Carbon in sedimentary rocks

Schidlowski, M. (1988). A 3 800 million year isotopic record of life from carbon in sedimentary rocks. Nature 333,313-318. 

See Rosing, M.T., 1999, 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland, Science 283 674-676 Shen, Y., Buick, R., and Canfield, D.E., 2001, Isotopic evidence for microbial sulphate reduction in the early Archaean era, Nature 410 77-81 and Shidlowski, M.A., 1988, A 3800-million-year isotopic record of life from carbon in sedimentary rocks, Nature 333 313-318. 

Table 4.1. Carbon in Sedimentary Rocks, Hydrosphere, Atmosphere, and Biosphere" ...

X 10 " g of reduced oxygen in the hydrosphere. Of the 2 X 10 g of carbon in sedimentary rocks, only about 20% is reduced carbon (Schidlowski,... 

From Eqn 6.6 it can be seen that the geological record of 813C for kerogens and carbonates reflects changes in the sizes of C reservoirs, in addition to changes in the relative importance of the various fractionation pro-cesses (such as C3 photosynthesis). Because the kerogen and carbonate in sedimentary rocks dominate the shallow reservoirs (>99.9%), the isotopic mass balance in Eqn 6.6 can be written simply as ... 

Table 11-3. Distribution of Carbon in Sedimentary Rocks, Excluding Volcanogenics... 

Evidence for the biological origin of reduced carbon in sedimentary rocks comes from two sources. One is the existence of chemofossils, that is, characteristic remnants of biologically important compounds more resistant to chemical degradation than others. Notable examples are the isoprenoids pristane (2,4,6,10-tetramethyl pentadecane) and phytane (2,4,6,10-tetramethyl hexadecane), which arise from the decay of chlorophyll (Eglin-ton and Calvin, 1967 Didyk et al., 1978 McKirdy and Hahn, 1982 Hahn,... 

The flux of reduced carbon should be consistent with the observed ratio for carbonate to organic carbon in sedimentary rocks. Recall that carbonate carbon is concentrated in limestones, organic carbon in shales, and assume for simplicity that both types of rocks are weathered at the same rate. For steady-state conditions, the flux balance equations then read... 

Figure 12-4 will now be discussed in more detail. The record of reduced carbon in sedimentary rocks and the associated, 3C/I2C isotope shift date back to the oldest deposits of 3.5 Gyr ago, and even further if one makes allowance for the metamorphic alterations of the still older Isua formation (see Fig. 11-2). The biological origin of the isotope shift has been convincingly demonstrated, as reviewed by Schidlowski et al (1983). The implication that life had been in full swing 3.5 billion yr ago is supported by several lines of fossil evidence stromatolites, microfossils, and banded iron formations.

The composition of the present atmosphere bears little resemblance to the composition of the early atmosphere. Most of the water vapor that outgassed from the Earth s interior condensed out of the atmosphere to form the oceans. The predominance of the C02 that outgassed formed sedimentary carbonate rocks after dissolution in the ocean. It is estimated that for each molecule of C02 presently in the atmosphere, there are about 105 C02 molecules incorporated as carbonates in sedimentary rocks. Since N2 is chemically inert, non-water-soluble, and noncondensable, most of the outgassed N2 accumulated in the atmosphere over geologic time to become the atmosphere s most abundant constituent. 

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