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Geological evolution from isotope proxy signals — sulfur
Chemical Geology 161 Ž1999. 89–101 www.elsevier.comrlocaterchemgeo
Geological evolution from isotope proxy signals — sulfur Harald Strauss
),1
Institut fur ¨ Geologie, Ruhr-UniÕersitat ¨ Bochum, 44780, Bochum, Germany Received 28 October 1997; accepted 15 October 1998
Abstract A currently emerging sulfur isotope record for Phanerozoic seawater, based on structurally substituted sulfate in stratigraphically well constrained biogenic carbonates, allows the detailed assessment of secular variations within the global sulfur cycle and the interaction between the sulfur and carbon cycles. It is superior to the evaporite-based dataset because it enables sampling of the entire biostratigraphic column. Discrete biological and environmental signals can be deciphered from a somewhat ‘‘noisy’’ sulfur isotope record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around y51‰ which appears to be constant throughout the entire Phanerozoic. Observable large spreads of d34 S sulfide for any given sedimentary unit are caused by environmental parameters, such as type and availability of organic carbon or availability of sulfate. In particular, the growing importance of land plants and their impact on the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Sulfur isotope record; Phanerozoic seawater; Sulfur cycles; Carbon cycles
1. Introduction The isotopic composition of sedimentary sulfur, both in its most oxidized form as dissolved seawater sulfate or marine evaporitic sulfate and its reduced form as sedimentary iron sulfide, provides important information on the global exogenic cycle and on variations of corresponding geological, geochemical andror biological parameters during Earth history. Recently, Strauss Ž1997. reviewed and critically
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E-mail: [email protected] Present address. Geologisch-Palaontologisches Institut, ¨ Westfalische Wilhelms-Universitat Corrensstrasse 24, ¨ ¨ Munster, ¨ 48149 Munster, Germany. ¨ 1
evaluated the available temporal records, pointing out the inadequacy of the existing datasets for detailed analysis of high-resolution isotope stratigraphy. The aim of this article is to discuss a currently emerging isotope record for seawater sulfate, based on structurally substituted sulfate in biogenic calcites, and to compare it to the available evaporitebased sulfur isotope record and eventually to the high-resolution isotope records of carbon, oxygen and strontium discussed in ŽVeizer et al., this volume.. An additional aim is a critical assessment of all parameters that affect the isotopic composition of sedimentary sulfides resulting from bacterial sulfate reduction. Reassessment of their importance affects the interpretion of changes in the global sulfur cycle.
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 0 8 2 - 0
H. Straussr Chemical Geology 161 (1999) 89–101
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2. Sulfur isotope systematics The geochemical interest in the sedimentary sulfur cycle has largely focussed on its two end-members, evaporitic sulfate and biogenic pyrite, with dissolved seawater sulfate being the central reservoir at any given time in Earth history. The fundamentals of sulfur isotope fractionation by abiological and biological processes were established more than 30 years ago Že.g., Thode et al., 1961; Kaplan and Rittenberg, 1964. and have been subsequently reviewed by, for example, Nielsen Ž1978., Chambers and Trudinger Ž1979. and Ohmoto Ž1992.. Marine evaporites Ži.e., gypsum and anhydrite., formed as purely inorganic precipitates, usually reflect the isotopic composition of the ambient seawater displaced only by a negligible isotope fractionation of 0 to q2.4‰ ŽAult and Kulp, 1959; Thode et al., 1961; Thode and Monster, 1965; Nielsen, 1978.. This, however, is not the case for minerals which have been formed during later stages of evaporation Že.g., K- and Mg salts. as documented by examples from ancient evaporite deposits Že.g., Nielsen and Ricke, 1964; Kampschulte et al., 1998; Peters, 1988. and by laboratory experiments ŽRaab and Spiro, 1991.. Primarily, this results from different fractionation factors for these minerals. Biologically controlled processes, on the other hand, can impose substantial fractionation of the isotopes between reactant and product, with dissimilatory sulfate reduction resulting in a major kinetic sulfur isotope effect. The latter is a key process in the sedimentary environment, ultimately resulting in the formation of pyrite via bacterial sulfate reduction, and can be described by the following equation: 8 Ž SO4 .
2y
q 2 Fe2 O 3 q 8 H2 O q 15C o r g
´ 4 FeS2 q 15CO 2 q 16 Ž OH .
y
The generally accepted assumption is that the resulting sedimentary pyrite typically acquires strongly negative d34 S values due to preferential utilization of 32 S by sulfate reducing bacteria under open system conditions. The magnitude of this isotope effect is quite variable between q4 and y46‰, based on laboratory experiments. Net isotopic fractionation reflects biological Že.g., the type of bacteria, type and concentration of organic substrate, re-
duction rates: Harrison and Thode, 1958; Kaplan and Rittenberg, 1964; Chambers and Trudinger, 1979. and environmental Že.g., availability of sulfate: Ohmoto, 1992; Habicht and Canfield, 1996. parameters. The biologically driven, kinetic isotope effects can be still further amplified via disproportionation of intermediate reaction products, such as thiosulfate andror elemental sulfur ŽJørgensen, 1990; Canfield and Thamdrup, 1994.. In the past, the existence of strongly variable and 34 S depleted sulfides has often been regarded as evidence of biogenicity, utilized as a criterion for characterisation of Žbio-.geochemical environments, and as a tracer of the antiquity of bacterial sulfate reduction. Such far reaching conclusions were frequently advocated without taking into account the complexity of parameters that affect isotope fractionation during sulfate reduction, a perspective that can significantly influence the interpretation of the geological record.
3. Secular variations in the isotopic composition of seawater sulfate Seawater sulfate represents the major reservoir within the sedimentary sulfur cycle. The most important output functions from this reservoir are deposition of evaporitic sulfate minerals and bacterial reduction of sulfate to hydrogen sulfide, the latter subsequently fixed as sedimentary iron sulfide. Replenishment of the seawater reservoir occurs via riverine input of dissolved sulfate generated from continental weathering of sulfates and sulfides. Additional input is derived from the mantle Žtrue magmatic sulfur., but this contribution has generally been neglected in mass balance calculations Že.g., Garrels and Perry, 1974; Garrels and Lerman, 1984; Kump, 1989.. The sedimentary sulfur cycle is intimately linked to the carbon cycle, both reflecting changing redox conditions in the sedimentary realm. These changes can be monitored and modelled through Earth history by utilizing the sulfur isotopic composition of seawater sulfate. For the modern ocean, this is readily measured on dissolved oceanic sulfate, but for the past it must be approximated from marine evaporitic sulfates. The latter has been performed under the
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assumption, that the sulfur isotopic composition of seawater is homogeneous laterally and vertically and that marine evaporitic sulfates reflect the isotopic composition of parental seawater. The spatial homogeneity of modern seawater sulfate has been fairly well documented Že.g., Rees et al., 1978; Longinelli, 1989.. The assumption that marine evaporites unequivocally reflect the original seawater isotopic composition requires further discussion. In principle, only a minor isotope effect accompanies the precipitation of calcium sulfate. Progressive evaporation, however, continually changes the geochemical conditions of the depositional environment, resulting in a substantial change in the sulfur isotopic composition of the precipitated sulfate minerals. This has been observed in natural evaporite deposits Že.g., Holser and Kaplan, 1966; Kampschulte et al., 1998., in experiments ŽRaab and Spiro, 1991., and in model calculations ŽAyora et al., 1994.. A careful assessment of depositional conditions Žsulfate vs. chloride vs. late-stage evaporite facies. is therefore required to avoid misinterpretations. In addition, laterally and vertically representative sampling and comparison of several time equivalent evaporite deposits from different regions of the world should be done in order to assure that the sulfur isotopic composition of seawater sulfate is representative for a given time interval ŽNielsen, 1989.. The Phanerozoic evaporite-based sulfur isotope record ŽFig. 1. displays clearly discernible secular variations, on differing time scales, as already noted by earlier authors Že.g., Holser and Kaplan, 1966; Claypool et al., 1980; Strauss, 1993a.. These include an isotopic maximum with d34 S values around q30 to q35‰ in the Vendian and Cambrian, minimum values around q11‰ for the Permian and a value of q21‰ for modern seawater sulfate. Superimposed on this 10 8-yr-scale trend are shorter term Ž10 7-yrscale. fluctuations. Rapid changes in ocean chemistry, some as short as 500,000 years, have been termed ‘‘catastrophic chemical events’’ Že.g., the FrasnianrFamennian or PermianrTriassic transition; Holser, 1977.. Assuming that the observed secular variations reflect primarily changes in the redox balance of the sulfur cycle, the long-term variations can be most easily explained by changes in the mass of net pyrite formation through bacterial sulfate reduction and burial in the sedimentary reservoir. As
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Fig. 1. Sulfur isotopic composition of Phanerozoic evaporites Žmodified and updated after Strauss, 1997; ages after Harland et al., 1990.. Horizontal bars indicate ranges of sulfur isotopes, vertical bars, positioned at the average isotopic composition, represent the age uncertainty.
