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Stable isotopes in evaporite systems: Part I: Sulphur

John Warren - Monday, April 30, 2018

 

Introduction

The sulphur isotopic composition of sulphate dissolved in modern seawater (SW), and the relationship with the associated modern and ancient sulphate precipitates, has been studied for more than five decades. An understanding of the controlling factors is fundamental in any interpretation of the origin of modern and ancient sedimentary calcium sulphates.

So, we shall look at the significance of sulphur isotopes, first by reviwing what is known in terms of the isotopic evolution of marine sulphate salts across the evaporation series from gypsum to the bitterns, and then across a time perspective via the evolution of oceanic sulphate and sulphide signatures from the Archean to the present.


Sulphur isotopes across the bittern series

The accepted d34S value of modern seawater-derived calcium sulphate (gypsum) is + 20.0 ±0.2‰ (Sasaki, 1972; Zak et al., 1980 and references therein). This is a average value, based on numerous analyses across the range ( +19.3 to +21.4‰). Notably, Rees et al. (1978) obtained a mean of +20.99 ± 0.09‰, using the SF6 method, which has a better reproducibility than the conventional S02 method. Mediterranean seawater gave a d34S value of +20.5‰ (Nielsen, 1978).

Measured values in natural gypsum from seawater show initial precipitates have a d34S value slightly higher than that of its source brine (Figure 1). The highest isotope differential for gypsum naturally precipitated from seawater, as recorded in the literature, is +4.2‰ (Laguna Madre, Texas, U.S.A.; Thode, 1964). Most reported d34Sgypsum-sw differentials lie in range from 0 to + 2.4‰ (Ault and Kulp, 1959; Thode et al., 1961; Thode and Monster, 1965; Holser and Kaplan, 1966).

Prior to Raab and Spirto (Figure 1; 1991), laboratory experiment data on d34Sgypsum-solution are scarce, especially for solutions mimicking initial precipitation of gypsum from natural seawater and passing into halite saturation. Harrison (1956) measured a d34Sgypsum-solution value of ~ + 2‰ for gypsum precipitated from an artificial solution, that was saturated with respect to gypsum. Thode and Monster (1965) calculated a K-value [ (32S/34S)solution/ (32S/34S)gypsum] of 1.00165 from a measured a d34Sgypsum-solution value of + 1.65‰ for a CaSO4.2H2O -saturated solution, evaporated under reduced pressure and allowed to age and equilibrate for 24 months at room temperature. An experiment using natural seawater was carried out by Holser and Kaplan (1966), who sampled the products of evaporating seawater in a tank with continuous refilling (green circles in Figure 1). The results show “only a small difference between brine and gypsum precipitated” (Holser and Kaplan, 1966, p.97), resulting in a mean value of d34Sgypsum-seawater = +1.7‰ (+19.4 to +21.1‰). Harrison (1956) calculated from experimental vibrational frequencies for S04 in solution and in crystalline CaSO4.2H2O, a constant K = 1.001 for the reaction:

(Ca34S04.2H2O + 32SO4)SOLID = (Ca32S04.2H2O + 34SO4)SOLUTION

which means a 1‰ increase of d34S in the solid fraction. Nielsen (1978, p. 16-B-20), using Rayleigh-type fractionation curves indicates that, “...the gypsum/anhydrite of the sulphate facies should be slightly enriched in 34S with respect to the unaffected seawater sulphate”

In the geological record the evaporites of the later Mg- and K-Mg- sulphate bittern facies are depleted in 34S relative to the earlier, basal Ca-sulphates, as rseen in the geological record. Nielsen and Ricke (1964, p.582) give a mean value of +2‰ for the depletion in 34S in later bittern evaporite sulphates relative to the basal Ca-sulphates in the Upper Permian Zechstein Series (Hattorf and Reyershausen, Germany) whereas Holser and Kaplan (1966, pp. 116 and 117) give a value of -1.0±0.8‰ (their d34Spotash-magnesia facies sulphates - d34Sgypsum/anhydrite facies) for the Zechstein Basin (Germany) and -0.8±0.5‰ for the Upper Permian Delaware Basin (U.S.A.) evaporites (green circles in Figure 1).

