Spatial and seasonal variations in depth profile of trace metals in saltmarsh sediments from Sapelo Island, Georgia, USA

Estuarine, Coastal and Shelf Science 72 (2007) 675e689 www.elsevier.com/locate/ecss

Spatial and seasonal variations in depth profile of trace metals in saltmarsh sediments from Sapelo Island, Georgia, USA Alakendra N. Roychoudhury Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Received 6 September 2006; accepted 2 December 2006 Available online 16 January 2007

Abstract This study was undertaken to elucidate the impact of early diagenetic processes on the accumulation of trace metals in Sapelo Island saltmarsh sediments as a function of time, space and sediment properties. Samples were collected from three sites in summer (May 1997) and winter (January 1998) along a transect from an unvegetated Creek Bank through a vegetated Tidal Levee to the vegetated midmarsh with evident lateral heterogeneity caused by hydrologic regime, macrophytes and microbial and macrofaunal activities. A suite of trace metals (As, Ba, Cr, Co, Cu, Cd, Mo, Ni, Pb, Th, Ti, U, V, Zn and Zr) was analyzed to obtain their depth-distribution at the three sites. Spatially marked differences were observed, that were primarily related to hydraulic flushing of trace metals away from the sites in high-energy regimes, rapid downward mixing and reworking of sediment via bioturbation, and below-ground degradation and production of Spartina biomass. Although sulfate reduction and the formation of acid volatile sulfide and pyrite were dominant processes throughout the marsh, the trace metal scavenging role of sulfides was not apparent. However, possible sulfurization of organic matter, leading to enhanced trapping of trace metals with organic carbon, may have played an important role in sequestration of trace metals. No similarity was observed visually between the depth trends of trace metals and sediment properties (grain size, iron-oxyhydroxide content, acid volatile sulfides and pyrite content) that are known to play a major role in trace metal partitioning. Only organic carbon content closely followed the trace metal profiles at all the three sites. Minor variation in depth-integrated sediment trace metal content was observed seasonally at each of the three sites. Furthermore, the depth trend of profiles of individual trace metals also did not vary significantly over the seasons either. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: trace metals; saltmarsh; early diagenesis; Spartina alterniflora; porewater; trace metal cycling

1. Introduction In sediments, trace metal concentrations are considered to vary with sediment properties such as organic carbon content, grain size, and carbonate, sulfide and FeeMn oxyhydroxide content (Adriano, 2001). The preferential partitioning among various phases is due to the surface properties of the mineral phases present within the sediments that allow surface sorption, as well as dominant chemical reactions, which result in precipitation and co-precipitation of trace metals. For example, El Bilali et al. (2002) show that different trace metals show different affinities to the organic and mineral fraction, or that trace metals seldom precipitate as a pure sulfide E-mail address: [email protected] 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.12.003

mineral in sediments, rather co-precipitation is normally the dominant pathway (Morse and Rickard, 2004). Trace metal studies often focus on: (1) their spatial distribution, speciation and partitioning in surficial sediments because of their pollution potential and to assess their bioavailability (e.g., Campbell, 1995; Monteiro and Roychoudhury, 2005; e.g., Tessier et al., 1979; Oakley et al., 1980; Rosental et al., 1986;Wangersky, 1986; Usero et al., 1996), (2) assessing pollution history (e.g., Allen and Rae, 1986; Zwolsman et al., 1993; Spencer, 2002; Kim and Jung, 2004) or (3) their use as proxies for palaeoenvironmental reconstruction of redox conditions and carbon sequestration (e.g., Arnaboldi and Meyers, 2003; Algeo and Maynard, 2004; e.g., Lapp and Balzer, 1993; Pailler et al., 2002; Sinninghe Damste et al., 2002; Tribovillard et al., 2004; Brumsack, 2006).

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2. Site description Sapelo Island is a barrier island off the coast of Georgia, USA, with large portions of the island covered by pristine saltmarsh (Fig. 1). The marshes and upland maritime forest that form the western and southern boundaries of the island fall within the Sapelo Island National Estuarine Research Reserve, which has the principal management objective of maintaining the area for research and educational purposes with a mandate to protect it from internal and external sources of stress which may alter or affect the nature of the ecosystems. Access to the island is restricted and, apart from a small marine research institute with few staff members living permanently on the island

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40 m

Tidal Range

Depth-distribution of trace metals in sediments is the primary focus of many palaeoenvironmental studies (Allen and Rae, 1986; Spencer, 2002; Audry et al., 2004; Kim and Jung, 2004; Tribovillard et al., 2004). Post-depositional conservative behavior of particulate trace metals is a common underlying assumption in several such ‘‘inverse interpretation’’ studies. However, diagenetic processes influence the sediment composition (Lapp and Balzer, 1993; van Santvoort et al., 1996; Thomson et al., 1996; Botto and Iribarne, 2000; Staubwasser and Sirocko, 2001; Riedinger et al., 2005) and, less information is available on the dynamics and ultimately the fate of trace metals with depths as a result of early diagenesis (e.g., Cooper and Morse, 1998; Douglas and Adeney, 2000; e.g., Lapp and Balzer, 1993). Furthermore, with anthropogenic imprints, it is often difficult to decipher if the observed variations in sediment trace metal content are related to natural biogeochemical processes or are the result of human activities. This is a cause of concern for accurately identifying controls over trace metal cycling and hence the interpretation of sediment memory record. Previous studies have shown that intertidal marshes on Sapelo Island are dynamic ecosystems, dominated by temporally and spatially changing biogeochemical processes that control microbial activity and cycling of major elements in the sedimentewater system (Howarth and Giblin, 1983; Roychoudhury et al., 1998; Kostka et al., 2002; Koretsky et al., 2003; Roychoudhury et al., 2003a; Roychoudhury et al., 2003b; Koretsky et al., 2005). The mobilization and fate of trace metals under such circumstances are often determined by primary biogeochemical reactions and their by-products that are strongly coupled to the trace metals. In this study, depth-distribution of trace metals from two seasons and three sites is assessed. The three sites lie across a short 40-m transect, but vary from each other in their biophysico-chemical characteristics. A high-resolution seasonal data set for trace metals, sediment characteristics and porewater chemistry spanning depths of up to 50 cm below the sedimentewater interface was generated with the main aim of identifying the controlling processes that can explain the observed spatial and temporal differences in the distribution pattern of trace metals in sediments and hence advance our understanding of trace metal dynamics in post-depositional diagenetic environments.

D

676

MSL

Levee Ponded Marsh

Creek Bank Fig. 1. Map showing the site location on Sapelo Island, GA, USA. A schematic cross-section across the marsh highlights the relative location of the three sites. During low-tide all of the three sites are exposed to the atmosphere and are inundated with tidal waters during high tide. Please note that the cross-section is not drawn to the scale.

and an equally small indigenous population living on the other end of the island, anthropogenic activity is minimal. This provides a unique opportunity to study sediment trace metal dynamics that is primarily influenced by natural processes. Sediment samples were collected from a saltmarsh mud flat at the southern end of Sapelo Island (Georgia, USA) (Fig. 1). A medium-size tidal creek flows through the southwest end of the marsh, and Spartina alterniflora constitutes the dominant vegetation. A sedimentation rate of 1e3 mm y1 has been estimated previously at Sapelo Island (Howarth and Giblin, 1983). The sediments were collected in summer (May 1997) and winter (January 1998) from the unvegetated tidal Creek Bank and from the vegetated portion of the marsh (Tidal Levee and Ponded Marsh), 10 m and 40 m away from the Creek Bank, respectively (Fig. 1). Apart from the differences in temperature that causes variation in macrofaunal and microbial activity, the seasonal sampling matched the summer time active growth (May) and winter time senescence (January) periods in the life-cycle of S. alterniflora. The sites were chosen based on an initial reconnaissance survey and differed from each other in the amount of tidal flushing, redox condition below the sedimentewater interface, carbon content, and solid phase iron-oxyhydroxide and iron sulfide content of the sediment. Furthermore, the Creek Bank site was completely devoid of vegetation and S. alterniflora flourished at the Levee and Ponded Marsh site. The growth of Spartina; however, was stunted (w0.3 m high) at the Ponded Marsh site with the taller phenotype (w1e1.5 m high) observed

