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In situ persistence of salt marshes in the face of sea-level rise relies on their ability to maintain substrate elevation through sufficient vertical accretion of sediment. However, sedimentation rates in salt marshes vary spatially and temporally, which complicates the assessment of their ability to keep up with sea-level rise. Here, we explore the spatial and temporal variation in sediment accumulation in a single back-barrier salt marsh site. Using one-time in situ measurements at the landscape scale, we obtained synoptic information of elevation and sediment thickness over the entire salt marsh in a chronosequence over centuries. Repeated measurements along short elevation transects (0.3–0.9 m +MHT) revealed decadal changes, complementing the broader marsh data with detailed information on elevation, thickness of the marsh deposits and accumulation rates. Thickness of the deposits was largely related to the elevation gradient: the sediment layer was thinner at the higher marsh (near the dunes and far away from the intertidal flats), and thicker at the lower marsh (near the intertidal flats). Moreover, the thickness of the layer increased with salt marsh age along the chronosequence, and age accounted for 72 % of variability in sediment accumulation. The rate of sediment accumulation was higher than the local rate of sea-level rise in the younger marsh, whereas it was equal to the rate of sea-level rise in the older marsh. In the older salt marsh, sediment accumulation was lower, possibly due to autocompaction in the thicker, older layers. Both at the landscape scale and along short elevation transects within individual drainage basins, sediment accumulation decreased with distance to sediment supply routes. However, their relative importance depended on the scale of observation. Distance to creeks accounted for 17 % of the variability in sediment accumulation at the landscape scale, compared to 4 % at the smaller scale. Similarly, the influence of distance to intertidal flats varied from 1 % at the landscape scale to 13 % at the smaller scale. Our main findings indicate that lower-elevation older marshes and higher-elevation younger marshes far away from sediment sources are at risk of not keeping pace with the local rate of sea-level rise and are potentially vulnerable to increased flooding. • Spatial and temporal scales are combined to understand salt-marsh sedimentation. • Marsh age is an important factor for sedimentation rates; other factors depend on spatial scale. • Low-elevation old marshes and high-elevation young marshes more at risk with sea-level rise. [ABSTRACT FROM AUTHOR]

Blue carbon stored in coastal wetlands can be highly impacted by extreme events, such as typhoons. However, the sediment carbon accumulation capacity in this circumstance, remains unclear. In this study, a field site crossing the bare mudflat-saltmarsh interface was established in Southern Hangzhou Bay, to investigate the geomorphological and subsequent sediment carbon accumulation changes during the typhoon season (2 months) and the succeeding calm weather period (10 months). Surface Elevation Tables (SETs) were used, together with GPS-RTK and UAV-LiDAR surveys, to observe surface elevation changes at different spatial scales. The elevation and sediment organic carbon density data were used to estimate the gain/loss of sediment carbon pool under the impact of two successive typhoons, "In-Fa" and "Chanthu". This revealed that during the typhoon season, 6.3 cm of surface erosion occurred on the bare mudflat whilst rapid depositions of 6.1 and 12.3 cm occurred in Scirpus mariqueter and Spartina alterniflora habitats, respectively. In the gap between the two typhoon events, the bare mudflat experienced deposition of 4.2 cm, signalling recovery from erosion, but deposition within the saltmarsh front slowed. During the calm weather period, sediment deposition increased landward, with magnitudes of 13.1 cm, 16.4 cm, and 18.2 cm. Sediment carbon budget estimates indicated that during the typhoon season, the bare mudflat lost ~3.4 Mg C ha−1 but saltmarshes gained ~4.9 Mg C ha−1 in their sediment organic carbon pools. These changes in sediment carbon accumulation rates over the typhoon season accounted for >30 % of annual change in the study area, and may be associated with tidal creek dynamics. This study suggests the important role of typhoons in mediating the sediment carbon pool across the mudflat-saltmarsh interface. With future climate change, increasingly frequent catastrophic events could cause considerable changes in coastal blue carbon sequestration capacity. • Sediment carbon accumulation rates were recorded at the mudflat-saltmarsh interface. • Differing patterns of carbon accumulation were found during typhoon and calm periods. • Typhoons caused a sediment carbon loss in mudflat but a gain in saltmarsh. • Typhoons accounted for >30 % of the annual change in the sediment carbon pool. • Tidal creeks could be crucial in mediating sediment carbon pools in typhoon season. [ABSTRACT FROM AUTHOR]

Creeks are essential for salt marshes by conveying water and sediment through this geomorphic system. In this paper, we investigate the mechanisms that determine the residual sediment flux using measurements conducted in tidal creeks in salt marshes of the Yangtze Estuary. A main creek and a secondary creek were studied to explore whether the mechanisms determining residual sediment fluxes through the main creek differ from those in the secondary creek. Measurements in creeks were carried out over 5 years, spanning different months. Sediment import was found during most tides, both in the main creek and the secondary creek, implying that creeks in Chongming generally function as a conveyor belt of sediment into the marsh. However, sediment export can occur during certain overbank tides. When comparing the role of creeks in drainage and sediment delivery, the main creek functions more in delivering sediment while the secondary creek primarily serves as a drainage conduit. To better understand the mechanisms behind sediment fluxes, the residual sediment flux was compared with the residual discharge and the sediment differential (differences in sediment concentration between flood and ebb). Overbank tides generally lead to a net outward discharge as more water from saltmarshes can be concentrated into the marsh creek during ebb tides. This net outward discharge tends to export more sediment during ebb tides. However, due to the sediment abundance during the flood phase in the turbid environment, sediment import can be expected even with the residual export of water. Export of sediment was only found for the few tides with a net outward discharge and a small positive sediment concentration differential. Large negative sediment differentials (larger averaged suspended sediment concentration during ebb tides) have not been observed because the sediment supply during ebb is limited. This paper unravels how the sediment differential and residual discharge contribute to the residual sediment flux, providing a better understanding of sediment dynamics in marsh creek systems. • The relative importance of asymmetries in sediment concentration and in flow for residual sediment fluxes was explored. • The main creek is mainly a conduit for sediment delivery whereas the secondary creek is mainly a drainage conduit. • Sediment supply limitation can even occur in a turbid system like Chongming. [ABSTRACT FROM AUTHOR]

The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and δ98Mo from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) 'iron-rich' sediments containing high concentrations of dissolved ferrous iron (up to 2 mM), (2) 'sulfide-rich' sediments containing very high concentrations of aqueous sulfide (up to 10 mM) and, (3) sediments that we consider to be intermediate between 'iron-rich' and 'sulfide-rich' conditions. In iron-rich sediments, we suggest that iron speciation and mineralogy controls the concentration and isotopic composition of Mo. Despite similar aqueous sulfide profiles, the intermediate and sulfide-rich pond sediment have different porewater Mo concentrations and δ98Mo. In the sulfide-rich pond sediment, we suggest that the concentration and isotopic composition of Mo is controlled by solubility equilibrium with an Fe-Mo-S mineral species (e.g. FeMoS 4) due to similarities in sediment and porewater δ98Mo throughout the sediment column. In the intermediate pond sediment, we conclude that active breakdown of iron oxides redistributes porewater Mo, observable as a peak of dissolved Mo (>100 ppb), which diffuses within the sedimentary porewaters. The sedimentary δ98Mo is higher in sulfide-rich and intermediate pond sediment (mean = 1.66‰, range = 0.98–1.92‰) than in iron-rich pond sediment (mean = 1.10‰, range = 0.28–1.65‰) with all ponds having sedimentary δ98Mo that is lower than seawater. The maximum sedimentary δ98Mo observed in these anoxic sediments, which is 0.5–0.7‰ lower than seawater, appears to be set by Fe-Mo-S equilibration with ambient thiomolybdate species. We suggest diagenetic overprinting can cause more efficient capture of pond water Mo and causes sediment δ98Mo of originally iron-rich pond sediment to evolve to higher values at progressively higher aqueous sulfide concentrations. [ABSTRACT FROM AUTHOR]

