Chapter 12
Environmental Controls on Iron Sulfide Mineral Formation in a Coastal Plain Estuary Jeffrey C. Cornwell and Peter A. Sampou Horn Point Environmental Laboratory, University of Maryland Center for Environmental and Estuarine Studies, Cambridge, MD 21613-0775
Salinity and organic matter loading gradients in estuaries can be used to examine iron sulfide mineral formation. Chesapeake Bay has low salinity, terrestrially-dominated sediments in the northern bay, seasonally anoxic mesohaline environments in the mid-bay region, and coarser-grained, bioturbated sediments in the more saline southern bay. High rates of sediment sulfur reoxidation and low inputs of labile organic matter limit the burial of authigenic sulfide minerals in northern bay sediments, while the activity of bioturbating organisms in south bay sediments enhances the reoxidation of sulfur. High iron sul fide δ S values in the northern bay are consistent with low sulfate concentrations. A strong bay-wide relationship between organic carbon and iron sulfide concentrations was evident when refractory terrestrial organic matter was considered. 34
Microbial sulfate reduction and the production of iron sulfide minerals are ubiquitous processes in estuarine and coastal marine sediments. There is considerable interest in the sedimentary sulfur cycle in coastal systems because a) sulfate reduction is often the dominant pathway for sediment metabolism and nutrient regeneration (1-3), b) sulfide production can maintain anoxia in the overlying water column (4), c) the conversion of iron oxides to sulfides can have a significant impact on phosphorus cycling (5,6), d) sulfur cycling influences the solution chemistry and solid phase speciation of trace metals (7-9), e) iron monosulfides are useful indicators for potential metal toxicity (10) and f) the form and quantity of iron sulfide minerals may be used as indicators of environments of deposition (11-13). A number of factors may limit sulfate reduction and the formation of iron sulfide minerals (Figure 1). In low sulfate waters, sulfate supply may limit the production of hydrogen sulfide, though in most marine and estuarine systems, sulfate is not limiting (14-16). In general, the limit to sulfate reduction in estuarine systems is organic matter supply. Hydrogen sulfide produced via sulfate reduction has three 0097-6156/95/0612-0224$12.00/0 © 1995 American Chemical Society
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Figure 1. Simplified diagram of the sediment sulfur cycle as it applies to the formation of iron sulfide minerals.
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main fates: reoxidation to sulfate or intermediate sulfur redox species such as thiosulfate, sulfite, elemental sulfur, polysulfides or polythionates (17-19); incorpora tion into iron monosulfides or pyrite (20-22); or incorporation into organic matter (23, 24). The first two pathways are quantitatively more important in estuarine sediments, with reoxidation dominating (2). In oxidizing or reducing horizons, sulfide oxidation can occur directly with oxygen or by interaction with metal oxides respectively (25, 26). The formation of pyrite requires a source of reduced sulfur which has been partially oxidized. Iron monosulfides or acid volatile sulfides (AVS) have often been considered a necessary precursor of pyrite formation (27), but pyrite synthesis from polysulfides and iron have demonstrated the feasibility of direct precipitation (27). The availability of "labile" iron mineral sources is a major limit to the formation of iron sulfide minerals (28) when sulfate reduction is a major metabolic process. The presence of pore water sulfide indicates that reactive iron phases are no longer available for sulfide mineral formation. Pyrite is the dominant iron sulfide mineral in estuarine sediments (22) and the degree of pyritization (DOP) may be calculated from pyrite and HCl-extractable iron concentrations (20): DOP = Pyrite-Fe/(pyrite-Fe + HCl-Fe) The use of DOP to indicate iron limitation of iron sulfide mineral formation may be valid only under moderate to high sulfate reduction rates. Canfield et al. (28) have cautioned that DOP can be an unreliable indicator of iron mineral reactivity. The formation of iron sulfide minerals on relatively short time scales (days to months) does not necessarily lead to their permanent burial. The preservation of iron sulfide minerals depends on several different physical and biogeochemical processes, including degree of overlying water oxygenation, intensity of physical and biological reworking of sediment, and the presence of oxidants within the sediment. Iron sulfide mineral preservation is high in permanently anoxic systems where reoxidation is minimized. Physical reworking of sediments in bioturbated sediments mixes sulfide minerals into more oxidized environments and transports oxidants such as oxygen, Mn(IV) oxide and Fe(III) into reducing zones (26). Sedimentary C/S ratios have been used to discriminate between freshwater and marine sediments (77). Freshwater sediments have high C/S ratios due to lower rates of sulfate reduction and high inputs of refactory organic carbon. Sediments deposited under oxygen-limited conditions exhibit considerably higher DOP (12). The sulfur isotopic composition of iron sulfide minerals reflects the deposition al environment. The microbial reduction of sulfate to hydrogen sulfide enriches the product in S, leaving the remaining sulfate enriched in S (29). Under high sulfate conditions, the sulfate pool is minimally enriched with heavier S because of exchange with overlying water sulfate. The resultant hydrogen sulfide is relatively light. Under sulfate limited conditions, virtually the whole sulfate pool is reduced and the hydrogen sulfide has an isotopic composition similar to overlying water sulfate (30). Consequently, iron sulfide minerals in fresh water sediments generally have a heavier 8 S than that found in marine sediments. In this paper, we examine both the factors which control the distribution of iron sulfide minerals in Chesapeake Bay and the utility of C/S ratios, DOP and sulfur isotopic composition as indicators of depositional environments. Chesapeake Bay is 32
