Stable sulfur isotope dynamics in an acid sulfate soil landscape following seawater inundation

Chemical Geology 439 (2016) 205–212

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Stable sulfur isotope dynamics in an acid sulfate soil landscape following seawater inundation C.A. Maher a,⁎, L.A. Sullivan b a b

Southern Cross GeoScience, Southern Cross University, Lismore, NSW, Australia Federation University, Ballarat, VIC, Australia

a r t i c l e

i n f o

Article history: Received 21 November 2014 Received in revised form 4 July 2016 Accepted 4 July 2016 Available online 05 July 2016 Keywords: Isotope geochemistry Tidal exchange Jarosite Pyrite oxidation

a b s t r a c t In 2002 a tidally driven seawater exchange remediation strategy was successfully implemented on a severely acidified tropical coastal landscape dominated by acid sulfate soils (ASS) in northern Australia. This study examined changes in the stable sulfur isotope signatures in a range of sulfide and sulfate (SO4) fractions at three sites with different levels of exposure to the tidally driven seawater exchange remediation. δ34S in the acid soluble SO4 fraction (e.g. jarosite) was less depleted in 34S than the corresponding sulfide, indicating a degree of fractionation during sulfide oxidation and jarosite precipitation. The δ34S of jarositic-SO4 was similar at all three sites indicating the appreciable stability of jarositic-SO4 even after extended exposure to seawater. δ34S of the water soluble, exchangeable and schwertmannitic-SO4 reflect conditions post remediation and indicate the relative contributions from two potential SO4 sources – a lighter SO4 derived from the oxidation of pyrite, and a heavier SO4 derived from the seawater. The δ34S of the contemporary surficial sulfide accumulations also reflect a SO4 contribution from seawater used for remediation and were isotopically different from the relict sulfides found at depth at all sites. δ34S of water soluble sulfate allowed the progress of the remediation to be traced down the soil profile. This study demonstrates the utility of stable sulfur isotope signatures in various sulfide and SO4 fractions to trace the sulfur geochemical pathways occurring in soils, in this case as a result of the introduction of tidally driven sea water. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The CRC CARE National Acid Sulfate Soil Demonstration Site is located on the eastern side of Trinity Inlet, near Cairns in far north Queensland (145°48′ E, 16°56′ S). The site, referred to as ‘East Trinity’ (Fig. 1), comprises 9.4 km2 of deeply stratified Holocene sediments down to 80 m overlying Pleistocene basement strata (Smith et al., 2004). This region has a tropical climate, with a summer wet-season, mean annual precipitation of 2000 mm and average daily maximum and minimum temperatures for all months exceeding 25 °C and 17 °C, respectively (Hicks et al., 2009). In the early 1970s the East Trinity property was developed for sugarcane production. This involved the construction of a bund wall to prevent tidal entry and the installation of pumps to assist with drainage of the intertidal wetland (Smith et al., 2004; Powell and Martens, 2005). The development of the site for agriculture by combined drainage and seawater exclusion severely degraded the environment. Firstly, the drainage of the land lowered the natural water table and allowed oxygen to enter the surface layers of the deep Holocene sediments ⁎ Corresponding author. E-mail address: [email protected] (C.A. Maher).

http://dx.doi.org/10.1016/j.chemgeo.2016.07.001 0009-2541/© 2016 Elsevier B.V. All rights reserved.

and oxidise the iron sulfide minerals. This initiated the acidification of the acid sulfate soil (ASS) materials, producing a variety of acidic iron precipitate minerals such as jarosite and schwertmannite (Russell and Helmke, 2002; Powell and Martens, 2005; Johnston et al., 2009a, 2009b). The exclusion of seawater from the site severely limited the supply of a readily available source of neutralisation namely bicarbonate (HCO− 3 ) in the tidally driven seawaters. Impacts associated with the oxidation of ASS at East Trinity as a result of this development include extremely low pH in the soil and surrounding creeks, mobilization of heavy metals (particularly iron and aluminium), diminished populations of aquatic biota, and significant fish kills (Russell and Helmke, 2002; Smith et al., 2004). In May 2000 the Queensland State Government purchased the East Trinity property and implemented a remediation plan to address the environmental hazards posed by this severely degraded site. In general the remediation plan at this site was guided by two strategies: firstly, to neutralise any existing acidity generated from the oxidation of pyritic material, and; secondly, to prevent further pyrite oxidation by reestablishing a higher water table to limit the introduction of oxygen into the soil profile. At East Trinity these strategies were achieved by lime assisted tidal exchange although the effect of the lime additions was negligible on the remediation of acidification compared to the