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only at the Levee site. Correspondingly, a well developed rhizosphere to depths of 40e45 cm was observed at the Tidal Levee site, whereas the root zone was confined to the depths of 10e15 cm at the Ponded Marsh site. Twice every day, the sites are inundated by tidal water for 3e4 h. Strong tidal flushing is observed at the Creek Bank site and to some extent at the Levee site; however, the water movement is restricted at the Ponded Marsh site. Oxygen concentration in the tidal water varies between 100 and 170 mM seasonally (Cai et al., personal communication). Fiddler crab (Uca sp.) burrows are found at each site, with the highest abundance observed at the Creek Bank site and the lowest at the Ponded Marsh site (Kostka et al., 2002). For example, fiddler crab burrow-aperture densities of 1040, 745 and 415 per m2 (Basan and Frey, 1977) and fiddler crab population densities of 65, 61 and 27 per m2 (Teal, 1958) have been reported previously for the Creek Bank, Tidal Levee and Ponded Marsh sites. In addition, the macro-invertebrates at the sites continuously rework the top 10e15 cm of sediment through bottom-feeding activities (Kostka et al., 2002) and the longest and deepest burrows are observed at the Creek Bank with the burrow network progressively found at shallower depths along the transect to the Ponded Marsh site. Due to bio-irrigation, enhanced oxygen flux of 8.1  1.1 to 117  115 mmol O2 m2 d1 to the sediments has been estimated (Meile et al., 2001); however, most of the oxygen is consumed within a depth of 2e4 mm below the sedimentewater interface (Cai et al., personal communication). Spartina roots at the Levee and Ponded Marsh sites provide an additional conduit for the diffusion of oxygen below the surface, creating oxic microcosms within the rhizosphere that is predominantly surrounded by anaerobic sediments (Luther et al., 1982; Dacey and Howes, 1984; Howes et al., 1986). 3. Materials and methods 3.1. Sediment collection and characterization The sediments were collected using a stainless-steel wedge corer (Inglett et al., 2004) that minimizes compaction. The cores were collected from an area within 1 m radius of the porewater samplers and were collected on the same day that the porewater samplers were retrieved. Once a core was retrieved, it was transferred to the field laboratory within 30 min and sectioned in less than 2 h inside a glove bag in a nitrogen atmosphere. One centimeter slices for the first 10 cm and then 2 cm slices for the rest of the length of the core were sectioned in each case. Sediment sections were double bagged under nitrogen and frozen for transportation to the home laboratory where a portion of each sediment section was freeze-dried within a week. Bulk sediment samples were analyzed for their trace metal content on freeze-dried sediments from 10 different depths in each core. Approximately 50 mg of sediment sample was treated with 4 ml of a 4:1 mixture of 28 M HF and 14 M HNO3 in sealed Savilex beakers. Samples were digested for 48 h at 50e60  C on a hot plate, with occasional agitation, before evaporation to complete dryness. A further 2 ml of 14 M

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HNO3 was added and samples digested at 50e60  C followed by evaporation to dryness at approximately 75  C. This latter procedure was repeated until complete dissolution of sediments was achieved. After cooling, samples were diluted 1000 times with an internal standard and were analyzed using an ICP-MS. A syenite geostandard, STM-1, from the United States Geological Survey and a reference lake sediment, SL1, from International Atomic Energy Agency (IAEA) were treated and analyzed in a similar fashion to determine the percentage systematic analytical error. Values for STM-1 and SL1 were published by IAEA (1979) and Gladney and Roelandts (1988), respectively. Analytical error was also calculated by analyzing a sample collected from Tidal Levee site in duplicate. Organic carbon, reactive iron and solid phase sulfide (acid volatile sulfides (H2S þ FeS); and chromium reducible sulfides (FeS2 þ S0); referred to henceforth as AVS and CRS, respectively) content of the sediments were determined as described previously (Roychoudhury et al., 2003a). Reactive iron is operationally defined as that fraction of iron-oxyhydroxides that is extracted by ascorbate extraction (see Roychoudhury et al., 2003a) and is believed to be released most easily from the solid phase. Grain size was determined by wet sieving followed by settling column method according to a British Standard procedure (Anonymous, 1963). 3.2. Porewater collection and characterization Porewater samples were collected using diffusion equilibrators or ‘‘peepers’’ (Hesslein, 1976). Peepers were fitted with 0.2 mm nylon membranes and after insertion into the sediment, were left for at least a month to equilibrate with the porewaters. Upon retrieval, the peepers were immediately transferred inside a nitrogen filled glove bag and transported to the field laboratory. Porewaters from each depth were retrieved using nitrogen flushed syringes fitted with stainless-steel needles. Subsequently, porewaters were filtered using 0.45 mm flow-through nylon filters and distributed in separate vials for further analyses. Porewaters were analyzed on-site using standard colorimetric methods for alkalinity (Sarazin et al., 1999), Fe2þ (Viollier et al., 2000) and SH2S (Cline, 1969), whereas pH was measured using a flow-through combination microelectrode (Cole-Parmer) connected to a bench-top pH/ MV/ion analyzer (Corning; Model 255). Porewater samples (100 mL) for sulfate analyses were preserved in 2 ml of 0.05 N HCl and were subsequently analyzed in the home laboratory in Atlanta within 15 days of collection by a turbidimetric method (Tabatabai, 1974). 4. Results 4.1. Sediment texture and composition Creek Bank sediments are texturally heterogeneous with an intermix of sand, silt and clay size particles (porosity ¼ 0.76  0.02; density ¼ 1.34  0.04 g cm3; 2s, n ¼ 10; Fig. 2). The sediments are brownish to grayish in color up to 25e30 cm below the sedimentewater interface, indicating a thick sub-oxic

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Creek Bank Grain Size (%)

2 3 4 5 6 7 8 9

20 40 60 80 100

0

50

100

150

0

20

40

60

CRS ( mol cm-3) 80

0 0

10

10

10

10

10

Silt + Clay

20 30 40 50

20 30 Summer Winter

40 50

20 30 40 50

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Sand

Depth (cm)

0

AVS ( mol cm-3)

Asc. Fe ( mol cm-3)

% Organic carbon

20 30 40

200 400 600 800

20 30 40 50

50

Tidal Levee Grain Size (%) 20 40 60 80 100

% Organic Carbon

Asc. Fe ( mol cm-3)

2 3 4 5 6 7 8 9

0

50

100

AVS ( mol cm-3) 0

150

20

40

60

CRS ( mol cm-3)

80

0 0

10

10

10

10

10

Clay

20 30 40

20 30 40 50

50

20 30 40

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Silt

Depth (cm)

0

20 30 40 50

50

200 400 600 800

20 30 40 50

Ponded Marsh Grain Size (%) 20 40 60 80 100

2 3 4 5 6 7 8 9

0

50

100

0

150

20

40

60

CRS ( mol cm-3) 80

0 0

10

10

10

10

10

Clay

20 30 40 50

20 30 40

20 30 40 50

50

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Silt

Depth (cm)

