Biogeochemistry of low‐ and high‐centered ice‐wedge polygons in wetlands in Svalbard

Arctic wetlands are a globally significant store of soil organic carbon. They are often characterized by ice‐wedge polygons, which are diagnostic of lowland permafrost, and which greatly influence wetland hydrology and biogeochemistry during summer. The degradation of ice‐wedge polygons, which can occur in response to climate change or local disturbance, has poorly understood consequences for biogeochemical processes. We therefore used geochemical analyses from the active layer and top permafrost to identify and compare the dominant biogeochemical processes in high‐centered (degraded) and low‐centered (pristine) polygons situated in the raised beach sediments and valley‐infill sediments of Adventdalen, Central Svalbard. We found similar organic‐rich sediments in both cases (up to 38 dry wt.%), but while low‐centered polygons were water‐saturated, their high‐centered counterparts had a relatively dry active layer. Consequently, low‐centered polygons showed evidence of iron and sulfate reduction leading to the precipitation of pyrite and siderite, whilst the high‐centered polygons demonstrated more oxidizing conditions, with decreased iron oxidation and low preservation of iron and sulfate reduction products in the sediments. This study thus demonstrates the profound effect of ice‐wedge polygon degradation on the redox chemistry of the host sediment and porewater, namely more oxidizing conditions, a decrease in iron reduction, and a decrease in the preservation of iron and sulfate reduction products.

At a smaller scale, ice-wedge polygons are widespread, ice-rich, and easily identifiable permafrost landforms, 5 playing a role in the site-specific hydrology of wetlands (e.g., Liljedahl et al. 6 ). Around 250,000 km 2 of the Arctic is covered by polygonal tundra. 7 There are two main types of ice-wedge polygon: low-centered and highcentered. 8 Low-centered polygons represent the first stage of icewedge development, while high-centered polygons occur only at a later stage. 8 The change from low-to high-centered polygons can be caused by climate, local changes in geomorphology, or hydrological changes. The local topography developing from ice-wedge polygonal activity regulates the landscape hydrology. 6 For example, the raised rims of low-centered polygons serve as hydrologic barriers, causing flooding of the centers of these polygons during summer. 9 Ice-wedge polygons are vulnerable to climate change. 10 In Alaska and several other Arctic sites, the degradation of ice wedges has increased as air temperatures have risen over the past 30 years. [11][12][13] Differential ground subsidence occurs as the near-surface permafrost thaws and ice-wedges melt, causing polygons to become flat-centered and then eventually high-centered. 9 The complete transformation of low-centered polygons to high-centered polygons has hydrologic implications, enabling the centers of high-centered polygons to drain via the connected troughs, causing drying of the landscape. 12,14 However, in Svalbard, the ice-wedge morphology in Adventdalen has been stable for the last seven decades, despite a large increase in mean annual air temperature. 15 Studies have shown that ice-wedge polygon morphology influences wetland biogeochemistry and hence organic carbon decomposition, with implications for greenhouse gas emissions from different polygon types. [16][17][18][19] For example, the reduction of ferric iron in wetlands increases carbon dioxide production relative to methane production, and ferrihydrite minerals suppress methanogenesis by acting as energetically favorable electron acceptors, while crystalline iron oxides are not as easily reduced in anaerobic respiration and do not decrease methane production. [20][21][22] However, few studies have investigated wetland biogeochemistry in areas with both low-and high-centered polygons by examining the variations with depth through the entire active layer and into the top permafrost.
Although the spatial variability of carbon dioxide and methane fluxes has been described, 15,23 and there is some previous work on the biogeochemistry of low-centered ice-wedge polygons, 24 there are no studies of high-centered polygons in Svalbard. The objective of this study is to characterize the dominant biogeochemical processes occurring in a high-centered polygon situated in a raised beach area in central Svalbard and compare this with low-centered polygons situated in a sediment-filled lowland valley area $14 km further inland. In so doing, we use space-for-time substitution to provide insights into the potential for changing biogeochemical processes within the active layer in an area of continuous permafrost where permafrost degradation may eventually commence in coastal lowlands. 15 25 Svalbard's climate is polar tundra. 26 At Svalbard Airport, which is close to Adventdalen, the linear trend of mean annual air temperature from 1899 to 2018 indicates an increase of 3.7 C during this period, which includes a warming of 1.7 C per decade since 1991. 27 In the lower Adventdalen area, mean annual ground temperature (MAGT) at the depth of zero annual amplitude ranges from À5.2 to À2.5 C, 28 and a recent increase in permafrost temperature has been linked to the rising air temperature. 29 Permafrost thawed during the Last Glacial Maximum, when Adventdalen was filled with a dynamic and erosive ice stream. [30][31][32] The subsequent glacial isostatic rebound of Svalbard meant that relative sea level fell, and a Gilbert-type delta prograded into Adventfjorden. 33,34 In the valley bottom, permafrost between 0 and $3 m depth formed syngenetically, as it aggraded concurrently with eolian sedimentation, whereas beneath this the permafrost aggraded epigenetically (following deposition of sediments, by downward freezing). 33 Here, the permafrost is now continuous, reaching a thickness of 80-100 m near the coast. 31,35 In lower Adventdalen, the active layer thickness is between 60 and 205 cm, although this varies according to snow depth, amount of vegetation, and sediment type. 28,36 This study is focussed on core-sampled material, down to 2 m depth, inclusive of the active layer and the top permafrost. Table 1 lists the sample locations with details of the sampling area and Figure 1 shows the sampling locations and a schematic of the polygon structure. The low-centered polygons situated at Adventdalen North are covered with Late Holocene loess (eolian) deposits overlying alluvial and deltaic deposits. 33,37 Adventdalen North is a water-saturated wetland, mainly fed by springs. The areas of the individual ice-wedge polygons studied at Adventdalen North were 90 m 2 (N2) and 270 m 2 (N1). The high-centered polygon studied is situated in peat deposits between raised beaches at Revneset. The area of this ice-wedge polygon was 99 m 2 (R1). The Adventdalen valley sediments are much thicker and more finegrained than those at Revneset. The primary water source to Revneset is summer precipitation, with a probable contribution of meltwater from a late-lying snow bank (visible in Figure 2b). The mean pH of active layer water was 6.0 at Adventdalen North (ranging from 4.7 to 6.5), while the mean pH of active layer water was 5.7 at Revneset (ranging from 5.3 to 5.9).