discussed above, this process is generally accompanied by a pronounced isotope fractionation, resulting from the withdrawal of 32 S from the marine sulfur cycle. Modeling approaches that utilize the isotopic composition of sulfate sulfur as a proxy indicate a level of pyrite burial that is twice as large as today for most of the Paleozoic, followed by a significant drop to values that were about half of today’s rate during the upper Carboniferous and Permian and by more or less constant rates for the last 180 Ma ŽKump, 1989.. A similar overall variation in the burial rate of pyrite is indicated by an alternative modeling approach based on the abundance of pyrite in sedimentary sequences through time ŽBerner and Canfield, 1989.. The experimentally derived negative correlation of d34 S sulfate and d13 C carbonate ŽVeizer et al., 1980. suggests that the global sulfur and carbon cycles are counterbalanced, thus precluding generation of unusual levels of atmospheric oxygen. However, this counterbalance appears to be limited only to time scales in excess of 10 7 years due to the long residence time of sulfate Ž7.9 = 10 6 years; Holland,
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1978. in the oceanic reservoir. On shorter time scales, large variations can be caused by regional or even local effects, such as a turnover that destroys water column stratification. Such local to regional scenarios could be particularly relevant for the so-called ‘‘catastrophic chemical events’’ ŽHolser, 1977.. Short-term substantial variations in d34 S of marine sulfates and ocean turnover as their proposed cause challenge the idea of vertical homogeneity of the water column with respect to the isotopic composition of dissolved sulfate. This could be particularly significant for the Paleozoic for which ocean stratification, bottom water anoxia and associated chemical and isotopic changes have been suggested Že.g., Berry and Wilde, 1978; Goodfellow and Jonasson, 1984.. This does not affect per se the conclusion that marine calcium sulfates reflect the sulfur isotopic composition of ancient sea water. It shows, however, that specific paleoceanographic conditions exert an influence on the isotopic composition of surface waters. Considering all interpretations and modeling approaches for the existing sulfate sulfur isotope record, two issues appear to be of most critical significance: Ø the assumption of a constant D34 S, the isotopic difference between seawater sulfate and biogenic pyrite linked to bacterial sulfate reduction of the former; and Ø the available time resolution that dictates the level of detail for any modeling result. The constancy of D34 S will be addressed later in the text. As far as time resolution is concerned, it is evident that the record for evaporitic sulfates not only harbours large age uncertainties but also substantial gaps, particularly during the Paleozoic. Although this pattern of discontinuity can plausibly be attributed to the unavailability of suitable evaporite sequences due to erosion or non-deposition, it nevertheless precludes generation of a high resolution time-series for d34 S sulfate of seawater sulfate. An alternative approach that could overcome these resolution limitations is the isotopic investigation of structurally substituted sulfate ŽSSS. in biogenic calcites. The sulfate ion, present in the carbonate lattice, reflects the sulfur isotopic composition of the seawater at the time of carbonate precipitation as demonstrated in recent biogenic calcites that yielded d34 S values close to the average value of q21‰ for dissolved oceanic sulfate ŽBurdett et al., 1989;
Kampschulte and Strauss, 1998.. The abundance of SSS ranges from several hundred to a few thousand ppm ŽStaudt and Schoonen, 1995., a level that requires a careful analytical approach during sulfur extraction in order to avoid contamination. The major advantage of this approach is that it enables sampling over the entire biostratigraphic column. Such a carbonate-based sulfur isotope record is already emerging, with data available for the Neogene ŽBurdett et al., 1989., the Mid-Jurassic to Cretaceous ŽKampschulte and Strauss, 1996. and for portions of the Paleozoic ŽKampschulte and Strauss, 1998.. The materials utilized were foraminifera, belemnites, brachiopods and inorganic carbonates. A comparison of the traditional evaporite-based d34 S record and the corresponding SSS record enables an assessment of the viability of this approach. The Mid-Jurassic to Cretaceous time interval ŽFig. 2. provides an example of such a comparison and shows that the overall agreement between both time records is quite good. This is also true at the more detailed resolution of an individual biostratigraphic stage ŽKampschulte and Strauss, 1996., although the often only crude time resolution of the evaporite-based record Že.g., Holser and Kaplan, 1966; Claypool et
Fig. 2. Sulfur isotopic composition for Mid-Jurassic and Cretaceous seawater derived from SSS and its comparison with timeequivalent evaporite data Žmodified after Kampschulte and Strauss, 1996..
H. Straussr Chemical Geology 161 (1999) 89–101
al., 1980. hinders such a detailed assessment. The SSS-approach represents an attractive alternative to the study of evaporites and will eventually result in a d34 S temporal record comparable to those for carbon, oxygen and strontium isotopes ŽVeizer et al., this volume.. The linkage between the global sulfur and carbon cycles has been based on the observed negative correlation between their respective seawater isotope records ŽVeizer et al., 1980; Holser, 1984.. The major drawback in these comparisons has always been the differing time resolution and the fact that analytical results were generated from different sets of samples. With the emerging high-resolution sulfur isotope record and a corresponding d13 C dataset obtained from the same biogenic carbonates ŽVeizer et al., this volume., it is tempting to address this question again ŽFig. 3.. The resulting regression line is fairly comparable Žy0.3 vs. y0.24. to the one published by Holser Ž1984., and its viability will improve with an expanding dataset. It must be pointed out that the data shown in Fig. 3 are from individual biogenic carbonates that have been assessed for their diagenetic alteration through petrographic, trace element and isotopic studies, and subsequently analysed for the isotopic compositions of their sulfate sulfur and carbonate carbon. This is in contrast to the average 10 Ma intervals used by Holser Ž1984. based on data derived from separate analyses of evaporites
Fig. 3. Correlation of d34 S and d13 C from biogenic carbonates Žsulfur isotope data from Kampschulte and Strauss, 1996; Kampschulte and Strauss, 1998; carbon isotope data from Veizer et al., this volume..
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and Žbiogenic. carbonates. The observed deviations from the regression line for the new approach are thus not due to poor time resolution but instead reflect true time-related differences in ocean chemistry likely caused by different responsermixingr residence times of carbon and sulfur and their isotopes in the global ocean. Once the Phanerozoic SSS-based d34 S sulfate record is completed, a much more detailed assessment of these and other aspects of the sulfur cycle should be possible. In summary, secular variations in pyrite burial and associated isotope effects are considered to be the prime cause for the observed variations in d34 S sulfate . Accepting this interpretation, we can now turn to the issue of an apparently constant D34 S from consideration of the corresponding sulfide sulfur isotope record.
4. The isotopic composition of sedimentary sulfide — a reflection of different signals 4.1. Systematics By analogy to carbon, reservoir sizes and redox reactions within the sedimentary sulfur cycle determine the observed variations in d34 S. An evaluation of the d34 S sulfide record — while even more discontinuous than the sulfate one — is therefore essential. In particular, it is necessary to test whether it parallels the d34 S sulfate record as would be expected from a constant D34 S. This has been the major assumption in most modeling approaches despite the large uncertainties Že.g., Garrels and Lerman, 1984.. Assessing the validity of this assumption requires that all biological and environmental effects are considered. The ultimate question is: ‘‘What determines the d34 S pyrite?’’ This question includes aspects that relate exclusively to the biology of sulfate reducers and the process of bacterial sulfate reduction, an issue beyond the scope of this article Žfor details see, for example, Postgate, 1984; Widdel, 1988., to parameters that are limiting the formation of sedimentary iron sulfide, notably the availability of metabolizable organic carbon Že.g., Morse and Berner, 1995. andror reactive iron Že.g., Canfield, 1989., and to
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aspects that affect the magnitude of the associated sulfur isotope fractionation Že.g., Harrison and Thode, 1958; Kaplan and Rittenberg, 1964; McCready, 1975; Habicht and Canfield, 1996.. In general, bacterial sulfate reduction is limited by the availability of metabolizable organic carbon. Commonly, this is expressed by a clear positive correlation between the abundances of sedimentary organic carbon and pyrite sulfur, as has been widely documented for marine environments Že.g., Raiswell and Berner, 1985, 1986.. Although the SrC ratio is strongly dependent on the degree of preservation of the host sediment, resulting in selective degradation and removal of either parameter, significant variations through Earth history do exist ŽFig. 4., possibly reflecting differences in the effectiveness of bacterial sulfate reduction. In addition, it must be kept in mind that the formation of sedimentary pyrite depends also on the availability of reactive iron ŽRaiswell and Canfield, 1998.. If this variable were limiting, H 2 S would not be fixed in the sediment, resulting in a partial loss of isotopic information. Thus, quantification of reactive iron for a given sediment, for example by the parameter termed ‘‘degree of pyritization’’ ŽDOP: Raiswell and Berner, 1985., is essential for any meaningful interpretation of d34 S values. The availability of sulfate as terminal electron acceptor and its reduction rate, the latter determined
for example by type and availability of organic compounds as electron donors, are the key factors affecting the sulfur isotopic fractionation associated with bacterial sulfate reduction. As a rule of thumb, the isotopic fractionation increases with decreasing reduction rate Že.g., Harrison and Thode, 1958; Kaplan and Rittenberg, 1964., although this proposition has recently been challenged ŽHabicht and Canfield, 1996.. The reactivity of organic matter is considered to be the most important parameter. Marine organic matter is a much more effective substrate than the terrestrial one because it contains a greater abundance of easily metabolizable organic compounds Že.g., Lyons and Gaudette, 1979.. The availability of sulfate and its impact on the magnitude of isotopic fractionation must be considered from the point of view of two different scenarios. Experimental studies have shown that isotopic fractionation during bacterial sulfate reduction is largely independent of absolute sulfate abundance down to a concentration of 1 mM, but decreases below this level Že.g., Ohmoto, 1992.. More important, however, is the possible diminution of sulfate with time due to ongoing bacterial sulfate reduction, an environmental situation termed ‘‘closed system’’. This continuous decrease in sulfate concentration is accompanied by increasingly more positive d34 S values resulting from preferential reduction of the 32 S isotope. Such closed system conditions can be a result of, for example, diminishing sulfate diffusion from the overlying water column due to increasing burial depth, high sulfate reduction rate exceeding sulfate replenishment Že.g., Zaback et al., 1993. due to favourable conditions for sulfate reducers or a combination of both factors. 4.2. The Phanerozoic sulfide record
Fig. 4. Sulfurrcarbon ratios from marine siliciclastic sediments of Neoproterozoic and Phanerozoic ages Ždata from Raiswell and Berner, 1986; Strauss, 1993b..
