Carbon in Precambrian

The question of the distribution of free carbon in Precambrian sedimentary rocks also needs special consideration. Ronov and Migdisov (1970) show that the content in Lower and Upper Proterozoic complexes remained at the same level, but at the boundary of the Proterozoic and Paleozoic there is observed a two-fold increase in the carbon content. However, quantitative data on the carbon content in different types of rocks, including the BIF, are very few. 

Schidlowski, M Eichmann, R. and Fiebiger, W., 1976. Isotopic fractionation between organic carbon and carbonate carbon in Precambrian banded ironstone series from Brazil. Neues Jahrb. Mineral. Monatsh., 8 344-353. 

Klein C (1974) Greenalite, stilpnomelane, minnesotaite, crocidolite, and carbonates in a very low-grade metamorphic Precambrian iron-formation. Can Min 12 475-498 Klein C (1978) Regional metamorphism of Proterozoic iron-formation, Labrador Trough, Canada. Am Min 63 898-912... 

Baker AJ, Fallick AE (1989) Heavy carbon in two-biUion-year-old marbles from Lofoten-Vesteralen, Norway implications for the Precambrian carbon cycle. Geochim Cosmochim Acta 53 1111-1115... 

Much of the chemical, mineralogical, and isotopic data on which our ideas of the evolution of Earth s surface environment are based come from analysis of sedimentary carbonates. Unfortunately because of the increased susceptibility of the sedimentary carbonate mass to alteration and destruction by the processes of weathering, subduction, and metamorphism, much original information is lost from the rock record with increasing age of the carbonate mass. We are left with a biased record of the preserved sedimentary carbonate rock mass. The Precambrian Eon, which includes more than 85% of Earth history, contains only about 25% of the mass of sedimentary carbonates in existence ... 

Vinogradov has pointed out that with the appearance of the biosphere somewhere on the verge of 3-10 yr ago, there was a major upheaval in the evolution of the Earth. Oxidizing processes were abruptly accelerated, a nitrogen atmosphere arose in which carbon dioxide predominated over methane, and free carbon was oxidized to CO2. After the carbon was oxidized or at the same time as that process, there began oxidation of divalent iron (at — 10 ), which led to subsequent wholesale deposition of the sediments of the Precambrian BIF. Free carbon in equilibrium with the atmosphere appeared only after complete oxidation of ferrous iron compounds in the hydrosphere and on the land surface. 

At the present time there are sufficiently reliable data on the distribution of the stable isotopes of oxygen, carbon, and sulfur in Precambrian sedimentary rocks. On the basis of analysis of these data, it is possible to obtain additional information on the geochemical history of the ocean and conditions of sedimentation. 

Sidorenko and Borshchevskiy (1977), who analyzed the general trend in the evolution of the isotopic composition of carbonates in the Precambrian and Phanerozoic, summarized all published data (up to 1976) and depicted them graphically (Fig. 25). In their opinion, the age effect in the oxygen isotopic composition is the result of regional metamorphism, as a global process. 

Fig. 27. Calculated dependence of the isotopic composition of sedimentary carbonates on CO2 and CH4 ratios in the atmosphere (after Galimov et al.) A = calculated values of isotopic composition of carbonates in the absence of an organic carbon cycle B = same, taking account of the organic carbon cycle. S C = 1 experimental values of investigated Precambrian carbonates.

Thus available data on the variations in the carbon isotopic composition in Precambrian carbonates, graphitites, and organic matter confirm the important role of organic processes in the formation of the BIF, especially in the Proterozoic. In the Archean, apparently, organic life was more limited and processes of chemical deposition of carbonates prevailed. 

Rankama (1957) wrote that finely divided carbon in crystalhne schists of mudstone origin is much more widespread in Precambrian rocks than previously believed. The origin of this carbon probably is organic. 

The regularities in the distribution of free carbon in the rocks of the BIF of the KMA have been studied in detail by Plaksenko (1966). The data obtained were fundamental for reconstructing the processes of sedimentation in the Precambrian (Strakhov, 1960) and the formation of ore mineral parageneses. 

In a new summary work by Dimroth and Kimberley (1976) devoted to a comparison of the distribution of carbon and some other elements in Precambrian and Phanerozoic sedimentary rocks, it is mentioned that in most Precambrian sequences the normal negative correlation between grain size and concentrations is established regardless of the depositional environment. That regularity indicates the great importance of the role of plankton in the accumulation of Garrels et al. (1973) came to the same conclusion. 

Investigations of the geochemistry of carbon in recent and ancient sedimentary rocks (Strakhov, 1960 Stashchuk et al., 1964) have demonstrated the special role of this element in the formation of iron minerals. The data obtained by S. Sidorenko (1971) for Precambrian sequences and especially by Plaksenko (1966) for the BIF of the KMA stimulated study of the distribution of carbon and some other elements in the iron-rich rocks, schists, and ore minerals of the Krivoy Rog deposits to a substantial extent. 

Eichmann, R. and Schidlowski, M 1975. Isotopic fractionation between coexisting organic carbon-carbonate pairs in Precambrian sediments. Geochim. Cosmochim. Acta, 39 585-595. 

Hinton M. J., Schiff S. L., and English M. C. (1998) Sources and flowpaths of dissolved organic carbon in two forested watersheds of the Precambrian Shield. Biogeochemistry 41, 175-197. 

It is likely that at least back to 3,500 Ma the principal environmental acid driving this hydrolytic reaction was carbonic acid dissolved in rain water and groundwater (Holland, 1984), as is the case in soils today (Nahon, 1991). Much soil CO2 may also have come from respiring organisms, which also could have contributed organic acids. Nitric and sulfuric acid may have been locally important in soils developed on particular parent materials, but nitrogen and sulfur salts are so far unreported in Precambrian paleosols, unlike modem soils of mine dumps (Borden, 2001), and hypothesized modem soils on Mars (Bell, 1996 Farquhar et al, 2002), and Venus (Barsukov et al, 1982 Basilevsky et al., 1985). 

There are two common stratigraphic occurrences of chert as bedded cherts associated with shales or iron formations and as nodules in carbonate rocks (Blatt et al., 1980). The bedded cherts are predominant in Precambrian time, reaching a maximum extent 2-3 Ga, when they represented as much as 15% of the sedimentary record. The Precambrian bedded cherts contain microspheres of quartz, suggesting that they may have precipitated inorganically. Commonly, bedded cherts are associated with ophiolite sequences, which may have hydrothermal or metasomatic sources of silicate. The co-occurrence of bedded cherts and shales (typically dark in color) suggests that many cherts form in a hemi-pelagic or deep-sea, open-ocean setting, far from sources of coarse clastic material. In... 

Veizer J. and Compston W. (1976) Sr/ Sr in Precambrian carbonates as an index of crustal evolution. Geochim. Cosmochim. Acta 40, 905 -914. 

Grotzinger J. P. and Knoll A. H. (1999) Stromatolites in Precambrian carbonates evolutionary mileposts or environmental dipsticks Ann. Rev. Earth Planet. Sci. 27, 313-358. 

Oehler, D.Z., Schopf, J.W. and Kvenvolden, K.A., 1972. Carbon isotopic studiesof organic matter in Precambrian rocks. Science, 175 1246—1248. 

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