The East Anglian salt marsh system (UK) has recently generated intriguing data with respect to sediment biogeochemistry. Neighbouring ponds in these salt marshes show two distinct regimes of redox cycling: the sediments are either iron-rich and bioturbated, or they are sulphide-rich and unbioturbated. No conclusive explanation has yet been given for this remarkable spatial co-occurrence. Here, we quantify the geochemical cycling in both pond types, using pore-water analyses and solid-phase speciation. Our results demonstrate that differences in solid-phase carbon and iron inputs are likely small between pond types, and so these cannot act as the direct driver of the observed redox dichotomy. Instead, our results suggest that the presence of bioturbation plays a key role in the transition from sulphur-dominated to iron-dominated sediments. The presence of burrowing fauna in marine sediments stimulates the mineralisation of organic matter, increases the iron cycling and limits the build-up of free sulphide. Overall, we propose that the observed dichotomy in pond geochemistry is due to alternative stable states, which result from non-linear interactions in the sedimentary iron and sulphur cycles that are amplified by bioturbation. This way, small differences in solid phase input can result in very different regimes of redox cycling due to positive feedbacks. This non-linearity in the iron and sulphur cycling could be an inherent feature of marine sediments, and hence, alternative stable states could be present in other systems. [ABSTRACT FROM AUTHOR]

Current tidal marsh elevations and their accretion rates are important predictors of vulnerability to sea-level rise. When tidal marshes are at risk, adaptation measures, such as sediment addition to increase elevations, can be implemented to prevent degradation and loss. In 2016, wildlife managers prescribed a thin-layer sediment addition of locally sourced dredged material from Anaheim Bay to mitigate plausible future impacts of sea-level rise to Seal Beach National Wildlife Refuge, an urbanized, subsiding, sediment limited, and low elevation tidal marsh in southern California, USA. In this study, our objectives were to determine how suspended sediment concentrations (SSC) and fluxes associated with sediment application affected an adjacent eelgrass bed and a more distant deep tidal channel at different time periods throughout the project and how sediment application affected tidal marsh elevation. Due to the sediment addition the tidal marsh elevation increased by 25.4 cm across 3.06 ha. Mean SSC levels at the eelgrass site increased from 4 mg/L during pre-augmentation to 16 mg/L during sediment application. SSC levels also increased up to 40 mg/L with the installation of sediment barriers. Winter storms during construction also had an influence on SSC, with an extreme storm event increasing SSC from 9 mg/L pre-storm levels to 16 mg/L during the storm. However, SSC at the deep site remained constant during all time periods with a mean of 7 mg/L. Our results show that little to zero sediment was exported out of Anaheim Bay during the study period, illustrating that any impacts were localized to the application area. These findings suggest that although long-range export of applied dredge material is unlikely from future thin-layer sediment application at this site, adjacent habitats could experience impacts if erosion control measures are not implemented effectively. Future projects will benefit from similar pre- and post-addition monitoring to establish the long-term efficacy of sea-level rise adaptation measures. [ABSTRACT FROM AUTHOR]

Highlights • Sediments are unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platforms. • Trapping capacity of sediment in different intertidal subdomains decreases exponentially with R, the ratio between advection length and the typical spatial length of channels and tidal flats. • The feedback between SLR and sediment trapping has a twofold effect: an increase in salt marsh resilience in the short term but a net loss of sediment in the long term that might jeopardize the entire intertidal system. Abstract Deposition of fine sediments on a marsh platform favors accretion that counteracts Sea Level Rise. However, it is difficult to assess the sediment trapping capacity of a marsh given the heterogeneity of sediment sources and the geometric complexity of the system, with a network of dendritic and meandering creeks dissecting the intertidal area. Here we use a numerical model to study the sediment trapping capacity of a marsh-dominated estuary, Plum Island Sound, USA, and its variations across the landscape. The results highlight the importance of the timing between sediment inputs and tidal phase and show that sediment discharged from tidal rivers deposit within the rivers themselves or in adjacent marshes. Most sediment is deposited in shallow tidal flats and channels and is unable to penetrate farther inside the marshes because of the limited water depths and velocities on the marsh platform. Trapping capacity of sediment in different intertidal subdomains decreases logarithmically with the ratio between advection length and the typical length of channels and tidal flats. Moreover, sediment deposition on the marsh decreases exponentially with distance from the channels and marsh edge. This decay rate is a function of settling velocity and the maximum value of water depth and velocity on the marsh platform. [ABSTRACT FROM AUTHOR]

Abstract In this research, sediments from the Sečovlje Salina (northern Adriatic, Slovenia) were geochemically investigated in order to decode the mobility of metal(oid)s in the hypersaline environment. The results demonstrated that the concentrations of As, Co, Cu, Mo, Ni, Pb, Sb, Sn and Zn were comparable to those detected in various similar coastal background areas, as well as in the Sečovlje salt marsh core sample. The estimated mobility potential of metal(oid)s in sediment decreases in the following order: Mo > As > Cu > Sb > Sn > Co > Pb > Ni > Zn. Since the hypersaline sediment (e.g. healing mud) is used as a healing factor (it can be applied directly on human skin), the study of metal(oid) bioavailability in an interaction between the hypersaline sediments and the artificial sweat was also performed. It is significant that the metal(oid) contents are leached in very low concentrations and are treated as nontoxic for humans according to international norms for cosmetic products. The study provides information in order to assess the role of metal mobility on the potential health impact of the application of natural healing mud. Graphical abstract Unlabelled Image Highlights • The transfer of metal(oid)s in a hypersaline environment was investigated. • Availability of metal(oid)s from a health risk perspective was conducted. • Sediment-water physical and geochemical properties induced limited mobility dynamics. • Extraction testing in artificial sweat indicate low mobility of metal(oid)s. [ABSTRACT FROM AUTHOR]

Salt marshes are ecosystems with significant economic and environmental value. However, the accelerating rate of sea-level rise is a significant threat to these ecosystems. Storms significantly contribute to the sediment budget of salt marshes, playing a critical role in salt marsh survival to sea-level rise. There are, however, uncertainties on the extent to which storms contribute sediments to different areas of marsh platforms (e.g., outer marsh vs marsh interior) and on the sediment sources that storms draw on (e.g., offshore vs nearshore). This study uses field analyses from an eight-month field campaign in the Ribble Estuary, North-West England, to understand storms' influence on the sediment supply to different marsh areas and whether storms can deliver new material onto the salt marsh platform which would otherwise not be sourced in fair-weather conditions. Field data from sediment traps indicate that storm activity caused an increase in inorganic sediment supply to the whole salt marsh platform, especially benefitting the marsh interior. Geochemistry and particle size distribution analysis indicate that the majority of the sediment supplied to the salt marsh platform during the stormy periods was generated by an increase in erosion and resuspension of mudflat and tidal creek sediments, while only a minimal contribution was given by the sediments transported from outside the intertidal system. This suggests that, in the long term, storms will promote salt marsh vertical accretion but might simultaneously reduce the overall larger-scale sediment availability with implications for the marsh lateral retreat. • Storms contribute significantly to salt marsh sediment budget. • There are uncertainties as to what extent this contribution benefits the marsh platform. • Field data showed that storms benefit marsh vertical accretion but limit lateral expansion. [ABSTRACT FROM AUTHOR]

Coastal vegetated habitats can be important sinks of organic carbon (C org ) and mitigate global warming by sequestering significant quantities of atmospheric CO 2 and storing sedimentary C org for long periods, although their C org burial and storage capacity may be affected by on-going sea level rise and human intervention. Geochemical data from published 210 Pb-dated sediment cores, collected from low-energy microtidal coastal wetlands in El Salvador (Jiquilisco Bay) and in Mexico (Salada Lagoon; Estero de Urias Lagoon; Sian Ka'an Biosphere Reserve) were revisited to assess temporal changes (within the last 100 years) of C org concentrations, storage and burial rates in tropical salt marshes under the influence of sea level rise and contrasting anthropization degree. Grain size distribution was used to identify hydrodynamic changes, and δ 13 C to distinguish terrigenous sediments from those accumulated under the influence of marine transgression. Although the accretion rate ranges in all sediment records were comparable, C org concentrations (0.2–30%), stocks (30–465 Mg ha −1 , by extrapolation to 1 m depth), and burial rates (3–378 g m −2 year −1 ) varied widely within and among the study areas. However, in most sites sea level rise decreased C org concentrations and stocks in sediments, but increased C org burial rates. Lower C org concentrations were attributed to the input of reworked marine particles, which contribute with a lower amount of C org than terrigenous sediments; whereas higher C org burial rates were driven by higher mass accumulation rates, influenced by increased flooding and human interventions in the surroundings. C org accumulation and long-term preservation in tropical salt marshes can be as high as in mangrove or temperate salt marsh areas and, besides the reduction of C org stocks by ongoing sea level rise, the disturbance of the long-term buried C org inventories might cause high CO 2 releases, for which they must be protected as a part of climate change mitigation efforts. [ABSTRACT FROM AUTHOR]