Interpretation of the sulfide sulfur isotope record has to consider therefore all the above aspects. The long-term trend for the entire Phanerozoic ŽFig. 5. broadly parallels the sulfate curve Žsee Fig. 1., with maximum values in the early Paleozoic, minimum values in the Permian and a shift back to heavier values in the Cenozoic. As with all other isotope records, the prime objective of this contribution is the detection of possible evolutionary patterns, either biological or environmental, thereby carefully distin-
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Ø The difference between d34 S sulfate and d S sulfide-max ŽD3. displays the greatest variability at y13 " 18.8‰. Most notable is an apparent difference between the early Paleozoic with its positive values Žsulfide sulfur isotope values heavier than the corresponding seawater sulfate. and the later part of the Phanerozoic. Although a definitive statement is hindered by the very limited dataset, the change appears to have occurred in the upper Devonian Žaround 370 Ma.. Prior to interpreting these observations, a few explanatory notes regarding the existing sulfide sulfur isotope record are necessary in order to demonstrate its limitations for deciphering evolutionary trends. Most data have been generated from whole rock samples of substantial size, likely masking any genetic and isotopic differences between individual pyrite grains ŽOhmoto, 1992.. For many studies, only very limited to no ‘‘background’’ information Žsuch as pyrite morphology, abundances of organic carbon, pyrite sulfur andror reactive iron. exists that would enable the genetic history of the sedimentary sulfides to be constrained which is particularly important for distinguishing local from global signals and syngenetic from later diagenetic patterns. These limitations result in substantial ‘‘noise’’ in the existing isotope record, enabling only very broad evolutionary pattern to be clearly discernible. In most studies, variable but generally negative d34 S values have been regarded as clear evidence that sedimentary pyrite formed by bacterial sulfate reduction. This qualitative conclusion, however, is insufficient to describe the observed variability in d34 S sulfide for individual time segments or more specifically for individual stratigraphicrsedimentary units. It is a pattern typical for the entire Phanerozoic and Proterozoic ŽStrauss, 1993b., yet its causes have generally not been discussed. Most promising in this respect appears to be a comparison of pyrite morphology and d34 S values ŽRaiswell, 1982; Strauss and Schieber, 1990; Nielsen, 1997.. Such consideration indicates that syngenetic to early diagenetic, commonly framboidal pyrite is frequently located at the 34 S depleted end of a given sulfur isotope range, whereas the late diagenetic morphotypes Ži.e., large idiomorphic cubes, concretions. almost always display 34 S enriched values. This reflects the continuous evolution of the geochemical environment due to 34
Fig. 5. Sulfur isotopic composition of Phanerozoic sedimentary sulfides Žmodified and updated after Strauss, 1997; ages after Harland et al., 1990.. Horizontal bars indicate ranges of sulfur isotopes, vertical bars, positioned at the average isotopic composition, represent the age uncertainty.
guishing global from local signals. Accepting the fragmentary nature of the presently existing record, I shall utilize average isotopic compositions of sulfide and sulfate sulfur for 10 Ma intervals ŽTable 1.. For the evaporite-based sulfate record, only the mean d34 S values are considered, while for sulfides the average, minimum and maximum are provided, resulting in three estimates for D34 S values ŽFig. 6.. Despite substantial gaps, particularly in the Paleozoic, several observations can be made that relate to both biological and environmental phenomena: Ø Individual time segments exibit variable and frequently substantial differences between minimum and maximum d34 S sulfide values, up to 75‰. This variability renders questionable the significance of an average sulfide sulfur isotopic composition for a given time segment or a sedimentary unit. Ø Acknowledging some variability, the isotopic difference between sulfate sulfur and the minimum sulfide sulfur value ŽD2. varies within y51‰" 8.3‰ throughout the entire Phanerozoic. Judged by its standard deviation in comparison to the other data, it is the best constrained D34 S value, possibly indicating some consistency with time in the initial kinetic isotope fractionation associated with bacterial sulfate reduction.
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Table 1 Average sulfur isotopic compositions for sulfate and sulfide in 10 Ma intervals Age ŽMa.
d 34 S avg
Standard deviation
d 34 S avg
Recent y5 y15 y25 y35 y45 y55 y65 y75 y85 y95 y105 y115 y125 y135 y145 y155 y165 y175 y185 y195 y205 y215 y225 y235 y245 y255 y265 y275 y285 y295 y305 y315 y325 y335 y345 y355 y365 y375 y385 y395 y405 y415 y425 y435 y445 y455 y465 y475 y485 y495 y505
20.3 20.2 22.1 16.5 21.5 19.8 17.2 18.2 16.2 18.2 15.0 15.6 14.5 15.6 15.5
0.1 1.7 0.9 5.5 1.2 2.6 0.5 1.1 1.6 1.1 1.1 2.1 1.2 1.3 0.6
7 10 4 19 19 3 2 4 11 5 32 8 9 3
17.1
1.4
23
16.8
1.1
16
11.7 11.8
0.6 1.7
2 21
16.4 17.1 25.9 10.9 12.4
2.7 0.2 4.6 1.3 1.2
82 2 160 144 9
14.6 15.8
0.7 2.1
9 8
15.9
1.1
37
27.6 20.5 20.2 19.7
2.4 1.6 4.1 2.4
43 12 45 20
28.2 24.8
1.0
1 4
25.4
0.5
13
24.1
5.0
5
N
Standard deviation
y18.2
N
d 34 S min
381
y54.0
d 34 S max
D1
D2
D3
28.0
y38.5
y56.7
y13.9
3.2
9.7
12
y18.5
15.1
y19.0
y40.6
y7.1
y25.3 y16.3 y24.3 y24.8
9.4 14.6 7.1 4.3
32 26 13 24
y38.5 y34.7 y37.1 y30.4
0.3 16.8 y8.1 y13.1
y43.5 y32.5 y42.4 y39.8
y56.7 y50.9 y55.3 y45.5
y17.9 0.6 y26.1 y28.1
y39.7 y31.5 y27.2
7.2 5.3 8.6
8 3 13
y48.6 y36.2 y36.1
y28.4 y24.1 y2.8
y54.2 y47.1 y42.7
y63.1 y51.8 y51.6
y42.9 y39.7 y18.3
y36.8
4.6
28
y41.0
y19.0
y53.9
y58.1
y36.1
y13.3
6.9
22
y25.8
y2.6
y20.8
9.3
39
y40.9
y2.9
y37.2
y57.3
y19.3
y39.0 y23.2
8.6 7.9
17 36
y51.9 y38.1
y25.2 y10.0
y49.9 y35.6
y62.8 y50.5
y36.1 y22.4
y29.0 y12.6
1.6 8.5
3 54
y31.0 y33.0
y27.0 9.0
y27.2
y47.6
y5.6
y14.7 y0.7 y4.8 7.4
10.7 9.1 11.1 9.5
114 11 43 43
y47.0 y13.0 y29.9 y24.0
26.7 20.0 29.8 26.0
y28.3 y25.3 y12.9
y40.6 y50.4 y44.2
y7.6 9.3 5.8
y2.7 y6.3 5.8
18.7 11.5 9.2
31 17 3
y36.6 y22.1 y1.4
38.8 14.1 18.8
y27.6
y61.4
14.0
y6.7
9.4
5
y23.0
1.5
H. Straussr Chemical Geology 161 (1999) 89–101
97
Table 1 Žcontinued. Age ŽMa. y 515 y525 y535 y545 y555 y565
d 34 S avg
30.3
Standard deviation
2.4
N
63
d 34 S avg
Standard deviation
N
d 34 S min
d 34 S max
y2.5 y1.4 16.0
5.4 7.6 6.0
7 8 68
y8.0 y13.0 y0.8
8.0 8.0 34.1
17.7
18.3
25
y20.1
52.8
progressing bacterial sulfate reduction. Resulting limitation of the sulfate supply at the site of bacterial reduction appears to be the most critical factor. The documented, frequently substantial, spread of d34 S sulfide values for a given sedimentary unit is thus best explained in terms of a ‘‘time series’’. Variations in the sulfur isotopic composition within a given sedimentary unit would reflect the diagenetic redox-Žre-.-cycling of sulfur phases including various stages from partial to complete closure of the system. The development of closed system conditions during diagenesis could well be triggered through high sedimentation rates, resulting in continuously decreasing diffusion of sulfate from the water column into the sediment. As a consequence, the diagenetic system would soon be exclusively dependent on porewater sulfate with no further replenishment. Goldhaber and Kaplan Ž1975., Maynard Ž1980., and others have noted a relationship between the sulfur isotopic composition of biogenic sulfides and sedimentation rate. Thereby, coarser grained sed-
Fig. 6. Isotopic difference between sulfate and sulfide sulfur during Phanerozoic time ŽD1: D34 S sulfate r sulfide - avg ; D2: D34 S sulfate r sulfide - min ; D3: D34 S sulfate r sulfide - max ..
