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Salt marshes are globally important ecosystems and thus their resilience to climate change holds societal importance. To date, studies addressing salt marsh responses to climate change have focused on sea‐level rise and biogeochemical feedbacks with increasing inundation. Less is known about how salt marsh sediment temperatures, which impact physical, biological, and chemical ecosystem processes, will respond to climate change. In this study, we present multi‐depth sediment temperature and porewater level data from low‐, mid‐, and high‐marsh sites at a New England salt marsh for a 1‐year period and investigate how salt marsh sediment temperatures respond to atmospheric and oceanic forcing. We use spectral analyses to identify the frequencies at which sediment temperatures vary and link the temperature variations to specific forcing mechanisms. We find that all sites across the marsh responded to air temperature with roughly equal amplitude whereas the responses to radiative heating and ocean tides varied spatially. The high‐marsh site is more sensitive to radiative heating than the mid‐ and low‐marsh sites. The low‐marsh is affected by tidal processes and inundation whereas the high‐ and mid‐marsh sites are not. In addition, we find that the bulk thermal diffusivity of the saturated sediments decreases with distance from the tidal channel. These factors contribute to considerable temporal and spatial variability in sediment temperatures with elevation, distance from the tidal channel, and time of year (season) being most important.

Groundwater discharge transports dissolved constituents to the ocean, affecting coastal carbon budgets and water quality. However, the magnitude and mechanisms of groundwater exchange along rapidly transitioning Arctic coastlines are largely unknown due to limited observations. Here, using first-of-its-kind coastal Arctic groundwater timeseries data, we evaluate the magnitude and drivers of groundwater discharge to Alaska’s Beaufort Sea coast. Darcy flux calculations reveal temporally variable groundwater fluxes, ranging from −6.5 cm d⁻¹(recharge) to 14.1 cm d⁻¹(discharge), with fluctuations in groundwater discharge or aquifer recharge over diurnal and multiday timescales during the open-water season. The average flux during the monitoring period of 4.9 cm d⁻¹is in line with previous estimates, but the maximum discharge exceeds previous estimates by over an order-of-magnitude. While the diurnal fluctuations are small due to the microtidal conditions, multiday variability is large and drives sustained periods of aquifer recharge and groundwater discharge. Results show that wind-driven lagoon water level changes are the dominant mechanism of fluctuations in land–sea hydraulic head gradients and, in turn, groundwater discharge. Given the microtidal conditions, low topographic relief, and limited rainfall along the Beaufort Sea coast, we identify wind as an important forcing mechanism of coastal groundwater discharge and aquifer recharge with implications for nearshore biogeochemistry. This study provides insights into groundwater flux dynamics along this coastline over time and highlights an oft overlooked discharge and circulation mechanism with implications towards refining solute export estimates to coastal Arctic waters.

Groundwater discharge is an important mechanism through which fresh water and associated solutes are delivered to the ocean. Permafrost environments have traditionally been considered hydrogeologically inactive, yet with accelerated climate change and permafrost thaw, groundwater flow paths are activating and opening subsurface connections to the coastal zone. While warming has the potential to increase land-sea connectivity, sea-level change has the potential to alter land-sea hydraulic gradients and enhance coastal permafrost thaw, resulting in a complex interplay that will govern future groundwater discharge dynamics along Arctic coastlines. Here, we use a recently developed permafrost hydrological model that simulates variable-density groundwater flow and salinity-dependent freeze-thaw to investigate the impacts of sea-level change and land and ocean warming on the magnitude, spatial distribution, and salinity of coastal groundwater discharge. Results project both an increase and decrease in discharge with climate change depending on the rate of warming and sea-level change. Under high warming and low sea-level rise scenarios, results show up to a 58% increase in coastal groundwater discharge by 2100 due to the formation of a supra-permafrost aquifer that enhances freshwater delivery to the coastal zone. With higher rates of sea-level rise, the increase in discharge due to warming is reduced to 21% as sea-level rise decreased land-sea hydraulic gradients. Under lower warming scenarios for which supra-permafrost groundwater flow was not established, discharge decreased by up to 26% between 1980 and 2100 for high sea-level rise scenarios and increased only 8% under low sea-level rise scenarios. Thus, regions with higher warming rates and lower rates of sea-level change (e.g. northern Nunavut, Canada) will experience a greater increase in discharge than regions with lower warming rates and higher rates of sea-level change. The magnitude, location and salinity of discharge have important implications for ecosystem function, water quality, and carbon dynamics in coastal zones.