Salt marsh sediments generally undergo steady accumulation over time and thus are widely used to reconstruct the depositional histories of various anthropogenic contaminants derived from atmospheric and fluvial sources. Major hurricanes can significantly affect coastal landscapes by eroding and re-distributing sediment. Thus, each major hurricane can leave distinct signals in coastal wetland sediments. On the other hand, early-diagenetic remobilization of Fe and Mn in organic rich marsh sediment is a common phenomenon. However, remobilization of Fe and Mn across the redox boundary can induce remobilization of other trace elements and thus can disturb their depositional histories. Four short (~ 1 m) sediment cores were collected from the fringing marshes of St. Louis Bay, Mississippi (located ~ 30 km east of Hurricane Katrina's track) during 2010–2011 to investigate possible impacts of Hurricane Katrina (2005), and early-diagenetic remobilization of Fe and Mn, on trace metal and dioxin depositional histories in these sediments. Results from 210 Pb, 137 Cs, stable Cs, particulate organic carbon (POC), sediment bulk density and grain size indicate significant impact of hurricane event layers on anthropogenic stable Cs, while deposition profiles of V, Ni and Cr are impacted by Fe and Mn remobilization to a limited extent. [ABSTRACT FROM AUTHOR]

In this study, lignin-derived phenols, stable carbon isotopes and bulk elemental compositions were determined along the length of two sediment cores (C1 and C2) from the Andong salt marsh, which is located southwest of Hangzhou Bay, China. The purpose of this study was to determine the short-term changes and their implications along sediment profiles. The 1997 high tide had caused an increase in the terrestrial organic matter (OM) signal from 1996/1997 to 2000 in both cores, which was indicated by a high Λ (total lignin in mg/100 mg OC), TOC, C/N and more negative δ 13 C values. The slight increases in terrestrial OM along the length of the cores between 2003 and 2006 were most likely attributable to the construction of the Hangzhou Bay Bridge. Both events have likely caused an increase in erosion, and thus, these events have increased the input of terrestrial OM to nearby areas. The effects of the distinctively dry year of 2006 can be observed along C2 between 2006 and 2008 in the steadily declining terrestrial OM signal. The overall slight decrease in terrestrial OM and the distinct increase in TOC along the length of both cores toward the present were most likely because of the overall reduced sediment caused by the trapping of materials within reservoirs. These results show that the reduction in terrestrial OM in the Andong salt marsh for the past 30 years was due to reservoirs and the 2006 drought, but this was counterbalanced by the 1997 high tide event and construction of the Hangzhou Bay Bridge, which resulted in increased erosion and terrestrial OM input. [ABSTRACT FROM AUTHOR]

Seasonal variability in the sediment dynamics of a Bay of Fundy tidal creek and salt marsh system was analyzed to better understand the ecomorphodynamics of a high suspended sediment concentration intertidal habitat. Data were collected over 62 tides for velocity, suspended sediment concentration, deposition, and grain size at four stations from the creek thalweg to the marsh surface. Five topographic surveys were also conducted throughout the 14-month study. Deposition rates per tide varied spatially from 56.4 g m -2 at the creek thalweg to 15.3 g m -2 at the marsh surface. Seasonal variations in deposition in the creek and marsh surface were from 38.0 g m -2 to 97.7 g m -2 and from 12.2 g m -2 to 19.6 g m -2 respectively. Deposition and erosion were greatest in late fall and winter. This seasonal change, led by higher suspended sediment concentrations, was observed in the creek and at the marsh bank but notably absent from the marsh edge and marsh surface. Sediments were predominantly deposited in floc form (76–83%). Because of high floc content, higher suspended sediment concentrations led to more rapid loss of sediment from suspension. With increasing sediment concentration, deposition increased in the tidal creek and at the marsh bank but not at the marsh edge or marsh surface. This suggests that in highly flocculated environments the water column clears fast enough that very little sediment remains in suspension when the water reaches the marsh and that the sediment concentration during marsh inundation is independent of the initial concentration in the creek. [ABSTRACT FROM AUTHOR]

Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession. This study presents the novel integration of 3D-computed X-ray microtomography to quantify sediment structure and porosity, with water level and geochemical data to understand the impact of pre-restoration land use and disturbance on the structure and functioning of restored saltmarshes. The study combines a broad-scale investigation of physical sediment characteristics in nine de-embanked saltmarshes across SE England, with an intensive study at one site examining water levels, sediment structure and the sediment geochemical environment. De-embankment does not restore the hydrological regime, or the physical/chemical framework in the saltmarshes and evidence of disturbance includes a reduction in microporosity, pore connectivity and water storage capacity, a lack of connectivity between the sub-surface environment and overlying floodwaters, and impeded sub-surface water flow and drainage. This has significant consequences for the sediment geochemical environment. This disturbance is evident for at least two decades following restoration and is likely to be irreversible. It has important implications for plant establishment in particular, ecosystem services including flood regulation, nutrient cycling and wild species diversity and for future restoration design. [ABSTRACT FROM AUTHOR]

Salt marshes are ecologically sensitive ecosystems where mercury (Hg) methylation and biomagnification can occur. Understanding the mechanisms controlling gaseous Hg flux from salt marshes is important to predict the retention of Hg in coastal wetlands and project the impact of environmental change on the global Hg cycle. We monitored Hg flux from a remote salt marsh over 9 days which included three cloudless days and a 4 mm rainfall event. We observed a cyclical diel relationship between Hg flux and solar radiation. When measurements at the same irradiance intensity are considered, Hg flux was greater in the evening when the sediment was warm than in the morning when the sediment was cool. This is evidence to suggest that both solar radiation and sediment temperature directly influence the rate of Hg(II) photoreduction in salt marshes. Hg flux could be predicted from solar radiation and sediment temperature in sub-datasets collected during cloudless days (R 2 = 0.99), and before (R 2 = 0.97) and after (R 2 = 0.95) the rainfall event, but the combined dataset could not account for the lower Hg flux after the rainfall event that is in contrast to greater Hg flux observed from soils after rainfall events. [ABSTRACT FROM AUTHOR]

Field investigations and indoor experiment were carried out to clarify the effect of temperature and chlorophyll a concentration of the sediment, photon flux density, and ground water level on the absorption and emission of CO 2 from the surface sediment of the artificial salt marsh. The flux of CO 2 absorption was closely related to sediment temperature, chlorophyll a and photon flux density. On the other hand, flux of CO 2 emission changed depending on the sediment temperature and ground water level. The ground water level was thought to be indispensable for estimating CO 2 emission from the sediment of the intertidal zone of the salt marsh. Using equations of CO 2 flux with sediment temperature, chlorophyll a, photon flux density and ground water level, the total CO 2 absorption and emission of the artificial north salt marsh of Osaka Nanko bird sanctuary was estimated as ca. 23 tons (4.3 g CO 2 m −2 day −1 ) for absorption and ca.14 tons (2.6 g CO 2 m −2 day −1 ) for emission in May to September 2014. These results suggest that a net of 9 tons of CO 2 (1.7 g CO 2 m −2 day −1 ) was absorbed, and that this salt marsh performs the function of a CO 2 sink in high temperature seasons. [ABSTRACT FROM AUTHOR]

