Geochemical Transformations of Sedimentary Sulfur - ACS Publications

Chapter 18

Sulfate Incorporation into Sedimentary Carbonates

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Wilfried J. Staudt and Martin A. A. Schoonen Department of Earth and Space Sciences, State University of New York, Stony Brook, NY 11794-2100

Modern and ancient sedimentary carbonate minerals (calcite, aragonite and dolomite) generally contain sulfate as a trace constituent with concentrations ranging from approximately 200 to 24,000 ppm. Particularly marine skeletal carbonate components, which account for most of the global calcium carbonate accumulation, contain considerable amounts of sulfate. It is estimated that marine calcium carbonate precipitation accounts for the removal of approximately 5% of the pre-anthropogenic annualriverinesulfate input. Carbonate shelves and platforms that account for 33% to 42% of the present-day calcium carbonate accumulation have estimated average sulfate concentrations of 5900 ppm. These environments, which represent only about 3.3% of the total area where calcium carbonate is produced, account for 55% to 77% of the present-day sulfate removal by calcium carbonate accumulation. Low sulfate content of burial calcites and dolomites is consistent with the release of sulfate during recrystallization of sulfate-rich shelf and platform carbonates under burial conditions. Release of carbonate-bound sulfate upon burial recrystallization of sedimentary carbonates may represent a significant sulfur source in sedimentary basins.

Carbonate rocks comprise approximately 15% of the sedimentary rock volume of the continental crust (7). Sedimentary carbonates, which have been formed throughout the geologic rock record, are mostly, although not exclusively, of marine origin (2) with calcium carbonate secreting organisms accounting for most of the modern carbonate production (3). In modern seawater sulfate is the third most abundant ion (after chloride and sodium) and the second most abundant anion. Incorporation of seawater sulfate in marine skeletal carbonates has been reported by Oomori et al. (4), Volkov and Rozanov (5) and Busenberg and Plummer (6). Sedimentary carbonates, which

0097-6156/95/0612-0332$12.00/0 © 1995 American Chemical Society In Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


18. STAUDT & SCHOONEN

Sedimentary Carbonates

333

were formed by direct precipitation from seawater or modified seawater, such as dolomites also can contain a considerable amount of sulfate (7, 8). How sulfate is present in sedimentary carbonates has received considerable attention. In biologically precipitated carbonates some of the sulfate may be present as an ester sulfate (9) or as lattice-bound sulfate. Lattice-bound sulfate in sedimentary carbonates may either substitute for the carbonate ion or it may in principle be present in interstitial sites. Because the sulfate ion is considerably larger than the carbonate ion and it is tetrahedral instead of planar, it has been conventional wisdom that the substitution of sulfate for carbonate is limited. Based on this notion several studies have therefore speculated that the sulfate in biologically precipitated carbonates is predominantly present as sulfate esters (10). However, infrared and Raman spectroscopic studies indicate that sulfate in corals, oysters and travertines is dominantly present as inorganic sulfate (11). In addition, studies by Busenberg and Plummer (6), Takano et al. (72), Staudt et al. (8) and Staudt et al. (13) support the notion that sulfate substitutes for the carbonate ion in synthetic calcite, travertine and natural dolomites. Reeder et al. (14) analyzed the Se K-edge XAFS spectra of selenate-bearing synthetic calcite crystals. Their study confirms that Se is present as the tetrahedral selenate complex anion substituting for the carbonate site of calcite. Because sulfate is isostructural with selenate but approximately 10% smaller it is reasonable to assume that sulfate also substitutes for the carbonate ion. Furthermore, based on analysis of the sulfur K-edge x-ray absorption near-edge structure (XANES) of natural carbonates Pingitore et al. (15) documented that sulfur is present in the 6+ oxidation state which is consistent with the presence of sulfate. The objective of this study is to evaluate the importance and geological implications of sulfate incorporation into sedimentary carbonates. To attain this objective more than 250 sedimentary carbonate samples including primary, dominantly biologically precipitated aragonites and calcites, as well as diagenetically formed calcites and dolomites were analyzed for their sulfate content. The results of this study indicate that modern marine calcium carbonate production represents a sink for oceanic sulfate. Upon burial and recrystallization of sulfate-rich sedimentary carbonates most of the initially incorporated sulfate may be released. This release of carbonate-bound sulfate may represent a significant sulfur source for the formation of H2S, elemental sulfur or metal sulfide deposits in the subsurface of sedimentary basins.

