Formation of Iron Sulfides in Modern Salt Marsh ... - ACS Publications

Chapter 11

Formation of Iron Sulfides in Modern Salt Marsh Sediments (Wallops Island, Virginia) 1

1,2

T. S. White , J. L. Morrison , and L. R. Kump

1

1

Department of Geosciences, Pennsylvania State University, University Park, PA 16802 Materials Research Laboratory, Pennsylvania State University, University Park, PA 16802

Downloaded by MIT on February 25, 2013 | http://pubs.acs.org Publication Date: June 29, 1990 | doi: 10.1021/bk-1990-0429.ch011

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Iron sulfide abundances and sediment pore fluid chemistries varied considerably between three closely spaced but environmentally distinct cores taken from evaporative marsh panne, low marsh, and tidal creek subenvironments within a Spartina salt marsh. From these cores, three depositional settings were discerned in the subsurface: marsh, tidal flat, and bay. The formation of iron sulfides in these sediments was found to be rapid, with pyrite occurring as the dominant sulfide phase. Extant marshchemistrieshave an overprinting effect on the underlying sediments producing relative high amounts of pyrite directly beneath the marsh sediments. Sulfide grain size and morphologies varied nonsystematically between cores. Wallops Island, Virginia, is a Holocene transgressive barrier island system off the eastern coast of the Delmarva Peninsula [I). Chincoteague and Assateague Islands lie to the north of Wallops Island, separated by Chincoteague Inlet which provides an inlet for open marine waters into Chincoteague Bay and an outlet for lagoonal and estuarine waters from the bay (Figure 1). The surface water drainage divide of the Delmarva Peninsula is very close to the ocean, therefore most of the precipitation runoff is directed to Chesapeake Bay. As a consequence, the bays between the Delmarva Peninsula and the offshore barrier islands are sediment starved with the majority of the sediments derived from adjacent eroding marshes and through distribution of tidal delta and washover sediments. A typical barrier-island system can be divided into three depositional environments (barrier beach, lagoonal/bay, and tidal channel-delta complex) each of which contains a number of smaller subenvironments (2). This work focused on a typical bay-fill sequence in which bay sediments are overlain by tidal flat deposits 0097-6156/90/0429-0204S06.00/0 © 1990 American Chemical Society

In Geochemistry of Sulfur in Fossil Fuels; Orr, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

Iron Sulfides in Modern Salt Marsh Sediments


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GEOChemISTRY OF SULFUR IN FOSSIL FUELS

which in turn are overlain by tidal flat deposits which in turn are overlain by a modern Spartina marsh. The main objective of this research effort was to characterize diagenetic transformations among the various sulfide phases and analyze pore fluid Chemistry with respect to depth. In addition, it also examined i f the overlying organic-rich Spartina marsh sediments affected the formation of iron sulfides in the underlying sediments.


Sample Collection and Processing The salt marsh can be divided into two distinct environments: high and low marsh (3). The high marsh is only subject to tidal flushing during the highest tides. This study focused on the low marsh where tidal flushing is regular. Cordgrass present throughout the marsh is Spartina alterniflora. Two forms of this plant are identifiable. The tall form occurs along the banks of the tidal creek and is tolerant of normal marine salinities; the short form is tolerant of higher salinities and occurs throughout the low marsh where such water may be encountered. Three subenvironments within the low marsh (Figure 2) were sampled and analyzed: (1) an evaporative or marsh panne, (2) the low marsh proper, and (3) a tidal creek. Sampling was performed (during March, 1988) using three inch aluminum tubing and a vibracoring unit (*)· Cores were immediately returned to the lab facilities of the Wallops Island Marine Science ConsortiuM. The cores were cut into smaller sections and immediately placed into a nitrogen atmosphere in glove boxes. Sub-sections were taken from various depths along the length of the core. Each sub-section was split; one half of the split was immediately stored under an inert nitrogen atmosphere and frozen for further analyses. One quarter was processed for sedimentological analysis and solid phase Chemistry. The remaining quarter was centrifuged to obtain pore fluids for analysis. Sample preparation was performed under a nitrogen atmosphere in the glove boxes. The frozen samples were returned to The Pennsylvania State University for preparation and pétrographie analysis. Preparation involved drying the sample at 105°C in a nitrogen atmosphere. The samples were pulverized to -16 mesh using a mortar and pestai, mixed with a Hexacol epolite resin and hardener, and then centrifuged to ensure complete mixing with the sediment. After curing, the resins were cut longitudally and placed in one inch stainless steel molds where additional resin was poured over the cured resin. The one inch pellets were then polished and viewed using reflected light microscopy. A twenty point reticule was used to better facilitate recording occurrences. Megascopic and microscopic analyses of the sediments provided the information for the correlated stratigraphie columns presented in Figure 3. Marsh, tidal f l a t , and bay sediments were discerned from each of the three cores. These sediments were differentiated on the basis of grain size, fossil content, and the relative abundance of Spartina root and rhizome debris.



