Anomalously fractionated sulfur isotopes in many sedimentary rocks older than 2.4 billion years have been widely believed to be the products of ultraviolet photolysis of volcanic sulfur dioxide in an anoxic atmosphere. Our laboratory experiments have revealed that reduced-sulfur species produced by reactions between powders of amino acids and sulfate at 150° to 200°C possess anomalously fractionated sulfur isotopes: Δ = +0.1 to +2.1 per mil and Δ = -1.1 to +1.1 per mil. These results suggest that reactions between organic matter in sediments and sulfate-rich hydrothermal solutions may have produced anomalous sulfur isotope signatures in some sedimentary rocks. If so, the sulfur isotope record of sedimentary rocks may be linked to the biological and thermal evolution of Earth in ways different than previously thought. 33 36

Applying square adsorption-well and Morse potential models for the adsorption of a sulfur specie on a solid surface, we have evaluated the fractionation of four sulfur isotopes (S-32, S-33, S-34 and S-36) during chemisorption reactions. Heterogeneous reactions between solid and aqueous (or gaseous) species are found to produce anomalous fractionations of sulfur isotopes when the adsorption energy is small (<0.3 eV; 30 kJ/mol) and a discontinuity occurs in the number of bound energy levels of different sulfur isotopes. We refer to "anomalous sulfur isotope fractionation" when the Delta S-33 and (33)theta' (approximate to delta S-33/delta S-34) values of a sample fall outside of 0 +/- 0.2 parts per thousand and 0.51 +/- 0.01 parts per thousand, respectively, and/or when the Delta S-36 and (33)theta' (approximate to delta S-36/delta S-34) values of a sample fall outside of 0 +/- 0.4 parts per thousand and 1.9 +/- 0.1 parts per thousand, respectively. The magnitude of anomalous sulfur isotope effects during a heterogeneous reaction increases with increasing temperature. Depending on the values of chernisorption energy and bond strength, the delta S-33, delta S-34, delta S-36, Delta S-33 and Delta S-36 values of chernisorption products may be as variable as those observed in Archean sedimentary rocks.Ab initio calculations for SO2 adsorption on a kerogen surface indicate the possibility of creating anomalous isotope signatures for the adsorbed species, such as: delta S-33/delta S-34 approximate to 1.08, delta S-36/delta S-34 approximate to 0.84, Delta S-33 = 7.0 to 13.parts per thousand, and Delta S-36= -13.0 to -25.2 parts per thousand in a temperature range of 0 to 200 degrees C) the magnitude of Delta S-33 and Delta S-36 increase with increasing temperature. These data, together with various geochemical data (e.g., organic C and sulfide S contents; hydrothermal alteration effects) on Archean sedimentary rocks with anomalous sulfur isotope fractionations, suggest that the anomalous sulfur isotope signatures in such rocks may have been created by heterogeneous reactions between organic matter (+ minerals) and sulfur-bearing aqueous solutions under hydrothermal conditions, rather than by UV photolysis of volcanic SO2 in an O-2-poor atmosphere. (C) 2008 Elsevier B.V. All rights reserved.

The presence of mass-independently fractionated sulphur isotopes (MIF-S) in many sedimentary rocks older than similar to 2.4 billion years (Gyr), and the absence of MIF-S in younger rocks, has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Gyr ago(1-9). This is because the only mechanism known to produce MIF-S has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen-poor atmosphere. Here we report the absence of MIF-S throughout similar to 100-m sections of 2.76-Gyr-old lake sediments and 2.92-Gyr-old marine shales in the Pilbara Craton, Western Australia. We propose three possible interpretations of the MIF-S geologic record: ( 1) the level of atmospheric oxygen fluctuated greatly during the Archaean era; ( 2) the atmosphere has remained oxic since similar to 3.8 Gyr ago, and MIF-S in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere; or ( 3) MIF-S in rocks was mostly created by non-photochemical reactions during sediment diagenesis, and thus is not linked to atmospheric chemistry.