Salt-marsh sediments are routinely used to reconstruct sea-level changes over past millennia. These reconstructions bridge an important gap between geological and instrumental sea-level records, and provide insights into the role of atmospheric, oceanic, climatic and anthropogenic sea-level drivers, thereby improving understanding of contemporary and future sea-level changes. Salt-marsh foraminifera, diatoms and testate amoebae are three of the proxies capable of accurately reconstructing former sea level over decadal to millennial timescales. Datasets of surface assemblages are collated along elevational gradients to provide modern analogues that can be used to infer former marsh-surface elevations from fossil assemblages. Testate amoebae are the most recently developed proxy and existing studies suggest that they are at least as precise as the two other proxies. This study provides a synthesis of sea-level research using testate amoebae and collates and analyses existing surface datasets of intertidal salt-marsh testate amoebae from sites throughout the North Atlantic. We test the hypothesis that intertidal testate amoebae demonstrate cosmopolitan intertidal zonation across wide geographical areas in a way that is unique to this proxy. Testate amoebae assemblages are harmonised under a unified taxonomy and standardised into a single basin-wide training set suitable for reconstructing sea-level changes from salt-marsh sediments across the North Atlantic. Transfer functions are developed using regression modelling and show comparable performance values to published local training sets of foraminifera, diatoms and testate amoebae. When used to develop recent (last 100 years) sea-level reconstructions for sites in Norway and Quebec, Canada, the testate amoebae-based transfer function demonstrated prediction uncertainties of ± 0.26 m and ± 0.10 m respectively. These uncertainties equate to 10% and 11% of the tidal ranges at each site, which is of comparable precision to other published sea-level reconstructions based on foraminifera or diatoms. There is great scope for further developing intertidal testate amoebae as precise sea-level indicators and their application should be tested at sites beyond the North Atlantic. [ABSTRACT FROM AUTHOR]

Tidal marshes and the ecosystem services they provide may be at risk from sea-level rise (SLR). Tidal marsh resilience to SLR can vary due to differences in local rates of SLR, geomorphology, sediment availability and other factors. Understanding differences in resilience is critical to inform coastal management and policy, but comparing resilience across marshes is hindered by a lack of simple, effective analysis tools. Quantitative, multi-metric indices are widely employed to inform management of benthic aquatic ecosystems, but not coastal wetlands. Here, we develop and apply tidal ma rsh r esilience to s ea-level rise (MARS) indices incorporating ten metrics that contribute to overall marsh resilience to SLR. We applied MARS indices to tidal marshes at 16 National Estuarine Research Reserves across the conterminous U.S. This assessment revealed moderate resilience overall, although nearly all marshes had some indication of risk. Pacific marshes were generally more resilient to SLR than Atlantic ones, with the least resilient marshes found in southern New England. We provide a calculation tool to facilitate application of the MARS indices to additional marshes. MARS index scores can inform the choice of the most appropriate coastal management strategy for a marsh: moderate scores call for actions to enhance resilience while low scores suggest investment may be better directed to adaptation strategies such as creating opportunities for marsh migration rather than attempting to save existing marshes. The MARS indices thus provide a powerful new approach to evaluate tidal marsh resilience and to inform development of adaptation strategies in the face of SLR. [ABSTRACT FROM AUTHOR]

Cryptic sulfur cycling is an enigmatic process in which sulfate is reduced to some lower-valence state sulfur species and subsequently quantitatively reoxidized; the rate and microbial energetics of this process and how prevalent it may be in the environment remain controversial. Here we investigate sulfur cycling in salt marsh sediments from Norfolk, England where we observe high ferrous iron concentrations with no depletion of sulfate or change in the sulfur isotope ratio of that sulfate, but a 5‰ increase in the oxygen isotope ratio in sulfate, indicating that sulfate has been through a reductive cycle replacing its oxygen atoms. This cryptic sulfur cycle was replicated in laboratory incubations using 18 O-enriched water, demonstrating that the field results do not solely result from mixing processes in the natural environment. Numerical modeling of the laboratory incubations scaled to represent the salt marsh sediments suggests that the uptake rate of sulfate during this cryptic sulfur cycling is similar to the uptake rate of sulfate during the fastest microbial sulfate reduction that has been measured in the natural environment. The difference is that during cryptic sulfur cycling, all of the sulfur is subsequently reoxidized to sulfate. We discuss mechanisms for this pathway of sulfur cycling including the possible link to the subsurface iron cycle. [ABSTRACT FROM AUTHOR]

Platinum is one of the least abundant elements in the earth crust with an average concentration of 0.51 ng g − 1 , but its use in several human activities (mainly automobile catalytic converters) is leading its enrichment in surface sediments. Whereas, previous studies have assessed the Pt behavior in soils from urban areas, natural systems are still poorly studied. Accordingly, this study is aimed to characterize the behavior of the Pt released to coastal environments in sediments, ascertain the role of vegetation on the biogeochemistry of this element and assess the transference of Pt into the biological compartments. Several sediment cores were sampled in salt marshes (in unvegetated and vegetated areas) of the Tagus Estuary under different traffic pressure. Platinum concentration was analyzed in solid sediment, interstitial water and roots, leaves and stems of Sarcocornia fruticosa , a typical plant in south European salt marshes. In addition, interpretative parameters (e.g. redox potential, dissolved oxygen, pH, total reduced sulfur species, salinity and dissolved Fe and Mn in interstitial waters) were determined to better understand the biogeochemical cycle of Pt in salt marsh sediments. Although surface enrichment of Pt exists in the sediments at Low Traffic Station (2.8 ng g − 1 ), the highest enrichment was found at High Traffic Station where concentration was, in some cases, up to 40 ng g − 1 , linked to traffic emissions. However, dissolved Pt in interstitial waters (from 0.14 to 0.70 ng L − 1 ) did not show this superficial maximum. This dissimilarity points out the unreactivity of traffic-borne Pt and the dissolution/precipitation cycle of natural Pt linked to Mn and O 2 cycle, depending on the redox conditions, highly controlled by the vegetation. Platinum concentration in roots (0.9 ± 0.6 ng g − 1 ) is reflecting the Pt concentration in the interstitial waters in each moment, even though at the two peaks of dissolved Pt (up to 2.5 ng L − 1 ) found at deeper layers, reflecting a low bioaccumulation. However, results pointed that a different Pt species (with different bioavailability) may exist in both stations. Besides, Pt mean value in stems and leaves (0.04 ± 0.05 ng g − 1 ) indicates a low translocation of Pt from the roots to aerial parts. Therefore, the role of vegetation is fundamental on the geochemical behavior of Pt in sediments, due to the control of the redox conditions by roots in the surrounding sediments (between the O 2 release and the organic matter degradation) and leading to Pt species of varying bioavailability. [ABSTRACT FROM AUTHOR]

The vertical growth of coastal wetlands is known to primarily be controlled by local tidal range and sediment availability as well as the occurrence of storm events. In estuaries, sediment availability additionally depends on riverine sediment input, the effect of which may be more pronounced in some parts of the estuary, thereby introducing a distinct spatial pattern that depends on the estuary's shape as well as the riverine sediment input and the hydro-meteorological regime. In the present study, we investigate how estuarine marshes along the whole Río de la Plata (RdlP) are affected by decadal and long-term variations in river discharge and storm activity. The El Niño Southern Oscillation (ENSO), in this context, appears to introduce a pronounced decadal variability on sediment loads brought into the RdlP. Based on 15 sediment cores, recovered along the RdlP and adjacent Atlantic coast, vertical marsh growth rates were studied using radionuclide dating ( 210 Pb and 137 Cs) and grain size distributions. By comparing these sedimentological records with historic river discharge and storm surge data, we spatially interpret the relative importance of temporal variations in river discharge and storm activity on estuarine marsh growth. By delivering the first estimates for vertical growth rates of the RdlP marshes, we conclude that with average vertical marsh growth rates between 0.4 and 2.6 cm year − 1 , the RdlP marshes are highly resilient against drowning under present and future sea-level rise (SLR) conditions. Furthermore, our results confirm a large spatial variability of the drivers for vertical marsh growth; extreme storm surges appear to play a role in the development of the outer RdlP marshes whereas the temporal variations in river discharge seem to be hierarchically more important for the marshes in the inner estuary. [ABSTRACT FROM AUTHOR]

Tidal patterns of the subtropical Paranaguá Estuarine Complex, in southern Brazil, are strongly affected by episodic cold fronts and by the coastal geometry and bottom topography, resulting in high temporal variability and marked gradients in flood regime. We delimit tolerance ranges of submersion and exposure for representative plant and animal species from local mangroves and salt marshes, through a quantitative analysis of flooding patterns in three estuarine sectors. Our results are consistent with flood regime being the leading factor on how species are distributed over the intertidal flats of the PEC. Subleading factors might be related to salinity, sediment composition and nutrient flow. [ABSTRACT FROM AUTHOR]