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Figure 2. Evaporative panne, marsh proper, and tidal creek subenvironments. In Figure 2a, an inundated evaporative panne is shown in the foreground with a Spartina marsh in the background. Figure 2b shows an exposed evaporative panne during low tide and the vibracoring unit. The tidal creek with high form Spartina alterniflora growing along its perimeter, and the short form growing towards the interior of the marsh is visible in Figure 2c.


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CORE I (marsh panne) (depth/cm) 25

m

Marsh Sediments


CORE II (low marsh)

CORE III (tidal creek)

TOP OF CORE

Dark gray clays, abundant Spartina roots

81

Tidal Flat Sediments

Dark gray clays, reworked Spartina

173

160 152

Bay Sediments

Dark gray clays with interbedded silts/very fine sands unit

Marine fossils

411

404 414

Figure 3.

Stratigraphie

columns of the cored subenvironments.


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WHITE E T A L .


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Analytical Methods Pore waters were analyzed for pH, alkalinity, salinity, and sulfate concentration following centrifugation. pH was measured with a Ross combination electrode standardized with commercial NIST traceable buffers. Alkalinity determinations were made by a single addition of 0.01 Ν HCL (5). Salinity was determined with a refractometer, and with an accuracy of +/- 0.5 o/oo. Sulfate was determined turbidimetrically (6). Solid phase Chemical analyses included determination of total organic C content, and the distribution of S between iron monosulfides (acid-volatile sulfur or AVS) and pyrite (the difference between total reducible sulfur and AVS). Total organic carbon was measured coulometrically following combustion at 1050°C (7). Acid-volatile sulfur and total reducible sulfur analyses followed the procedure of Canfield et a l . (8). A microbiological assay of the abundance of sulfate reducing bacteria was performed according to

(£). Results Pore fluid Chemistries of the various samples are summarized in Figure 4. The distribution of salinities (Figure 4A) within each core is fairly regular but distinct from each of the other cores. The tidal creek core (core 3) has pore water salinities closest to that of normal seawater, perhaps reflecting a constant supply of seawater in the tidal creek. The evaporative marsh panne (core 1) has salinities greater than normal sea water. Marine waters may reach the evaporative pannes via shallow subsurface hydrologie communication with the tidal creeks and through surface flooding during high tides (10). Standing water in the evaporative panne is subject to evaporation which concentrates the sea water salts. The "plumbing" which allows for elevated salinity at depth is not fully understood. Fluctuations in near-surface salinities do occur seasonally (lj.). The highest alkalinity pore fluids correspond to the most saline waters (Figure 4B); core 2 most closely matches seawater with the upper portion of the other curves trending towards it. In Figure 4C, the pH of the pore fluids is shown to be in the range of 7-7.8 in all three cores. Low marsh and tidal creek sediments (cores 2 and 3, respectively) show a gradual increase in pH with depth while the evaporative panne shows a decrease in pH with depth followed by an increase. The results of dissolved sulfate concentrations are summarized in Figure 4D. Sulfate concentrations are highest in the top of the evaporative panne (core 1) sediments. Sediments of the evaporative panne and tidal creek (cores 1 and 3 respectively) do not reproduce the anticipated closed system behavior for sediments undergoing sulfate reduction, i.e. progressive decrease with depth (12). Instead, initial depletions in the top meter are followecTEy sulfate increases deeper in the cores. The marsh core (core 2) does not contain detectable amounts of dissolved sulfate at any depth. A microbiological assay of the abundance of sulfate reducing bacteria was performed to determine the distribution of colony


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0

1000

2000

3000

A b u n d a n c e of S u l f a t e R e d u c e r s (CPV/g d r y ) • • •

E v a p o r a t i v e P a n n e ( C o r e 1) L o w M a r s h ( C o r e 2) T i d a l C r e e k ( C o r e 3)

Figure 4. Pore fluid/sediment Chemistries and abundance of sulfate reducers from the various cores.