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Fig. 1. Location of the East Trinity site in northern Queensland, Australia. Sampling Sites 1, 2 and 3 in this study correspond to Sites T2.1, T2.3 and T2.4 respectively (Johnston et al., 2011b).

contribution of the seawater (Johnston et al., 2012). The ongoing remediation also provided an excellent opportunity to investigate the likely response of a severely degraded ASS during climate change induced sea level rise. There have been several studies examining the consequent geochemical changes. Some of these studies examined the hydrology and water chemistry of the site (Johnston et al., 2009a, 2011a), contemporary pedogenesis (Johnston et al., 2009b) and the abundance and reactivity of aluminium, iron and trace metals (Johnston et al., 2010; Keene et al., 2010; Burton et al., 2011a, 2011b; Claff et al., 2011). Although stable sulfur isotope geochemistry has been investigated previously in field and experimental studies, only a few preliminary studies having been undertaken to date in ASS, and none in relation to the likely response of these sediments to climate change impacts. For example, Johnston et al. (2009b) used sulfur isotopes solely to help identify the depth that seawater inundation was impacting the soil profile and consuming acidity at the East Trinity site. Their results indicate that tidal inundation was only affecting the surficial sulfuric horizon and not the underlying sulfidic material. Boman et al. (2008) used sulfur isotopes to identify the possible sources and causes of sulfate and acidity in waters draining from Finnish ASS. Fractionation of stable isotopes occurs because the strength of the chemical bonds varies slightly with the mass of the isotope. The principle reaction in the formation of sulfide minerals is the reduction of SO4 to produce H2S (Rees, 1973; Dent and Pons, 1993; Sammut et al., 1996; Burton et al., 2011a, 2011b). This bacterially mediated process results in isotopic fractionation because the rate at which 34SO24 − transforms to H34 2 S is significantly slower than the rate at which 32SO24 − transforms to H32 2 S. Thus the result of sulfidisation is lighter sulfide and heavier SO4 (Fry et al., 1995; Brownlow, 1996; Wijsman et al., 2001; Fry, 2006; Hatzinger et al. 2012). Some of the factors that can affect the degree of isotopic fractionation include SO4 concentration, SO4 reduction rates, substrates, depositional environment, temperature, pH, bacterial species and growth conditions (Kaplan and Rittenberg, 1964; Chambers and Trudinger, 1979; Habicht and Canfield, 1997; McConville et al., 2000; Farquhar et al., 2007; Stam et al., 2011). The aim of this study is to examine the

utility of sulfur isotopes in examining the geochemical processes occurring in ASS landscapes, using a formerly severely degraded ASS landscape that has been subject to varying degrees of remediation by tidally driven seawater inundation. 2. Methodology 2.1. Sampling site Three sites were examined across the East Trinity property with varying surface elevations. Site 1 has a surface elevation of 0.5 m AHD and has not been affected by seawater inundation. Site 2 is at a slightly lower elevation (0.1 m AHD) and receives intermittent seawater flushing (Johnston et al., 2011a). Site 3 is again topographically lower (− 0.1 m AHD) and remains inundated by seawater for most of the time (Johnston et al., 2009b). Site 3 has been receiving tidal inundation with seawater since 2002. Sites 1, 2, and 3 in this study correspond to Sites T2.1, T2.3, and T2.4, respectively as described by Johnston et al. (2011b)). 2.2. Sample collection and preservation Soil samples were collected using a gouge auger to a depth of 1.5 m and sectioned into 0.1 m depth increments. At each site multiple cores were taken and these increments bulked. Soil samples were placed in thick plastic bags, squeezed to exclude oxygen, sealed to reduce oxidation, and immediately frozen. Samples were kept frozen and analysed within 2 years of collection. 2.3. Chemical analysis Subsamples were oven dried at 105 °C for 7 days, then reweighed for gravimetric moisture content determination (θg) (Rayment and Lloyds, 2011). Where applicable, results are reported on an oven-dried basis. pH and electrical conductivity (EC) were measured immediately after thawing of the samples, in a 1:5 soil:water suspension (Rayment and

