CARBON, NITROGEN, AND SULFUR GEOCHEMISTRY OF ARCHEAN AND PROTEROZOIC SHALES FROM THE KAAPVAAL CRATON, SOUTH-AFRICA

The C, N, and S contents and delta(13)C and delta(34)S values were ana lyzed for 100 shale samples from ten formations, 3.0 to 2.1 Ga in age, in the central and eastern regions of the Kaapvaal Craton, South Afri ca. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06-2.79 wt%, average 0.47 wt%), N (range <0.01-0.0 9 wt%, average 0.1 wt%), and S (range <0.01-1.63 wt%, average 0.1 wt%) . The low N/C (<0.005) and H/C (mostly similar to 0.2) atomic ratios i n kerogens from the shales indicated that the Kaapvaal shales lost con siderable amounts of N, C, S, and H during diagenesis and regional met amorphism (up to the greenschist facies). From the theoretical relatio nships between the H/C ratios of kerogen and organic C contents of sha les, the original C contents of the Archean and Proterozoic shales fro m the Kaapvaal Craton are estimated to be on average similar to 2 wt%. These values are similar to the average organic C content of modern m arine sediments. This suggests that the primary organic productivity a nd the preservation of organic matter in the ocean during the period o f 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of elastic sediments to the ocean was similar. This would al so imply that the rate of O-2 accumulation in the atmosphere-ocean sys tem, which has equaled the burial rate of organic matter in sediments, has been the same since similar to 3.0 Ga. The delta(34)S values of b ulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4 parts per th ousand for seven sulfide-rich samples of similar to 2.9 Ga to similar to 2.6 Ga. These values are consistent with a suggestion by Ohmoto (19 92) and Ohmoto et al. (1993) that most pyrite crystals in Archean shal es were formed by bacterial reduction of seawater sulfate with delta(3 4)S values between +2 and +10 parts per thousand, and that the Archean seawater was sulfate rich. Changes in the delta(13)C(org) values duri ng maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan (1983), a nd from the observations by Simoneit et al. (1981) on natural samples. These evaluations suggest that the magnitudes of delta(13)C(org) incr ease are much less than those estimated by Hayes et al. (1983) and Des Marais et al. (1992), and only about 2 to 3 parts per thousand for th e kerogens that decreased their H/C ratios from 1.5 to less than 0.3. Based on the relationships among sulfide-S contents, organic-C content s, and delta(13)C(org) values, four different types of depositional en vironments are identified for the Archean and early Proterozoic shales in the Kaapvaal Craton: (I) euxinic marine basins, characterized by n ormal marine organisms with delta(13)C(org) = -33 +/- 3 parts per thou sand; (II) near-shore, oxic marine environment, characterized by norma l marine organisms with delta(13)C(org) = -31 +/- 3 parts per thousand ; (III) hypersaline, low-sulfate lakes, characterized by organisms wit h delta(13)C(org) = -26 +/- 3 parts per thousand; and (IV) euxinic, ma rine basins which supported the activity of methanogenic and methanotr ophic bacteria and accumulated organic matter with delta(13)C(org) = - 43 +/- 3 parts per thousand. In contrast to the currently popular mode l positing a global anoxic ocean prior to similar to 2.2 Ga (e.g., Des Marais et al, 1992; Hayes, 1994; Logan et al., 1995), this study sugg ests that the development of anoxic basins, which accumulated Group II and IV sediments, occurred only regionally and episodically during th e period between 3.0 Ga and 2.1 Ga. This further suggests that the nor mal ocean has been oxic since at least similar to 3.0 Ga Diversificati ons of environments, as well as of biological species, had already occ urred similar to 3.0 Ga. The carbon isotope mass balance calculation s uggests that the removal rates of organic C and carbonate C from the o cean and the weathering rates of organic C and carbonate C on the cont inents during the 3.0-2.1 Ga period were basically the same as those i n the Phanerozoic era. This would have been possible only if the atmos pheric P-O2 level had been basically constant since at least 3.0 Ga. T he results of this study, therefore, add to a growing list of evidence that the atmosphere has been oxic (i.e., Po, >1%PAL) since at least 3 .0 Ga. The list of evidence includes the sulfur isotope data on Archea n sedimentary rocks (Ohmoto and Felder, 1987; Ohmoto et al., 1993), th e Fe3+/Ti ratios of paleosols (Ohmoto, 1996), and the paragenesis of m inerals in the ''detrital'' gold-uranium ores in pre-2.0 Ga quartz peb ble beds that suggests nondetrital origins for uraninite and pyrite in these deposits (Barnicoat et al., 1997). Copyright (C) 1997 Elsevier Science Ltd.