D1
D2
D3
y14.3
y31.1
3.8
iments are characterized by more positive d34 S values. Clearly, any given average isotopic composition reflects how complete this diagenetic system has been sampled. In that regard, any additional information characterizing the depositional andror geochemical environment of pyrite formation will assist in interpreting observed sulfur isotope variations. The above qualitative statement relating d34 S values to biogenicity has always been complemented by a quantitative component, specifically the observed maximum isotopic fractionation of y46‰ for culture experiments ŽKaplan and Rittenberg, 1964. compared to even larger displacements of up to y60‰ in recent natural settings ŽChambers and Trudinger, 1979.. This long-standing discrepancy has not been understood until the importance of thiosulfate andror elemental sulfur disproportionation and their associated isotope effects were demonstrated ŽJørgensen, 1990; Canfield and Thamdrup, 1994; Canfield and Teske, 1996.. Canfield and Teske Ž1996. in particular interpret the Phanerozoic sulfide sulfur isotope record as indicative of a combination of bacterial sulfate reduction and additional internal, oxidative recycling of intermediate sulfur species. Thereby, associated isotope discrimination would enlarge the overall isotopic difference between sea water sulfate and biogenic sulfide. It should be pointed out, however, that only a very limited initial database on isotopic fractionation during bacterial sulfate reduction was generated in the 1950s and 1960s ŽHarrison and Thode, 1958; Kaplan and Rittenberg, 1964; Kemp and Thode, 1968.. The renewed interest in experimental work in that respect Že.g., Habicht and Canfield, 1996; Smock et al., 1998. will advance our understanding with respect to sulfur isotope fractionation. In summary, the true magnitude of sulfur isotopic fractionation associated
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with bacterial sulfate reduction, i.e., the kinetic isotope effect, is still largely unknown for most natural environments and for many of the environmentally important sulfate reducing bacteria. Nevertheless, the fact that the existing sulfur isotope records display a more or less equal displacement between parental seawater sulfate and the most 34 S depleted pyrite Ž d34 S sulfide-min . throughout the entire Phanerozoic argues for some biological consistency in the initial isotopic fractionation. This observation suggests that the same group of sulfate reducing bacteria Žand its biochemical pathway. were important in the geological past and in modern marine settings ŽJørgensen, 1982.. The total spread of d34 S for a given sedimentary unit and the relative proportion of 34 S depleted versus 34 S enriched values for biogenic pyrite is strongly dependent on the method of sampling Ži.e., whole rock vs visible sulfide vs microdrilling.. Assuming that this ‘‘bias’’ is similar for all available datasets, an evaluation of the isotope difference between d34 S sulfate and d34 S sulfide-max through time ŽD3 in Fig. 6. offers the base for some interesting speculations. Although associated with some ‘‘noise’’, it appears that the early Paleozoic time segments are all characterized by positive to strongly positive d34 S sulfide-max values and, more importantly, that these maximum d34 S sulfide values exceed the d34 S sulfate values Ži.e., a positive D3.. A similar pattern can be observed also for the average ŽD1. trend. This lower Paleozoic feature extends into the Proterozoic ŽStrauss, 1993b. and likely reflects a secular trend. Considering that the availability of sulfate and the rate of its reduction, the latter reflecting the availability of easily metabolizable organic material, are the most important factors that control d34 S sulfide , it is interesting to note that the shift in Dsulfatersulfide-max appears to have occurred around 370 Ma ago. This was the time of rapidly increasing importance of land plants on Earth Že.g., Algeo et al., 1995; Algeo and Scheckler, 1998., providing additional substrate available to marine sulfate reducers. The less-easily metabolizable terrestrial compounds resulted in a less efficient mineralisation of sedimentary organic matter by sulfate reducers, an observation consistent with the higher SrC ratios for normal marine sediments ŽFig. 4. in the early Paleozoic ŽRaiswell and Berner, 1986.. The observed sulfur isotope pattern for sedimentary pyrite
would be consistent with a scenario where, due to the more reactive purely marine organic matter, sulfate reduction in the early Paleozoic would have been rapid and efficient, quickly exhausting sulfate at the site of bacterial reduction. The availability of easily metabolizable organic matter would have been even greater in anoxic waters which might have been more frequent in early Paleozoic oceans, as suggested, for example by Berry and Wilde Ž1978., Goodfellow and Jonasson Ž1984. and Goodfellow Ž1987.. With the growing input of less metabolizable land plant detritus into the nearshore, marine environment, bacterial sulfate reduction in the younger Phanerozoic would utilize the rapidly diminishing easily-metabolizable carbon source, while sulfate was still in unlimited supply. In summary, the available fragmentary d34 S sulfide record is consistent with the operation of bacterial sulfate reduction characterized by a more or less constant kinetic isotope fractionation around y51‰ throughout the entire Phanerozoic. The overall spread in d34 S sulfide for any given stratigraphic unit is a reflection of the interplay of several parameters, such as the quantity of easily metabolizable organic matter, sulfate availability and abundance of reactive iron Ži.e., reservoir effects.. The evolution of land plants and their subsequent impact on the pool of marine sedimentary organic matter may have affected the global sulfur cycle, as indicated by the sulfur isotope record, further supporting the interdependancy of geochemical cycles of sulfur and carbon.
5. Summary The utility of the sulfur isotope record as a proxy for the evolution of the global sulfur cycle through time is strongly limited because of its fragmentary nature. This is the case for both the sulfate and sulfide records. The currently emerging seawater sulfate isotope record, derived from structurally substituted sulfate from stratigraphically well constrained biogenic calcites, represents a fundamental improvement in that respect. The available data confirm the overall temporal trend based on evaporite samples but also suggest the existence of frequent shorter-term fluctuations that could not have been
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previously resolved due to insufficient time resolution. The existing sulfide isotope record, despite its noise, still appears to retain vestiges of initial biological and environmental signals. Bacterial sulfate reduction displays a constant maximum fractionation of around y51‰ throughout the entire Phanerozoic. Whether this magnitude of fractionation is solely a product of sulfate reduction or includes isotope effects linked to disproportionation reactions needs to be clarified through further systematic culture experiments. The large d34 S sulfide variability for any given sedimentary unit is best explained as a time-series of pyrite formation. Strongly positive maximum d34 S sulfide values in early Paleozoic indicate that bacterial sulfate reduction proceeded in a rapid and efficient fashion, substantially diminishing the available sulfate pool. In the younger Phanerozoic, evolution of land plants led to a change in organic substrate from purely marine algal matter to growing input from land plants, the latter were more difficult to metabolize. This transition appears to have occurred in the Mid-rLate Devonian, as indicated also by a change in the mean SrC ratio of normal marine sediments. Renewed and growing interest in the isotopic composition of sedimentary sulfur and its evolution through time is greatly improving the existing fragmentary isotope records. Once completed, their time resolution will be comparable to that of other isotope systems, yielding a detailed proxy signal for the geochemical, biological and geological evolution of the SYSTEM EARTH.
Acknowledgements This study represents a contribution to the Earth System Evolution Program of the Canadian Institute for Advanced Research ŽCIAR.. Inspiring discussions with program members over the years as well as support through CIAR are gratefully acknowledged. Part of this research has been funded through the Deutsche Forschungsgemeinschaft ŽGrants: Ve112r8-1; Str281r7-1.. Unpublished sulfur isotope data from biogenic carbonates were kindly provided by A. Kampschulte. This is also a contribution
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to IGCP Project 386 ‘‘Response of the oceanratmosphere system to past global changes’’. References Algeo, T.J., Scheckler, S.E., 1998. Terrestrial–marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events. Phil. Trans. R. Soc. Lond. B 353, 113–130. Algeo, T.J., Berner, R.A., Maynard, J.B., Scheckler, S.E., 1995. Late Devonian oceanic anoxic events and biotic crisis: rooted in the evolution of vascular land plants. GSA Today 5 Ž45., 64–66. Ault, W.U., Kulp, J.L., 1959. Isotopic geochemistry of sulphur. Geochim. Cosmochim. Acta 16, 201–235. Ayora, C., Taberner, C., Pierre, C., Pueyo, J.J., 1994. Refining the d34 S and d18 O values of sulphate in ancient seas. Mineral. Mag. 58A, 32–33. Berner, R.A., Canfield, D.E., 1989. A model for atmospheric oxygen over Phanerozoic time. Am. J. Sci. 289, 333–361. Berry, W.B.N., Wilde, P., 1978. Progressive ventilation of the oceans. Am. J. Sci. 278, 257–275. Burdett, J.W., Arthur, M.A., Richardson, M., 1989. A Neogene seawater sulfate isotope age curve from calcareous pelagic microfossils. Earth Planet. Sci. Lett. 94, 189–198. Canfield, D.E., 1989. Reactive iron in marine sediments. Geochim. Cosmochim. Acta 53, 619–632. Canfield, D.E., Teske, A., 1996. Late Proterozoic rise in atmospheric oxygen concentration inferred from phylogenetic and sulphur-isotope studies. Nature 382, 127–132. Canfield, D.E., Thamdrup, B., 1994. The production of 34 S-depleted sulfide during bacterial disproportionation of elemental sulfur. Science 266, 1973–1975. Chambers, L.A., Trudinger, P.A., 1979. Microbiological fractionation of stable sulfur isotopes: a review and critique. Geomicrobiology J. 1, 249–293. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H., Zak, I., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol. 28, 190– 260. Garrels, R.M., Lerman, A., 1984. Coupling of sedimentary sulfur and carbon cycles — an improved model. Am. J. Sci. 284, 989–1007. Garrels, R.M., Perry, E.A., 1974. Cycling of carbon, sulfur, and oxygen through geologic time. In: Goldberg, E.D. ŽEd.., The Sea, Vol. 5, Wiley, New York, pp. 303–336. Goldhaber, M.B., Kaplan, I.R., 1975. Controls and consequences of sulfate reduction rates in recent marine sediments. Soil Sci. 119, 42–55. Goodfellow, W.D., 1987. Anoxic stratified oceans as a source of sulphur in sediment-hosted stratiform zinc–lead deposits ŽSelwyn Basin, Yukon, Canada.. Chem. Geol. 65, 359–382. Goodfellow, W.D., Jonasson, I.R., 1984. Ocean stagnation and ventilation defined by 34 S secular trends in pyrite and barite Selwyn basin, Yukon. Geology 12, 583–586. Habicht, K.S., Canfield, D.E., 1996. Sulphur isotope fractionation
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Handbook of Geochemistry. Springer, Berlin, 16-B-1–16-B40. Nielsen, H., 1989. Local and global aspects of the sulphur isotope age curve of oceanic sulphate. In: Brimblecombe, P., Lein, A.Yu. ŽEds.., Evolution of the Global Biogeochemical Sulphur Cycle. Wiley, New York, pp. 57–64. Nielsen, J.K., 1997. Pyrite in the Alum Shale Formation: indications of palaeoenvironment and burial temperature. Ann. Meet. Dansk Sedimentologisk Forskningsgruppe, March 1997 ŽAbstr... Nielsen, H., Ricke, W., 1964. Schwefel-Isotopen-Verhaltnisse von ¨ Evaporiten aus Deutschland; ein Beitrag zur Kenntnis von S 34 im Meerwasser-Sulfat. Geochim. Cosmochim. Acta 28, 577– 591. Ohmoto, H., 1992. Biogeochemistry of sulfur and the mechanisms of sulfide–sulfate mineralization in Archean oceans. In: Schidlowski, M., Golubic, S., Kimberley, M.M., McKirdy, D.M., Trudinger, P.A. ŽEds.., Early Organic Evolution, Springer, Berlin, pp. 378–397. Peters, H., 1988. Stoffbestand und Genese des Kaliflozes ¨ Riedel ŽK3Ri. im Salzstock Wathlingen-Hanigsen, Werk Niedersach¨ sen-Riedel. Unpubl. PhD thesis, Universitat Ger¨ Gottingen, ¨ many. Postgate, J.R., 1984. The Sulphate-reducing Bacteria. 2nd edn., Cambridge Univ. Press, Cambridge. Raab, M., Spiro, B., 1991. Sulfur isotopic variations during seawater evaporation with fractional crystallization. Chem. Geol. 86, 323–333. Raiswell, R., 1982. Pyrite texture, isotopic composition and the availability of iron. Am. J. Sci. 282, 1244–1263. Raiswell, R., Berner, R.A., 1985. Pyrite formation in euxinic and semi-euxinic sediments. Am. J. Sci. 285, 710–724. Raiswell, R., Berner, R.A., 1986. Pyrite and organic matter in Phanerozoic normal marine shales. Geochim. Cosmochim. Acta. 50, 1967–1976. Raiswell, R., Canfield, D.E., 1998. Sources of iron for pyrite formation in marine sediments. Am. J. Sci. 298, 219–245. Rees, C.E., Jenkins, W.J., Monster, J., 1978. The sulphur isotopic composition of ocean water sulphate. Geochim. Cosmochim. Acta. 42, 377–381. Smock, A.M., Bottcher, M.E., Cypionka, H., 1998. Fractionation ¨ of sulfur isotopes during thiosulfate reduction by DesulfoÕibrio desulfuricans. Arch. Microbiol. 169, 460–463. Staudt, W.J., Schoonen, M.A.A., 1995. Sulfate incorporation into sedimentary carbonates. In: Vairavamurthy, M.A., Schoonen, M.A.A. ŽEds.., Geochemical Transformations of Sedimentary Sulfur. Am. Chem. Soc., Washington, pp. 332–345. Strauss, H., 1993a. The sulfur isotopic record of Precambrian sulfates: new data and a critical evaluation of the existing record. Precamb. Res. 63, 225–246. Strauss, H., 1993b. Geochemische und isotopengeochemische Untersuchungen zum Schwefelkreislauf in prakambrischen Sedi¨ menten. Unpubl. Habilitationsschrift, Ruhr-Universitat ¨ Bochum, Germany. Strauss, H., 1997. The isotopic composition of sedimentary sulfur through time. Paleogeogr., Paleoclimat., Paleoecol. 132, 97– 118.