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Lloyds, 2011). Measurements of pH were gained using a TPS LabCHEM pH meter and Ionode IJ44 pH electrode calibrated with DIN 19266 pH buffer solutions of 4.01 and 7.00. EC readings were also taken from the suspension using a TPS conductivity sensor. Total carbon, sulfur and nitrogen contents were determined on previously frozen soil, dried for 48 h at 65 °C. Soil was finely ground in a mortar and pestle and analyses were conducted in a LECO™ CNS 2000 induction furnace analyser. Water soluble (WS) Cl and SO4 was extracted in a 1:10 soil:water suspension. In accordance with recommendations by Maher et al. (2004) analyses were conducted on frozen soil. Extracts were filtered through a 0.45 μm filter and 10 mL analysed on a Perkin Elmer Elan 9000 inductively coupled plasma optical emission spectrometer (ICPOES) according to APHA Method 3120 (APHA, 2005). Quality control standards were run after every tenth sample. Duplicate sample analysis gave a precision of ±9% for chloride and ±7% for sulfur with a detection limit of 0.05 mg/L. The remaining extract was retained for isotope analysis. Exchangeable and acid soluble SO4 were extracted using a sequential extraction procedure. Exchangeable SO4 was measured using a 1 M KCl extract in a 1:40 soil:solution ratio (McElnea and Ahern, 2004) on frozen soil that was allowed to thaw (Maher et al., 2004). The supernatant was filtered (0.45 μm) and analysed for sulfur by ICP-OES and the remaining soil rinsed with Milli-Q water. Acid soluble SO4 was extracted from the remaining soil using 4 M HCl in a 1:40 soil:solution ratio (McElnea and Ahern, 2004) and the filtered supernatant was analysed by ICP-OES. Duplicate analysis gave a precision of ±6.5% and ±8% for KCl and HCl SO4 respectively. These extracts are reported as KCl-SO4 and HCl-SO4 for exchangeable and acid soluble SO4, respectively. A sequential extraction procedure was used to differentiate readily available and less available forms of Fe. Readily available Fe (termed reactive Fe) was extracted using 1 M HCl from recently thawed frozen soil (Wallmann et al., 1993). Extracts were analysed for Fe2+ and total Fe using the 1,10 phenanthroline method (APHA 3500) (APHA, 2005). Both Fe2+ and total Fe trapping solutions contained a phenanthroline ammonium acetate buffer solution. Total Fe traps also contained hydroxyl ammonium chloride as a reducing agent. Samples were analysed by HACH Spectrophotometer. Fe3 + is represented by total Fe minus Fe2+. Reactive Fe species are reported as FeR2+ and FeR3+ for ferrous and ferric Fe species respectively. Residual soil samples were rinsed with 1 M MgCl2 and the less available forms of Fe were then extracted using a citrate dithionite extraction procedure (Kostka and Luther, 1994). Aliquots were pipetted into phenanthroline ammonium acetate traps and analysed by HACH Spectrophotometer. Results are reported as FeCDE. Duplicate analysis gave a precision of ±7% with a detection limit of 0.05 mg/L Fe. Reduced inorganic sulfur species include iron monosulfides, operationally defined as acid volatile sulfides (AVS), elemental sulfur (ES) and iron disulfides defined here as chromium reducible sulfur (CRS). A procedure to sequentially extract these species was employed. AVS analysis was conducted on frozen soil (Maher et al., 2004) with values operationally defined following the diffusion method of Hsieh et al. (2002) and a modified apparatus described by Burton et al. (2007). 6 M HCl was used to evolve H2S which was trapped in a zinc acetate trapping solution and quantified by iodometric titration. Residual soil samples were rinsed with Milli-Q water before ES was extracted using toluene (Burton et al., 2011a, 2011b) and analysed by HPLC on a Dionex UltiMate 3000 system. CRS analysis was conducted according to the method of Sullivan et al. (2000). Duplicate analysis gave a precision of ±10% with a detection limit of 0.001% S. 2.4. Isotope analysis Stable sulfur isotope analyses were conducted on the sulfide and SO4 fractions of selected samples. The sulfide fraction was extracted using the CRS procedure. When used a stand-alone technique this process

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extracts all reduced inorganic sulfur species (Sullivan et al., 2000). ZnS precipitates contained in the trapping solutions were centrifuged, rinsed three times with Milli-Q water then dried at 105 °C for 24 h. Three SO4 fractions were examined including WS-SO4 (1:5 soil:water), KCl-SO4 (1:10 soil:1 M KCl) and HCl-SO4 (1:10 soil:4 M HCl). The suspensions were filtered through a 0.45 μm filter directly into a solution of 1 M BaCl2 to precipitate BaSO4. Precipitates were rinsed three times with Milli-Q water then dried at 105 °C for 24 h. ZnS and BaSO4 precipitates were analysed for δ34S by Continuous Flow Isotope Ratio Mass Spectrometry (CF-IRMS) using a Thermo Flash EA 1112 coupled to a Thermo Delta V Plus IRMS. Reference material 8555 (NIST) was used for calibration. A sulphanilamide standard (IVA Analysentechnik e.L., Dusselforf) was used to check for any changes due to sample preparation and isotopic drift. Replicates of reference material indicated a standard deviation of 0.06‰. Duplicate sample analysis gave a precision of ±0.3‰. Results are presented as δ34S(CRS) for the sulfide fraction, δ34S(WS-SO4) for H2O-soluble SO4, δ34S(KCl- SO4) and δ34S(HCl-SO4) for exchangeable and HCl-soluble SO4 respectively.