Salt marshes are key ecosystems for biogeochemical cycling of nutrients in coastal areas, providing essential sequestration of contaminants and elemental organic forms while recycling these last to more bioavailable forms. Extracellular phosphatase-mediated phosphorus biogeochemical cycling acquires this way a reinforced role. In terms of speciation, salt marsh sediments exhibited a large pool of P in a bioavailable form (> 60%), being this pool characterized by being readily accessible to plants by weak chemical reactions. Regarding phosphatase activity, the acid isoform was found to be the most abundant pH-isoform present in salt marsh rhizosediments. A positive correlation could be found between humic acid bound P and total and acid phosphatase activities, possibly driven by the action of acid phosphatase on the acidic molecules of humic acids, using organic P as substrate, and releasing inorganic P into the desorbable P-pool. This process is more evident during the cold seasons, where the largest necromass input is observed entering the sediments, due to seasonal plant senescence. It could also be observed a positive correlation between inorganic phosphorus forms and phosphatase activity, consistent with phosphatases-driven inorganic phosphorus production from the recycling of organic forms, and thus a possible direct effect of phosphatase, as it was described for other ecosystems. This biogeochemical cycling is key for the ecosystem nutrient recycling, while passive remediator of eutrophication, but also for maintaining the salt marsh areas high primary productivity and their role as habitat, shelter and feeding ground of many animal species. • Salt marsh sediments exhibited a large pool of P in a bioavailable form. • Acid phosphatase was the most abundant isoform present in salt marsh sediments. • Acid phosphatase activity is responsible for releasing inorganic P into the desorbable P-pool. • This process is more active during the cold seasons due to higher necromass input. [ABSTRACT FROM AUTHOR]

As the first effort in literature, this paper conducts pore scale modeling on the methane hydrate dissociation and transportation in the reconstructed three-dimensional models of the MH-bearing sediment. The porous MH sample is synthesized using excess-gas method and imaged by micro-CT, which is used as input for the reconstructed mesh models. The real-time distribution of MH & water & methane, velocity and temperature is investigated. The effects of the temperature, pressure and flow rate of the injected water on MH dissociation and transportation are simulated and discussed. The results indicate that: 1) The hydrate generated by the excess - gas method is mainly cementing and mineral-coating on the sands surface, and occupies the small pores firstly. 2) The heterogeneity of the porous MH sediments is one of the key factors which influences the dissociation and the transportation process of the MH. 3) A lack of heat supply will restrict the dissociating rate of the MH reaching the maximum under the given PT conditions. 4) The gathering of the gas will decrease the flowing capacity of both water and methane. This study provides a new method to predict the multiple physicochemical and thermodynamical properties of the porous MH bearing sediments. • Pore scale MH dissociation and transportation in the real-shape porous media is modeled. • Real-time phase-change, multiphase flow, heat & mass transfer in porous media are studied. • Absolute and dynamic permeability of water & methane in the MH samples are predicted. • New method to predict petrophysical-thermal properties of MH sediments is provided. [ABSTRACT FROM AUTHOR]

A field study was conducted on a tidal flat intersected by small tidal channels (depth <0.1 m, width <2 m) within a tidal marsh. Data were collected in the channels, and on the adjacent tidal flat that encompasses approximately 1600 m 2 in planform area. Hydrodynamic processes and sediment suspension between the channels and adjacent flat were compared. Shear stress and turbulent kinetic energy were computed from high frequency velocity measurements. Maximum water depth at the field site varied from 0.11 m during the lowest neap high tide to 0.58 m during a storm event. In the channel intersecting the tidal flat, the shear stress, turbulence and along-channel velocity were ebb dominant; e.g. 0.33 m/s peak velocity for ebb compared to 0.19 m/s peak velocity for flood. Distinct pulses in velocity occurred when the water level was near the tidal flat level. The velocity pulse during flood tide occurred at a higher water level than during ebb tide. No corresponding velocity pulse on the tidal flat was observed. Sediment concentrations peaked at the beginning and end of each tidal cycle, and often had a secondary peak close to high tide, assumed to be related to sediment advection. The influence of wind waves on bed shear stress and sediment suspension was negligible. Water levels were elevated during a storm event such that the tidal flat remained inundated for 4 tidal cycles. The water did not drain from the tidal flat into the channels during the storm, and no velocity pulses occurred. Along-channel velocities, turbulent kinetic energy, and shear stresses were therefore smaller in the channels during storm conditions than during non-storm conditions. [ABSTRACT FROM AUTHOR]

Artificial coastal dykes have been widely built to prevent erosion, or to accelerate sedimentation processes in saltmarshes. To investigate the sediment dynamic changes induced by construction of dykes, three observation stations were set around a dyke within an expanding Scirpus mariqueter saltmarsh, in southern Hangzhou Bay. The hydrodynamics of the study area was dominated by tidal currents. The presence of dyke, in combination with the adjacent tidal creek, altered the hydrodynamic boundary conditions, thereby affecting the flow field. When the tidal flows approaching the dyke, the speed was accelerated at the head of the dyke and then decreased at the rear, due to the narrowing effect of the dyke head. Turbulence intensity around the dyke showed a similar pattern to that of the tidal flows. The net sediment input behind the dyke is approximately 2.3 times of that at the dyke head, illustrating sediment deposition in the back region of the dyke. As a result, flows entrained the sediments and then deposited behind the dyke to cause a local accretion to promote saltmarsh vegetation succession. The construction of dykes should be taken into account for future coastal wetland management. • In situ sediment dynamic observation was conducted around a dyke in saltmarsh. • Dyke altered tidal flows and caused sediment transport towards the rear region. • Construction of the dyke drove vegetation succession in the saltmarsh. [ABSTRACT FROM AUTHOR]

Sediment microbial communities are responsible for several ecosystem key-processes such as decomposition. However, these communities depend on aerobic respiration making them a source of CO 2 to the atmosphere. Since sediments are a known important carbon sink, it becomes important to address the factors that modulate sediment respiration and therefore CO 2 efflux to the atmosphere. Therefore, the present work aimed to assess the main factors controlling sediment respiratory activity in salt marshes. Sediment respiration and several sediment abiotic characteristics were assessed in two salt marshes from Ria de Aveiro coastal lagoon with contrasting environmental conditions. Sediment respiration had significant differences across seasons and salt marshes, and different patterns of activity were found for each salt marsh. Statistical analysis and modelling by Generalized Linear Model (GLM) revealed that sediment respiration is mostly influenced by organic matter quality (C:N ratio), sediment temperature and sediment pH. Nevertheless, as temperature appeared to be one of the most important factors influencing CO 2 effluxes from the sediments, its influence during possible global warming scenarios was focused. The simulations produced by the GLM using the IPCC scenarios projections, indicated that salt marshes will tend do decrease their CO 2 emissions with the increasing temperatures, reinforcing their role as important carbon sinks. This can be interpreted as an ecosystemic counteract measure toward a reduction of the increasing temperature by reducing the amounts of greenhouse gas emissions, namely CO 2 . [ABSTRACT FROM AUTHOR]

An understanding of erosional and depositional processes in intertidal wetland environments is of great importance to coastal geomorphologists, ecologists, and engineers. To evaluate the morphodynamic response of intertidal mudflats and saltmarshes to tidal inundation, we measured water depth, wave activity, current velocity profiles, suspended sediment concentration (SSC), and sediment composition at a dynamic mudflat–saltmarsh transition on eastern Chongming Island, at the Yangtze delta front, China. Based on these data, we calculated bed shear stresses generated by the combined current–wave action ( τ cw ), and the critical shear stress required to erode the surface sediments ( τ ce ), and so were able to calculate the erosional ( E ) and depositional ( D ) fluxes. The erodibility parameter ( M E ) and settling velocity ( w s ) used in the calculations of E and D were calibrated using daily measurements of bed-level change. Our results showed that the mudflat experienced alternating phases of net erosion and net deposition during tidal inundation. The burst-based changes in bed level ranged from −0.92 (net erosion) to +0.43 mm/10 min, and the cumulative bed-level changes over an entire tidal cycle ranged from approximately 0 to −5.4 mm (net erosion), with an average change of −3.4 mm/tide (net erosion) over five consecutive spring tides. In contrast, only net deposition was recorded on the saltmarsh during our observations. The burst-based changes in bed level ranged from around 0 to +0.56 mm/10 min, and the cumulative changes over a tidal cycle ranged from around 0 to +5 mm, with an average of +2.6 mm/tide for the five consecutive spring tides. We conclude that net erosion and net deposition during tidal cycles alternate on the mudflat, but that only net deposition occurs on the saltmarsh. [ABSTRACT FROM AUTHOR]