| Coring and core subdivision
A detailed methodology for the coring, core subdivision, pore water extractions, and chemical analyses is available in Jones et al. 24 Briefly, coring was undertaken in the polygon centers using hand drilling (a Stihl BT 130 drilling engine with a cylindrical drill head and rods) to a depth of 2 m in late winter before the onset of thaw, core segments were extruded into sterile Whirl Pak ® bags, and transport and subdivision were conducted while maintaining frozen conditions. Cores were subdivided by sawing into 2-cm-depth slices while frozen. The freshly cut surfaces were scraped with a scalpel, and the outer 2 cm was removed with a hollow brass tube (3 cm in diameter), to prevent contamination. The sawblade, scalpel, and brass tube were cleaned with 70% isopropanol between slices.
In addition, in late summer 2017, thaw depths were measured three times in each polygon center using an active layer probe. The water table was measured once in late summer 2017 in the polygon centre.

| Pore water extractions
A pore water extraction method was adapted from Spence et al. 38 Vials containing samples were transferred to a Coy Vinyl Anaerobic chamber with an N 2 atmosphere (0 ppm oxygen). Each vial was weighed to determine the sample mass. Nitrogen-sparged de-ionized water (Milli-Q) was added to fill each vial. The vials were reweighed to determine the mass and volume of water added. A 3-mL volume of water was subsequently removed from the top of the vial to create a headspace. The vials were crimp-capped, inverted and stored for 5 days at 4 C whilst submerged in water (to prevent gas diffusion across the septa). This storage time enabled the de-ionized water to equilibrate with the sediment pore water (e.g., Spence et al. 38 ). Seven days after first saturation of the sample, the vials were centrifuged at 7,750 rpm for 5 min and transferred back to the anaerobic chamber. The equilibrated supernatant was filtered (0.22-μm nylon syringe filter) for chemical analysis and the sediment remaining in the vials was weighed after drying at 105 C for T A B L E 1 Site and sampling locations (coordinates in decimal degrees). Cores R1a and R1b are two different cores extracted from the center of the same ice-wedge polygon (R1).   The following solid phase analyses were conducted on cores N1

| Solid phase analyses
and R1a only. Acid-volatile sulfur (AVS) and chromium-reducible sulfur (CRS) were determined via a two-step distillation method applied to freeze-dried and milled sediment samples, first using 6 M HCl and then using boiling 3 M CrCl 2 solution. 41,42 In each extraction, H 2 S was precipitated as Ag 2 S, filtered and dried, and sulfide was determined gravimetrically. The stoichiometry of the phase was used to convert the mass to weight per cent (FeS for AVS; FeS 2 for CRS).
Different operationally defined iron mineral phases were targeted with a four-step sequential extraction procedure applied to 100 mg of freeze-dried and milled sediment from one core at each site, following procedures from Raiswell et al., 43

| Spatial analyses
The approximate proportion of high-and low-centered polygons between the raised beaches was estimated by visually classifying the polygons along the Revneset coastline, using high-resolution aerial imagery (where 1 mm on the photo represents 1,000 mm in reality) from 2009 from the Norwegian Polar Institute. A total of 139 polygons at Revneset and 1,212 polygons at Adventdalen North were classified by eye from these aerial images.

