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doi: 10.7185/geochemlet.1911 Global sea level rise exposes terrestrially derived natural organic matter to elevated salinities, which may alter the complex biogeochemical cycling of halogens in coastal wetland sediments. Here we show that sea level rise increases the natural production of organobromines in submerged soils and wetland sediments. We compared the concentrations and speciation of sedimentary chlorine and bromine along a salinity gradient in low-lying coastal forested wetlands in Winyah Bay (South Carolina, United States). Sorption differences between chloride and bromide were not observed, but up to 80 % of total retained bromine is organically bound, with the highest fraction of organically bound bromine found in formerly freshwater wetlands inundated by seawater. Wet/dry cycling of soils and the abundance of aromatic-rich natural organic matter in these salt-affected dieback forested wetlands promote bromination of organic matter, as demonstrated by laboratory simulations. Bromination of soil organic matter caused by continued sea level rise thus may be a major source of organobromines in coastal environments and possibly volatile halomethanes. Received 18 January 2019 | Accepted 26 March 2019 | Published 23 April 2019 1. Department of Chemistry, Princeton University, Princeton, NJ 08544, USA 2. Department of Geosciences, Princeton University, Princeton, NJ 08544, USA 3. Biogeochemistry and Environmental Quality Research Group, Clemson University, Georgetown, SC 29440, USA † Current address: Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA * Corresponding author (email: joewongc@stanford.edu; drs3@princeton.edu) Introduction Global warming and associated rises in sea level threaten to alter the biogeochemistry of coastal ecosystems. Flooding of coastal freshwater wetlands exposes their vegetation and soilbound natural organic matter (NOM) to seawater and ultimately results in conversion to salt marshes (Doyle et al., 2007; Hilton et al., 2008; Krauss et al., 2009). The increased salinity alters the abundance and composition of NOM in salt-affected wetlands (Goñi and Thomas, 2000; Neubauer, 2013), due to changes in vegetal inputs (Tuxen et al., 2011) and microbial activity (Morrissey et al., 2014). Conversely, reactions with previously inaccessible terrestrially derived NOM may also alter the biogeochemical cycling of halogens. Both chlorine (Cl) and bromine (Br) undergo complex biogeochemical cycling in marine and soil systems. During NOM decomposition, inorganic Cland Brreact with NOM both abiotically and biotically to form halogenated organic compounds (organochlorines and organobromines, respectively) (Leri and Myneni, 2010, 2012). Understanding the natural formation of organohalogens is important in order to evaluate the stability of these compounds and their potential breakdown and release as volatile halomethanes. Many halomethanes are known to catalytically destroy stratospheric ozone (Molina and Rowland, 1974), and coastal ecosystems such as salt marshes contribute as much as 10 % of halomethane emissions in the atmosphere (Rhew et al., 2000). It is thus crucial to determine how global sea level rise may affect the biogeochemistry of halogen cycling in coastal ecosystems. In this study, we evaluate the impacts of seawater intrusion on the production of organohalogens in wetland sediments as a first step towards predicting future halomethane emissions. The study site is located on the southeastern coast of the United States (Winyah Bay, South Carolina, United States) (Fig. S-1) (Titus and Richman, 2001). Winyah Bay is a tidally influenced forest-marsh, backwater-dominated, low-lying wetland ecosystem that has experienced severe saltwater intrusion, successively converting forested freshwater wetland to a salt-affected wetland and salt marsh over a short distance (~5 km) (Krauss et al., 2009). The freshwater and salt-affected wetlands experience seasonal wet and dry cycles based on precipitation. In contrast, the salt marsh is always saturated, although water depth varies with daily tidal cycles. Vegetation varies significantly with salinity (Krauss et al., 2009; Chow et al., 2013). To assess the importance of NOM origin and composition, we compared the effects of increased salinity on freshwater wetland soils from Winyah Bay and on temperate forest soils from pine lands in New Jersey. Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2019) 10, 31-35 | doi: 10.7185/geochemlet.1911 32 Methods Methodological details are available in the Supplementary Information. In brief, three sites at Winyah Bay spanning a range of salinities were sampled along a transect from the freshwater wetland to the salt-affected wetland and the salt marsh (Fig. S-1). 18-24 cm soil cores were collected between July 2012 and December 2016. At least two cores were taken at each site to assess variability. Additionally, a 30 cm soil core was collected from the Pine Barrens in New Jersey to determine whether halogenation reactions occur in other ecosystems. The functional group composition of organic carbon was assessed using Fourier transform infrared (FTIR) spectroscopy and solid state 13C nuclear magnetic resonance (NMR) spectroscopy. Cl and Br concentrations and speciation were determined using X-ray fluorescence (XRF) and synchrotron-based 1s X-ray absorption near edge structure (XANES) spectroscopy respectively. Results and Discussion Both the concentrations and the speciation of sediment-bound Cl and Br vary between the different sites, as expected. The average concentration of Cl (Cltotal) increases linearly with salinity: from 0.14 (±0.07) g.kg-1 in freshwater wetland sediments to 6 (±1) and 27 (±2) g.kg-1 in the salt-affected wetland and salt marsh, respectively (Table S-1). The variability shown in the parentheses arises from differences between the two sampled cores, which were minimal, and between different depths and seasons. Cl concentrations decrease with depth in the freshwater and salt-affected wetlands, consistent with soil studies where Cl concentrations are reported to decrease sharply below the top organic-rich horizons (Leri and Myneni, 2010; Krzmarzick et al., 2012). The absence of such a trend with depth in the salt marsh may be caused by homogenisation via tidal action. The speciation of Cl also varies with salinity, as shown by Cl XANES spectroscopy (Fig. 1). Most Cl in the top layers of freshwater sediments is organically bound (Clorg), although Cl speciation in the lower layers (>~14 cm) could not be determined due to the low concentration of Cl. In contrast, salt-affected wetland and salt marsh samples at all depths show predominantly inorganic Cl(Fig. 1). The presence of Clorg cannot be ruled out in the latter samples but is expected to be less than 6 % of Cltotal (Leri et al., 2006). Variability in Cl speciation between the two cores collected at each site was also minimal. The abundance and speciation of Br significantly differ from the behaviour of Cl. Total Br concentrations (Brtotal) averaged over depth and between two separately collected cores increase from 13 (±10) mg.kg-1 in the freshwater wetland to 120 (±50) in the salt-affected wetland but increase only marginally to 160 (±14) mg.kg-1 in the salt marsh despite the roughly threefold increase in water salinity. Similar to Cl, most Br in the freshwater sediments is in the form of organically bound Br (Brorg) (Fig. 2, Table S-1), although variations between wet and dry seasons are observed (Fig. S-3), which may be due to both temporal and spatial heterogeneity. More than 80 % of Br in the salt-affected wetland and 50 % of Br in the salt marsh is present as Brorg, in contrast to the predominance of inorganic Clat these two sites. The large difference in Br:Cl ratios for seawater and sediments is striking. Br:Cl ratios in all sediments are greater than the Br:Cl ratio in seawater by a factor ranging from a maximum of 26.5 in the freshwater wetland to a minimum of 1.5 in the salt marsh. Cland Brions are considered inert and Figure 1 (a) Cl and (b) Br XANES spectra of top layer of sediments (~0-4 cm) on top (black) and standards (grey) at the bottom. Cl standards are chlorophenol red, chlorodecane, and NaCl (aq) for aromatic, aliphatic, and inorganic Cl respectively. Br standards are bromophenol blue, 1-bromoeicosane, and KBr for aromatic, aliphatic, and inorganic Br respectively. Grey dotted and dashed lines show the spectral maxima for aliphatic and aromatic organohalogen standards respectively. Figure 2 The concentration and speciation of Br (left, blue y axis) and Cl (right, black y axis) in sediments as a function of salinity (x axis) in Winyah Bay, South Carolina. All concentrations are the average of samples from different depths and two cores. Vertical error bars show the standard error and are in some cases smaller than the points.

Sea level rise produces abundant organobromines in salt-affected coastal wetlands