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an excellent system for the study of sulfur biogeochemistry because of gradients in salinity, organic production and bottom-water oxygenation. Considerable information on the biology, chemistry, physics and geology of Chesapeake Bay has been developed in the past two decades because of the need to effectively manage this heavily impacted system; this information also provides useful background on the biogeochemical processes which influence the sedimentary sulfur cycle. Chesapeake Bay Biogeochemical Setting Chesapeake Bay, a >300 km long coastal plain estuary, is characterized by large landderived inputs of nutrients, high rates of plankton production, seasonal bottom water anoxia in the mesohaline region, and high rates of sediment metabolism. The mainstem Chesapeake Bay can be divided into three major regions: 1) the oligohaline upper bay, 2) the mesohaline mid-bay region characterized by high rates of organic production and seasonal anoxia, and 3) the shallow lower bay. High turbidity in the upper bay limits organic productivity and Susquehanna River-derived nutrients fuel primary production in the mid-bay region (57). Chesapeake Bay sediments are generally fine-grained in the upper bay and at greater depths in the mid-bay region, while shallow water mid-bay and most southern bay sediments are sandy (32, 33). Sediment accretion rates are high in the northern bay because of high inputs of fluvial particulates. Sand from the continental shelf is a major source of sediment for the southern bay while the lower rates of sedimenta tion in the mid-bay region are largely derived from shoreline erosion (34). Upper bay sediments have the highest concentrations of organic matter, most of which is fluvially-derived (32, 33). Deep-water organic carbon concentrations in the mid-bay are generally in the 2-4% range, with low concentrations in the lower bay. Sediment metabolism is low in the northern bay, high in the mid-bay region, and moderate in the southern bay (35). Rates of sulfate reduction in mid-bay sediments are high (15-20 mol m yr ) (36) and summer fluxes of hydrogen sulfide contribute to bottom water anoxia (4). 2
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Methods In June 1988 and August 1989 cores were collected at 18 sites to obtain a systemwide description of solid phase and pore water chemistry (Figure 2). Seasonal measurements of sulfate reduction were made at many of these sites in 1988 and 1989. In 1989, spring, summer and fall profiles of sulfur species were measured at 5 sites on an east-west transect in the mesohaline region of the bay. Two cores for geochronological studies were collected in 1991 (37). Sediments were collected using box corers. Cores were extruded in N -filled glove bags and pore water was obtained by centrifugation. Water samples were filtered (0.4 urn) and hydrogen sulfide (£H S) and iron were analyzed colorimetrically (38, 39). Sulfate analysis was via colorimetry (40) or ion chromatography. Sediment for acid volatile sulfide (AVS) and total reduced sulfur (TRS) analysis was stored frozen and analyzed using cold 6 N HC1 for AVS extraction (41) and acidic Cr(II) for TRS (42). The TRS analysis was not sequential and includes AVS, elemental S and pyrite-S. Hydrogen sulfide was analyzed via Pb titration (41). 2
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Figure 2. Map of Chesapeake Bay showing locations of sample sites. In the vicinity of site 7, five additional sites on a west to east transect (Ml, M2, M3, M4, M6) were used for pore water and solid phase chemistry.
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Oxalate-Fe was extracted using ammonium oxalate/oxalic acid (43) on frozen samples and HCl-Fe was extracted from dried sediments using 6 N HC1 for 1 hr (22). Organic carbon was calculated as the difference between total C (CHN analyzer) and carbonate C analyses (44). The isotopic composition of TRS was analyzed via mass spectrome try at Coastal Science Laboratories (Austin, Texas) on AgS precipitates (45). Sulfate reduction was estimated by sediment incubation (46) of samples from 0-2, 5-7 and 1214 cm core intervals. Homogenized sediment was packed into 50 mL centrifuge tubes and the sulfate decrease was determined after 1-2 weeks. Results and Discussion Pore Water and Solid Phase Profiles. Pore water sulfur and iron chemistry exhibited high spatial and temporal variability in Chesapeake Bay sediments (Figure 3). Low sulfate concentrations, rapid sulfate depletion, undetectable £H S concentrations, and large vertical dissolved iron gradients were observed at the north bay site. In the seasonally anoxic mid-bay region sulfate was depleted below 10 cm during summer months, similar to other coastal sediments (47). High rates of sulfate reduction are found in these mesohaline sediments (36) leading to high concentrations of XH S. During summer, low concentrations of XH S (