0

AVS ( mol cm-3)

Asc. Fe ( mol cm-3)

% Organic Carbon

20 30 40 50

200 400 600 800

20 30 40 50

Fig. 2. The depth profiles illustrate physico-chemical characteristics of the sediments and their seasonal variation at the three sampling sites. Please note that the individual profiles are plotted on the same scale for the respective sites for easy comparison.

not change significantly between seasons (Table 1). A high concentration (100e160 mmol cm3) of ascorbate-extractable iron is present down to a depth of 10e15 cm, below which it decreases to 40e60 mmol cm3 by a depth of 30e40 cm (Fig. 2). At Creek Bank, depth-integrated (0e35 cm) AVS is low (0.11e0.38 mmol (S) cm2; Table 1). A gradual increase in the AVS concentrations with depth is observed in May (Fig. 2). Surprisingly, AVS accumulates more in January

zone. A blackish color is observed below those depths. Except for a few intermittent peaks, organic carbon content was constant with depth in summer (3.71  0.85%; 2s, n ¼ 10), but in winter, organic carbon content decreased from 4.4% at the surface to 2.3% at a depth of 43 cm from the sedimente water interface (Fig. 2). Carbonate carbon content was low (0.20%). At Creek Bank, ascorbate-extractable iron (May e 3.05 mmol cm2, January e 3.03 mmol cm2) did

Table 1 Depth-integrated (0e35 cm) concentrations of trace metals (mg cm2) and other chemical phases (mmol cm2) in Sapelo Island saltmarsh sediments Site and season

Ti

V

Cr

Co

Ni

Cu

Zn

As

Zr

Mo

Cd

Ba

Pb

Th

U

Org-Ca

AVS

CRS

Asc-Feb

Winter Creek Bank Tidal Levee Ponded Marsh

128 457 158 869 141 421

2916 4125 3625

2021 2863 2404

248 326 325

604 817 810

439 609 599

3132 4067 3627

453 568 928

1785 2165 2061

70 81 169

5 4 17

6138 6303 5955

680 893 796

297 368 331

90 118 123

122 153 137

381 217 183

4094 3544 10028

3034 3469 2013

Summer Creek Bank Tidal Levee Ponded Marsh

122 671 148 682 143 388

2654 3925 3726

1857 2661 2280

228 332 342

536 812 858

411 565 647

2915 3771 3408

442 553 792

1778 2007 2011

69 80 171

4 3 4

5435 6218 6060

628 886 813

252 357 352

82 110 125

137 162 153

118 691 72

6987 9971 16461

3052 2678 483

a b

Organic carbon. Ascorbate-extractable iron.

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when sulfate reduction rates are lower (see Kostka et al., 2002) and porewaters are comparatively more oxygenated than in May (see Roychoudhury et al., 2003a). A peak is observed around 22 cm, below which AVS decreases rapidly (Fig. 2). Even though depth-integrated sulfate reduction rates are highest in summer at the Creek Bank site; CRS concentrations are substantially lower than those observed at the Tidal Levee or the Ponded Marsh sites (Fig. 2). Most of the sulfur at the Creek Bank site is tied as CRS (4e7 mmol (S) cm2). Tidal Levee sediments are comprised primarily of clay-rich silt (porosity ¼ 0.83  0.02; density ¼ 1.26  0.03 g cm3; 2s, n ¼ 10; Fig. 2). The sub-oxic zone, with mostly grayish color sediment, is thinner compared to the Creek Bank site and extends only to about 10e15 cm below the surface. Aboveground, tall phenotype of Spartina flourishes at the Tidal Levee site with its root zone extending as deep as 40e45 cm below the surface. However, the organic carbon content remains low and nearly constant at all depths (4.0e4.5%; Fig. 2) compared to the values reported in the literature (11e43%) from other east-coast saltmarshes (Barnes et al., 1973). Negligible carbonate content (0.15%) is also measured at the Tidal Levee site. Ascorbate-extractable iron concentration remains constant at approximately 130 mmol cm3 down to 12 cm depth in May and January (Fig. 2). Below this, a rapid decrease in reactive iron oxides is observed. The gradient is much sharper during summer, compared to that in winter. At the Levee site, the highest AVS concentrations are observed in May with an AVS peak forming at a depth of 8 cm (Fig. 2). In winter, the AVS concentration remains less than 20 mmol cm3 at all depths; however, the AVS peak forms at 20 cm depth, suggesting sub-oxic conditions prevailing to greater depths in winter. CRS concentrations increase in general with depth for both seasons at the Levee site. Down to a depth of 12 cm below the sedimentewater interface, CRS concentrations remain constant at around 150 mmol cm3; however, the concentrations increase markedly to about 400 mmol cm3 below the transition zone (Fig. 2). At the Ponded Marsh site, sediments are black in color and are homogeneous with mostly silt and clay size particles (porosity ¼ 0.82  0.04; density ¼ 1.26  0.09 g cm3 2s, n ¼ 10; Fig. 2). Occasional sand lenses occur within the sediment. An extensive root zone extends to depths of 15e20 cm. Similar to the Tidal Levee site, the organic carbon content of the sediment is not very high (4.85%) at the Ponded Marsh site and a negligible amount of inorganic carbon (0.15%) is found in the sediments. For both seasons, organic carbon content decreases downcore by as much as 30% with depth (Fig. 2). At the Ponded Marsh site, ascorbate-extractable iron content in the month of May is relatively low (w80 mmol cm3) at the surface. A rapid decrease to about 10 mmol cm3 is observed within the first 6 cm, below which values remain constant (Fig. 2). In January, concentrations are lower in the surface sediments and, except for one anomalous peak at 36 cm, the concentrations are nearly constant with depth (Fig. 2). The highest concentration of AVS (w120 mmol cm3) is observed in the top 1 cm, below which concentrations decrease exponentially at the Ponded Marsh site (Fig. 2). The peak AVS concentration shifts downwards in January and again an

679

exponential decrease is observed below the maxima (Fig. 2). Most of the sulfur is tied in the CRS form and concentrations increase gradually with depth (Fig. 2). Intermittent peaks are observed between 5 and 8 cm below the sedimentewater interface (Fig. 2) and are mostly associated with the Spartina roots. 4.2. Trace metal distribution in bulk sediment Measured trace metal concentrations at the three sites are shown in Table 2 and the seasonal depth profiles of the individual trace metals are shown in Fig. 3. All of the trace metal concentrations are normalized to Al in order to remove the lithogenic effect on trace metal mobility. However, note that the normalization did not change the general trend with depth for each of the plotted profiles. For the geostandards (STM-1 and SL1) treated in the same manner as the saltmarsh sediment samples, except for Zn and Li in SL1, analytical error was below 10% for all of the elements for which certified values were available. A percent difference of 26 and 133 was measured for Zn and Li, respectively. In addition, replicate analysis (n ¼ 21) of a basalt standard (BHVO-1), following the same digestion procedure and instrument in our laboratory, typically gave an overall procedural error of better than 3% (RSD) (see le Roex et al., 2001). Analytical error for each of the measured trace element at 95% confidence interval based on replicate analysis of surface sample (0e1 cm) from the Tidal Levee site is shown in Table 2. At the Creek Bank site, except for Pb, Th, Ti, Zr and Ba profiles, that show random fluctuations with depth, trace metal concentrations do not change significantly either with depth or with seasons (Fig. 3 and Table 1). A minor increase in depthintegrated trace metal concentration was observed for winter, as opposed to that in summer for all of the measured trace metals at the site (Table 1). In addition, except for Ti, Zr, Cd and Mo, surface (0e1 cm) enrichment of trace metals is observed in winter compared to that in summer at the Creek Bank site and the surface concentrations for most trace metals were highest at the Creek Bank site compared to the other two sites (Fig. 3). At the Tidal Levee site, concentrations of individual trace metals did not change significantly with depth (Fig. 3); however, similar to the Creek Bank site, depth-integrated trace metal concentrations were slightly higher in winter compared to those in summer (Table 1). Although, for each trace metal, measured concentrations in the surface sediments were lower at the Tidal Levee site compared to the Creek Bank site, the depth-integrated concentrations were among the highest measured at the three sites for both seasons (Table 1). Sediment trace metal profiles at the Ponded Marsh site show uniquely different characteristics from the other two sites (Fig. 3). For each of the trace metals, a sub-surface maximum is observed between 5 and 10 cm below the sedimente water interface that coincides with the depth of well developed rhizosphere. The trace metal concentrations then decrease almost exponentially with depth (Fig. 3). At the Ponded Marsh site, depth-integrated concentrations of only Cr, Zn, As, Zr,