Geological and geochemical characteristics of banded iron formations (BIFs) suggest that they formed by mixing locally (or regionally) discharged submarine hydrothermal fluids with local seawater, rather than by upwelling deep ocean water. Submarine hydrothermal fluids typically evolved from local seawater by acquiring heat, metals, and sulfur during deep circulation through a variety of rocks (e.g., volcanics, evaporites) in greenstone terranes that developed under a variety of tectonic settings. In general, when the fluids were heated above ̃350 °C, they may have produced Cuand Zn-rich volcanogenic massive sulfi dedeposits (VMSDs), whereas those heated less than ̃200 °C were generally poor in H S and heavy metals, except Fe, and may have subsequently produced BIFs. Depending on the salinity contrast between discharging hydrothermal fluids (evolved seawater) and local seawater, hydrothermal fluids may (1) mix rapidly with local seawater to form smoker-type BIFs or (2) create a metal-and silica-rich brine pool, mix slowly with the overlying water body, and form brine pool-type BIFs. BIFs associated with VMSDs and volcanic rocks generally belong to smoker-type BIFs; many formed at seawater depths >2.5 km. Large BIFs, including the 2.6-2.4 Ga BIFs in the Hamersley Basin, Australia, the 2.5 Ga Kuruman IF in South Africa, and the 1.87 Ga BIFs in the Lake Superior region, United States-Canada, belong to brine pool-type BIFs. The Hamersley Basin and possibly other large BIF-hosting basins were probably land-locked seas (like the Black Sea) where river waters diluted the surface water zone and the underlying water bodies were anoxic. During the accumulation of a BIF sequence, the dominant Fe mineralogy frequently changed from ferric (hydr)oxides (oxide BIFs) to siderite (carbonate BIFs) and to pyrite (sulfide BIFs). Such changes were probably caused by changes in the relative amounts of dissolved O (DO), ΣCO , and ΣS in local seawater. From the Fe -O mass balance calculations for the formation of iron oxides in smoker-type BIFs, and the relationship between the atmospheric pO and oceanic O depth profile, we conclude that the atmosphere and oceans have been fully oxygenated since ca. 3.8 Ga, except in local anoxic basins. Thermodynamic analyses of the formational conditions of siderite and analyses of the carbon isotopic composition of siderite associated with major BIFs suggest that the pre-1.8 Ga atmosphere was CO -rich (pCO >100 PAL) and CH -poor (pCH ≈10 ppm); therefore, CO , rather than CH , was the major greenhouse gas throughout geologic history. After a decline of hydrothermal fluid flux, BIF-hosting basins generally became euxinic (H S-rich) because of the increased activity of sulfate-reducing bacteria (SRB) and SO -rich seawater, and thereby accumulated organic carbon-rich and pyriterich black shales (sulfide-type BIFs). The SO contents and SRB activity in the oceans have been essentially the same since ca. 3.8 Ga. The Archean oceans were most likely poor in both Fe and silica, much like modern oceans. Our study also suggests that diverse communities of organisms, including cyanobacteria, SRB, methanogens, methanotrophs, and eukaryotes, evolved very early in Earth's history, probably by the time the oldest BIFs (ca. 3.8 Ga) formed. BIFs have been found in rocks of all geologic age. Therefore, they cannot be indicators of an anoxic atmosphere and/or anoxic oceans as suggested by many previous researchers. Instead, BIFs indicate that the atmosphere and ocean chemistry have been regulated at present compositions (except pCO ) through geologic history by interactions with the biosphere. The general trend of declining size and abundance of BIFs with geologic time reflects the cooling history of Earth's interior. ©2006 Geological Society of America. 2 2 3 2 2 2 2 2 4 4 2 4 2 4 4 2 2- 2- 2+ 2- 2- 2+

It is generally thought that, in order to compensate for lower solar flux and maintain liquid oceans on the early Earth, methane must have been an important greenhouse gas before similar to2.2 billion years (Gyr) ago(1-5). This is based upon a simple thermodynamic calculation that relates the absence of siderite (FeCO3) in some pre-2.2-Gyr palaeosols to atmospheric CO2 concentrations that would have been too low to have provided the necessary greenhouse effect(1). Using multi-dimensional thermodynamic analyses and geological evidence, we show here that the absence of siderite in palaeosols does not constrain atmospheric CO2 concentrations. Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O-2 concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H-2-rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today. We therefore conclude that CO2 alone ( without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.