Low‐elevation coastal areas are increasingly vulnerable to seawater flooding as sea levels rise and the frequency and intensity of large storms increase with climate change. Seawater flooding can lead to the salinization of fresh coastal aquifers by vertical saltwater intrusion (SWI). Vertical SWI is often overlooked in coastal zone threat assessments despite the risk it poses to critical freshwater resources and salt‐intolerant ecosystems that sustain coastal populations. This review synthesizes field and modeling approaches for investigating vertical SWI and the practical and theoretical understanding of salinization and flushing processes obtained from prior studies. The synthesis explores complex vertical SWI dynamics that are influenced by density‐dependent flow and oceanic, hydrologic, geologic, climatic, and anthropogenic forcings acting on coastal aquifers across spatial and temporal scales. Key knowledge gaps, management challenges, and research opportunities are identified to help advance our understanding of the vulnerability of fresh coastal groundwater. Past modeling studies often focus on idealized aquifer systems, and thus future work could consider more diverse geologic, climatic, and topographic environments. Concurrent field and modeling programs should be sustained over time to capture interactions between physical processes, repeated salinization and flushing events, and delayed aquifer responses. Finally, this review highlights the need for improved coordination and knowledge translation across disciplines (e.g., coastal engineering, hydrogeology, oceanography, social science) to gain a more holistic understanding of vertical SWI. There also needs to be more education of communities, policy makers, and managers to motivate societal action to address coastal groundwater vulnerability in a changing climate.

Tidal marshes are valuable global carbon sinks, yet large uncertainties in coastal marsh carbon budgets and mediating mechanisms limit our ability to estimate fluxes and predict feedbacks with global change. To improve mechanistic understanding, we assess how net carbon storage is influenced by interactions between crab activity, water movement, and biogeochemistry. We show that crab burrows enhance carbon loss from tidal marsh sediments by physical and chemical feedback processes. Burrows increase near-creek sediment permeability in the summer by an order of magnitude compared to the winter crab dormancy period, promoting carbon-rich fluid exchange between the marsh and creek. Burrows also enhance vertical exchange by increasing the depth of the strongly carbon-oxidizing zone and reducing the capacity for carbon sequestration. Results reveal the mechanism through which crab burrows mediate the movement of carbon through tidal wetlands and highlight the importance of considering burrowing activity when making budget projections across temporal and spatial scales.

Low‐lying coastlines are vulnerable to sea‐level rise and storm surge salinization, threatening the sustainability of coastal farmland. Most crops are intolerant of salinity, and minimization of saltwater intrusion is critical to crop preservation. Coastal wetlands provide numerous ecosystem services, including attenuation of storm surges. However, most research studying coastal protection by marshes neglects consideration of subsurface salinization. Here, we use two‐dimensional, variable‐density, coupled surface‐subsurface hydrological models to explore how coastal wetlands affect surface and subsurface salinization due to storm surges. We evaluate how marsh width, surge height, and upland slope impact the magnitude of saltwater intrusion and the effect of marsh migration into farmland on crop yield. Results suggest that along topographically low coastlines subject to storm surges, marsh migration into agricultural fields prolongs the use of fields landward of the marsh while also protecting groundwater quality. Under a storm surge height of 3.0 m above mean sea level or higher and terrestrial slope of 0.1%, marsh migration of 200 and 400 m protects agricultural yield landward of the marsh‐farmland interface compared to scenarios without migration, despite the loss of arable land. Economic calculations show that the maintained yields with 200 m of marsh migration may benefit farmers financially. However, yields are not maintained with migration widths over 400 m or surge height under 3.0 m above mean sea level. Results highlight the environmental and economic benefits of marsh migration and the need for more robust compensation programs for landowners incorporating coastal wetland development as a management strategy.

Surface effects of sea‐level rise (SLR) in permafrost regions are obvious where increasingly iceless seas erode and inundate coastlines. SLR also drives saltwater intrusion, but subsurface impacts on permafrost‐bound coastlines are unseen and unclear due to limited field data and the absence of models that include salinity‐dependent groundwater flow with solute exclusion and freeze‐thaw dynamics. Here, we develop a numerical model with the aforementioned processes to investigate climate change impacts on coastal permafrost. We find that SLR drives lateral permafrost thaw due to depressed freezing temperatures from saltwater intrusion, whereas warming drives top‐down thaw. Under high SLR and low warming scenarios, thaw driven by SLR exceeds warming‐driven thaw when normalized to the influenced surface area. Results highlight an overlooked feedback mechanism between SLR and permafrost thaw with potential implications for coastal infrastructure, ocean‐aquifer interactions, and carbon mobilization.