Theoretical calculations of the behaviour of the sulphur isotopic fractionation during the late evaporation stages were made by Holser and Kaplan (1966, pp. 116 and 117, fig. 4) and by Nielsen (1978, p. 16-B-20, fig. 16-B-12) applying the Rayleigh distillation equation and using the same fractionation factor calculated from the initial gypsum (1.00165). Their curves are thus in a continuous line with those calculated for the Ca-sulphates. These show an increasing degree of depletion in 34S in the sulphates precipitated in the course of the progressive evaporation in a closed basin, relative to the first Ca-sulphate precipitated, up to the end of the carnallite facies. They explain it by the continuous depletion in 34S in the brines. Thus their calculated d34Scrystal-initial gypsum at the end of the halite facies is ~ -0.6‰, at the end of the Mg-sulphate facies -1.0‰, and at the beginning of the carnallite facies -3.8‰, and relative to the original seawater (their 34SC) the differences are +1.0, +0.4 and -2.2‰, respectively. Nielsen (1978) also plotted an extrapolated fractionation curve for the residual brines in a closed reservoir, indicating that the brine is constantly depleted by 1.65‰ relative to the associated precipitate.

Prior to the laboratory work of Raab and Spiro (1991), no experimental data pertaining to the isotopic behaviour of sulphate sulphur in the late evaporative stages of seawater was available in the literature. Raab and Spiro evaporated seawater, stepwise and isothermally at 23.5°C, for 73 days, up to a degree of evaporation of 138x by H2O weight. At various stages of evaporation the precipitate was totally removed from the brine and the brine was allowed to evaporate further. The sulfur isotopic compositions of the precipitates and related brines showed the following characteristics (Figure 1) where the initial d34S of the original seawater is +20‰. The d34S of both precipitates and associated brines decrease gradually across the gypsum field nd aup to the end of the halite field, where d34Sprecipitate = + 19.09‰ and d34Sbrine = + 18.40‰. The precipitates are always enriched in 34S relative to the associated brines in these fields, but the enrichment becomes smaller towards the end of the halite field. A crossover, where the d34S value of the brines becomes higher than those of the precipitates, occurs at the beginning of the Mg-sulfate field. The d34Sprecipitate increases from + 19.09‰ at the end of the halite field through +19.35‰ in the Mg-sulfate field to + 19.85‰ in the K-Mg-sulfate field, whereas the d34Sbrine increased from +18.40‰, through +20.91‰ to +20.94‰, respectively.

This evolution implies different values of fractionation factors (a) for the minerals precipitated in the late halite, Mg-sulphate and K-Mg-sulphate fields, other than that for gypsum (1.00165). The value of aprecipitate-residual brine would then be very slightly >1 in the late halite field and >1 in the two later fields.

The experimental pattern of evolution of the d34S-values of the precipitates from their experiment is in good agreement with data for natural anhydrites interbedded in halites, where d34S-values are lower relative to basal gypsum (and secondary anhydrite), and of primary minerals of the Mg- and K-Mg-sulfate facies, reported in evaporitic sequences, such as those of the Delaware (U.S.A.) and of the Zechstein (Germany) basins and so can be used to better interpret a marine origin of the sulphate bitterns.


Ancient oceanic sulphate

The element sulphur is an important constituent of the Earth’s exogenic cycle. During the sulphur cycle, 34S is fractionated from 32S, with the largest fractionation occurring during bacterial reduction of marine sulphate to sulphide. Isotopic fractionation is expressed as d34S, in a manner similar to that used for carbon isotopes and the longterm carbon curves related to the sulphur isotope curve across deep time (see next article). Sedimentary sulphates (mostly measured on anhydrite, but also baryte) typically are used to record the isotopic composition of sulphur in seawater (Figure 2). Mantle d34S is near 0‰, and bacterial reduction of sulphate to sulphides (mostly as pyrite) strongly prefers 32S, thus reducing d34S in organic sulphides to negative values (≈ -18‰), so leaving oxidized sulphur species with approximately equivalent positive values (+17‰; Figure 3).


Historically, the sulphur cycle has been interpreted as being largely controlled by the biosphere and in particular by sulphate-reducing bacteria that inhabit shallow marine waters (Strauss, 1997). Typically, sulphur occurs in its oxidized form as dissolved sulphate in seawater or as evaporitic sulphate and in its reduced form as sedimentary pyrite. The isotopic compositions of both redox states are sensitive indicators for changes of the geological, marine geochemical or biological environments in the past (Figure 2). The isotope record of marine sedimentary sulphate through time has been used successfully to determine global variations of the composition of seawater sulphate.