A similar to 17-m paleosol sequence at Schagen, South Africa, which developed on a serpentinized dunite intrusion in a granite-gneiss terrain similar to 2.6 Ga ago, is characterized by an alternating succession of thick (similar to 1-3 m) carbonate-rich (dolomite and calcite) zones and silicate-rich (serpentines, talc, and quartz) zones; the upper similar to 8 m section is especially rich in organic C (up to similar to 1.4 wt.%). Petrologic and geochemical data suggest the tipper similar to 8 m section is composed of at least three soil profiles that developed on: (i) silicate-rich rock fragments (and minerals) that were transported from local sources (serpentinite and granite) by fluvial and/or eolian processes: and (ii) dolomite and calcite zones that formed by locally discharged groundwater. The Mg and Fe in the paleosol sequence were largely supplied from local sources (mostly serpentinite), but the Ca, Sr, and HCO3- were supplied by groundwater that originated from a surrounding granite-gneiss terrain. In the uppermost soil profile, the (Fe is retained, the Fe3+/Fe2+ ratio increases, and ferri-stilpnomelane is abundant. These data suggest the atmospheric pO(2) was much greater than similar to 10(-3.7) atm (>0.1% present atmospheric level [PAL]).The carbonaceous matter in the soils is intimately associated with clays (talc, chlorite, and ferri-stilpnomelane) and occurs mostly as seams (20 mun to 1 mm thick) that parallel the soil horizons. These occurrences, crystallographic structures, H/C ratios, and delta(13)C(org) values (-17.4 to -14.4parts per thousand PDB) suggest that the carbonaceous matter is a remnant of in situ microbial mats, originally similar to1 to similar to20 mm thick. The microbial mats developed: (a) mostly on soil surfaces during the formation of silicate-rich soils, and (b) at the bottom of an evaporating, anoxic, alkaline pond during the precipitation of the Fe-rich dolomite. These delta(13)C(org) values are difficult to be explained by a current popular idea of a methane- and organic haze-rich Archean atmosphere (Pavlov et al., 2001); these values, however, can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment. Copyright (C) 2004 Elsevier Ltd

To determine the initial rates and effects of silica in solution on the dissolution kinetics of smectite, short- and long-term batch experiments (0.5 hour to 30 days) were completed at three temperatures (T = 25, 50, and 75degreesC) using stock solutions pH adjusted by NaOH (pH = 12, 13, and 13.5) with varying initial silica concentrations (0, 30, 60, and 100 ppm). The following important characteristics were observed at pH = 12: (1) The concentrations of Al, Si, Mg, Fe, and Ti in solutions increase rapidly during the first similar to2 hours and reach steady state (equilibrium) within similar to5 days. (2) The concentration ratios of Al, Si, Fe, Mg, and Ti in solutions during the early (<2 hours) reaction phase differ significantly from those of smectite, indicating initial dissolution proceeds non-stoichiometrically; Al dissolves much faster than Si, Mg, Fe, and Ti. (3) Further dissolution of smectite proceeds nearly stoichiometrically, including Fe and Ti. (4) The high solubility of Ti in highly alkaline solutions may be due to the formation of aqueous complexes, such as TiO(OH)(3)(-) and TiO2(OH)(2)(2-), similar to aqueous silica species. (5) The initial rate of smectite dissolution increases with increasing pH, T, and initial silica content of solution. (6) The silica in solution acts as a promoter and a catalyst, rather than an inhibitor, of smectite dissolution in high-alkaline solutions. This role is easily recognizable when the solubility of smectite and amorphous silica are very high, i.e., at pH >similar to9.

Microorganisms have flourished in the oceans since at least 3.8 billion years (3.8 Gyr) ago(1,2), but it is not at present clear when they first colonized the land. Organic matter in some Au/U-rich conglomerates and ancient soils of 2.3-2.7 Gyr age has been suggested as remnants of terrestrial organisms(3-5). Some 2.7-Gyr-old stromatolites have also been suggested as structures created by terrestrial organisms(6-7). However, it has been disputed whether this organic matter is indigenous or exogenic, and whether these stromatolites formed in marine or fresh water. Consequently, the oldest undisputed remnants of terrestrial organisms are currently the 1.2-Gyr-old microfossils from Arizona, USA(8). Unusually carbonaceous ancient soils-palaeosols-have been found in the Mpumalanga Province (Eastern Transvaal) of South Africa(9). Here we report the occurrences, elemental ratios (C, H, N, P) and isotopic compositions of this organic matter and its host rocks. These data show that the organic matter very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 Gyr ago. This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported.