Research Strategy and Methods To evaluate the importance of sulfate incorporation into sedimentary carbonates we analyzed a large variety of skeletal and non-skeletal carbonate minerals. These samples which represent sedimentary carbonates of various originsfromthe Qrdovician to the Recent generally contain sulfate as a trace constituent. Sulfate incorporation into sedimentary carbonates can be best evaluated using modern marine skeletal components because these have not been altered by diagenetic processes, such as recrystallization. Hence, all marine skeletal carbonates analyzed for sulfate were collectedfromliving organisms. The marine carbonate secreting organisms selected for this study account for the majority of the sediment production in modern carbonate accumulating environments. Non-biogenic carbonates such as calcite cements and

In Geochemical Transformations of Sedimentary Sulfur; Vairavamurthy, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


334

GEOCHEMICAL TRANSFORMATIONS OF SEDIMENTARY SULFUR

dolomites account for only a small fraction of the modern carbonate accumulation. To constrain the amount of sulfate incorporated into calcite cements and dolomites we analyzed samples that were formed by fluids covering a range of sulfate concentrations. The depositional environment and diagenetic history of these dolomites and calcites has been well established based on detailedfieldand pétrographie studies. Using average sulfate concentrations of skeletal and non-skeletal carbonates, their relative abundance in carbonate accumulating environments and an estimate of the global calcium carbonate accumulation we estimated the total removal of oceanic sulfate by calcium carbonate precipitation. The fate of sulfate in sedimentary carbonates was evaluated by comparing an early-diagenetic dolomite of the Burlington-Keokuk Formation with its later-diagenetic dolomite replacement. Dolomites of the Burlington-Keokuk Formation were chosen because their depositional environment, diagenetic history and relative timing of dolomitization have been well documented. Origin of Samples. More than 250 sulfate analyses of modern marine skeletal and non-skeletal calcites, aragonites, as well as calcites and dolomites from the geologic record (Ordovician to Recent) form the basis for this study. Carbonate skeletal components (58) were provided from the collections of P. Bretsky and W.J. Meyers. All modern marine skeletal carbonates were collectedfromliving organisms at several locations in the Caribbean Sea and along the east coast of the United States. Sulfate data of modern marine carbonates were supplemented by 20 analyses published by Oomori et al. (4), Volkov and Rozanov (5), and Busenberg and Plummer (6). Previously well characterized dolomites (141) and calcites (43) were also analyzed for their sulfate content. Their location, age and presumed depositional environment are summarized in Table 1. Sulfate Analysis. A Dionex 2000i Ion Chromatograph (IC) with an AS4A column was used to determine sulfate concentrations in carbonate samples following the method described by Staudt et al. (5). For dolomites a 10 mg portion, and for calcites and aragonites a 2 mg portion of the powdered and rinsed sample material was dissolved in dilute nitric acid and subsequently diluted with deionized water by a factor of 1000 (dolomites) or 2000 (calcites and aragonites). Matrix matching standards for IC analysis were prepared by gravimetric dilution from single stock solutions. Detection limit for sulfate determination in carbonates, determined as three standard deviations of the blank for 63

SO4

Barnacles

ppm

Average

Sulfate content and relative abundance (wt.%) of modern marine carbonate components in carbonate-accumulating environments

Table 2.



