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Volcanic Gases, Black Smokers, and the Great Oxidation Event

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Volcanic Gases, Black Smokers, and the Great Oxidation Event Geochimica et Cosmochimica Acta, Vol. 66, No. 21, pp. 3811–3826, 2002 Copyright © 2002 Elsevier Science Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/02 $22.00 ϩ .00 PII S0016-7037(02)00950-X Volcanic gases, black smokers, and the Great Oxidation Event HEINRICH D. HOLLAND* Harvard University, Department of Earth and Planetary Sciences, 20 Oxford Street, Cambridge, MA 02138, USA (Received September 7, 2001; accepted in revised form January 30, 2002) Abstract—This paper proposes that gradual changes in the composition of volatiles that have been added to the atmosphere-ocean system are responsible for the Great Oxidation Event (G.O.E.) ca. 2.3 Ga. Before ca. 2.3 Ga, the composition of these volatiles was probably such that 20% of the carbon gases could be reduced to organic matter and all of the sulfur gases could be reduced to pyrite. Since 2.3 Ga, the composition of these volatiles has been such that 20% of the carbon gases could be reduced to organic matter, but only a fraction of the sulfur gases could be reduced to pyrite. This change led to the oxygenation of the atmosphere and to Ϫ2 a large increase in the SO4 concentration of seawater. A considerable body of observational data supports these proposals. Copyright © 2002 Elsevier Science Ltd 1. INTRODUCTION genic to an oxygenic state is much smaller than 2 log units and that there is no inconsistency between the required change and The evidence for a major change in the oxidation state of the that allowed by other indicators of the f of volcanic gases in O2 atmosphere between ca. 2.3 and 2.1 Ga was summarized briefly the past. The hypothesis explains a wide range of geologic and in 1999 (Holland, 1999). Since then, the discovery of the mass geochemical observations. independent fractionation of sulfur, its variation with time in the geologic record (Farquhar et al., 2000), and the confirma- 2. THEORETICAL FRAMEWORK tion of this finding by Mojzsis et al. (2001), Farquhar et al. (2001a), and Bekker et al. (2002) now make a major change in The oxidation state of the atmosphere at any given time the oxidation state of the atmosphere during the Paleo-protero- depends in large part on the composition of the contemporary volcanic gases. With the exception of N and the heavier rare zoic a virtual certainty. The appearance of O2 in the atmosphere 2 ca. 2.3 Ga could be explained easily if cyanobacteria evolved at gases, all of the constituents of volcanic gases are removed that time. However, this explanation has been rendered very from the atmosphere-ocean system into sediments and into unlikely by the discovery of biomarkers in 2.7- to 2.8-Ga interplanetary space on a geologically short time scale. CO2 sedimentary rocks that are characteristic of cyanobacteria and CO are removed from the atmosphere-ocean system by the (Brocks et al., 1999). An alternative explanation involves a deposition of organic matter and carbonate minerals. SO2 and change in the redox state of volcanic gases as the trigger for the H2S are removed by the deposition of sulfide and sulfate change in the oxidation state of the atmosphere. This was minerals. H2 is removed by the reduction of CO2 and CO to proposed by Kasting et al. (1993). They pointed out that the organic matter, by the reduction of SO2 to sulfides, and by loss into interplanetary space. H loss to interplanetary space from loss of H2 from the top of a reducing atmosphere would have 2 increased the overall oxidation state of the Earth and almost volcanic gases is significant only when the quantity of H2 in the certainly that of the mantle. This in turn would have led to an volcanic gases exceeds the quantity required to convert ϳ20% ϩ increase in the f of volcanic gases and to a change in the of the total quantity of (CO2 CO) to organic matter and all O2 ϩ redox state of the atmosphere. They proposed that an increase of the (SO2 H2S) to a constituent of pyrite. ϩ in reduced gas emissions by a factor of ϳ4 would be enough The proposition that the fraction of (CO2 CO) that is removed as organic carbon has always been close to 20% is today to preclude the presence of a significant quantity of O2 in the atmosphere, even if organic carbon was being buried at the counterintuitive. We have long been taught that the burial same rate as today. In a more detailed analysis of the changes proportions of organic matter in sediments are influenced or in