The isotopic composition of sedimentary (biogenic) pyrite reflects geochemical conditions during its formation via bacterial sulphate reduction. Sedimentary pyrite is, thus, an important record of evolutionary (microbial) processes of life on Earth. Both time records (anhydrite and pyrite) have been combined in an isotope mass balance calculation, and changes in burial rates of oxidized vs. reduced sulphur can be determined (Strauss, 1997). This, in turn, yields important information for the overall exogenic cycle (i.e. the earth's oxygen budget as discussed in the next article).

And so, values preserved in ancient marine sulphate evaporites are part of the broader world sulphur cycle across deep time that includes movements in and out of marine sulphides (dominantly pyrite) and marine baryte precipitates (Figure 2). Values based on evaporitic CaSO4 are consistent with the ranges seen in modern gypsum (Figure 3). A plot of ancient marine CaSO4 evaporites shows the oxxidised sulphur curve for seawater has varied across time from +30‰ in the Cambrian, to around +10‰ in the Permian and that it increased irregularly in the Mesozoic to its present value of +20‰ (Figure 4). Oxygen values show much less variability and will be discussed in more detail in the next article in this series. Time-consistent variations are reflected in all major marine sulphate evaporite deposits and were most likely controlled by major input or removal of sulphides from the oceanic reservoirs during changes driven by longterm variations in tectonic activity and weathering rates.

Historically, simple removal of oceanic sulphate via an increase in the volume of megasulphate deposition in a saline giant was not thought to be accompanied by dramatic isotopic effects. Rather, variations within the global sulphur cycle were thought to be controlled by a redox balance with stored sulphides and organics in more reducing environments, which are also linked to the carbon cycle and the atmospheric oxygen budget.

In this scenario the oxidative part of the global sulphur cycle is largely governed by continental weathering (especially of marine black shale), riverine transport and evaporite deposition, while the reduced part of the sulphur cycle is controlled by levels of fixation of reduced sulphur-bearing compounds in the sediment column, mostly as pyrite via bacterial sulphate reduction (Figure 2.). The latter process preferentially removes isotopically light sulphur from seawater and so increases the d34S value in the ocean, and any consequent precipitate.

However, more recent work question aspects of this older sulphur cycle/pyrite/organics model. As just discussed, variations in d34Ssulphate across the Phanerozoic are traditionally interpreted to reflect changes in the total amount of sulphur buried as pyrite in ocean sediments — a parameter referred to as fpyr and defined as (Hurtgen, 2012);

fpyr = [(pyrite Sburial)/(pyrite Sburial + evaporite S burial)].

However, Wortmann and Paytan (2012) conclude that the 5‰ negative d34Ssulphate shift in ~120-million- year-old rocks was caused by massive seawater sulphate removal, which accompanied large-scale evaporite deposition during the opening of the South Atlantic Ocean (Figure 4). In their model, the negative d34Ssulphate shift is driven by lower pyrite burial rates that result from substantially reduced marine sulphate levels in the world ocean, tied to megasulphate precipitation. The authors attribute a 5‰ positive d34Ssulphate shift in the world’s oceans about 50 million years ago to an abrupt increase in marine sulphate concentrations as a result of large-scale dissolution of freshly exposed evaporites; they argue that the higher sulphate concentrations in the ocean in turn led to more pyrite burial.


Likewise, Halevy et al. (2012 ) studied past sulphur fluxes to and from the ocean, but over a longer time-frame (the Phanerozoic). They quantified sulphate evaporite burial rates through time, then scaled these rates to obtain a global estimate of variation in sulphur flux. Their results indicate that sulphate burial rates were higher than previously estimated, but also greatly variable. When Halevy et al. (2012) integrated these improved evaporite burial fluxes with seawater sulphate concentration estimates and sulphur isotope constraints, their calculations implied that Phanerozoic fpyr values (fpyr = fraction of sulphur removed from the oceans as pyrite) were ~100% higher on average than previously recognized. These surprisingly high and constant pyrite burial outputs must have been balanced by equally high and constant inputs of sulphate to the ocean via sulphide oxidation (weathering). These relatively high and constant rates of pyrite weathering and burial over the Phanerozoic, as identified by Halevy et al. (2012, suggest that the consumption and production of oxygen via these processes played a larger role in regulating Phanerozoic atmospheric oxygen levels than previously recognized, perhaps by as much as 50%.

Both studies recognize the importance of episodic evaporite burial on the sulphur cycle, while Wortmann and Paytan (2012) clearly show that large-scale deposition and dissolution of sulphate evaporites over relatively short geologic time scales can have an enormous impact on marine sulphate concentrations, pyrite burial rates, and the carbon cycle and so probably play a more important role than previously recognised in regulating the chemistry of the ocean atmosphere system.