340


Figure 3. Contribution (wt.%) of each carbonate-accumulating environment to the estimated total global calcium carbonate accumulation of 3.2* 10 moles per year (after Milliman [79]). The most important carbonate accumulating environment is the deep sea accounting for 34% followed by carbonate reefs (22%) and slopes (18%). 13

Figure 4. Contribution (wt.%) of each carbonate-accumulating environment (see Fig. 3) to the estimated total removal of oceanic sulfate of 1.5* 10 moles per year. Note that reefs, slopes and tropical shelves account for 73% of the total annual sulfate removal, whereas the deep sea, with 34% of the total carbonate accumulation, accounts for only 8% of the sulfate removal. 11





341

Our estimate for the removal of sulfate by calcium carbonate precipitation is higher than earlier estimates which range from 0% (20) over 0.6-1.2% (23) to 1.4% (5) of the annualriverinesulfate input. Crucial for the estimation of sulfate removal by calcium carbonate precipitation is which of the estimates of the global calcium carbonate precipitation and accumulation is used. We chose Milliman's (19) estimate because, to our knowledge, it represents the most recent and detailed estimate that gives areas and rates of calcium carbonate production and accumulation for various environments. It should be mentioned, however, that Holser (24) and Morse and Mackenzie (16) calculated significantly different rates of calcium carbonate production. These differences in production rates and their effect on the estimated sulfate removal are summarized in Table 3. The removal of seawater sulfate by calcium carbonate precipitation represents only a minor sink compared to several major sulfur sinks. In estimates of sulfur sinks, pyrite formation is generally thought to be the largest sedimentary sink (5, 27, 25, 26) with up to 66% of the annual naturalriverinesulfate input (24). The formation of evaporitic sulfate minerals is generally considered to be an important sedimentary sulfate sink with estimates ranging from 20% (27) to 50% (20), whereas the importance of sulfate removal by hydrothermal circulation through mid-oceanic ridges remains controversial (28). Hence, sulfate removal by carbonates is less important than other sinks, but it is not trivial as suggested before. Furthermore, it is important to note that the rate of sulfate removal by biological calcium carbonate precipitation is likely to have varied through geologic time, as it is dependent on factors such as latitude of continents, ocean temperature, sea level, PC0 , and changes in taxonomic makeup of carbonate precipitating organisms. 2

Fate of Sedimentary Sulfate upon Burial. In the geologic record there are numerous examples of buried carbonate shelves and platforms. Modern carbonate shelves and platforms are suggested to account for 33% (16) to 42% (19) of the present-day calcium carbonate accumulation. The estimated average sulfate concentrations of modern carbonate shelves and platforms is 5900 ppm. Importantly, these environments, which represent only about 3.3% (19) of the total area where calcium carbonate is produced, account for 55% to 77% of the present-day sulfate removal by calcium carbonate accumulation (depending on which estimate is used). Hence, carbonate shelves and platforms represent relatively confined areas of sulfate enrichment To evaluate the fate of sulfate-rich buried carbonate buildups we use regional extensive dolomitesfromthe Burlington-Keokuk Formation as an example. The stratigraphy, petrography, cathodoluminescence, major and trace element geochemistry, 0-,C-, Sr isotope geochemistry, and relativetimeframeworkof the Mississipian Burlington-Keokuk dolomites have been discussed in great detail (29, 30, 31, 32). Burlington-Keokuk dolomites are doininated (> 95%) by two regionally extensive dolomite generations, referred to as Dolomite I and Π. The earlier diagenetic Dolomite I was presumably formed by mixing of freshwater with moderately concentrated seawater at the near-surface (33). Sulfate concentrations of 26 Dolomite I samples range from 1000 to 3500 ppm. Pétrographie and cathodoluminescence evidence indicates replacement of Dolomite I by Dolomite Π (32). This later diagenetic Dolomite Π was presumably formed at a sedimentary burial depth of not more than 500



4

12

12

moles C / year

MO

1 1

3%

moles S /year

moles C / year

moles C / year

: Based on estimates of the pre-anthropogenic annualriverinesulfate input into the oceans (20, 21, 22).

4

11

3

5%

1.5-10 moles S / year

12

1 1 · 1 0 moles C / year

: Assuming preservation of 30% of the CaC0 deep sea production.