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680

Table 2 Trace metal concentration (in mg/kg) in bulk saltmarsh sediment samples from Sapelo Island Sample #

Depth (cm)

Al

Ti

V

Cr

Co

Ni

Cu

Zn

As

Zr

Mo

Cd

Ba

Pb

Th

U

Creek Bank CB0-1 CB2-3 CB6-7 CB8-9 CB14-15 CB19-21 CB25-29 CB33-37 CB41-42 CB45-49

(winter) 0.5 2.5 6.5 9 14.5 20 27 35 43 47

61302 79676 69341 74462 68562 64107 68042 60179 52092 66095

2741 4036 3887 3936 3759 3560 3676 3685 3519 3672

88 97 85 91 85 82 83 74 63 81

62 66 59 62 58 57 58 51 48 57

8.4 8.2 7.4 7.6 7.2 6.8 7.0 6.5 5.4 6.9

21 20 17 18 17 17 18 15 14 18

20 14 12 14 13 12 12 11 9 12

145 99 88 96 91 84 88 84 76 89

15 15 14 13 12 14 12 12 9 12

49 56 51 62 52 44 50 54 45 52

1.94 2.26 1.98 2.19 1.88 2.14 1.95 1.84 1.63 2.18

0.11 0.51 0.13 0.13 0.12 0.14 0.12 0.12 0.12 0.16

179 187 181 190 188 157 178 174 164 166

20 22 20 21 20 18 20 18 16 19

8.1 9.1 10.0 8.2 9.6 8.2 7.6 8.7 7.8 8.9

2.52 2.80 2.68 2.70 3.01 2.30 2.52 2.49 2.44 2.74

Creek Bank CB1 CB3 CB5 CB7 CB9 CB15 CB19 CB25 CB29 CB35

(summer) 0.5 2.5 4.5 6.5 8.5 14 18 24 28 34

66155 67908 49959 63604 70893 69838 79586 49738 56105 70449

3844 358 3068 3633 3837 3774 3961 3173 3934 3530

85 81 60 78 88 89 94 64 69 82

61 56 42 55 64 61 65 45 48 57

7.0 6.7 5.0 7.6 7.4 7.5 7.9 5.5 5.9 7.3

17 16 12 16 18 18 19 13 13 18

15 12 9 12 13 14 15 10 10 13

91 87 66 84 93 94 100 72 87 87

15 14 11 14 15 14 15 11 11 13

54 66 47 46 51 51 68 49 47 51

2.04 1.76 1.43 1.97 2.20 2.36 2.18 1.75 1.94 2.54

0.11 0.10 0.08 0.16 0.11 0.11 0.11 0.10 0.11 0.12

165 166 150 157 160 152 171 148 179 171

20 18 15 19 21 20 22 16 17 20

7.6 8.3 6.4 7.0 8.9 7.9 8.7 5.8 7.0 7.6

2.36 2.45 1.92 2.25 2.72 2.64 2.90 2.10 2.28 2.58

Tidal Levee TS0-1 TS2-3 TS4-5 TS6-7 TS8-9 TS12-16 TS20-24 TS28-32 TS32-36 TS36-40

(winter) 0.5 96291 2.5 97120 4.5 97430 6.5 100194 8.5 97801 14 98840 22 98217 30 93772 34 98061 38 97792

4634 4659 4531 4566 4528 4582 4646 4678 4522 4623

115 118 118 119 120 122 121 118 118 118

81 81 83 81 81 82 82 89 82 86

9.6 9.8 9.7 9.8 10.2 10.0 8.9 8.8 9.2 9.5

23 23 24 23 24 23 23 25 24 26

17 17 17 17 17 18 18 17 18 19

121 120 134 121 118 118 117 113 115 112

20 17 18 15 16 17 17 16 15 19

63 65 56 60 58 60 67 64 67 63

2.03 2.33 1.89 2.30 2.33 2.32 2.46 2.29 2.64 2.81

0.21 0.10 0.10 0.08 0.10 0.15 0.10 0.11 0.09 0.12

185 185 189 179 177 185 188 175 181 179

25 25 26 25 26 26 27 26 25 27

10.0 10.6 10.5 10.7 10.6 11.0 10.9 10.2 10.5 11.0

2.82 3.18 3.06 3.27 3.39 3.36 3.62 3.54 3.43 3.94

Tidal Levee TS1 TS3 TS5 TS7 TS10 TS14 TS20 TS24 TS30 TS38

(summer) 0.5 2.5 4.5 6.5 9 13 19 23 29 37

94929 94351 92879 89760 92940 99325 88209 94729 93262 97966

4429 4456 4619 4443 4523 4185 4148 4293 4242 4286

119 117 115 109 118 118 109 113 112 114

78 77 78 79 78 78 75 76 77 78

9.8 9.5 10.3 9.6 10.2 9.8 9.1 9.9 9.2 9.8

22 22 23 23 23 25 23 24 24 24

16 17 17 16 17 17 16 17 16 17

110 110 115 105 115 108 99 112 103 139

15 17 17 14 16 16 16 17 15 16

64 59 61 54 59 59 56 64 54 56

2.00 2.88 2.49 1.94 2.00 2.42 2.49 2.62 1.93 2.46

0.09 0.09 0.10 0.07 0.10 0.11 0.09 0.09 0.10 0.10

184 174 179 161 169 180 170 205 180 189

25 25 25 24 25 27 24 27 26 26

10.2 10.1 10.2 10.0 10.3 10.6 9.9 10.6 10.5 10.6

2.94 3.32 3.14 3.08 3.20 3.22 3.04 3.41 3.16 3.33

81618 86755 82427 80384 75929 89223 87463 95419 91629 96016

4176 4331 4319 4241 4031 3962 3817 4288 3971 4347

101 104 104 101 97 102 105 109 107 108

71 72 76 70 83 67 68 69 66 67

8.5 8.6 8.5 8.7 9.6 10.6 9.8 9.1 9.7 9.8

20 20 22 20 22 24 24 24 25 24

16 15 17 15 15 17 18 19 18 18

106 105 107 104 101 99 103 108 109 106

16 14 16 23 41 35 41 22 19 18

70 70 73 74 74 59 51 55 57 59

2.53 2.31 3.30 5.45 12.20 8.65 5.94 3.22 2.75 3.21

2.34 0.34 1.54 0.47 1.53 0.45 0.32 0.17 0.22 0.16

197.22 197.93 182.51 193.01 205.24 169.73 155.24 169.99 160.79 170.70

23.50 24.78 23.91 23.10 22.38 22.67 21.42 24.84 21.29 22.31

9.4 10.2 9.9 9.3 8.8 9.5 9.1 10.0 9.6 10.2

2.98 3.15 3.27 3.52 4.43 4.27 3.92 3.34 3.00 3.17

(summer) 0.5 80496 2.5 81471 4.5 72586 6.5 91246 9 99334

4119 3946 3630 4172 4219

100 95 92 102 112

66 66 60 63 66

8.3 8.0 9.1 9.6 9.6

20 21 20 24 25

15 14 14 18 19

100 97 92 98 99

14 17 42 26 20

69 70 64 60 63

1.93 3.08 11.98 8.42 4.25

0.11 0.10 0.15 0.13 0.10

197.24 196.42 176.10 177.51 175.49

23.40 22.74 21.71 24.51 23.67

10.0 9.3 8.4 10.4 10.6

2.85 3.25 4.42 3.95 3.71

Ponded Marsh (winter) SS0-1 0.5 SS2-3 2.5 SS4-5 4.5 SS6-7 6.5 SS8-9 8.5 SS10-12 11 SS14-18 16 SS22-26 24 SS34-38 36 SS42-48 45 Ponded Marsh SS1 SS3 SS5 SS7 SS10