H. Straussr Chemical Geology 161 (1999) 89–101 Strauss, H., Schieber, J., 1990. A sulfur isotope study of pyrite genesis: the mid-proterozoic newland formation, Belt Supergroup, Montana. Geochim. Cosmochim. Acta 54, 197–204. Thode, H.G., Monster, J., 1965. Sulfur isotope geochemistry of petroleum, evaporites, and ancient seas. Am. Assoc. Pet. Geol. Mem. 4, 367–377. Thode, H.G., Monster, J., Dunford, H.B., 1961. Sulphur isotope geochemistry. Geochim. Cosmochim. Acta 25, 159–174. Veizer, J., Holser, W.T., Wilgus, C.K., 1980. Correlation of 13 Cr12 C and 34 Sr32 S secular variations. Geochim. Cosmochim. Acta 44, 579–587.
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Viezer, J., Ala, D., Azmy, K., Buckschen, P., Bolh, D., Brohn, F., Calden, G.A.F., Diener, A., Ebneth, S., Godderis, Y., Jasper, T., Korle, C., Pawellek, F., Podlaha, O.G., Strauss, H., this volume. 87 Srr86 Sr, d13 C and d18 O evolution of Phanerozoic seawater. Chem Geol. Widdel, F., 1988. Microbiology and ecology of sulfate- and sulfur-reducing bacteria. In: Zehnderm, A.J.B. ŽEd.., Biology of Anaerobic Microorganisms. Wiley, New York, pp. 469–585. Zaback, D.A., Pratt, L.M., Hayes, J.M., 1993. Transport and reduction of sulfate and immobilization of sulfide in marine black shales. Geology 21, 141–144.
Geological evolution from isotope proxy signals — sulfur Harald Strauss
),1
Institut fur ¨ Geologie, Ruhr-UniÕersitat ¨ Bochum, 44780, Bochum, Germany Received 28 October 1997; accepted 15 October 1998
Abstract A currently emerging sulfur isotope record for Phanerozoic seawater, based on structurally substituted sulfate in stratigraphically well constrained biogenic carbonates, allows the detailed assessment of secular variations within the global sulfur cycle and the interaction between the sulfur and carbon cycles. It is superior to the evaporite-based dataset because it enables sampling of the entire biostratigraphic column. Discrete biological and environmental signals can be deciphered from a somewhat ‘‘noisy’’ sulfur isotope record for sedimentary biogenic pyrite. These include a maximum isotopic fractionation around y51‰ which appears to be constant throughout the entire Phanerozoic. Observable large spreads of d34 S sulfide for any given sedimentary unit are caused by environmental parameters, such as type and availability of organic carbon or availability of sulfate. In particular, the growing importance of land plants and their impact on the amount of metabolizable organic substrate affects the sulfide sulfur isotopic composition. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Sulfur isotope record; Phanerozoic seawater; Sulfur cycles; Carbon cycles
1. Introduction The isotopic composition of sedimentary sulfur, both in its most oxidized form as dissolved seawater sulfate or marine evaporitic sulfate and its reduced form as sedimentary iron sulfide, provides important information on the global exogenic cycle and on variations of corresponding geological, geochemical andror biological parameters during Earth history. Recently, Strauss Ž1997. reviewed and critically
)
E-mail: [email protected] Present address. Geologisch-Palaontologisches Institut, ¨ Westfalische Wilhelms-Universitat Corrensstrasse 24, ¨ ¨ Munster, ¨ 48149 Munster, Germany. ¨ 1
evaluated the available temporal records, pointing out the inadequacy of the existing datasets for detailed analysis of high-resolution isotope stratigraphy. The aim of this article is to discuss a currently emerging isotope record for seawater sulfate, based on structurally substituted sulfate in biogenic calcites, and to compare it to the available evaporitebased sulfur isotope record and eventually to the high-resolution isotope records of carbon, oxygen and strontium discussed in ŽVeizer et al., this volume.. An additional aim is a critical assessment of all parameters that affect the isotopic composition of sedimentary sulfides resulting from bacterial sulfate reduction. Reassessment of their importance affects the interpretion of changes in the global sulfur cycle.
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 0 8 2 - 0
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2. Sulfur isotope systematics The geochemical interest in the sedimentary sulfur cycle has largely focussed on its two end-members, evaporitic sulfate and biogenic pyrite, with dissolved seawater sulfate being the central reservoir at any given time in Earth history. The fundamentals of sulfur isotope fractionation by abiological and biological processes were established more than 30 years ago Že.g., Thode et al., 1961; Kaplan and Rittenberg, 1964. and have been subsequently reviewed by, for example, Nielsen Ž1978., Chambers and Trudinger Ž1979. and Ohmoto Ž1992.. Marine evaporites Ži.e., gypsum and anhydrite., formed as purely inorganic precipitates, usually reflect the isotopic composition of the ambient seawater displaced only by a negligible isotope fractionation of 0 to q2.4‰ ŽAult and Kulp, 1959; Thode et al., 1961; Thode and Monster, 1965; Nielsen, 1978.. This, however, is not the case for minerals which have been formed during later stages of evaporation Že.g., K- and Mg salts. as documented by examples from ancient evaporite deposits Že.g., Nielsen and Ricke, 1964; Kampschulte et al., 1998; Peters, 1988. and by laboratory experiments ŽRaab and Spiro, 1991.. Primarily, this results from different fractionation factors for these minerals. Biologically controlled processes, on the other hand, can impose substantial fractionation of the isotopes between reactant and product, with dissimilatory sulfate reduction resulting in a major kinetic sulfur isotope effect. The latter is a key process in the sedimentary environment, ultimately resulting in the formation of pyrite via bacterial sulfate reduction, and can be described by the following equation: 8 Ž SO4 .
2y
q 2 Fe2 O 3 q 8 H2 O q 15C o r g
´ 4 FeS2 q 15CO 2 q 16 Ž OH .
y
The generally accepted assumption is that the resulting sedimentary pyrite typically acquires strongly negative d34 S values due to preferential utilization of 32 S by sulfate reducing bacteria under open system conditions. The magnitude of this isotope effect is quite variable between q4 and y46‰, based on laboratory experiments. Net isotopic fractionation reflects biological Že.g., the type of bacteria, type and concentration of organic substrate, re-
duction rates: Harrison and Thode, 1958; Kaplan and Rittenberg, 1964; Chambers and Trudinger, 1979. and environmental Že.g., availability of sulfate: Ohmoto, 1992; Habicht and Canfield, 1996. parameters. The biologically driven, kinetic isotope effects can be still further amplified via disproportionation of intermediate reaction products, such as thiosulfate andror elemental sulfur ŽJørgensen, 1990; Canfield and Thamdrup, 1994.. In the past, the existence of strongly variable and 34 S depleted sulfides has often been regarded as evidence of biogenicity, utilized as a criterion for characterisation of Žbio-.geochemical environments, and as a tracer of the antiquity of bacterial sulfate reduction. Such far reaching conclusions were frequently advocated without taking into account the complexity of parameters that affect isotope fractionation during sulfate reduction, a perspective that can significantly influence the interpretation of the geological record.