The C, N, and S contents and δ C and δ S values were analyzed 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 Africa. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06-2.79 wt%, average 0.47 wt%), N (range <0.01-0.09 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 ∼0.2) atomic ratios in kerogens from the shales indicated that the Kaapvaal shales lost considerable amounts of N, C, S, and H during diagenesis and regional metamorphism (up to the greenschist facies). From the theoretical relationships between the H/C ratios of kerogen and organic C contents of shales, the original C contents of the Archean and Proterozoic shales from the Kaapvaal Craton are estimated to be on average ∼2 wt%. These values are similar to the average organic C content of modern marine sediments. This suggests that the primary organic productivity and the preservation of organic matter in the ocean during the period of 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of clastic sediments to the ocean was similar. This would also imply that the rate of O accumulation in the atmosphere-ocean system, which has equaled the burial rate of organic matter in sediments, has been the same since ∼3.0 Ga. The δ S values of bulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4‰ for seven sulflde-rich samples of ∼2.9 Ga to ∼2.6 Ga. These values are consistent with a suggestion by Ohmoto (1992) and Ohmoto et al. (1993) that most pyrite crystals in Archean shales were formed by bacterial reduction of seawater sulfate with δ S values between +2 and +10‰, and that the Archean seawater was sulfate rich. Changes in the δ C values during maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan (1983), and from the observations by Simoneit et al. (1981) on natural samples. These evaluations suggest that the magnitudes of δ C increase are much less than those estimated by Hayes et al.(1983) and Des Marais et al. (1992), and only about 2 to 3‰ for the 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 contents, and δ C values, four different types of depositional environments are identified for the Archean and early Proterozoic shales in the Kaapvaal Craton: (I) euxinic marine basins, characterized by normal marine organisms with δ C = -33 ± 3‰ ; (II) near-shore, oxic marine environment, characterized by normal marine organisms with δ C = -31 ± 3‰; (III) hypersaline, low-sulfate lakes, characterized by organisms with δ C = -26 ± 3‰; and (IV) euxinic, marine basins which supported the activity of methanogenic and methanotrophic bacteria and accumulated organic matter with δ C = -43 ± 3‰. In contrast to the currently popular model positing a global anoxic ocean prior to ∼2.2 Ga (e.g., Des Marais et al, 1992; Hayes, 1994; Logan et al., 1995), this study suggests that the development of anoxic basins, which accumulated Group II and IV sediments, occurred only regionally and episodically during the period between 3.0 Ga and 2.1 Ga. This further suggests that the normal ocean has been oxic since at least ∼3.0 Ga. Diversifications of environments, as well as of biological species, had already occurred ∼3.0 Ga. The carbon isotope mass balance calculation suggests that the removal rates of organic C and carbonate C from the ocean and the weathering rates of organic C and carbonate C on the continents during the 3.0-2.1 Ga period were basically the same as those in the Phanerozoic era. This would have been possible only if the atmospheric P level had been basically constant since at least 3.0 Ga. The results of this study, therefore, add to a growing list of evidence that the atmosphere has been oxic (i.e., P >1%PAL) since at least 3.0 Ga. The list of evidence includes the sulfur isotope data on Archean sedimentary rocks (Ohmoto and Felder, 1987; Ohmoto et al., 1993), the Fe /Ti ratios of paleosols (Ohmoto, 1996), and the paragenesis of minerals in the "detrital" gold-uranium ores in pre-2.0 Ga quartz pebble beds that suggests nondetrital origins for uraninite and pyrite in these deposits (Baraicoat et al., 1997). Copyright © 1997 Elsevier Science Ltd. 13 34 34 34 13 13 13 13 13 13 13 3+ 2 org org org org org org org O2 O2