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Table 2 (continued ) Sample # SS14 SS20 SS24 SS30 SS40

Depth (cm)

Al

Ti

V

Cr

Co

Ni

Cu

Zn

As

Zr

13 19 23 29 39

98507 102910 108518 95933 101300

4105 4288 4380 4075 4340

112 114 113 107 106

65 70 67 66 67

11.6 9.6 10.9 9.6 9.9

27 26 27 25 25

21 21 21 18 19

104 103 101 93 97

35 17 16 22 21

56 54 57 56 55

Analytical error (%P)a

1.39

4.94

0.11

4.42

2.31

3.40

3.07

0.54

2.99

12.39

Mo

Cd

Ba

Pb

Th

U

4.84 4.07 3.38 5.11 4.16

0.11 0.07 0.12 0.10 0.09

165.10 171.96 177.72 173.97 178.48

24.04 23.30 26.22 22.29 22.37

10.3 10.3 11.0 10.0 10.4

3.83 3.48 3.57 3.45 3.47

16.12

6.84

0.99

2.43

0.55

0.59

a Analytical error for digested sediments at 95% confidence limit (%P ¼ (2s/mean)100) based on duplicate surface samples (0e1 cm) analyzed from the Tidal Levee site.

Cd, AVS and ascorbate-extractable Fe were higher in winter, whereas other measured trace metals were more enriched during summer (Table 1). Also, in general, trace metal concentration in surface sediments (0e1 cm) at the Ponded Marsh site was higher than those measured at the Tidal Levee site but lower than the Creek Bank site (Fig. 3). 4.3. Porewater chemistry Porewater profiles measured at the three sites are illustrated in Fig. 4. The profiles show distinct seasonal and spatial differences. Although the general trend with depth for each parameter is similar, there are visible differences in the magnitude of the measured concentrations seasonally and from site to site suggesting that porewater concentrations are far more sensitive to the lateral heterogeneity of the saltmarsh sediments. At the Creek Bank site, a sharp decline in pH below the sedimentewater interface is observed for both seasons (Fig. 4). Further down, the pH stabilizes and remains constant. A similar drop in pH is observed at the other two vegetated sites for summer whereas in winter the pH did not vary significantly with depth (Fig. 4). The porewaters of the Tidal Levee and the Ponded Marsh sites are slightly more acidic compared to those of the Creek Bank site. Alkalinity increases with depth at all the three sites with the highest increase observed at the Ponded Marsh site for both seasons (Fig. 4). At the Tidal Levee, a sharp increase in the alkalinity is observed for summer compared to winter, when only a gradual increase is observed. Alkalinity remains low at the Creek Bank site for both seasons (Fig. 4). Significant differences in the dissolved iron concentration are observed at the three sites. A broad peak of Fe2þ spanning all of the measured depths is observed at the Creek Bank site in summer (Fig. 4). In winter the peak is small and is closer to the sedimentewater interface (Fig. 4). As oppose to this, at the Tidal Levee site, a sharp peak of Fe2þ is observed in summer compared to winter when measurable iron was detected at all depths (Fig. 4). At the Ponded Marsh site, very little dissolved iron is present and very sharp peaks are detected closest to the sedimentewater interface during summer and winter, compared to the other two sites (Fig. 4). Dissolved sulfide concentrations remain near or below detection limit at all depths at the Creek Bank and Tidal Levee

sites (Fig. 4). At the Ponded Marsh site, dissolved sulfide concentrations increase to millimeter levels close to those of the sedimentewater interface for both seasons, with higher concentrations measured in the summer (Fig. 4). In general, sulfate concentrations decrease with depth at the three sites. Only a minor change in sulfate concentration is detected with depth at the Tidal Levee site over winter, though. The downcore depletion is much more rapid in summer compared to that in winter for all of the three sites (Fig. 4). At the Ponded Marsh site, a very sharp gradient is observed in summer and sulfate concentrations reach to below detection levels at a depth of 15 cm. 5. Discussion 5.1. Spatial and temporal changes in trace metal content In the absence of major anthropogenic sources on Sapelo Island, trace metals are primarily brought by tidal waters flowing through the large creek and are deposited either on the creek bottom or on the marsh mud flat, when tidal water percolates over the marsh after filling creeks during high tides. Tidal water being the common source and assumingly similar mobility behavior of individual trace metals would mean that trace metal concentration should not vary over short distances within the depositional zones. However, at Sapelo Island, trace metal content of saltmarsh sediments varied considerably over a 40-m long transect and to some extent with seasons. Post-depositional early diagenetic processes, whether physical and biogeochemical, in isolation or in combination may have affected the rate of accumulation/release and ultimately the concentration of trace metals in sediments. 5.2. Creek Bank At the Creek Bank site, pH near the sedimentewater interface varies by more than a unit during the two seasons. Consequently, due to high pH, most trace metals are enriched during winter in the surface sediments at the Creek Bank (Fig. 3). With depth, the pH drops during both seasons and then stabilizes, however, the trace metal profiles do not follow the trend dictated by the pH. Rather, random peaks are

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Creek Bank 2.4

10 20 30 40

Summer Winter

0.6 0.7 0.8 0.9 0

1.1

10

0.09 0

20 30 40 50

50

Ni/Al

Co/Al 1

0.11

0.13

10

0.22 0

20 30 40 50

0.28

10

Cu/Al 0.34

Depth (cm)

Cr/Al 2

Depth (cm)

1.6

Depth (cm)

Depth (cm)

1.2

Depth (cm)

Zn/Al 0.8 0

20 30 40 50

0.16 0

0.24

0.32

10 20 30 40 50

Tidal Levee Zn/Al

Cr/Al

Co/Al

0

0

0

0

10

10

10

10

10

30 40

30 40 50

50

0.22

20 30 40 50

0.28

0.34

Depth (cm)

0

20

0.13

Cu/Al

0.09

20

0.11

Ni/Al

0.6 0.7 0.8 0.9 1 1.1

Depth (cm)

2.4

Depth (cm)

2

Depth (cm)

Depth (cm)

0.8 1.2 1.6

20 30 40

0.16

0.24

0.32

20 30 40 50

50

Ponded Marsh Zn/Al

Cr/Al

0

10

10

10

10

10

40 50

40 50

30 40 50

20 30 40 50

0.28

0.34

Depth (cm)

0

20

0.22

Depth (cm)

0

Depth (cm)

0

30

0.13

Cu/Al

0

20

0.11

Ni/Al

0.09

30

2.4

Co/Al

0.6 0.7 0.8 0.9 1 1.1

20

2

Depth (cm)