In this study, a bio-entrapped membrane reactor (BEMR) and a salt marsh sediment membrane bioreactor (SMSMBR) were evaluated to study the organic treatment performance of pharmaceutical wastewater. The influences of hydraulic retention time (HRT) and salinity were also studied. The conventional biomass in the BEMR cannot tolerate well of the hypersaline conditions, resulting in total chemical oxygen demand (TCOD) removal efficiency of 54.2–68.0%. On the other hand, microorganisms in the SMSMBR, which was seeded from coastal shore, strived and was able to degrade the complex organic in the presence of salt effectively, achieving 74.7–90.9% of TCOD removal efficiencies. Marine microorganisms able to degrade recalcitrant compounds and utilize hydrocarbon compounds were found in the SMSMBR, which resulted in higher organic removal efficiency than the BEMR. However, specific nitrifying activity decreased and inhibited due to the saline effect that led to poor ammonia nitrogen removal. [ABSTRACT FROM AUTHOR]

Although nitrogen has become a major concern for wetlands scientists dealing with eutrophication problems, phosphorous represents another key element, and consequently its biogeochemical cycling has a crucial role in eutrophication processes. Microbial communities are a central component in trophic dynamics and biogeochemical processes on coastal systems, since most of the processes in sediments are microbial-mediated due to enzymatic action, including the mineralization of organic phosphorus carried out by acid phosphatase activity. In the present work, the authors investigate the biogeochemical sediment drivers that control phosphatase activities. Authors also aim to assess biogeochemical factors' influence on the enzyme-mediated phosphorous cycling processes in salt marshes. Plant rhizosediments and bare sediments were collected and biogeochemical features, including phosphatase activities, inorganic and organic phosphorus contents, humic acids content and pH, were assessed. Acid phosphatase was found to give the highest contribution for total phosphatase activity among the three pH-isoforms present in salt marsh sediments, favored by acid pH in colonized sediments. Humic acids also appear to have an important role inhibiting phosphatase activity. A clear relation of phosphatase activity and inorganic phosphorous was also found. The data presented reinforces the role of phosphatase in phosphorous cycling. [ABSTRACT FROM AUTHOR]

Thin layer application (TLA) of dredged sediments has emerged as an adaptive management option for marshes that are not increasing in elevation fast enough to keep pace with local relative sea level rise. While there have been multiple previous demonstrations of TLA, there is limited pre-and post-application monitoring data available from past projects to develop guidance concerning optimal siting, design and implementation of projects. To address this gap, we conducted a controlled TLA experiment in a Spartina alterniflora marsh adjacent to the Atlantic Intracoastal Waterway in central North Carolina. The project was informed by the results of a long-term marsh monitoring and research program which had identified the project area as particularly vulnerable to sea level rise. The experimental design included triplicate meso-scale (24m2) treatment and control cells and quarterly (during the first year) to annual monitoring efforts to document trends in marsh surface elevation and vegetative biomass. In addition, sampling was conducted to characterize porewater chemistry and sediment characteristics. Thin layer application of dredged sediments resulted in an average elevation gain of 6 cm in treatment plots. Monitoring results indicate that the gains in elevation achieved by TLA have been maintained over five growing seasons. The response of vegetative biomass to the added elevation was well-predicted by the elevation-biomass relationship of the surrounding marsh platform. Treatment cells exhibited continued increases in surface elevation after placement, while control cell elevation remained constant. These data illustrate the potential of TLA as a tool for increasing the resilience of low-lying marshes to sea level rise and illustrate the value of the surface elevation-marsh biomass relationship for providing guidance for its use in S. alterniflora -dominated marshes. • The impact of adding a thin layer of dredged sediments to a low-lying coastal salt marsh was assessed. • Increasing the elevation of the marsh platform by 6 cm resulted in a doubling of vegetative biomass • Increased biomass led to increased ability to build further elevation after sediment application was completed. • Thin layer sediment application provides a mechanism for enhancing marsh resilience to future sea level rise. [ABSTRACT FROM AUTHOR]

Thin Layer Placement (TLP) is an increasingly used management technique to promote coastal resilience to accelerated sea level rise, but it is unclear how differential thickness of dredged sediment application alters plant-soil carbon dynamics in northeastern Atlantic coast salt marshes. We initiated a field experiment in a microtidal Spartina alterniflora (smooth cordgrass) dominated salt marsh in Connecticut (USA) to examine how a silt-loam dredge applied at different thicknesses (low: +5 cm, medium: +10 cm, and high: +15 cm) altered biological and biogeochemical responses. Over two growing seasons we monitored plant responses (above and belowground biomass, stem height, stem density, leaf area) and soil-biogeochemical parameters (EC, pH, redox, NH 4 +, sulfides, C:N, decomposition, bulk density, carbon fluxes). We found adding 5–7 cm of dredged sediment promoted rapid revegetation, alleviated phytotoxic sulfides, enhanced CO 2 uptake, and reduced CH 4 emissions. Low treatments reached similar vegetation cover to controls after one growing season, though sediment treatments greater than 10-cm thick showed delayed growth. Low levels of sediment addition stimulated the highest root biomass in the underlying soil, promoting CO 2 efflux, but this was offset by aboveground biomass productivity, promoting net CO 2 uptake. Phytotoxic sulfides were reduced in all sediment addition treatments, but our data indicate that coastal managers aiming to promote rapid vegetative recovery while increasing elevation in the short term should target conservative depths (< 10 cm) when using fine-grained dredged material. Collectively, our work will guide wetland managers as they develop restoration specifications for perpetuating coastal marshes in the face of rising seas. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The organic production related to minerogene salt marsh deposits represents a challenge to all attempts to model the development of these areas, and evaluate their chances of survival under different sea level scenarios. Salt marsh deposits on a typical temperate backbarrier saltmarsh area at the Skallingen barrierspit (Denmark) were investigated for autochthonous or below ground organic production, which was found to have a mean value of 0.14kgm−2 y−1 or about 0.1mmy−1. This production is concentrated in the upper approximately 5cm of the salt marsh. Below this level the organic material decomposes with a decreasing organic content until about 15cm below the surface. Hereunder the decomposition of organic material seems to stabilize at a very low level. The constant rate of the below ground organic production results in a larger concentration of this type of organic matter in the slowest accreting salt marsh deposits. The root activity is the primary reason for differences in the bulk dry density of salt marsh surface layers, therefore the surface layers of slowly accreting salt marshes have the lowest bulk dry density. During autocompaction, the bulk dry density increases down core. This process is only to a small degree (<10%) directly affected by the space loss due to decomposition of organic matter. The decay of organic matter, however, is most likely indirectly part of the autocompaction process by weakening the internal strength of the root fabric and thus the capability of resistance against compaction from the overburden of depositing sediments. [Copyright &y& Elsevier]

Abstract: Methane, a powerful greenhouse gas, is both produced and consumed in anoxic coastal sediments via microbial processes. Although the anaerobic oxidation of methane (AOM) is almost certainly an important process in coastal freshwater and salt marsh sediments, the factors that control the rates and pathways of AOM in these habitats are poorly understood. Here, we present the first direct measurements of AOM activity in freshwater (0PSU) and brackish (25PSU) wetland sediments. Despite disparate sulfate concentrations, both environments supported substantial rates of AOM. Higher sulfate reduction (SR) rates were measured in the freshwater site and SR at both sites was of sufficient magnitude to support the observed AOM activity. Laboratory incubations of freshwater and brackish tidal, wetland sediments amended with either nothing [control], sulfate, nitrate, manganese oxide (birnessite) or iron oxide (ferrihydrite) and supplied with a methane headspace were used to evaluate the impact(s) of electron acceptor availability on potential AOM rates. Maximum AOM rates in brackish slurries occurred in the sulfate amendments. In contrast, addition of sulfate and several possible electron acceptors to the freshwater slurries decreased AOM rates relative to the control. High ratios of AOM activity relative to SR activity in the nitrate, birnessite, and ferrihydrite treatments of both the brackish and freshwater slurries provided evidence of AOM decoupled from SR. This study demonstrates that both freshwater and brackish coastal wetland sediments support considerable rates of anaerobic methanotrophy and provides evidence for sulfate-independent AOM that may be coupled to nitrate, iron, or manganese reduction in both environments. [Copyright &y& Elsevier]