3. Results Sites 1, 2 and 3 represent a sequence of the degree of remediation across the site. Site 1 has not been affected by seawater inundation and remains in a well-drained and severely acidified state. Site 3 was previously well-drained and acidified but has been constantly inundated with seawater since the program commenced in 2002. Site 2 is between Sites 1 and 3 and has been inundated by seawater only intermittently. Figs. 2–5 show some of the geochemical changes across the site from an unremediated post-drainage state (Site 1) to long term remediated (Site 3). Site 1 is typical of an ASS profile that is well oxidised and severely acidified as a result of the oxidation of sulfide minerals caused by drainage of the landscape (Fig. 2). The pH at the surface was ~ 4.0 (Fig. 2a) and the Cl:SO4 ratios (Fig. 3) are below 1 throughout the majority of the profile demonstrating the strong influence of a pedogenic source of SO4 at this site (i.e. the Cl:SO4 ratio of seawater is ~ 7 to 8): in this case the oxidation of sulfide minerals. The spike in SO4 concentration at approximately 0.8 m depth (Fig. 2d) indicates the current sulfide oxidation front where appreciable pyrite concentrations are first encountered with depth (Fig. 2f). The peak in HCl-SO4 at 0.2 to 0.6 m depth (Fig. 2e) results from the accumulation of jarosite as observed by both Johnston et al. (2009b, 2010) and Keene et al. (2010, 2011). Site 2 has clearly been affected by the remediation practices (Fig. 4). Relative to the untreated Site 1, the pH has increased to around 6.0 down the profile to ~1.2 m depth (Fig. 4a). There has also been an increase in EC (Fig. 4a) at the surface and the Cl:SO4 ratio has risen to 2 (Fig. 3). Below 0.9 m the Cl:SO4 ratio falls below 1 and this layer appears to be unaffected by the seawater applied to the surface during the remediation (Johnston et al., 2009b). Fig. 4f indicates contemporary formation of sulfidic minerals in the upper 0.5 m of the profile at this site – a process noted in other studies (Johnston et al., 2009b, Burton et al., 2011a, 2011b). Fig. 4c shows the surficial accumulation of relatively poorly crystalline iron minerals (including ferrihydrite, lepidocrocite, goethite and schwertmannite) as noted for that location by Johnston et al. (2011a). Site 3 has higher concentrations of contemporary sulfides in the upper 0.5–0.6 m than Site 2 (Fig. 5f). In particular there is a greater abundance in both monosulfides (as measured by AVS) and elemental sulfur (ES) as also observed at this site by Burton et al. (2011a, 2011b) and Johnston et al. (2011b, 2012). pH has risen to circum-neutral (Fig. 5a) and the Cl:SO4 ratio is approaching that of seawater (~ 7, Mulvey, 1993) (Fig. 3). There is considerably less total carbon at the surface of this site (Fig. 5b) compared to the other two sites (Figs. 2b and 4b).

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b

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ES 0.0

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S (%)

Fig. 2. Soil characteristics for Site 1 (no seawater influence), including: (a) pH and EC; (b) total C, S and N; (c) reactive ferrous and ferric Fe (FeR2+, FeR3+) and citrate dithionate extractable Fe (FeCDE); (d), H2O-soluble Cl and SO4; (e), KCl- and HCl-SO4, and; (f) acid volatile sulfur (AVS), elemental sulfur (ES), chromium reducible sulfur (CRS).

The relatively high concentration of KCl-SO4 observed at a depth of between 0.9 and 1.2 m at both Sites 1 and 2 (Figs. 2e and 4e, respectively) is not evident at Site 3 (Fig. 5e). The concentration of HCl-SO4 at a depth of between 0.3 m and 0.6 m at Site 3 is also smaller than at the corresponding depth on the other two sites, and there is a significant spike in FeR3 + at the surface (Fig. 5c). Johnston et al. (2010) found that long term inundation with seawater allowed the establishment of reducing conditions which favour the dissolution of jarosite from these sub-surface layers, accompanied by the mobilization of Fe3+ that accumulates as iron-rich surficial layers at this site. Thus the HCl-SO4 of the surficial layer is largely derived from the relatively poorly crystalline iron minerals (including ferrihydrite, lepidocrocite, goethite and schwertmannite) noted for this location by Johnston et al. (2011a, 2011b), whereas the HCl-SO4 of the subsurface layers between 0.1 m and 0.6 m especially, are largely derived from jarosite.