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20.0

30.0

0

500

0.00

• A •

1.00

40.0 50.0 A l k a l i n i t y (meq/l)

1000 Sulfate

1500 (ppm)

60.0

2000

2.00 3.00 4.00 5.00 6.00 O r g a n i c C a r b o n (wt % C)

Evaporative Panne (Core L o w M a r s h (Core 2) T i d a l C r e e k (Core 3)

7.00

1)

Figure 4. Continued.


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forming units (CFU) with respect to depth (Figure 4E). The sediments of the marsh core (core 2) contain the highest CFU counts. All three cores show an abundance of sulfate reducers near the top of the core trending towards zero near the middle, and then increasing towards the bottom of the core. In general, the abundance of sulfate reducers is low, but not unexpectly, so given the time of sampling (March). Organic carbon profiles (Figure 4F) in all three subenvironments show the same decreasing profile with depth (13). The evaporative panne (core 1) contain the highest amounts of organic carbon which corresponds to an abundance of Spartina roots and filamentous algae visible in hand specimen. Iron monosulfides show an antithetic relationship with pyritic sulfur (Table I). The highest amounts of acid volatile sulfides (which remained small compared to S in pyrite) occur directly beneath the marsh sediments within the upper portions of the tidal flat deposits. The high value for iron monosulfides in the upper part of the tidal creek sediments may be related to high rates of sulfate reduction at these depths. High amounts of pyritic sulfur occur throughout our cores suggesting rapid formation of pyrite, either directly or through an iron - monosulfide precursor. The relatively minor AVS content of these sediments suggest that i f AVS acts as a precursor, its existence is as a transient intermediate in a fast reaction to pyrite. A variety of pyrite sizes and morphologies are recognized (Figure 5). In Figure 5A, SEM photomicrographs show fine-grained framboidal pyrite. Framboidal pyrite and an irregular mass of inequigranular pyrite are shown in Figure 5B. An irregular shaped mass of pyrite in inertinite is presented in Figure 5C. All of these morphologies and associations presented in Figure 5 are very common in the stratigraphie record, particularly in coal-bearing strata. A record was maintained of the size of each pyrite occurrence. Core 1 shows a gradual increase in the percentage of pyrite particles less than six microns in size whereas core 3 shows a decrease in the percentage of the same size pyrite particles (Table I). Core 2 shows no consistent pattern. The tidal creek with an abundance of sulfate in a reducing environment appears to produce more pyrite of smaller sizes. On the marsh panne, larger pyrite particles are formed in this hypersaline, high alkalinity environment. A record of morphology classes for each pyrite occurrence was kept during pétrographie analyses. Monocrystalline pyrite includes euhedral and subhedral pyrite crystals. This morphology class is always more prevalent than framboidal pyrite except at the top of core 1 and the bottom of core 3. Discussion Organic Carbon and Pyrite Relationships. Organic carbon enters most marine sediments as detritus at the sediment-water interface and then goes progressive decomposition with burial. However, in salt marshes, where the subsurface production of organic matter (in the form of roots and rhizomes) can be as great as eight times the above-ground production (14), a major addition of organic matter



0.200 0.001 0.030 0.005

0.140 0.060 0.040 0.000

0.060 0.100 0.001 0.020 0.030

Acid Volatile Sulfides (mg S/g dry sediment)

18.400 23.000 10.370 7.100

8.310 13.660 5.910 8.450

4.540 18.230 18.130 6.280 9.140

Pyritic Sulfur (mg S/g dry sediment)

*Percentage of the observed pyrite.

4.6 11.5 14.3 3.0

12.4 5.0 2.9 8.5

0.0 19.2 11.9 20.0 16.4

Monocrystalline* Pyrite

Sulfide characterization data.

Evaporative panne subenvironment (core 1) = I 36-386 Low marsh subenvironment (core 2) = II 51-376 Tidal creek subenvironment (core 3) = III 10-381

Tidal Flat Tidal Flat Bay Bay

III -10 111-107 111-244 III-381

Marsh Tidal Flat Bay Bay Bay

Marsh/Tidal Flat Tidal Flat Bay Bay

-36 -102 -203 -305 -386

Environment

II -51 II -168 II -269 II -376

I I I I I

Sample (depth/cm)

Table I.