The 18O content in seawater sulphate fluctuates less than sulphur values over geologic time (see next article for detailed discussion). The isotopic composition of sulphate minerals varied only slightly from the Neoproterozoic to the Palaeozoic decreasing from +17 to +14‰ (Figure 4). Values then rose during the Devonian to reach +17‰ during the Early Carboniferous (Mississippian). Values then fell to =+10‰ during the Permian, mimicked by a similar decline in sulphur values in the Late Permian to Early Triassic. Since the rise to +15‰ in the Early Triassic, values of marine sulphate minerals have remained close to +14‰ (add 3.5‰ to mineral determined value to give ambient seawater value). Overall, oxygen values show little correlation with marine sulphate variation and are perhaps are more controlled by sulphide weathering reactions.

What is also significant is that, given the now well established sulphur isotope age curve, a comparison of a measured d34S value from an anhydrite or gypsum of known geological age to the curve allows an interpretation of a possible marine origin to the salt. A value which differs from the marine signature does not necessarily mean a nonmarine origin, but, at the least, it does mean diagenetic reworking or, more likely, a groundwater-induced recycling of sulphate ions into a nonmarine saline lake (Pierre, 1988). Such oxygen and sulphur isotopic crossplots have been used to establish the continental (nonmarine) origin of the Eocene gypsum of the Paris Basin and the upper Miocene gypsum of the Granada basin, with sulphate derived from weathering of uplifted Mesozoic marine evaporites (Fontes and Letolle, 1976; Rouchy and Pierre, 1979; Pierre, 1982).

Sulphur is largely resistant to isotopic fractionation during burial alteration and transformation of gypsum to anhydrite (Figure 5; Worden et al., 1997). For example, primary marine stratigraphic sulphur isotope variation is preserved in anhydrites of the Permian Khuff Formation, despite subsequent dehydration to anhydrite during burial (≈1,000m) and initial precipitation as gypsum from Permian and Triassic seawater. Gypsum dehydration to anhydrite did not involve significant isotopic fractionation or diagenetic redistribution of material in the subsurface. At depths greater than 4300 m, the same sulphur isotope variation across the Permian-Triassic boundary is still present in elemental sulphur and H2S, both products of the reaction of anhydrite with hydrocarbons via thermochemical sulphate reduction (Figure 5). Clearly, thermochemical sulphate reduction did not lead to sulphur isotope fractionation. Worden et al. also argues that significant mass transfer has not occurred in the system, at least in the vicinity of the Permian-Triassic boundary, even though elemental sulphur and H2S are both fluid phases at depths greater than 4300 m. Primary differences in sulphur isotopes have been preserved in the rocks and fluids, despite two major diagenetic overprints that converted the sulphur in the original gypsum into elemental sulphur and H2S by 4300 m burial and the potentially mobile nature of some of the reaction products. That is, all reactions occurred must have occurred in situ; there was no significant sulphur isotope fractionation, and only negligible sulphur was added, subtracted, or moved internally within the system.


The resistance to fractionation of sulphur isotopes in subsurface pore waters can also be utilised to determine the origin of saline thermal pore waters. In a study of sulphur isotopic compositions of waters in saline thermal springs, Risacher et al. (2011) came to the interesting conclusion that dissolution of continental sedimentary gypsum from the Tertiary-age Salt Cordillera was the dominant supplier of sulphate (Figure 6). The sulphate in the springs was not supplied by the reworking of volcanic sulphur in this active volcanic terrain. d34S values from 3 to 11‰ in continental gypsum and this also encompasses the range of d34S in pedogenic gypsum (5 to 8‰) and in most surface waters (3.4 to 7.4‰) including salt lakes (Rech et al., 2003). Frutos and Cisternas (2003) found isotope ratios ranging from 1.5 to 10.8‰ in five native sulphur samples. Figure 6 presents the sulphur isotope ratio of dissolved sulphate in thermal waters sampled by Risacher et al. (2011) and references therein. The d34S of sulphate in northern thermal springs is within the range of salt lakes waters and continental gypsum. In an earlier paper Risacher et al. (2003) showed that salar brines leak through bottom sediments and are recycled in the hydrologic system. Deep circulating thermal waters are dissolving continental gypsum in sedimentary layers below the volcanics associated with the present day salars. The exception to this observation is the sulphur in Tatio springs where Cortecci et al. (2005) proposed a deep-seated source for the sulphate, related to magma degassing (Figure 6).