3. Secular variations in the isotopic composition of seawater sulfate Seawater sulfate represents the major reservoir within the sedimentary sulfur cycle. The most important output functions from this reservoir are deposition of evaporitic sulfate minerals and bacterial reduction of sulfate to hydrogen sulfide, the latter subsequently fixed as sedimentary iron sulfide. Replenishment of the seawater reservoir occurs via riverine input of dissolved sulfate generated from continental weathering of sulfates and sulfides. Additional input is derived from the mantle Žtrue magmatic sulfur., but this contribution has generally been neglected in mass balance calculations Že.g., Garrels and Perry, 1974; Garrels and Lerman, 1984; Kump, 1989.. The sedimentary sulfur cycle is intimately linked to the carbon cycle, both reflecting changing redox conditions in the sedimentary realm. These changes can be monitored and modelled through Earth history by utilizing the sulfur isotopic composition of seawater sulfate. For the modern ocean, this is readily measured on dissolved oceanic sulfate, but for the past it must be approximated from marine evaporitic sulfates. The latter has been performed under the
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assumption, that the sulfur isotopic composition of seawater is homogeneous laterally and vertically and that marine evaporitic sulfates reflect the isotopic composition of parental seawater. The spatial homogeneity of modern seawater sulfate has been fairly well documented Že.g., Rees et al., 1978; Longinelli, 1989.. The assumption that marine evaporites unequivocally reflect the original seawater isotopic composition requires further discussion. In principle, only a minor isotope effect accompanies the precipitation of calcium sulfate. Progressive evaporation, however, continually changes the geochemical conditions of the depositional environment, resulting in a substantial change in the sulfur isotopic composition of the precipitated sulfate minerals. This has been observed in natural evaporite deposits Že.g., Holser and Kaplan, 1966; Kampschulte et al., 1998., in experiments ŽRaab and Spiro, 1991., and in model calculations ŽAyora et al., 1994.. A careful assessment of depositional conditions Žsulfate vs. chloride vs. late-stage evaporite facies. is therefore required to avoid misinterpretations. In addition, laterally and vertically representative sampling and comparison of several time equivalent evaporite deposits from different regions of the world should be done in order to assure that the sulfur isotopic composition of seawater sulfate is representative for a given time interval ŽNielsen, 1989.. The Phanerozoic evaporite-based sulfur isotope record ŽFig. 1. displays clearly discernible secular variations, on differing time scales, as already noted by earlier authors Že.g., Holser and Kaplan, 1966; Claypool et al., 1980; Strauss, 1993a.. These include an isotopic maximum with d34 S values around q30 to q35‰ in the Vendian and Cambrian, minimum values around q11‰ for the Permian and a value of q21‰ for modern seawater sulfate. Superimposed on this 10 8-yr-scale trend are shorter term Ž10 7-yrscale. fluctuations. Rapid changes in ocean chemistry, some as short as 500,000 years, have been termed ‘‘catastrophic chemical events’’ Že.g., the FrasnianrFamennian or PermianrTriassic transition; Holser, 1977.. Assuming that the observed secular variations reflect primarily changes in the redox balance of the sulfur cycle, the long-term variations can be most easily explained by changes in the mass of net pyrite formation through bacterial sulfate reduction and burial in the sedimentary reservoir. As
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Fig. 1. Sulfur isotopic composition of Phanerozoic evaporites Žmodified and updated after Strauss, 1997; ages after Harland et al., 1990.. Horizontal bars indicate ranges of sulfur isotopes, vertical bars, positioned at the average isotopic composition, represent the age uncertainty.
discussed above, this process is generally accompanied by a pronounced isotope fractionation, resulting from the withdrawal of 32 S from the marine sulfur cycle. Modeling approaches that utilize the isotopic composition of sulfate sulfur as a proxy indicate a level of pyrite burial that is twice as large as today for most of the Paleozoic, followed by a significant drop to values that were about half of today’s rate during the upper Carboniferous and Permian and by more or less constant rates for the last 180 Ma ŽKump, 1989.. A similar overall variation in the burial rate of pyrite is indicated by an alternative modeling approach based on the abundance of pyrite in sedimentary sequences through time ŽBerner and Canfield, 1989.. The experimentally derived negative correlation of d34 S sulfate and d13 C carbonate ŽVeizer et al., 1980. suggests that the global sulfur and carbon cycles are counterbalanced, thus precluding generation of unusual levels of atmospheric oxygen. However, this counterbalance appears to be limited only to time scales in excess of 10 7 years due to the long residence time of sulfate Ž7.9 = 10 6 years; Holland,
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1978. in the oceanic reservoir. On shorter time scales, large variations can be caused by regional or even local effects, such as a turnover that destroys water column stratification. Such local to regional scenarios could be particularly relevant for the so-called ‘‘catastrophic chemical events’’ ŽHolser, 1977.. Short-term substantial variations in d34 S of marine sulfates and ocean turnover as their proposed cause challenge the idea of vertical homogeneity of the water column with respect to the isotopic composition of dissolved sulfate. This could be particularly significant for the Paleozoic for which ocean stratification, bottom water anoxia and associated chemical and isotopic changes have been suggested Že.g., Berry and Wilde, 1978; Goodfellow and Jonasson, 1984.. This does not affect per se the conclusion that marine calcium sulfates reflect the sulfur isotopic composition of ancient sea water. It shows, however, that specific paleoceanographic conditions exert an influence on the isotopic composition of surface waters. Considering all interpretations and modeling approaches for the existing sulfate sulfur isotope record, two issues appear to be of most critical significance: Ø the assumption of a constant D34 S, the isotopic difference between seawater sulfate and biogenic pyrite linked to bacterial sulfate reduction of the former; and Ø the available time resolution that dictates the level of detail for any modeling result. The constancy of D34 S will be addressed later in the text. As far as time resolution is concerned, it is evident that the record for evaporitic sulfates not only harbours large age uncertainties but also substantial gaps, particularly during the Paleozoic. Although this pattern of discontinuity can plausibly be attributed to the unavailability of suitable evaporite sequences due to erosion or non-deposition, it nevertheless precludes generation of a high resolution time-series for d34 S sulfate of seawater sulfate. An alternative approach that could overcome these resolution limitations is the isotopic investigation of structurally substituted sulfate ŽSSS. in biogenic calcites. The sulfate ion, present in the carbonate lattice, reflects the sulfur isotopic composition of the seawater at the time of carbonate precipitation as demonstrated in recent biogenic calcites that yielded d34 S values close to the average value of q21‰ for dissolved oceanic sulfate ŽBurdett et al., 1989;
Kampschulte and Strauss, 1998.. The abundance of SSS ranges from several hundred to a few thousand ppm ŽStaudt and Schoonen, 1995., a level that requires a careful analytical approach during sulfur extraction in order to avoid contamination. The major advantage of this approach is that it enables sampling over the entire biostratigraphic column. Such a carbonate-based sulfur isotope record is already emerging, with data available for the Neogene ŽBurdett et al., 1989., the Mid-Jurassic to Cretaceous ŽKampschulte and Strauss, 1996. and for portions of the Paleozoic ŽKampschulte and Strauss, 1998.. The materials utilized were foraminifera, belemnites, brachiopods and inorganic carbonates. A comparison of the traditional evaporite-based d34 S record and the corresponding SSS record enables an assessment of the viability of this approach. The Mid-Jurassic to Cretaceous time interval ŽFig. 2. provides an example of such a comparison and shows that the overall agreement between both time records is quite good. This is also true at the more detailed resolution of an individual biostratigraphic stage ŽKampschulte and Strauss, 1996., although the often only crude time resolution of the evaporite-based record Že.g., Holser and Kaplan, 1966; Claypool et
Fig. 2. Sulfur isotopic composition for Mid-Jurassic and Cretaceous seawater derived from SSS and its comparison with timeequivalent evaporite data Žmodified after Kampschulte and Strauss, 1996..
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al., 1980. hinders such a detailed assessment. The SSS-approach represents an attractive alternative to the study of evaporites and will eventually result in a d34 S temporal record comparable to those for carbon, oxygen and strontium isotopes ŽVeizer et al., this volume.. The linkage between the global sulfur and carbon cycles has been based on the observed negative correlation between their respective seawater isotope records ŽVeizer et al., 1980; Holser, 1984.. The major drawback in these comparisons has always been the differing time resolution and the fact that analytical results were generated from different sets of samples. With the emerging high-resolution sulfur isotope record and a corresponding d13 C dataset obtained from the same biogenic carbonates ŽVeizer et al., this volume., it is tempting to address this question again ŽFig. 3.. The resulting regression line is fairly comparable Žy0.3 vs. y0.24. to the one published by Holser Ž1984., and its viability will improve with an expanding dataset. It must be pointed out that the data shown in Fig. 3 are from individual biogenic carbonates that have been assessed for their diagenetic alteration through petrographic, trace element and isotopic studies, and subsequently analysed for the isotopic compositions of their sulfate sulfur and carbonate carbon. This is in contrast to the average 10 Ma intervals used by Holser Ž1984. based on data derived from separate analyses of evaporites
Fig. 3. Correlation of d34 S and d13 C from biogenic carbonates Žsulfur isotope data from Kampschulte and Strauss, 1996; Kampschulte and Strauss, 1998; carbon isotope data from Veizer et al., this volume..
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and Žbiogenic. carbonates. The observed deviations from the regression line for the new approach are thus not due to poor time resolution but instead reflect true time-related differences in ocean chemistry likely caused by different responsermixingr residence times of carbon and sulfur and their isotopes in the global ocean. Once the Phanerozoic SSS-based d34 S sulfate record is completed, a much more detailed assessment of these and other aspects of the sulfur cycle should be possible. In summary, secular variations in pyrite burial and associated isotope effects are considered to be the prime cause for the observed variations in d34 S sulfate . Accepting this interpretation, we can now turn to the issue of an apparently constant D34 S from consideration of the corresponding sulfide sulfur isotope record.