| Data analyses
Data analyses were performed in R 45 (Table 3), and the amounts of Fe oxalate (primarily magnetite) T A B L E 2 Thaw and water table depths in the polygon centers during late summer at Adventdalen North (N1 and N2) and Revneset (R1a and R1b), and summary statistics for the organic carbon content in all cores. Thaw depths are presented as a mean of three measurements within each polygon center, with the minimum and maximum thaw depth of each polygon in parentheses, and water table is one measurement, from the polygon center. Organic carbon content is presented as a mean of n measurements, with the minimum and maximum organic carbon content in parantheses.

Site Date Thaw depth (cm)
Water  at both sites were variable, with the Adventdalen North sediments containing slightly more overall. Concentrations of AVS were low at both sites (Table 3).
The concentration of Fe (aq) in cores R1a and R1b was low (<0.7 mmol L À1 ; Figure 4b). In R1a, the concentration of Fe (aq) was lowest below 117 cm, whereas in R1b, the concentration was lowest below 75 cm. The concentrations of Fe (aq) were in general higher at Adventdalen North, with Fe (aq) in N1 reaching over 1.5 mmol L À1 ($150 cm depth; Figure 4a). N2 had the highest concentrations, reaching 3 mmol L À1 at the base of the core ( Figure 4a).

| Biogeochemical processes driving carbon dioxide production
Lipson et al. 22 showed that the reduction of ferric iron increased carbon dioxide production while suppressing methane production and resulted in siderite formation in an Arctic peat soil in Alaska. The large amount of siderite relative to ferrihydrite at Adventdalen North is indicative of iron reduction, which suggests production of bicarbonate, leading to a high carbon dioxide emission potential (Table 3).
Jones 49 reported up to 6462 μmol L À1 carbon dioxide (as carbon dioxide, excluding bicarbonate) in active layer pore water of core N1 from Adventdalen North. In contrast, less iron reduction is evidenced at Revneset. Relatively low concentrations of siderite and pyrite are found at Revneset (Table 3) ) under steady-state conditions. 51 Overall, however, we expect that dissimilatory manganese reduction is of minor importance and a negligible contribution to carbon oxidation in these sediments. The pore water profiles provide stronger evidence that the oxidation of pyrite by manganese oxides was the mechanism which produced the observed Mn (aq) concentrations (e.g., Schippers and Jorgensen 52 ). This mechanism of manganese reduction does not produce carbon dioxide because it is not directly linked to the oxidation of organic carbon. N1 shows a significant negative correlation between Mn (aq) and organic carbon content (ρ = À0.51, p < 0.01; ) concentrations. This proposed mechanism is supported by the absence of gypsum from the bedrock. 54 In addition, the release of sulfur from organic matter via microbial processing (e.g., Bartlett et al. 55 ) may contribute to the high sulfate to chloride ratio observed in these samples. ) concentration and organic carbon content for all cores (ρ = À0.66, p < 0.0001), demonstrating a probable organic carbon control upon sulfate reduction. The control of organic carbon content on sulfate reduction has also been found in salt marsh sediments. 57

| Summary and predictions of the impact of permafrost thaw
In Adventdalen, organic carbon content and the degree of water saturation exert an important control on the prevailing biogeochemical processes, as has been observed in saturated active layer sediments (this study and Jones et al. 24  Permafrost in Svalbard is relatively warm for its northerly latitude, 58 and there has been a warming of 1.7 C per decade at Svalbard Airport since 1991. 27 Despite this, occurrences of extremely high summer temperatures have so far been limited by the maritime climate on Svalbard. 15 This may explain the absence of ice-wedge degradation in Adventdalen, as extremely high summer temperatures have been a trigger of ice-wedge degradation in Alaska. 11,12,15 In addition, the acceleration of wind channelized along the Adventdalen valley provides reduced snow thickness and efficient ground cooling in winter. 59 Matsuoka et al. 60 identified that present ice-wedge activity in Adventdalen is largely controlled by cold winter spells, thus enabling ongoing ice-wedge formation even during warming conditions. Pirk et al. 15 suggest that as temperature increases markedly in at Revneset, and hence potentially more concerning for greenhouse gas emissions under future warming and permafrost thaw, the particularly organic-rich sediments are mainly confined to the upper 2 m. 37 The data presented in this study demonstrate the profound effect of ice-wedge polygon drainage on the sediment and porewater biogeochemistry: polygon drainage results in more oxidizing conditions, a decrease in iron reduction, and a decrease in the preservation of the products of iron and sulfate reduction.