References

Cortecci, G., Boschetti, T., Mussi, M., Herrera Lameli, C., Mucchino, C. and Barbieri, M., 2005. New chemical and original isotopic data on waters from El Tatio geothermal field, northern Chile. Geochemical Journal 39: 547-571.

Fontes, J.C. and Letolle, R., 1976. 18O and 34S in the upper Bartonian gypsum deposits of the Paris Basin. Chemical Geology, 18(4): 285-295.

Frutos, J. and Cisternas, M., 2003. Isotopic Differentiation in Volcanic-Epithermal Surface Sulfur Deposits of Northern Chile: d34S < 0‰ in “Fertile” Systems (Au-Ag-Cu Ore Deposits below), versus d34S ≥ 0‰ for “Barren” Systems. Short Papers - IV South American Symposium on Isotope Geology (Salvador, Brazil, 2003): 733-735.

Halevy, I., Peters, S.E. and Fischer, W.W., 2012. Sulfate Burial Constraints on the Phanerozoic Sulfur Cycle. Science, 337(6092): 331-334.

Holser, W.T. and Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulfates. Chemical Geology: 93-135.

Hurtgen, M.T., 2012. The Marine Sulfur Cycle, Revisited. Science, 337(6092): 305-306.

Nielsen, H., 1978. Sulfur isotopes in nature. In: K.H. Wedepohl (Editor), Handbook of Geochemistry Section 16B, pp. B1 - B40.

Nielsen, H. and Ricke, W., 1964. Schwefel-lsotopenverhältnissen von Evaporiten aus Deutschland; Ein Beitrag zur Kenntnis von d34S im Meerwasscr-Sulfat. Geochimica et Cosmochimica Act, 28: 577-591.

Pierre, C., 1982. Teneurs en isotopes stables (18O, 2H, 13C, 34S) et conditions de genese des evaporites marines; application a quelques milieux actuels et au Messinien de la Mediterranee. Doctoral Thesis, Orsay, Paris-Sud.

Raab, M. and Spiro, B., 1991. Sulfur isotopic variations during seawater evaporation with fractional crystallization. Chemical Geology: Isotope Geoscience section, 86(4): 323-333.

Rech, J.A., Quade, J. and Hart, W.S., 2003. Isotopic evidence for the source of Ca and S in soil gypsum, anhydrite and calcite in the Atacama Desert, Chile. Geochimica et Cosmochimica Acta 67(4): 575-586.

Rees, C.E., Jenkins, W.J. and Monster, J., 1978. The sulfur isotopic composition of ocean water sulphate. Geochimica et Cosmochimica Acta, 43: 377-381.

Risacher, F., Fritz, B. and Hauser, A., 2011. Origin of components in Chilean thermal waters. Journal of South American Earth Sciences, 31(1): 153-170.

Rouchy, J.M. and Pierre, C., 1979. Donnees sedimentologiques et isotopiques sur les gypses des series evaporitiques messiniennes d'Espagne meridionale et de Chypre. Rev. Geogr. Phys. Geol. Dyn., 21(4): 267-280.

Sasaki, A., 1971. Variation in sulfur isotope composition of oceanic sulfate. 14th Int. Geol. Congr. Sect. 1: 342-345.

Strauss, H., 1997. The isotopic composition of sedimentary sulfur through time. Palaeogeography Palaeoclimatology Palaeoecology, 132: 97-118.

Thode, H.D., 1964. Stable isotopes a key to our understanding of natural processes. Bulletin Canadian Petroleum Geologists, 12: 246-261.

Thode, H.G. and Monster, J., 1965. Sulfur-Isotope Geochemistry of Petroleum, Evaporites, and Ancient Seas, Fluids in Subsurface Environments. AAPG Memoir 4, pp. 367-377.

Worden, R.H., Smalley, P.C. and Fallick, A.E., 1997. Sulfur cycle in buried evaporites. Geology, 25(7): 643-646.

Wortmann, U.G. and Paytan, A., 2012. Rapid Variability of Seawater Chemistry Over the Past 130 Million Years. Science, 337(6092): 334-336.

Zak, I., Sakai, H. and Kaplan, R., 1980. Factors controlling the 18O/16O and 34S/32S isotopic ratios of ocean sulfates and interstitial sulfates from modern deep sea sediments. In: E.D. Goldberg, Y. Horibe and K. Saruhaki (Editors), Isotope Marine Chemistry. Geochem. Res. Assoc, Tokyo, pp. 339-373.


 


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