3

12

12

1 4 · 1 0 moles C / year

24·10

: Calculated using average sulfate content and abundance of carbonate sediments in modern marine environments.

1.6%

5.2· 10 moles S /year

10

12

Milliman (19) 29· 10 moles C / year

3

3

12

12· 10 moles CI year

6·10

12


12


12

Morse and Mackenzie (16)

2

3

1.5· 10 moles CI year

12

2

1


5·10


: Assuming preservation of 50% of the CaC0 shelf production. 3

Total Marine

Open Ocean

Reef-Bank-Shelf

Open Ocean

Reef-Bank-Shelf

1

% of Annual Riverine Input

4

Estimated S 0 Removal

3

CaC0 Accumulation:

3

CaC0 Production:

12

Holser et al. (24)

Comparison of selected calcium carbonate production rate estimates and their effect on the estimated sulfate removal

Table 3.




343

m (32) by an extraformational subsurface fluid (30). Sulfate concentrations of 14 Dolomite Π samples are generally less than 500 ppm. Hence, during the replacement of the sulfate-rich Dolomite I by a sulfate-depleted Dolomite Π a significant amount of sulfate must have been released. Because recrystallization of sedimentary carbonates is a common process in carbonate diagenesis, it is reasonable to assume that significant amounts of sulfate may be released upon burial of carbonate platforms.


Geological Implications Although the incorporation of sulfate into sedimentary carbonates is not the most important sulfate sink with approximately 5% of the annualriverflux, the process has important geological implications. Upon burial, sulfate-rich carbonate shelves and platforms are likely to release sulfate (see above). The generally low sulfate content (< 500 ppm) of carbonates which were formed at depth indicates that initially sulfate-rich sediments (e.g. reef, bank and tropical shelf sediments) release most of their carbonatebound sulfate upon burial. Sulfate released at depth may subsequently be reduced to H S, elemental sulfur, incorporated in organic matter or metal sulfides. This release of carbonate-bound sulfate may be particularly important in sedimentary basins with thick carbonate shelf or reef deposits. For example, burial of the Bahama Platform and subsequent recrystallization under burial conditions would release a significant amount of sulfate. Assuming that Bahama sediments have an average density of 2.83 g/cm and a porosity of 60%, the total sediment accumulation since the last sea level low stand (~ 4000 years) is approximately 7.9* 10 moles CaC0 . These sediments, containing on the average 5900 ppm sulfate, represent a significant sulfur reservoir of 4.8* 10 moles. If, after burial recrystallization, the average sulfate content of Bahama sediments would be only 500 ppm, a total of 4.5* 10 moles would have been released, which is equivalent to approximately 50% of the sulfur present as ZnS in U.S. resources (34). 2

3

13

3

11

11

Summary The determination of the sulfate concentrations of a wide range of skeletal and nonskeletal calcites, aragonites and dolomites show that sedimentary carbonates generally contain sulfate as a trace constituent. Particularly modern marine skeletal carbonates, which account for most of the present-day calcium carbonate production, contain considerable amounts of sulfate (200 to 24,000 ppm). Based on these data we estimate that the rate of present-day oceanic sulfate removal by biological calcium carbonate precipitation accounts for approximately 5% of the naturalriverinesulfate input Upon sedimentary burial and recrystallization under reducing subsurface conditions, initially sulfate-rich sedimentary carbonates probably release most of their sulfate. The amount of sulfate that can be released by sulfate-rich shelf and reef deposits may represent a significant source of sulfur in sedimentary basins. Given the geological importance of the fate of carbonate-bound sulfate upon burial it is clear that further research is warranted.


344


Acknowledgments


Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for providingfinancialsupport for this research. We are grateful to P. Bretsky, WJ. Meyers and W.B. Ward for providing samples for this study. We also thank L. Kump, M.A. Arthur (Perm State Univ.), T. Rasbury (SUNY Stony Brook), and an anonymous reviewer for helpful suggestions and discussions.

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3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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Geochemical Transformations of Sedimentary Sulfur - ACS Publications

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