the composition of volcanic gases that are required to main- even controlled by the exogenic system, the rates of carbon tain a reduced atmosphere Kump et al. (2001) concluded that fixation, and the preservation of organic matter in sediments. the average f of volcanic gases had to be 2 log units below However, although these processes do exert some effect on the O2 that of the FMQ buffer at and before the transition from a burial proportions of organic matter, their effect seems to have reducing to an oxidizing atmosphere. Such a low value of f in been relatively minor. With a few notable exceptions, the O2 average volcanic gases is inconsistent with the data of Delano isotopic composition of carbon in organic carbon and in car- (2001) and Canil (1997, 1999) for the redox history of the bonate minerals has been roughly constant during the past 3.5 Earth’s mantle. This paper reexamines the issue. It proposes Ga (e.g., Schidlowski, 1988; Strauss et al., 1992). Estimates of that the change in the average f of volcanic gases required to the average percentage of organic carbon in the total carbon of O2 account for the transition of the atmosphere from an anoxy- sedimentary rocks range from 15 to 23% (Holser et al., 1988a). The figure 20% used here is a reasonable average of the reported estimates. The demonstrated changes in biota, the * Author to whom correspondence should be addressed likely changes in ocean productivity and in the preservation ([email protected]). fraction of organic matter, and the almost certain changes in the 3811 3812 H. D. Holland composition of the atmosphere and of seawater during the past and igneous rocks, these elements are released to a significant 3.5 Ga have apparently had only a relatively minor effect on the extent. Their release apparently accounts for a good deal of ratio of carbon burial as a constituent of organic matter to that their quantity in volcanic gases and hydrothermal solutions. As in carbonate minerals. long as the redox state of volcanic gases is such that H2 is not The quantity of H2 that is needed to convert volcanic (CO2 present in excess, the redox state of subducted sediments is apt ϩ CO) to organic carbon (CH2O) is, therefore, and has been to be similar to that of volcanic gases. If, however, H2 in during much of the geologic time the amount required to reduce volcanic gases is present in excess, some of this gas escapes ϳ 20% of the total volcanic carbon gas content to CH2O. This into interplanetary space, and subducted sediments are more quantity is independent of the reduction path. In the presence of oxidized than the volcanic gases from which they were derived. excess H2, it makes no difference whether reduction is carried These qualitative considerations need to be converted into a out directly by organisms using photosystem I: quantitative treatment of the relationship between the compo- sition of volcanic gases and the disposal of their constituents ϩ 3 ϩ CO2 2H2 CH2O H2O (1) among the phases of sediments and into interplanetary space. A or indirectly by organisms using photosystem II: convenient approach consists of deriving a relationship be- tween the composition of volcanic gases and the fraction, f, of ϩ 3 ϩ CO2 H2O CH2O O2 (2) their contained sulfur, which can be precipitated as a constitu- ent of pyrite. Let us first convert all of the CO in a volcanic gas In the latter case, the O2, which is generated by green-plant to CO2 via the reaction photosynthesis, is subsequently consumed by reaction with H2 ϩ 3 ϩ to yield H2O: CO H2O CO2 H2 (4) ϩ 3 If the initial concentration of CO was m , that of CO m , 2H2 O2 2H2O (3) o CO 2o CO2 and that of H2omH , the concentration of CO2 at the end of the The sum of Eqn. 2 and 3 is the same as Eqn. 1; two moles of ϩ2 ϩ reaction is (omCO omCO), and that of H2 is (omH omCO). H are always needed to convert one mole of CO to CH O. 2 2 2 2 2 Next let 20% of the total amount of CO2 be converted to CH2O H2 which remains in a batch of volcanic gas after the via the reaction ϩ reduction of 20% of its contained (CO2 CO) to CH2O, is ϩ 3 ϩ available to convert the contained sulfur gases into a constitu- CO2 2H2 CH2O H2O (5) ent of sulfide minerals. The dominant sulfide in marine sedi- At the end of this reaction ments is pyrite. The quantity of H2 required to remove the volcanic sulfur gases as a constituent of sulfides is therefore ϭ ϩ Ϫ mH2 omH2 0.6 omCO 0.4 omCO2 (6) determined by the quantity of H2 required to convert the Next let all of the H S be converted to SO via the reaction volcanic sulfur into FeS2.
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