Depth (cm)

0.8 1.2 1.6

0.16

0.24

0.32

20 30 40 50

Fig. 3. Seasonal depth-distribution of trace metal content measured at the three sites. Trace metal concentrations are normalized to Al to account for the accumulation affected by the variation in the mobility of different trace metals. Please note that the individual profiles are plotted on the same scale for the respective sites for easy comparison.

observed in the profiles of trace metals (Th, Ti and Zr) that are often associated with terrigenous sediment fraction. Recently it has been shown that enzymes released in the gut of bottom feeders can preferentially enrich metals from the sediment (Turner, 2006). Therefore, sediment reworking activities of macrofauna like Uca may produce such a signal. Note that the fluctuations are more prominent during summer when macrofaunal activity is high. Redox-sensitive trace metals (Co, Ni, Cu, Zn, As, Mo, Cd, Pb, V, Cr and U); however, do not show a similar signature. Rather, their profiles are homogenized at all depths (Fig. 3). Note that the deepest and highest concentration of macrofaunal burrows were observed at the Creek Bank site (Kostka et al., 2002). In addition, twice daily the influx of tidal water laterally transports oxygen to all depths that oxidizes excess sulfide and replenishes ironoxyhydroxides (Roychoudhury et al., 2003a). It is believed that increased intrusion of oxidants to greater depths, due to burrowing and lateral mixing of water, also mixes the redox fronts, thereby homogenizing the profiles of redox-sensitive trace elements.

The depth-integrated trace metal content of all of the measured trace metals at the Creek Bank site is lower than that measured at the other two sites. Trace metals often partition with fine-grained organic-rich sediments or with ironoxyhydroxide and sulfide phases (Adriano, 2001). The organic carbon content does not vary significantly between the three sites; however, at the Creek Bank site, the sediments contain up to 30% sand plus nearly equal amounts of silt and clay fraction, as opposed to the other two sites, where silt and clay make up the sediments (Fig. 2). This makes the Creek Bank a less favorable site for trace metal deposition. In addition, at the Creek Bank, although iron-oxyhydroxide content is high (Table 1), Fe2þ concentration in the porewaters (Fig. 4) suggests prevailing iron reduction, which would tend to release trace metals associated with the iron-oxyhydroxides. This, combined with active lateral and tidal flushing, would result in scavenging of trace metals away from the Creek Bank site. Furthermore, despite the highest depth-integrated sulfate reduction rates measured at the Creek Bank site (Kostka et al., 2002), no excess sulfide is measured in the porewaters

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Creek Bank

20 30

2.5

Cd/Al 3

0

3.5

10

Depth (cm)

Depth (cm)

Depth (cm)

10

2

20 30

0.015

Mo/Al 0.03

0

0

0

10

10

20 30

U/Al

0.05 0.1 0.15 0.2

0.02 0

Depth (cm)

Ba/Al 1.5 0

Depth (cm)

As/Al 0.1 0.2 0.3 0.4 0.5 0.6 0

20 30

0.04

0.06

10 20 30

40

40

40

40

40

50

50

50

50

50

Tidal Levee

20 30

2.5

Cd/Al 3

3.5

10

0

Depth (cm)

Depth (cm)

Depth (cm)

10

2

20 30

0.015

Mo/Al 0.03

0

0

0

10

10

20 30

U/Al

0.05 0.1 0.15 0.2

0.02 0

Depth (cm)

Ba/Al 1.5 0

Depth (cm)

As/Al 0.1 0.2 0.3 0.4 0.5 0.6 0

20 30

0.04

0.06

10 20 30

40

40

40

40

40

50

50

50

50

50

Ponded Marsh

30

10 20 30

2.5

Cd/Al 3

3.5

0

0.015

0

0

0

10

10

20 30

U/Al

Mo/Al 0.03

Depth (cm)

20

Depth (cm)

Depth (cm)

10

2

20 30

0.05 0.1 0.15 0.2

0.02 0

Depth (cm)

Ba/Al 1.5 0

Depth (cm)

As/Al 0.1 0.2 0.3 0.4 0.5 0.6 0

0.04

0.06

10 20 30

40

40

40

40

40

50

50

50

50

50

Fig. 3 (continued ).

(Fig. 4) and the sediment sulfide fraction (AVS and CRS) remains low (Table 1 and Fig. 2), thereby reducing the extent of entrapment of trace metals with sulfide phases at the Creek Bank site. Seasonally, a minor increase in depth-integrated trace metal content was observed in winter compared to summer. Two possibilities exist. (1) Trace metals are preferentially associated with iron-oxyhydroxides and are retained in the solid phase due to the low rate of iron reduction in winter as inferred from the lower dissolved Fe2þ in the porewaters (see Fig. 4). However, low depth-integrated iron-oxyhydroxide concentrations in winter suggest otherwise (Table 1). Furthermore, Koretsky et al. (2003) have shown that in Sapelo Island saltmarsh sediments, iron reducers are more active during winter. (2) Trace metals are associated with organic carbon but are mobilized during degradation and trapped in the AVS fraction or are directly partitioned with the AVS fraction. At the Creek Bank site, more of AVS is formed during winter, when sulfate reduction rates are low and porewaters are more oxygenated (Kostka et al., 2002; Roychoudhury et al., 2003a). In laboratory experiments, formation of zero-valent sulfur was

observed when limited oxygen was supplied to the sulfide solutions (Wilkin and Barnes, 1996). Moreover, enhanced entrapment of trace metals on dominant sulfide minerals has been observed when they are reduced via interaction with zero-valent sulfur (polysulfide ions and molecular sulfur) (Vorlicek et al., 2004). 5.3. Tidal Levee At the Tidal Levee site, the pH profiles show an almost opposite seasonal trend compared to the Creek Bank site (Fig. 4). The pH at the sedimentewater interface was higher during the summer by almost 1.5 units, compared to that measured in the winter. However, this did not impact the surface accumulation of trace metals, as was the case for Creek Bank site. In fact, the surface concentration of trace metals remains nearly constant over the seasons at the Tidal Levee site. More importantly, the surface concentration of most of the measured trace metals was lower at the Tidal Levee site compared to the Creek Bank site (Fig. 3). As discussed later, this is true for the Ponded Marsh site as well. It is believed that the

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Creek Bank

20 30

0.1 0

0.12

0.14

10

40 0

20 30

50

60

V/Al 70

80

1

Zr/Al

1.1 1.2 1.3 1.4 1.5

0.5 0.6 0.7 0.8 0.9 0

0

10 20 30

10

Depth (cm)

10

Ti/Al

Th/Al 0.34

Depth (cm)

0.3

Depth (cm)

Depth (cm)

0.26

Depth (cm)

Pb/Al 0.22 0

20 30

1

10 20 30

40

40

40

40

40

50

50

50

50

50

Tidal Levee

20 30

0.1 0

0.12

0.14

10

40 0

20 30

50

60

V/Al 70

80

1

Zr/Al

1.1 1.2 1.3 1.4 1.5

0.5 0.6 0.7 0.8 0.9 0

0

10 20 30

10

Depth (cm)

10

Ti/Al

Th/Al 0.34

Depth (cm)

0.3

Depth (cm)

Depth (cm)

0.26

Depth (cm)

Pb/Al 0.22 0

20 30

1

10 20 30

40

40

40

40

40

50

50

50

50

50

Ponded Marsh 0.1 0 10 20 30

0.12

Ti/Al 0.14

40 0

50

60

Zr/Al

V/Al 70

80

1

0.5 0.6 0.7 0.8 0.9 0

1.1 1.2 1.3 1.4 1.5

0

10 20 30

10 20 30

Depth (cm)