0.0

Depth (m)

0.3 0.6

Site 1

0.9

Site 2

1.2

Site 3

1.5 0.0

2.0

4.0

6.0

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Cl:SO4 ratio Fig. 3. Cl:SO4 ratios for Sites 1–3. Site 1 is unaffected by seawater inundation, Site 2 has received only intermittent seawater inundation, and Site 3 has been constantly inundated by seawater since 2002.

3.1. Stable sulfur isotope signatures The results indicate there have also been considerable changes to the stable sulfur isotope signatures across Sites 1–3 (Fig. 6). At Site 1 δ34S(CRS) ranges from −28.4‰ to −33.6‰ and shows a maximum fractionation from seawater SO4 of 54.2‰ (Fig. 6a). This maximum value occurs at 0.8 m, corresponding with the oxidation front. δ34S(WS-SO4) values are also strongly negative throughout the profile suggesting the dominant source of WS-SO4 is from the oxidation of isotopically negative pyrite. The most negative δ34S(WS SO4) also corresponds with the most negative δ34S(CRS) suggesting the WS-SO4 fraction has been largely derived from the oxidation of the sulfide minerals formed prior to drainage and remediation. At Site 1, most of the δ34S values for the three soluble SO4 fractions are very similar, (Fig. 6a) apart from the δ34S(KCl-SO4) fraction for three layers. These three layers are the only occurrences in this study where the KCl-SO4 fraction had a markedly different isotope signature to the corresponding WS-SO4 fraction. The reason for this departure is not evident. At Site 2, the introduction of seawater has affected the sulfur isotope signatures of especially the WS- and HCl-SO4 fractions in the soil layers down to ~ 1.0 m depth (Fig. 6b). The δ34S(CRS) values are similar to Site 1 with a range of − 27.6‰ to − 32.2‰, however the WS-SO 4 fraction shows a marked increase in δ 34S, up to + 3.8‰ near the surface. In the jarositic 0.2 m to 0.6 m depth layer at Site 2, the δ34S(HCl-SO4) values were within the range of − 25‰ to − 15‰ measured for the jarositic subsurface layer at Site 1. The surficial layer at Site 2 contains accumulations of relatively poorly crystalline iron minerals (including schwertmannite) (Johnston et al., 2011a, 2011b) where the δ34S(HCl-SO4) value was considerably heavier than for the underlying jarositic layers (~− 10‰). This indicates an appreciable influence of seawater derived SO4 in the formation of schwertmannite in this layer. At Site 3 the geochemical shifts in the sulfur isotope signatures were more pronounced than for Site 2 (Fig. 6c). In the upper 1.0 m of the

C.A. Maher, L.A. Sullivan / Chemical Geology 439 (2016) 205–212

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Fig. 4. Soil characteristics for Site 2 (intermittent seawater inundation), including: (a) pH and EC; (b) total C, S and N; (c) reactive ferrous and ferric Fe (FeR2+, FeR3+) and citrate dithionate extractable Fe (FeCDE); (d), H2O-soluble Cl and SO4; (e), KCl- and HCl-SO4, and; (f) acid volatile sulfur (AVS), elemental sulfur (ES), chromium reducible sulfur (CRS).

profile the δ34S(WS-SO4) values were positive with a maximum value of + 25.6‰. This is higher than the δ34S of seawater SO4 (+ 20.6‰ – Bottcher et al., 2004) and may be a function of light SO4 removal through SO4 reduction (Wijsman et al., 2001; Farquhar et al., 2008). The δ34S(CRS) for the contemporary sulfide minerals in the remediated surficial horizons were less negative than the older sulfide minerals in

a

b

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the unoxidised subsoil layers at Site 3. In the upper 0.7 m the δ34S(CRS) averaged −22.5‰ (SD = 2.6, n = 7), whereas in the lower half of the profile the δ34S(CRS) averaged − 27.6‰ (SD = 0.8, n = 8). At this site the sole sample of δ34S(HCl SO4) had a value within the range of −25‰ to − 15‰ measured for the jarositic subsurface layers at both Sites 1 and 2.