The distribution and accumulation of rare earth elements (REE) in the labile fraction of sediment cores collected from salt marshes in the Patos Lagoon estuary from southern Brazil were investigated. Sediment cores (ca. 40 cm) were obtained from three locations within the estuary to capture possible changes in REE content across the salinity gradient (i.e., where saline, brackish, and freshwater dominate). Salt marsh sediments from all three coring locations were enriched in the light REE (LREE) over the heavy REE (HREE) when normalized to the North American shale composite (NASC). Shale normalized values for the LREE of marsh sediments from sites M2 and M3 in the mid- and upper estuary commonly approximates unity indicating these sediments chiefly exhibit a terrigenous signature. In contrast, all 14 naturally occurring REE are depleted in the sediments from the M1 coring location in the lower estuary compared to shale. Sediments from the mid-estuary (M2 core location) where typical salinity values are ca. 10 practical salinity units, exhibit the greatest shale normalized LREE enrichments. The higher LREE contents of the M2 sediments likely reflect preferential removal of LREE from the water column owing to salt-induced coagulation of river-borne colloids that occurs during estuarine mixing processes. Sediments samples collected from the salt marsh in the lower estuary nearer the Atlantic Ocean (i.e., M1), have substantially lower REE contents than salt marsh sediments from sites M2 and M3 in the mid- and upper estuary, respectively. The more sand-rich lower estuary sediments are exposed to higher salinity water from the South Atlantic compared to the generally finer grained sediments from the mid- and upper estuary, which are dominated by brackish and freshwater conditions, respectively. Negative Ce-anomalies (i.e., Ce/Ce* < 1) in marsh sediments from depths generally greater than 27 cm support the occurrence of early diagenetic reactions consistent with suboxic to anoxic conditions whereby reductive dissolution of ferromanganese oxide/oxyhydroxide carrier phases, and/or Ce(IV) phases, release Ce(III) to the marsh pore waters. The negative Ce anomalies suggests that a suboxic or anoxic environment predominates to a depth of 9 cm, and then again below 27 cm depth in sediments from the M2 salt marsh core, and below a depth of 13 cm in the upper estuary marsh sediments (i.e., M3). In contrast, positive Ce anomalies (Ce/Ce* > 1) for much of the sediment from the M1 core, the mid depths of core M2 (i.e., 9 cm to 27 cm), and shallower than 13 cm in sediments from core M3 support oxic conditions where Ce is enriched in sediments from these depths by preferential scavenging from pore waters onto Fe/Mn oxides/oxyhydroxides. These trends in Ce/Ce* with depth in each sediment core are also consistent with known bioturbation that occurs within these marsh sediments. The relatively high REE contents of the sediments from location M3 are also related to the fact that these sediments are dominantly composed of clay-silt deposits that are enriched in organic matter. Finally, we find no conclusive evidence that the studied salt marsh sediments have been impacted by addition of REE from the local fertilizer manufacturing industry. • ƩLREE/ƩHREE ratios were generally lower in saltmarshes sediments closer to the sea. • REE enrichment in freshwater dominate part of the estuary is due to high fine-grained and organic contents. • Positive Ce-anomalies are supported by oxic conditions in salt marsh sediment. [ABSTRACT FROM AUTHOR]

Estuaries are considered one of the most dynamic ecosystems in the world, ensuring high production and transfer of large quantities of organic matter (OM) from continents to the ocean. Despite the large influence of estuaries in the functioning of coastal ecosystems, our understanding of OM dynamics in these buffering coastal environments remains fragmented. In this frame, our study was carried out to describe and understand the main drivers of OM distribution in two estuaries (Canche, Authie) of the eastern English Channel, based on descriptive and quantitative approaches. For this purpose, in February and March 2018, surface sediments were sampled at low tide along cross-shore transects in the intertidal zone of each estuary. Analysis of sediment grain size, porosity, proportion of aggregated particles, total carbonate content and organic geochemical characteristics (total OM content, atomic TOC/TN ratio, δ13C value) of surface sediments allowed identifying four biosedimentary facies: sand flat, mud flat, low and high salt marshes. In parallel, the elemental and isotopic characterization of the suspended particulate OM inputs by the rivers, the surface biofilms and the halophytic vegetation was performed to trace the three major sources contributing to sedimentary OM in both estuaries. Spatial distribution of OM in surface sediments (ranging between 0.6 and 26.9% dry weight) evidenced higher contents along marsh platforms than in sand and mud flats. and the OM pool was mainly constituted of a mixing of C 3 terrestrial vascular plants, marine and riverine phytoplankton. The comparison between both estuaries highlighted that the geomorphology impacted by natural and anthropic factors plays a preponderant role in the distribution and the sources of sedimentary OM by influencing vegetation communities and hydrodynamic processes. In the context of sea level rise, an improved understanding of the OM dynamics within each biosedimentary facies is crucial to better constrain and assess the role of macrotidal estuaries in carbon cycle. • Field observation and physico-chemical analyses of exhaustive surface sediment samples used for determining biosedimentary facies. • Source of sedimentary organic matter defined by elemental and isotopic proxies. • Influence of the structure and texture of surface sediments on OM distribution. • Principal Components Analysis highlighted two contrasted geomorphological stages. [ABSTRACT FROM AUTHOR]

Abstract: Coastal sediment cells reflect processes operating at a range of scales, but it is the medium spatial and temporal scales (decades to centuries) that are of greatest interest for coastal management. This paper focuses on coastline position change within a single sediment cell over 150years where the geomorphology includes cliffs, beaches and saltmarshes. The focus is the east coast of England from Flamborough Head to Gibraltar Point. Although the updrift sector of this sediment cell has been studied for well over a century, the downdrift sector has attracted significantly less attention. Using topographic profiles, bathymetric profiles, aerial photographs and historical maps we mapped coastline erosion and accretion using the Digital Shoreline Analysis System (DSAS) and calculated volumetric changes for different morphometric units. Rapid erosion of the updrift Holderness cliffs has been counterbalanced with accretion on beaches along the downdrift Lincolnshire coast. The amount of accretion in Lincolnshire corresponds to around 29% of the volume of sediment eroded from Holderness. Much of the eroded cliff material is likely to be deposited temporarily into nearshore and offshore sand banks before being redistributed by cross-shore currents. An exploration of storm surge impact on long-term erosion and accretion rates showed no clear relationship between storm surge frequency and change in coastline position, however this may be in part due to the relative timing of storm occurrence and data acquisition. The Jenkinson daily weather type classification was found to be a reasonable proxy for the occurrence of strong onshore winds which may offer scope for further investigation of the role of forcing factors over time periods beyond the length of the meteorological and tidal station records. Winter North Atlantic Oscillation phase was not a good indicator of storminess on the east coast of England but may be a useful proxy for quiescence. [Copyright &y& Elsevier]