30

Th/Al 0.34

Depth (cm)

20

0.3

Depth (cm)

Depth (cm)

10

0.26

Depth (cm)

Pb/Al 0.22 0

1

10 20 30

40

40

40

40

40

50

50

50

50

50

Fig. 3 (continued ).

surface concentrations at the two vegetated sites are influenced by the uptake of trace metals by Spartina. Although the difference was minor, most trace metal concentrations in the surface sediments were lower in summer when Spartina growth is at its peak compared to those in winter when majority of the Spartina plants die. Note; however, that even in winter, sparse but healthy stalks of Spartina were observed at the Tidal Levee site. Evidence for trace metal infusion in root, stem and Spartina leaf tissues on Sapelo Island and other east-coast saltmarsh sites has been documented previously (Pellenbarg, 1984; Alberts et al., 1990). In addition, release of trace metals through salt glands from the leaves of standing strands has also been proven (Burke et al., 2000; Weis et al., 2002). Once the plants die; however, the organic material decomposes, releasing the trace metals back into the sediments. This usually takes place close to the plant germination site in the low-energy environment, where the standing strands of Spartina result in restricted water movement. Nevertheless, some loss of trace metals occurs, due to the removal of organic litter by diurnal tidal flow. At the Tidal Levee site, only a minor change in trace metal concentration is observed with depth (Fig. 3). Trace metal

concentrations either remain nearly constant (e.g., As, Ba, Cd, Mo, Zn, Cr and Cu) or a slight increase (e.g., Pb, Th, Zr, Ni and U) in their concentration is observed for both seasons at the Tidal Levee site (Fig. 3). A minor decrease in the concentration of V, Ti and Co with depth is observed, though (Fig. 3). Because none of the profiles follow the profiles of solid phases (organic carbon, AVS, CRS, iron-oxyhydroxide and grain size) with which trace metals commonly associate, it is not clear as to why minor differences in the behavior of trace metals were observed under a similar set of environmental conditions at the Tidal Levee site. Some possibilities are discussed below. Sharp gradients, observed in the solid phase iron, AVS and CRS profiles rule out homogenization of the sub-surface redox fronts and trace metal profiles at the Tidal Levee due to bioturbation or lateral tidal flushing, as was the case for the Creek Bank site. Except for As, Mo, U, and to some extent Cd, depth-integrated trace metal concentrations were highest at the Tidal Levee site in winter or remained high and similar to the Ponded Marsh site in summer (Table 1). If the sediment characteristics of the two sites are compared, the depth-integrated clay and CRS contents of Ponded Marsh sediment were higher, whereas AVS, organic carbon and iron-oxyhydroxide contents were lower compared

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Creek Bank pH 7

8

9

Fe(II) ( M)

0 10 20 30 40 50 60

10

0

SO42- (mM)

H2S ( M)

200 400 600 800

0

1

2

3

4

5

0

0

0

10

10

10

10

10

20 30 Summer Winter

40 50

20 30

20 30

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

Depth (cm)

6

Alkalinity (mM)

20 30

10

20

30

40

20 30

40

40

40

40

50

50

50

50

Tidal Levee Alkalinity (mM)

pH 7

8

9

10

0

0 10 20 30 40 50 60

SO42- (mM)

H2S ( M) 0

200 400 600 800

50 100 150 200 250

0

0

10

10

10

10

10

20 30

20 30

20 30

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

Depth (cm)

6

Fe(II) ( M)

20 30

0

10

0

10

20

30

40

20 30

40

40

40

40

40

50

50

50

50

50

Ponded Marsh pH 7

8

Alkalinity (mM) 9

10

0

SO42- (mM)

H2S (mM)

Fe(II) ( M)

0 10 20 30 40 50 60

0

200 400 600 800

2

4

6

8

10

0

0

10

10

10

10

10

20 30 40 50

20 30

20 30

40

40

50

50

20 30 40 50

Depth (cm)

0

Depth (cm)

0

Depth (cm)

0

Depth (cm)

Depth (cm)

6

20

30

40

20 30 40 50

Fig. 4. Porewater chemistry and its seasonal variation are depicted as depth profiles for the three sampled sites. Please note that dissolved sulfide is plotted on different concentration scale for the three sites due to large observed variation.

to the Tidal Levee site (Table 1). This would indicate that, at the Tidal Levee site, trace metals are preferentially associated with either the organic carbon fraction, and/or AVS and iron-oxyhydroxide phases. Note; however, that the profiles of both solid phase AVS and iron-oxyhydroxide show almost an exponential decrease below a depth of 10e20 cm for both seasons, but the organic carbon profile does not change significantly with depth (Fig. 2). As noted above, except for the minor decrease observed for V, Ti and Co, trace metal profiles remain either fairly constant with depth or a minor increase is observed. This would suggest that the trace metals are either predominantly associated with the organic carbon fraction, or if they are associated with AVS and iron-oxyhydroxide, they are mobilized and subsequently trapped below a depth of 20 cm leading to their preferential enrichment, although minor, in the pyritic phase at greater depths. Albeit no free sulfide was observed in the porewaters, disappearance of Fe2þ and increase in alkalinity with depth does suggest active formation of a more stable form of sulfidic solid phase (CRS) at greater depths (see Figs. 2 and 4). A note must be made here on the lack of gradient in the organic carbon profile, despite evidence for prolific iron and sulfate reduction below the sedimentewater interface (see

Figs. 2 and 4; Kostka et al., 2002). The Tidal Levee site is vegetated with tall phenotype of Spartina with a root zone that extends almost 40 cm deep below the surface. This results in considerable below-ground production (as roots and rhizomes) that can often exceed the above-ground production (Gallagher and Plumley, 1979; Schubauer and Hopkinson, 1984). Consequently, even with ongoing degradation, organic carbon is constantly replenished at all measured depths. 5.4. Ponded Marsh At the Ponded Marsh site, the porewater pH profile did not change significantly with depth in winter; however, in summer a sharp decrease is observed below the sedimentewater interface that eventually stabilizes below a depth of 5 cm (Fig. 4). No effect of pH was displayed in the sediment trace metal content, probably because the pH at the Ponded Marsh site was consistently close to near-neutral values. Surface concentrations of trace metals at the Ponded Marsh site were similar (Ba, Pb, Ti, V and Zr) or slightly lower (As, Cd, Mo, U, Zn, Cr, Co, Ni and Cu) during summer, the Spartina growth season. However, for most trace metals the surface concentration