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Fig. 5. Soil characteristics for Site 3 (long term seawater inundation), including: (a) pH and EC; (b) total C, S and N; (c) reactive ferrous and ferric Fe (FeR2+, FeR3+) and citrate dithionate extractable Fe (FeCDE); (d), H2O-soluble Cl and SO4; (e), KCl- and HCl-SO4, and; (f) acid volatile sulfur (AVS), elemental sulfur (ES), chromium reducible sulfur (CRS).

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Site 1

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WS SO4 KCl SO4

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HCl SO4

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WS SO4

approximately 1.2 m depth the Cl:SO4 ratios are very similar suggesting the influence of the introduced seawater is limited at these depths (Johnston et al., 2009b). There have also been marked changes in the abundance of the different iron species between Sites 1–3. Following long term tidal inundation there is a decrease in FeCDE below 0.5 m and an increase in FeR2+ above 0.5 m. Studies by Keene et al. (2010, 2011) and Burton et al. (2011a, 2011b) showed an abundance of FeR2+ in the surface and subsurface layers, whereas this study shows an accumulation of FeR3+ particularly at the surface of Site 3. Johnston et al. (2011b) found that this accumulation consists of poorly crystalline iron minerals including ferrihydrite, lepidocrocite, goethite and schwertmannite. The accumulations of sulfide minerals in the upper layers of Sites 2 and 3 are contemporary formations (Johnston et al., 2009b; Keene et al., 2010, 2011; Burton et al., 2011a). Both of these sites underwent considerable and severe oxidation due to drainage, and sulfide formation only occurred following a geochemical shift in the redox conditions from oxidising to reducing following remediation using seawater inundation (Smith et al., 2003, Johnston et al., 2011a). The relative abundance of AVS and ES fractions suggests the current conditions favour the accumulation of monosulfides and elemental sulfur. Similar results were returned by Burton et al. (2011a, 2011b) and Keene et al. (2011) with the AVS mineral identified as greigite. Below the former oxidation boundary, pyrite is the dominant sulfide mineral.

KCl SO4

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Site 3

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WS SO4 KCl SO4

1.2

HCl SO4

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-20

0

20

40

Content (%) Fig. 6. δ34S of the sulfide and SO4 fractions for: a) Site 1, b) Site 2 and c) Site 3. Sulfide minerals are represented by the chromium reducible sulfur fraction (CRS). The SO4 fractions include water soluble SO4 (WS-SO4), exchangeable SO4 (KCl-SO4) and acid soluble SO4 (HCl-SO4).

4. Discussion 4.1. The effect of seawater inundation on soil geochemistry The remediation strategy using tidally driven seawater inundation is having considerable effect on the geochemistry of the East Trinity property. The difference in surface elevation between Sites 1–3 means that this toposequence can be used as a proxy temporal sequence of tidal inundation. pH has increased considerably at Sites 2 and 3 with seawater exchange, particularly in the upper soil layers, indicating the seawater has provided sufficient buffering capacity to neutralise existing acidity and raise the pH to nearly neutral (Johnston et al., 2009a, 2011a). There has also been a significant change in the Cl:SO4 ratio at these sites with an increase in values from b 1 at Site 1 to N6 at Site 3. Below

4.2. Stable sulfur isotope dynamics following seawater inundation In this study, δ34S of the sulfide fraction is measured as chromium reducible sulfur (CRS) and includes the acid volatile sulfur, elemental sulfur and disulfide fractions combined. At Sites 1, 2 and 3, the δ34S(CRS) values are all negative and indicative of sulfides formed from bacterial reduction of SO4 (McConville et al., 2000;Wijsman et al., 2001; Stam et al., 2011). At Site 1 the maximum fractionation from seawater SO4 (54.2‰) was recorded in the current oxidation zone (i.e. ~ 0.8 m depth). Such enrichment of 32S at the oxidation boundary of ASS has also been reported at several other sites in south east Australia by Maher (2013) and likely resulted from sulfur cycling processes (Canfield and Thamdrup, 1994; Cyprionka et al., 1998; Habicht et al., 1998; Bush, 2000). In these niche zones, conditions cycle between oxidising and reducing depending on the height of the water table. Oxidation of pyrite provides a source of isotopically light SO4 which is utilized during SO4 reduction and produces sulfides with greater enrichment in 32S Maher, 2013. At Site 3, δ34S(CRS) in the upper half of the profile (−22.5‰, SD = 2.6, n = 7) is different from the δ34S(CRS) in the lower half of the profile (−27.6‰ SD = 0.8, n = 8). At this site the sulfide minerals in the surficial layers are isotopically heavier than the original sulfide minerals at depth. This distinction indicates the divide between the contemporary sulfide minerals found in the upper layers (Johnston et al., 2009b; Keene et al., 2010, 2011; Burton et al., 2011a), and those in the lower original Holocene sediments (Smith et al., 2003; Powell and Martens, 2005; Johnston et al., 2010). This is likely primarily due to the re-introduction of the seawater which contains isotopically heavy SO4 at this site. The contemporary sulfides formed in the upper 0.4 m of Site 2 gave an average δ34S(CRS) of −30.5‰ (SD = 1.5, n = 3) which is more negative than that of the corresponding layers at Site 3. At this site, which lies topographically between Sites 1 and 3, the tidal exchange is only intermittent and redox conditions regularly oscillate between reducing and oxidising (Johnston et al., 2009b; Burton et al., 2011a, 2011b). Such conditions provide two sources of SO4 – seawater and pyrite oxidation generated – which can be reduced by bacteria (Poulton et al., 1998; Johnston et al., 2009b). Site 3 experiences constant seawater inundation and under these conditions the dominant SO4 source for contemporary SO4 reduction in the surficial layers is likely to be seawater. This relatively 34S enriched seawater has likely lead to the formation of the less