4. The isotopic composition of sedimentary sulfide — a reflection of different signals 4.1. Systematics By analogy to carbon, reservoir sizes and redox reactions within the sedimentary sulfur cycle determine the observed variations in d34 S. An evaluation of the d34 S sulfide record — while even more discontinuous than the sulfate one — is therefore essential. In particular, it is necessary to test whether it parallels the d34 S sulfate record as would be expected from a constant D34 S. This has been the major assumption in most modeling approaches despite the large uncertainties Že.g., Garrels and Lerman, 1984.. Assessing the validity of this assumption requires that all biological and environmental effects are considered. The ultimate question is: ‘‘What determines the d34 S pyrite?’’ This question includes aspects that relate exclusively to the biology of sulfate reducers and the process of bacterial sulfate reduction, an issue beyond the scope of this article Žfor details see, for example, Postgate, 1984; Widdel, 1988., to parameters that are limiting the formation of sedimentary iron sulfide, notably the availability of metabolizable organic carbon Že.g., Morse and Berner, 1995. andror reactive iron Že.g., Canfield, 1989., and to
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aspects that affect the magnitude of the associated sulfur isotope fractionation Že.g., Harrison and Thode, 1958; Kaplan and Rittenberg, 1964; McCready, 1975; Habicht and Canfield, 1996.. In general, bacterial sulfate reduction is limited by the availability of metabolizable organic carbon. Commonly, this is expressed by a clear positive correlation between the abundances of sedimentary organic carbon and pyrite sulfur, as has been widely documented for marine environments Že.g., Raiswell and Berner, 1985, 1986.. Although the SrC ratio is strongly dependent on the degree of preservation of the host sediment, resulting in selective degradation and removal of either parameter, significant variations through Earth history do exist ŽFig. 4., possibly reflecting differences in the effectiveness of bacterial sulfate reduction. In addition, it must be kept in mind that the formation of sedimentary pyrite depends also on the availability of reactive iron ŽRaiswell and Canfield, 1998.. If this variable were limiting, H 2 S would not be fixed in the sediment, resulting in a partial loss of isotopic information. Thus, quantification of reactive iron for a given sediment, for example by the parameter termed ‘‘degree of pyritization’’ ŽDOP: Raiswell and Berner, 1985., is essential for any meaningful interpretation of d34 S values. The availability of sulfate as terminal electron acceptor and its reduction rate, the latter determined
for example by type and availability of organic compounds as electron donors, are the key factors affecting the sulfur isotopic fractionation associated with bacterial sulfate reduction. As a rule of thumb, the isotopic fractionation increases with decreasing reduction rate Že.g., Harrison and Thode, 1958; Kaplan and Rittenberg, 1964., although this proposition has recently been challenged ŽHabicht and Canfield, 1996.. The reactivity of organic matter is considered to be the most important parameter. Marine organic matter is a much more effective substrate than the terrestrial one because it contains a greater abundance of easily metabolizable organic compounds Že.g., Lyons and Gaudette, 1979.. The availability of sulfate and its impact on the magnitude of isotopic fractionation must be considered from the point of view of two different scenarios. Experimental studies have shown that isotopic fractionation during bacterial sulfate reduction is largely independent of absolute sulfate abundance down to a concentration of 1 mM, but decreases below this level Že.g., Ohmoto, 1992.. More important, however, is the possible diminution of sulfate with time due to ongoing bacterial sulfate reduction, an environmental situation termed ‘‘closed system’’. This continuous decrease in sulfate concentration is accompanied by increasingly more positive d34 S values resulting from preferential reduction of the 32 S isotope. Such closed system conditions can be a result of, for example, diminishing sulfate diffusion from the overlying water column due to increasing burial depth, high sulfate reduction rate exceeding sulfate replenishment Že.g., Zaback et al., 1993. due to favourable conditions for sulfate reducers or a combination of both factors. 4.2. The Phanerozoic sulfide record
Fig. 4. Sulfurrcarbon ratios from marine siliciclastic sediments of Neoproterozoic and Phanerozoic ages Ždata from Raiswell and Berner, 1986; Strauss, 1993b..
Interpretation of the sulfide sulfur isotope record has to consider therefore all the above aspects. The long-term trend for the entire Phanerozoic ŽFig. 5. broadly parallels the sulfate curve Žsee Fig. 1., with maximum values in the early Paleozoic, minimum values in the Permian and a shift back to heavier values in the Cenozoic. As with all other isotope records, the prime objective of this contribution is the detection of possible evolutionary patterns, either biological or environmental, thereby carefully distin-
H. Straussr Chemical Geology 161 (1999) 89–101
Ø The difference between d34 S sulfate and d S sulfide-max ŽD3. displays the greatest variability at y13 " 18.8‰. Most notable is an apparent difference between the early Paleozoic with its positive values Žsulfide sulfur isotope values heavier than the corresponding seawater sulfate. and the later part of the Phanerozoic. Although a definitive statement is hindered by the very limited dataset, the change appears to have occurred in the upper Devonian Žaround 370 Ma.. Prior to interpreting these observations, a few explanatory notes regarding the existing sulfide sulfur isotope record are necessary in order to demonstrate its limitations for deciphering evolutionary trends. Most data have been generated from whole rock samples of substantial size, likely masking any genetic and isotopic differences between individual pyrite grains ŽOhmoto, 1992.. For many studies, only very limited to no ‘‘background’’ information Žsuch as pyrite morphology, abundances of organic carbon, pyrite sulfur andror reactive iron. exists that would enable the genetic history of the sedimentary sulfides to be constrained which is particularly important for distinguishing local from global signals and syngenetic from later diagenetic patterns. These limitations result in substantial ‘‘noise’’ in the existing isotope record, enabling only very broad evolutionary pattern to be clearly discernible. In most studies, variable but generally negative d34 S values have been regarded as clear evidence that sedimentary pyrite formed by bacterial sulfate reduction. This qualitative conclusion, however, is insufficient to describe the observed variability in d34 S sulfide for individual time segments or more specifically for individual stratigraphicrsedimentary units. It is a pattern typical for the entire Phanerozoic and Proterozoic ŽStrauss, 1993b., yet its causes have generally not been discussed. Most promising in this respect appears to be a comparison of pyrite morphology and d34 S values ŽRaiswell, 1982; Strauss and Schieber, 1990; Nielsen, 1997.. Such consideration indicates that syngenetic to early diagenetic, commonly framboidal pyrite is frequently located at the 34 S depleted end of a given sulfur isotope range, whereas the late diagenetic morphotypes Ži.e., large idiomorphic cubes, concretions. almost always display 34 S enriched values. This reflects the continuous evolution of the geochemical environment due to 34
Fig. 5. Sulfur isotopic composition of Phanerozoic sedimentary sulfides Žmodified and updated after Strauss, 1997; ages after Harland et al., 1990.. Horizontal bars indicate ranges of sulfur isotopes, vertical bars, positioned at the average isotopic composition, represent the age uncertainty.
guishing global from local signals. Accepting the fragmentary nature of the presently existing record, I shall utilize average isotopic compositions of sulfide and sulfate sulfur for 10 Ma intervals ŽTable 1.. For the evaporite-based sulfate record, only the mean d34 S values are considered, while for sulfides the average, minimum and maximum are provided, resulting in three estimates for D34 S values ŽFig. 6.. Despite substantial gaps, particularly in the Paleozoic, several observations can be made that relate to both biological and environmental phenomena: Ø Individual time segments exibit variable and frequently substantial differences between minimum and maximum d34 S sulfide values, up to 75‰. This variability renders questionable the significance of an average sulfide sulfur isotopic composition for a given time segment or a sedimentary unit. Ø Acknowledging some variability, the isotopic difference between sulfate sulfur and the minimum sulfide sulfur value ŽD2. varies within y51‰" 8.3‰ throughout the entire Phanerozoic. Judged by its standard deviation in comparison to the other data, it is the best constrained D34 S value, possibly indicating some consistency with time in the initial kinetic isotope fractionation associated with bacterial sulfate reduction.
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Table 1 Average sulfur isotopic compositions for sulfate and sulfide in 10 Ma intervals Age ŽMa.