686

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was less than that observed at the Creek Bank site but slightly higher than the Tidal Levee site (Fig. 3). This could be due to: (1) limited transport of particulates with the tidal waters from the inner marsh that is in the lowest-energy regime among the three sites; and (2) stunted growth of Spartina at the Ponded Marsh site. The short phenotypes depict growth under adverse conditions, such as a highly sulfidic eco-toxic environment (Stribling, 1997; Chambers et al., 1998), resulting in a lower uptake of trace metals and nutrients by the plants. Except for relatively more enrichment of As in winter, the depth-integrated trace metal content at the Ponded Marsh site increased in summer (Table 1). Both AVS and ironoxyhydroxide in the Ponded Marsh sediment decreased, whereas organic carbon and CRS increased over the same period (Table 1), indicating partitioning of trace metals, either in the organic or the CRS fraction. Furthermore, at the Ponded Marsh site, the highest clay and CRS content and the lowest AVS and iron-oxyhydroxide content were measured among the three sites (Table 1). However, the depth-integrated trace metal content at the Ponded Marsh site was lower than that measured at the Tidal Levee site but higher than that present at the Creek Bank site e a signature similar to the organic carbon content of the sediments (Table 1). This further corroborates the possibility that at the Ponded Marsh site, trace metals are primarily associated with the organic carbon. Depth profiles of trace metals at the Ponded Marsh site were distinctly different from those observed at the other two sites. For both seasons, each profile had a sub-surface maximum, below which the concentrations decreased almost exponentially with depth (Fig. 3). The depth profiles of trace metals at the Ponded Marsh site also follow the organic carbon profile closely (see Figs. 2 and 3). In winter a sub-surface maximum for organic carbon and trace metals was observed between 10 and 12 cm. In summer, the maximum shifts closer to the surface at around 4 and 5 cm below the sedimentewater interface (Fig. 3). Furthermore, enhanced trapping of trace metals around these maxima is observed compared to any depths from any of the three sites. These seasonal shifts in the depth of the trace metal maximum closely match the sulfate reduction rates measured at the Ponded Marsh site (see Kostka et al., 2002). However, the trace metal profiles do not show any resemblance to the AVS or CRS profiles, ruling out the possibility that the trace metals are primarily partitioned as solid phase sulfide. Enhanced trapping of trace metals by sulfurized organic matter has been shown previously, where trace metal enrichment was positively correlated with the amount of sulfurized organic matter, but not with pyrite (CRS) abundance in the sediments (Tribovillard et al., 2004). Sulfurized organic matter is formed in quantitatively significant amounts when reactive iron-oxyhydroxide concentrations are low and excess hydrogen sulfide is formed in the porewaters (Zaback and Pratt, 1992; Pratt et al., 1993; Suits and Arthur, 2000). This is primarily because iron-oxyhydroxides are active scavengers of hydrogen sulfide. However, in estuarine environments with land-derived organic matter, simultaneous formation of pyrite and organic sulfur has also been observed (Bruchert and Pratt, 1996). At the Ponded

Marsh site, excess sulfide in the porewaters (Fig. 4) and low reactive iron (Fig. 2) were observed for both seasons, favoring sulfurization of organic matter and hence the possibilities of enhanced entrapment of trace metals. As organic matter degrades with depth; however, trace metals are released back into the porewater resulting in a simultaneous exponential decrease in the particulate fraction observed (Fig. 3). Even though free sulfide is sustained at all depths where trace metals are released back into the porewaters, their small concentration in the porewater do not favor nucleation of pure phases, i.e., the formation of respective trace metal sulfides (Morse and Rickard, 2004). Furthermore, iron-oxyhydroxide concentrations were limited, especially in summer, at these depths, therefore significant scavenging by pyrite-forming phases can also be ruled out. However, trace metal concentrations in winter were slightly higher than in summer at all depths (10e47 cm) where active degradation of organic matter is observed (Fig. 1). Note that in winter, measurable reactive ironoxyhydroxide was present at those depths (Fig. 1), which may have contributed to minor enrichment of trace metals during the formation of the pyritic phase. 6. Summary and conclusions Comparative data collected from three sites on Sapelo Island, suggest that sediment trace metal content within the saltmarsh environment is influenced by diurnal tidal flushing, bioturbation and sediment mixing, vegetation pattern and degradation and production of below-ground biomass. A combination of the suggested processes acted at the three sites (Fig. 5); however, dominant processes controlling the trace metal cycling varied from one site to another and are believed to be coupled to the lateral heterogeneity of the saltmarsh. At the unvegetated Creek Bank site, trace metal distribution is primarily controlled by tidal flushing and bioturbation. Tidal flushing results in a lower accumulation of trace metals and homogenization of the depth profiles of redox-sensitive elements. To some extent, bioturbation also homogenizes the profiles by downward mixing of sediments and by providing a conduit for oxygen diffusion to sub-oxic zones. But more importantly, through bottom-feeding activities and subsequent defecation, localized enrichment of trace metals having affinity to lithogenic fraction is observed. At the vegetated Tidal Levee site, Spartina play a role, although minor, in uptake of trace metals from the sediments. The slightly lower trace metal content in the surface sediments, compared to the Creek Bank site, is primarily due to the diurnal removal of organic debris with the tides rather than via plant uptake. At the Tidal Levee, trace metal profiles do not change significantly with depth despite development of distinct redox fronts. Furthermore, the highest depth-integrated trace metal and organic carbon contents were measured at this site for both seasons. It is suggested that trace metals are primarily retained with the organic fraction and despite ongoing microbially mediated degradation, significant belowground primary production maintains a high organic content, thereby retaining the trace metals at all depths. Although,

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Fig. 5. A conceptual figure summarizing different biogeochemical processes that control the trace metal distribution at the three sites. Note that the cross-section is not drawn to scale; however, it replicates the observed bio-physiographical variation observed across the three sites.

minor, seasonal recycling of trace metals between the organic and the sulfide phase cannot be ruled out. In contrast, at the Ponded Marsh site, a sub-surface maximum was evident below which trace metal concentrations decrease with depth in accordance with the organic carbon profile (Figs. 2 and 3). At the Ponded Marsh site, belowground production is limited to the top 10e15 cm, as sulfide toxicity restricts the root zone and microbial biomass to these depths. Therefore, with progressive degradation of organic matter, coupled to sulfate reduction at deeper depths, trace metals are released into the porewaters at greater depths. Due to iron limitation, however, co-precipitation with the pyrite mineral phase is negligible at these depths and consequently upwardly diffusing trace metals are entrapped in the organic-rich root zone. Excess sulfide at these depths results in sulfurization of organic matter and Spartina roots deliver oxygen in otherwise sulfidic environment forming zero-valent sulfur, both ultimately responsible for trace metal sequestration. Only minor variations in the total trace metal content and depth profiles were observed seasonally, despite drastic changes in the microbial activity (Roychoudhury et al., 1998; Kostka et al., 2002; Koretsky et al., 2003; Roychoudhury et al., 2003b), major ion porewater chemistry (Fig. 4) and, to some extent, in the vegetation pattern. Furthermore, variation in organic carbon, iron-oxyhydroxide and solid phase sulfide content in the sediment was also observed seasonally (Fig. 2). Thus, rapid recycling of trace metals among the solid phases is suggested. Moreover, at Sapelo Island, initially trace metals are preferentially partitioned in the organic fraction; however, when excess dissolved sulfide and iron-oxyhydroxides are present sequestration of trace metals with the CRS phases at greater depths occurs.

The downcore trends of trace metals with positive or negative anomalies are often utilized for assessing anthropogenic intervention or major climate change. The effectiveness of trace metals as a proxy depends on their immobility upon sequestration. However, little is known about the fate of trace metals under early and post-diagenetic situations. Data obtained in this study show that in dynamic coastal ecosystems, such as a saltmarsh, upon burial, a number of processes can play a critical role in the enrichment and rapid recycling of trace metals among various reservoirs at a range of depth scales. Therefore an assumption of post-depositional static conditions can lead to erroneous reconstruction of sediment memory record that may have great implications on our understanding of palaeo-climatic processes or anthropogenic activities. Acknowledgement This work was supported by grants from NRF, South Africa (2053191, FA2004041200002). The author would like to thank the Sapelo working group (Philippe van Cappellen, Eric Viollier, Joel Kostka, Suvasis Dixit, Kim Hunter, Christof Meile and Patrick Inglett) for their help in the field and for the analysis of porewaters as part of a bigger project, and John Rogers for his help with grain size analysis. John Compton provided critical comments on an earlier draft of the manuscript. Comments from two anonymous reviewers helped to improve the manuscript. References Adriano, D.C., 2001. Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability, and Risks of Metals. Springer, London, 867 pp.

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