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negative δ34S(CRS) in the contemporary sulfide minerals in the surficial layers at this site relative to either the contemporary sulfide minerals observed in the surficial layers at Site 2 where there is an additional strong source of relatively light SO4 derived from sulfide mineral oxidation, or the deeper unoxidised Holocene sulfidic layers at each site. This relatively 34S enriched seawater has also likely lead to the formation of the less negative δ34S(HCl-SO4) signatures for the poorly crystalline iron mineral surface layer at Sites 2 and 3. The greatest isotopic changes were seen in the δ34S(WS SO4) between Sites 1, 2 and 3. At Site 1, δ34S(WS SO4) are negative indicating the dominant SO4 source is the oxidation of isotopically negative sulfides (Mayer et al., 2010; Kilminster and Cartwright, 2011; Unland et al., 2012). This is further supported by the Cl:SO4 ratios which are b1 throughout the majority of the profile. The Cl:SO4 ratio is useful for identifying when additional SO4 sources are likely (Mulvey, 1993), whereas the isotope signature provides evidence of the source of the SO4 (Mayer et al., 2010; Kilminster and Cartwright, 2011; Unland et al., 2012). When used in combination, the two techniques provide an enhanced understanding of the nature and extent of the contribution of different sources of SO4 within ASS landscapes. At Site 1 the lowest δ34S(WS-SO4) correspond with the most recently oxidised layers. At the oxidation boundary the redox conditions change and SO4 reduction again begins to take place. In these zones the dominant source of SO4 is isotopically light, which bacteria can reduce to produce sulfides with even greater fractionation (relative to sea water) than the original sulfides (Poulton et al., 1998; Johnston et al., 2009b). At Site 2, δ34S(WS-SO4) values show a distinct increase from the values recorded at Site 1, particularly above 1.1 m depth. This indicates a shift to a different source of SO4 – in this case SO4 derived from the implementation of the seawater exchange strategy. At this intermittently seawater inundated site, the SO4 signature is likely due to a combination of pyrite derived SO4 and seawater derived SO4. Below ~ 1.3 m the δ34S(WS-SO4) at Site 2 is similar to Site 1 suggesting this depth represents the limit of seawater influence at Site 2 to date (Johnston et al., 2009b). The δ34S(WS-SO4) at Site 3 shows even greater influence from seawater. Above 1.0 m δ34S(WS-SO4) values are positive and range from +3.1‰ to + 29.3‰ indicating the influence of pyrite derived SO4 was much lower at Site 3 than at the other sites. In the upper 0.3 m of this profile δ34S(WS-SO4) are higher than seawater SO4 (+ 20.6‰ – Bottcher et al., 2004). In other studies where strongly positive isotope signatures were recorded in the SO4 fraction of ASS, the values corresponded with the formation of contemporary sulfides (Maher, 2013). Under these circumstances the process of SO4 reduction results in the removal of light SO4, with an abundance of heavy SO4 remaining (Brownlow, 1996; Wijsman et al., 2001; Farquhar et al., 2008). The δ34S signatures at Site 3 allow the progress of the remediation strategy to be tracked. As the seawater penetrates the soil profile the dominate SO4 source shifts from pyrite derived to seawater derived. Given the seawater provides a strong source of the acid neutralizing capacity needed for remediation, tracking the progress of seawater through the soil profile provides a measure of the effectiveness of the remediation strategy. In time, it may be assumed the δ34S(WS-SO4) values at Site 3 will be similar to seawater SO4 throughout the entire soil depth. The δ34S(HCl-SO4) was less negative at all sites than the corresponding 34 δ S(CRS). HCl-SO4 is a measure of secondary minerals arising from pyrite oxidation in ASS and is most likely either jarosite or schwertmannite (McElnea and Ahern, 2004). Dowuona et al. (1992) found similar sulfur isotope signatures in the jarosite and pyrite fractions of ASS samples from Canada and the United States of America, indicating pyrite oxidised to jarosite without major isotopic fractionation of sulfur. However in this study, the δ34S(HCl-SO4) was always less negative than δ34S(CRS) suggesting there was an appreciable source of heavier δ34S(WS-SO4) - most likely the pre-existing pore waters - in addition to the lighter δ34S(WS-SO4) derived from pyrite oxidation, available for jarosite precipitation. Fractionation between δ34S(CRS) and δ34S(HCl-SO4) in jarositic layers averaged ~12‰. As previously discussed, the surface