d 34 S avg
Standard deviation
d 34 S avg
Recent y5 y15 y25 y35 y45 y55 y65 y75 y85 y95 y105 y115 y125 y135 y145 y155 y165 y175 y185 y195 y205 y215 y225 y235 y245 y255 y265 y275 y285 y295 y305 y315 y325 y335 y345 y355 y365 y375 y385 y395 y405 y415 y425 y435 y445 y455 y465 y475 y485 y495 y505
20.3 20.2 22.1 16.5 21.5 19.8 17.2 18.2 16.2 18.2 15.0 15.6 14.5 15.6 15.5
0.1 1.7 0.9 5.5 1.2 2.6 0.5 1.1 1.6 1.1 1.1 2.1 1.2 1.3 0.6
7 10 4 19 19 3 2 4 11 5 32 8 9 3
17.1
1.4
23
16.8
1.1
16
11.7 11.8
0.6 1.7
2 21
16.4 17.1 25.9 10.9 12.4
2.7 0.2 4.6 1.3 1.2
82 2 160 144 9
14.6 15.8
0.7 2.1
9 8
15.9
1.1
37
27.6 20.5 20.2 19.7
2.4 1.6 4.1 2.4
43 12 45 20
28.2 24.8
1.0
1 4
25.4
0.5
13
24.1
5.0
5
N
Standard deviation
y18.2
N
d 34 S min
381
y54.0
d 34 S max
D1
D2
D3
28.0
y38.5
y56.7
y13.9
3.2
9.7
12
y18.5
15.1
y19.0
y40.6
y7.1
y25.3 y16.3 y24.3 y24.8
9.4 14.6 7.1 4.3
32 26 13 24
y38.5 y34.7 y37.1 y30.4
0.3 16.8 y8.1 y13.1
y43.5 y32.5 y42.4 y39.8
y56.7 y50.9 y55.3 y45.5
y17.9 0.6 y26.1 y28.1
y39.7 y31.5 y27.2
7.2 5.3 8.6
8 3 13
y48.6 y36.2 y36.1
y28.4 y24.1 y2.8
y54.2 y47.1 y42.7
y63.1 y51.8 y51.6
y42.9 y39.7 y18.3
y36.8
4.6
28
y41.0
y19.0
y53.9
y58.1
y36.1
y13.3
6.9
22
y25.8
y2.6
y20.8
9.3
39
y40.9
y2.9
y37.2
y57.3
y19.3
y39.0 y23.2
8.6 7.9
17 36
y51.9 y38.1
y25.2 y10.0
y49.9 y35.6
y62.8 y50.5
y36.1 y22.4
y29.0 y12.6
1.6 8.5
3 54
y31.0 y33.0
y27.0 9.0
y27.2
y47.6
y5.6
y14.7 y0.7 y4.8 7.4
10.7 9.1 11.1 9.5
114 11 43 43
y47.0 y13.0 y29.9 y24.0
26.7 20.0 29.8 26.0
y28.3 y25.3 y12.9
y40.6 y50.4 y44.2
y7.6 9.3 5.8
y2.7 y6.3 5.8
18.7 11.5 9.2
31 17 3
y36.6 y22.1 y1.4
38.8 14.1 18.8
y27.6
y61.4
14.0
y6.7
9.4
5
y23.0
1.5
H. Straussr Chemical Geology 161 (1999) 89–101
97
Table 1 Žcontinued. Age ŽMa. y 515 y525 y535 y545 y555 y565
d 34 S avg
30.3
Standard deviation
2.4
N
63
d 34 S avg
Standard deviation
N
d 34 S min
d 34 S max
y2.5 y1.4 16.0
5.4 7.6 6.0
7 8 68
y8.0 y13.0 y0.8
8.0 8.0 34.1
17.7
18.3
25
y20.1
52.8
progressing bacterial sulfate reduction. Resulting limitation of the sulfate supply at the site of bacterial reduction appears to be the most critical factor. The documented, frequently substantial, spread of d34 S sulfide values for a given sedimentary unit is thus best explained in terms of a ‘‘time series’’. Variations in the sulfur isotopic composition within a given sedimentary unit would reflect the diagenetic redox-Žre-.-cycling of sulfur phases including various stages from partial to complete closure of the system. The development of closed system conditions during diagenesis could well be triggered through high sedimentation rates, resulting in continuously decreasing diffusion of sulfate from the water column into the sediment. As a consequence, the diagenetic system would soon be exclusively dependent on porewater sulfate with no further replenishment. Goldhaber and Kaplan Ž1975., Maynard Ž1980., and others have noted a relationship between the sulfur isotopic composition of biogenic sulfides and sedimentation rate. Thereby, coarser grained sed-
Fig. 6. Isotopic difference between sulfate and sulfide sulfur during Phanerozoic time ŽD1: D34 S sulfate r sulfide - avg ; D2: D34 S sulfate r sulfide - min ; D3: D34 S sulfate r sulfide - max ..
D1
D2
D3
y14.3
y31.1
3.8
iments are characterized by more positive d34 S values. Clearly, any given average isotopic composition reflects how complete this diagenetic system has been sampled. In that regard, any additional information characterizing the depositional andror geochemical environment of pyrite formation will assist in interpreting observed sulfur isotope variations. The above qualitative statement relating d34 S values to biogenicity has always been complemented by a quantitative component, specifically the observed maximum isotopic fractionation of y46‰ for culture experiments ŽKaplan and Rittenberg, 1964. compared to even larger displacements of up to y60‰ in recent natural settings ŽChambers and Trudinger, 1979.. This long-standing discrepancy has not been understood until the importance of thiosulfate andror elemental sulfur disproportionation and their associated isotope effects were demonstrated ŽJørgensen, 1990; Canfield and Thamdrup, 1994; Canfield and Teske, 1996.. Canfield and Teske Ž1996. in particular interpret the Phanerozoic sulfide sulfur isotope record as indicative of a combination of bacterial sulfate reduction and additional internal, oxidative recycling of intermediate sulfur species. Thereby, associated isotope discrimination would enlarge the overall isotopic difference between sea water sulfate and biogenic sulfide. It should be pointed out, however, that only a very limited initial database on isotopic fractionation during bacterial sulfate reduction was generated in the 1950s and 1960s ŽHarrison and Thode, 1958; Kaplan and Rittenberg, 1964; Kemp and Thode, 1968.. The renewed interest in experimental work in that respect Že.g., Habicht and Canfield, 1996; Smock et al., 1998. will advance our understanding with respect to sulfur isotope fractionation. In summary, the true magnitude of sulfur isotopic fractionation associated
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with bacterial sulfate reduction, i.e., the kinetic isotope effect, is still largely unknown for most natural environments and for many of the environmentally important sulfate reducing bacteria. Nevertheless, the fact that the existing sulfur isotope records display a more or less equal displacement between parental seawater sulfate and the most 34 S depleted pyrite Ž d34 S sulfide-min . throughout the entire Phanerozoic argues for some biological consistency in the initial isotopic fractionation. This observation suggests that the same group of sulfate reducing bacteria Žand its biochemical pathway. were important in the geological past and in modern marine settings ŽJørgensen, 1982.. The total spread of d34 S for a given sedimentary unit and the relative proportion of 34 S depleted versus 34 S enriched values for biogenic pyrite is strongly dependent on the method of sampling Ži.e., whole rock vs visible sulfide vs microdrilling.. Assuming that this ‘‘bias’’ is similar for all available datasets, an evaluation of the isotope difference between d34 S sulfate and d34 S sulfide-max through time ŽD3 in Fig. 6. offers the base for some interesting speculations. Although associated with some ‘‘noise’’, it appears that the early Paleozoic time segments are all characterized by positive to strongly positive d34 S sulfide-max values and, more importantly, that these maximum d34 S sulfide values exceed the d34 S sulfate values Ži.e., a positive D3.. A similar pattern can be observed also for the average ŽD1. trend. This lower Paleozoic feature extends into the Proterozoic ŽStrauss, 1993b. and likely reflects a secular trend. Considering that the availability of sulfate and the rate of its reduction, the latter reflecting the availability of easily metabolizable organic material, are the most important factors that control d34 S sulfide , it is interesting to note that the shift in Dsulfatersulfide-max appears to have occurred around 370 Ma ago. This was the time of rapidly increasing importance of land plants on Earth Že.g., Algeo et al., 1995; Algeo and Scheckler, 1998., providing additional substrate available to marine sulfate reducers. The less-easily metabolizable terrestrial compounds resulted in a less efficient mineralisation of sedimentary organic matter by sulfate reducers, an observation consistent with the higher SrC ratios for normal marine sediments ŽFig. 4. in the early Paleozoic ŽRaiswell and Berner, 1986.. The observed sulfur isotope pattern for sedimentary pyrite
would be consistent with a scenario where, due to the more reactive purely marine organic matter, sulfate reduction in the early Paleozoic would have been rapid and efficient, quickly exhausting sulfate at the site of bacterial reduction. The availability of easily metabolizable organic matter would have been even greater in anoxic waters which might have been more frequent in early Paleozoic oceans, as suggested, for example by Berry and Wilde Ž1978., Goodfellow and Jonasson Ž1984. and Goodfellow Ž1987.. With the growing input of less metabolizable land plant detritus into the nearshore, marine environment, bacterial sulfate reduction in the younger Phanerozoic would utilize the rapidly diminishing easily-metabolizable carbon source, while sulfate was still in unlimited supply. In summary, the available fragmentary d34 S sulfide record is consistent with the operation of bacterial sulfate reduction characterized by a more or less constant kinetic isotope fractionation around y51‰ throughout the entire Phanerozoic. The overall spread in d34 S sulfide for any given stratigraphic unit is a reflection of the interplay of several parameters, such as the quantity of easily metabolizable organic matter, sulfate availability and abundance of reactive iron Ži.e., reservoir effects.. The evolution of land plants and their subsequent impact on the pool of marine sedimentary organic matter may have affected the global sulfur cycle, as indicated by the sulfur isotope record, further supporting the interdependancy of geochemical cycles of sulfur and carbon.
5. Summary The utility of the sulfur isotope record as a proxy for the evolution of the global sulfur cycle through time is strongly limited because of its fragmentary nature. This is the case for both the sulfate and sulfide records. The currently emerging seawater sulfate isotope record, derived from structurally substituted sulfate from stratigraphically well constrained biogenic calcites, represents a fundamental improvement in that respect. The available data confirm the overall temporal trend based on evaporite samples but also suggest the existence of frequent shorter-term fluctuations that could not have been
H. Straussr Chemical Geology 161 (1999) 89–101
previously resolved due to insufficient time resolution. The existing sulfide isotope record, despite its noise, still appears to retain vestiges of initial biological and environmental signals. Bacterial sulfate reduction displays a constant maximum fractionation of around y51‰ throughout the entire Phanerozoic. Whether this magnitude of fractionation is solely a product of sulfate reduction or includes isotope effects linked to disproportionation reactions needs to be clarified through further systematic culture experiments. The large d34 S sulfide variability for any given sedimentary unit is best explained as a time-series of pyrite formation. Strongly positive maximum d34 S sulfide values in early Paleozoic indicate that bacterial sulfate reduction proceeded in a rapid and efficient fashion, substantially diminishing the available sulfate pool. In the younger Phanerozoic, evolution of land plants led to a change in organic substrate from purely marine algal matter to growing input from land plants, the latter were more difficult to metabolize. This transition appears to have occurred in the Mid-rLate Devonian, as indicated also by a change in the mean SrC ratio of normal marine sediments. Renewed and growing interest in the isotopic composition of sedimentary sulfur and its evolution through time is greatly improving the existing fragmentary isotope records. Once completed, their time resolution will be comparable to that of other isotope systems, yielding a detailed proxy signal for the geochemical, biological and geological evolution of the SYSTEM EARTH.
Acknowledgements This study represents a contribution to the Earth System Evolution Program of the Canadian Institute for Advanced Research ŽCIAR.. Inspiring discussions with program members over the years as well as support through CIAR are gratefully acknowledged. Part of this research has been funded through the Deutsche Forschungsgemeinschaft ŽGrants: Ve112r8-1; Str281r7-1.. Unpublished sulfur isotope data from biogenic carbonates were kindly provided by A. Kampschulte. This is also a contribution
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