responsibility for balancing variations in sedimentary sulfate would go to the fixed oxygen because of its overwhelming size, repercussions on the free oxygen can by no means be excluded. If we accept a variation of free oxygen proportionate to that of the sum of the two reservoirs as a minimum requirement, then atmospheric PO2 can be expected to have fluctuated during the Phanerozoic within some ±20% of its present level.

The separation factor, aM-0= (18O/16O) magnetite/' (18O/16O) atmospheric oxygen, between the magnetite crust of iron meteorites and atmospheric oxygen has been determined to be 0.9946 ± 0.0005. It is concluded that this fractionation of the oxygen isotopes is the consequence of an equilibrium isotope effect at high temperatures. It can be assumed that this is also valid for cosmic spherules, which are mainly ablation products of iron meteorites. As these spherules are found in sediments of different geological ages, their oxygen isotope ratio can give information on the development of atmospheric oxygen. The difference of the oxygen isotope ratios between magnetite from the lithosphere and airborne magnetite can be used to distinguish between terrestrial and extraterrestrial material.

As suggested by the title of this volume, there is a widespread suspicion that certain types of Precambrian sedimentary mineral deposits may in part owe their origin to the activity of contemporaneous microorganisms. If this view is indeed correct, it is of both economic and academic importance.

Reduced carbon, commonly a residuum of biological activity, has been a conspicuous constituent of sedimentary rocks since the start of the rock record 3.8 × 109 yr ago. The 36,000 Corg assays presently available for Phanerozoic sediments indicate that the average organic carbon content of sedimentary rocks has oscillated around a mean of 0.5 – 0.6% during the last 600 million years. Lack of detailed correlation between observed Corg variations and the isotope age curve of Phanerozoic carbonates raises doubt as to whether all of the variations reported are real. However, the higher organic carbon content of Carboniferous and younger rocks seems to be reflected by more positive levels of the δ13Ccarb which is to be expected from mass balance considerations. Corg assays for Precambrian rocks fall within the scatter of the Phanerozoic data, suggesting that the organic carbon content of Precambrian sediments does not differ significantly from that of geologically younger formations. The constancy of the isotopic fractionation observed between reduced and oxidized carbon throughout the record is best interpreted as the signature of biological activity during the past 3.5 × 109 yr (or possibly 3.8 × 109 yr).

Carbonate constituents from 36 samples of spilitized basalts, dolerites, possible lamprophyres, pyro-clastic tuffs and hydrothermal vein fillings which occur in the sedimentary fram of the Almaden mercury deposit show consistently low δ13C values of — 6.6 ± 2.2 %o vs. PDB, and a corresponding δ18O average of + 17.9 ± 2.5%o vs. SMOW. With the δ13C values of the rare intercalations of sedimentary limestones and dolomites lying in the normal range (- 0.2 ± 0.3%o vs. PDB), the igneous carbonate constituents are unlikely to be of sedimentary pedigree, but rather represent genuinely “hypogenic” material derived from primary magmatic sources.

An investigation of the gold fraction of the Witwatersrand conglomerates from the Orange Free State goldfield (South Africa) has confirmed that the bulk of this metal is concentrated within the bottom parts of the “reefs”, mostly near the footwall contacts. This, together with other criteria, suggests that the distribution of the metal was governed by sedimentary processes. On the other hand, the shape of the gold grains, as well as the general fabric of the gold-enriched layers, often lack conspicuous sedimentary characteristics. Subsequent metamorphism has obviously produced marked structural modifications and a limited textural rearrangement of the gold and its host-rock, thus obliterating to a certain degree the primary alluvial character of the metallization. The large-scale reconstitution of the gold fraction was effected either by a mechanical reshaping of the malleable and ductile gold particles, or by an intermediate solution process followed by reprecipitation. These processes took place more or less in situ. Accordingly, the original sedimentary distribution pattern was not profoundly affected, whereas the individual allogenic gold grains were transformed into authigenic ones or sometimes even into major gold aggregates. Electron-probe microanalysis established that the gold contains on the average between 9.9 and 12.4% silver, which is also suggestive of an alluvial origin. In spite of some uncertainties arising from the camouflaging effect of “pseudohydrothermal” reconstitution processes and from the absence of hydraulic equilibrium between gold and several demonstrably detrital heavy minerals, an integration of all available data strongly supports a modified placer theory for the origin of the Witwatersrand gold.

Ablation products of iron meteorites recovered from Tertiary and Devonian sediments have preserved the 18O/16O ratio of atmospheric oxygen from the respective time periods. While the Tertiary atmosphere was characterized by a rather modern δ18O value (~23.3% o vs. SMOW), the δ-value of the Devonian free oxygen reservoir was markedly lower (~17.3‰) which can be explained in terms of a reduced 18O release by contemporary land photosynthesis.

DURING mineralographic work on Witwatersrand gold conglomerates from the Orange Free State goldfield, I encountered several peculiar micro-structures strongly resembling so-called ‘mineralized bacteria’1–4 and, in certain respects, some types of ‘organized elements’ recently under discussion as possible primitive life-forms in meteorites5–7. These structures are preserved in some of the minute detrital pyrite grains of the Elsburg A1 and A3 horizons occurring near the top of the Witwatersrand system, the deposition of which is believed to have been completed about 2,150 million years ago8. As the organized elements are indigenous to allogenic components of the rock, they are certainly considerably older.