3.1 6.2 4.8 9.1

5.7 4.3 0 4.2

14.1 9.2 10.2 0 7.6

Framboidal* Pyrite

88.5 77.0 78.1 66.7

80.7 80.6 89.1 74.5

60.6 70.6 78.0 80.0 81.0

0-6

10.7 16.8 12.4 15.2

14.9 12.9 7.9 14.9

26.7 18.4 15.3 16.3 15.1

6-12

0.8 6.2 9.5 18.1

4.4 6.5 3.0 10.6

12.7 11.0 6.7 3.7 3.9

>12

Grain Size (ym)*



214

Figure 5.

Photomicrographs of Iron Sulfides.



11. WHITE ET AL.

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occurs at shallow depths in the sediments. If this material is digestible, i t should stimulate the activity of anaerobic sulfate reducing bacteria in this largely anoxic environment. Thus one might expect an association between pyrite (the mineralogical end-product of sulfate reduction) and the remains of Spartina roots and rhizomes. The iron sulfides found intimately associated with fresh plant tissues in near surface sediments were in most cases classified as framboidal. In addition, irregular pyrite morphologies were identified deeper in the cores, i n f i l l i n g organic tissues that would be classified by coal petrographers as inertinite. The pyrite found at depth associated with inertinite probably originated near the surface. It is believed that inertinite (as the remains of vascular plants) does not form a good substrate for sulfate-reducing bacteria (15). Thus, extensive and continued formation of pyrite at deptlTin these sediments is unlikely. This conclusion is made despite the apparent increase in sulfate-reducing bacteria and sulfate concentrations at depth in these sediments. This increase in sulfate concentration is coincident with a decrease in alkalinity, and thus might represent the oxidation of pyrite at depth: FeS + 2

7/2 0 + 2 HCO3" " 2

_ >

F e 2 +

+

2

S 0

4 ~ 2

+

2

C0

2

+

H

2°

( 1 )

The supply of CL would thus imply deep circulation of oxidizing pore fluids in these sediments. The continued decrease of organic carbon might be tied to this excess supply of oxidants at depth. Diagenetic Overprinting. Pye (14) noted that the nature of authigenic iron minerals in salt marsh sediments is dependent on accretion rates, initial reactive iron content, marsh vegetation, and fluctuations in sulfate-reducing bacterial activity. Given that the Chincoteague Bay marshes are eroding, accretion rates appear to be negligible; the sediments may be subject to exhumation. Therefore, the zone of sulfate reduction will not migrate through the sediment column since the sediment-water interface is not being elevated. In fact, the zone of sulfate reduction may be maintained at a given depth for a considerable period of time, and may remigrate through underlying sediments which have already been subject to these processes. The aforementioned occurrence of pyrite infilling inertinite could be a manifestation of such a stage of pyrite formation. However, a more metabolizable organic material is necessary for bacterial consumption and sulfate reduction. This energy source is not readily available in these sediments. Thus, pyrite associated with inertinite was probably emplaced near the surface. The superposition of marsh sediments on older bay and tidal flat sediments has likely increased their pyrite content. Environmental overprinting has been reported in both recent sediments (16) and in the rock record {17). Pyrite Morphology. Pyrite formed during early diagenesis in marsh sediments has essentially two modes of occurrence, as small single crystals and as framboids (18). The processes that lead to one form or the other remain unclear. Euhedra may form by direct