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layer of Site 2 contained considerable HCl-SO4, but this was due to the accumulation of recently formed poorly crystalline iron minerals (including schwertmannite) (Johnston et al., 2011b), rather than jarosite. At Site 1, the δ34S(HCl-SO4) was generally very similar to δ34S(KCl-SO4) and δ34S(WS-SO4), indicating that these three SO4 fractions are in sulfur isotopic equilibrium, most likely due to the dominant source of SO4 for each of these three fractions being pyrite oxidation and the lack of fractionation of sulfur during the transitions between WS-SO4, KCl-SO4, and the formation of jarosite. However at Sites 2 and 3 the WS- and KCl-SO4 fractions are more enriched in 32S than the HCl-SO4 in the jarositic layers. Jarositic SO4, being a structural component of this mineral, is far more stable than the KCl- or WS-SO4 fractions which exhibit a strong sulfur isotopic influence from the intermittently applied seawater. The δ34S(HCl-SO4) signatures for all jarositic layers in this study reflect conditions that pre-date the introduction of seawater, whereas the δ34S(KCl-SO4) and δ34S(WS-SO4) signatures and the δ34S(HCl-SO4) signatures for the poorly crystalline iron mineral surface layer at Site 2, tend to reflect the current environmental conditions (relative to the supply of SO4) especially in regard to the degree of exposure to seawater. This data demonstrates that the various SO4 fractions can be employed to elucidate and understand the relative influence of various sources of SO4 during periods of geochemical change such as the introduction of seawater to a previously freshwater dominated environment. 5. Conclusion This study indicates that: 1) There was an appreciable source of heavier sulfate - most likely the pre-existing pore waters - in addition to the heavier sulfate derived from pyrite oxidation for jarosite precipitation. 2) Jarositic-SO4 has considerable remnance in the soil profile even after extended exposure to seawater. 3) The δ34S of the water soluble, exchangeable and schwertmanniticSO4 reflect conditions post remediation and indicate the relative contributions from two potential SO4 sources – the lighter SO4 derived from the oxidation of pyrite, and the heavier SO4 derived from the seawater. 4) The δ34S of surficial contemporary sulfide accumulations were isotopically different from the relict sulfides found at depth reflecting an appreciable influence of SO4 from seawater, and 5) The δ34S of the water soluble SO4 allowed the progress of the seawater remediation to be traced down the soil profile. Overall, this study demonstrates the utility of examining the stable sulfur isotope signatures in various sulfide and SO4 fractions to trace sulfur geochemical pathways occurring in soils, in this case as a result of introduction of tidally driven sea water. Acknowledgements This study benefitted from financial assistance from the Co-operative Research Centre for Contamination, Assessment and Remediation of the Environment (CRC CARE) as part of the East Trinity Project 6-601-06/07. The authors also acknowledge Scott Johnston and Annabelle Keene for assistance with field sampling and Matheus Carvalho de Carvalho for the sulfur isotope analysis. References APHA, 2005. In: 21st Edition (Ed.), Standard Methods for the Examination of Water and Wastewater. American Public Health Association – American Water Works Association, Baltimore, United States of America. Boman, A., Astrom, A., Frojdo, S., 2008. Sulfur dynamics in boreal acid sulfate soils rich in metastable iron sulfide—the role of artificial drainage. Chem. Geol. 255, 68–77. Bottcher, M.E., Khim, B., Suzuki, A., Gehre, M., Wortmann, U.G., Brumsack, H., 2004. Microbial sulfate reduction in deep sediments of the Southwest Pacific (ODP Leg 181, sites 1119-1125): evidence from stable sulfur isotope fractionation and pore water modelling. Mar. Geol. 205, 249–260.

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