216


precipitation where pore waters are supersaturated with respect to pyrite but undersaturated with respect to mackinawite or greigite, or by the reaction between mackinawite and elemental sulfur. Framboids may also develop inorganically during the reaction of iron monosulfides with elemental sulfur. Giblin and Howarth (19) observed pyrite occurring predominantly as small single crystals an3 framboids. They believed that framboids result from iron monosulfide precursors whereas the small single crystals arise when pyrite precipitates directly. In all three cores of our study, the framboidal pyrite shows an overall gradual decrease with depth followed by an increase at the bottom (Table I). This trend may be related to the same trend visible in the plots of abundance of sulfate reducers and iron monosulfides. The top of core 3 is the only outlier to the decreasing then increasing trend in percentage framboids. This outlier corresponds to the most reducing Eh value, the lowest alkalinity, the highest amount of iron monosulfides, and the second highest amounts of sulfate, sulfate reducers, and pyrite obtained from any of the cores. Apparently, framboids do not form as readily in the tidal creek as compared to the marsh panne (core 1). The small amount of AVS in the upper portion of core 1 may be a function of uptake through framboid formation; core 1 has the most framboids of any of the cores. Cores 1 and 3 show an antithetic relationship between framboidal and monocrystalline pyrite whereas core 2 has the same decreasing then increasing with depth profile as observed for framboids. The genetic relationship between these pyrite morphologies is evident. Where AVS content is high and pyritic sulfur content is low, the monocrystalline morphologies are more prevalent than framboids. However, where framboid content is high, monocrystalline pyrite and AVS content are low, whereas pyritic sulfur content is high. Framboid formation through iron monosulfide conversion to pyrite appears to boost the total pyrite content of the sediments, presumably beyond levels attained through direct pyrite precipitation. Conclusions The pore water profiles, solid phase Chemistry, and pyrite petrography in three closely spaced cores from salt marsh subenvironments show complex trends and rapid variations. Marsh overprinting on the underlying sediments produced relatively high amounts of pyritic sulfur at shallow depths (100 to 203 cm). The iron sulfides were very fine-grained with the 0 to 6 micron size fraction most common. The reducing environment of the tidal creek produced more pyrite of smaller grain sizes, whereas larger pyrite particles were formed in the hypersaline, high alkalinity sediments of the marsh panne. Monocrystalline pyrite morphologies were more prevalent than framboidal pyrite where AVS-content was relatively high and pyritic sulfur content was low. However, where framboid content was high, monocrystalline pyrite and AVS-content was low, and pyritic sulfur content was high. Framboids appear to form more readily in the marsh panne as compared to the tidal creek.


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Framboid formation may boost the total pyrite content of the sediments beyond levels attained through direct precipitation. Acknowledgments The authors acknowledge the efforts of the members of Geosc. 597B during the Spring semester of 1988 which provided the majority of the geochemical data used throughout this publication, and the Department of Geosciences at The Pennsylvania State University which provided the necessary logistical support for this research endeavor. The National Aeronautical and Space Administration (NASA) provided accessibility to Wallops Island. We also thank two anonymous reviewers for their helpful suggestions.


Literature Cited 1. Dade, W.B. M.S. Thesis, The Pennsylvania State University, Pennsylvania, 1983. 2. Reinson, G.E. In Facies Models; Walker, R.G., Ed.; Geol. Assoc. Canada, 1984; pp 119-140. 3. Leatherman, S.P. Barrier Island Handbook; University of Maryland, 1982;p109. 4. Sanders, J.E.; Imbrie, J. Geol. Soc. Amer. Bull. 1963, 74, 1287-92. 5. Strickland, J.D.H.; Parsons, T.R. A Practical Handbook of Seawater Analysis; Strickland, J.D.H.; Parsons, T.R., Eds.; Alger Press Ltd., 1977; pp. 11-34. 6. Standard Methods for the Examination of Waste and Wastewater; Port City Press, MD, 1985, 16th edition. 7. Ramirez-Rojas, A.J. Ph.D. Thesis, The Pennsylvania State University, Pennsylvania, 1988. 8. Canfield, D.E.; Raiswell, R.; Westrich, J.T.; Reaves, C.M.; Berner, R.A.Chem.Geol. 1986, 54, 149-55. 9. Postgate, J.R. The Sulphate-Reducing Bacteria; Cambridge University Press, New York, 1984. 10. Nuttle, W.K.; Hemond, H.F. Global Biogeochem. Cycles. 1988, 2, 91-114. 11. Casey, W.H.; Guber, Α.; Bursey, C.; Olsen, C.R. EOS Trans. Amer. Geoph. Union 1986, 67, 1305. 12. Berner, R.A. Geochim. Cosmochim. Acta. 1984, 48, 605-15. 13. Bluth, V.S. M.S. Thesis, The Pennsylvania State University, Pennsylvania, 1989. 14. Pye, K. Nature 1981, 294, 650-52. 15. Lyons, W.B.; Gaudette, H.E. Org. Geochem. 1979, 1, 151-55. 16. Cohen, A.D.; Spackman, W.; Dolsen, P. International Journal of Coal Geology 1984, 4, 73-96. 17. Williams, E.G.; Keith, M.L. Economic Geology 1963, 58, 720-729. 18. Raiswell, R. Am. Jour. Sci. 1982, 282, 1244-1263. 19. Giblin, A.E.; Howarth, R.W. Limnol. Oceanogr. 1984, 29, 47-63. RECEIVED

April 11, 1990


Formation of Iron Sulfides in Modern Salt Marsh ... - ACS Publications

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