Abstract: Environmental changes during the last 2 millennia in the Minho River tidal marsh (NW Portugal–Spain border) were reconstructed. Changes in the sources of organic matter (OM) delivered to the marsh were evaluated from elemental, isotopic and molecular composition using a 1m sediment core. Carbon isotopic composition (δ13C) and organic carbon to total nitrogen ratio (Corg/N) provided valuable information concerning the origin of the OM. These parameters indicated a major input from land plants, reaching a maximum at 1100–1200 and 1750–1850AD. These periods match with major flood events in the NW of the Iberian Peninsula, as reported by several authors. A significant reduction in the terrestrial signature occurred at 6–4cm (ca. 1960–1985AD), which is contemporaneous with the construction of several major dams on the Minho River. The distribution of selected lipid biomarkers, including n-alkanes, n-fatty acids and n-alkan-2-ones and specific parameters derived from the molecular distributions, were useful for refining bulk geochemical results. Long chain n-alkanes with an odd number of carbons are indicators of soil-and vascular plant-derived terrestrial OM and were dominant throughout the core. In addition, a greater contribution of plankton-derived lipids was observed in the sections corresponding to ca. 1960–1985AD (6–4cm) and ca. 100–200AD (96–94cm). Although different degradation rates for individual compounds might have partly affected biomarker assemblages, the variations could be attributed to a sharp decrease in the freshwater contribution to the Minho River estuary (dam construction) and a possible marine highstand, respectively. In addition, several parameters suggested changes in land use (including deforestation and farming) and probably the effects of mining exploitation during the Roman occupation of the area. [Copyright &y& Elsevier]

Empirical data derived from a network of surface elevation tables established on the Hunter River, Australia, in 2000 was used to model sediment accretion within estuarine wetlands using factorial analysis of variance. As surface elevation change did not differ significantly from accretion over the 10 year study period in the mangrove and saltmarsh (p =0.4104), the accretion model was regarded as a reliable estimate of elevation change. Using the current rate of sea-level rise (3.65mmy−1), a rate deemed to be relatively moderate, a landscape elevation model was developed by applying the accretion model to a LiDAR-derived digital elevation model at annual increments to 2050. Based on current rates of sea-level rise and the intertidal elevation that currently supports mangrove and saltmarsh, the landscape elevation model projected a 16% increase in the area within the elevation range suitable to support mangrove and saltmarsh. This contrasts ‘bathtub modelling’, which projected a 6% decline in wetland extent. Bathtub modelling fails to account for the ability of mangrove and saltmarsh to accommodate sea-level rise through processes of accretion, shrink-swell of sediments and the accumulation of organic material. Results from the landscape elevation model suggest that planning for sea-level rise should be directed towards facilitating wetland adaptation by promoting tidal exchange to mangrove and saltmarsh and providing land for wetland migration. [Copyright &y& Elsevier]

Abstract: The salt marsh grass Spartina anglica is an important habitat-modifying ecosystem engineering agent that facilitates large-scale salt marsh formation by enhancing sediment accretion. It dominates many European tidal environments and is invasive in many other parts of the world. We question (1) to what extent the ecosystem engineering ability of patchy Spartina vegetation depends on large-scale abiotic processes, and (2) whether tussock shape provides an indicator for future lateral salt marsh development. Analysing the topography of 83 individual tussocks in contrasting environments revealed that there are 6 clearly distinguishable tussock shapes, and that the classical example of a sediment-accumulating dome-shaped tussock only occurs under a limited set of abiotic conditions. The outcome of habitat modification by S. anglica is shown to be conditional, depending on large-scale morphodynamics and sediment grain size. Resulting tussock shape provides a clear indication for the long-term development of the pioneer zone. Understanding of the conditional outcome of ecosystem engineering is highly relevant in this era of climate change and ongoing anthropogenic influences on coastal ecosystems. [Copyright &y& Elsevier]

Abstract: The stability of cohesive sediments of a saltmarsh in Southern England was measured in the field and the laboratory using a Cohesive Strength Meter (CSM) and a shear vane apparatus. Cores and sediment samples were collected from two tidal creek banks, covered by Atriplex portulacoides (Sea Purslane) and Juncus maritimus (Sea Rush), respectively. The objectives of the study were to examine the variation of sediment stability throughout banks with cantilevers present and investigate the influence of roots and downcore consolidation on bank stability. Data on erosion threshold and shear strength were interpreted with reference to bank depth, sediment properties and biological influences. The higher average erosion threshold was from the Sea Purslane bank whilst the Sea Rush bank showed higher average vane shear strength. The vertical variation in core sediment stability was mainly affected by roots and downcore consolidation with depth. The data obtained from the bank faces revealed that vertical variations in both erosion threshold and vane shear strength were affected primarily by roots and algae. A quantitative estimate of the relative contributions of roots and downcore consolidation to bank sediment stability was undertaken using the bank stability data and sediment density data. This showed that roots contributed more to the Sea Purslane bank stability than downcore consolidation, whilst downcore consolidation has more pronounced effects on the Sea Rush bank stability. [Copyright &y& Elsevier]

Abstract: The 2010 BP oil spill became the worst environmental disaster in US history with unprecedented damages to the environment and human health. Ecosystem recovery from the oil spill largely depends on the microbial degradation of oil contaminants. In this study benzene was selected as a representative of petroleum hydrocarbons to assess its anaerobic biodegradation potential in salt marsh sediment from the Louisiana Gulf coast. The experiment was conducted in two separate phases of incubation of the sediment slurries to determine the benzene biodegradation kinetics (phase-1), and to determine if any stimulation of benzene degradation occurred after benzene exposure (phase-2). Benzene biodegradation was assumed to follow Michaelis–Menten kinetics, and the V max was determined to be 1.66mgg−1 week−1 and the K m to be 1.17mgg−1. According to this model, it would take 11 weeks for 1m2 of the salt marsh to degrade 1L of benzene. Benzene degradation rates were found to have a minimum increase after the sediment had been exposed to benzene for 4 weeks, which suggests that the sediment has almost reached its natural maximum biodegradation potential. [Copyright &y& Elsevier]

Abstract: This study examined sediment deposition and retention in a section of salt marsh in the St. Jones River estuary in Delaware. Sediment traps, siphon samplers, a LISST particle sizer, and four water level sensors were deployed in a 100m by 200m grid over the course of a spring tide in June 2007. The objective was to determine the relative influences of suspended sediment concentration, settling velocity, tidal hydroperiod, and vegetation density on sediment deposition and trapping efficiency within a small section of marsh. Hydroperiod is not a major contributor to sediment deposition in the study plot. Spatial patterns in depositional processes on tidal timescales are instead primarily controlled by overmarsh SSC, settling velocity, and distance from the tidal channel. Trapping efficiency, or the ratio of measured deposition and the amount of sediment available for deposition, decreased with distance away from the tidal channel, indicating that less of the available suspended sediment was deposited in the marsh interior. The suspended particles were flocculated, but floc sizes decreased with distance away from the tidal channel. [Copyright &y& Elsevier]

Abstract: Ice rafting is an important secondary sedimentation process that redistributes sediment form tidal flats, channel beds, and ponds to the vegetated marsh surface in northern temperate climates. Source location of ice-rafted sediment is identifiable based on distinct sediment properties. In New England salt marsh systems, ice raft thickness and entrained sediment load vary both during the season and interannually as a function of severity and duration of winter conditions; however, 97% of ice rafts carry measurable sediment loads. Thick rafts move sand or peat up to 100m from source areas, whereas thinner rafts tend to transport mud still further onto the marsh platform, sometimes reaching the upland border. Based on these observations, we present relationships defining the theoretical sediment-carrying potential of ice rafts as well as empirical parameterizations for ice-rafted sediment with respect to ice volume. Our results suggest that ice-rafting deposits a volume of sediment contributing up to 5% of annual vertical accretion, an important input in a region where rates of vertical accretion barely compensate for sea-level rise. We provide conceptual models of ice-raft formation and sediment entrainment linking these processes to the general geomorphic evolution of northern temperate marshes, which must be understood in light of the modern acceleration in rates of sea-level rise. [Copyright &y& Elsevier]

Abstract: In order to further elucidate the early diagenetic behavior of Mo, we examined the redox speciation of dissolved Mo in organic rich sediment from a back-barrier salt marsh environment in eastern Long Island (Flax Pond, NY). Total dissolved Mo (ΣMo) was ~80nM in surface nonsulfidic porewater, dominantly as Mo(VI). ΣMo increased up to 150nM at a depth of 3.5cm with low sulfide content (ΣH2S: 25–100μM), and Mo(V) reached 20nM. ΣMo decreased to ~70nM at a depth of 7.5cm in highly sulfidic deep sediment porewater (ΣH2S: >100μM) with Mo(V) accounting for ~10%. Mo(VI) dominated residual ΣMo, likely as MoS4 2−. Averaged in situ Mo speciation patterns are complicated by mixed redox conditions created by biogenic structures and reworking. Serial anoxic incubation of surface sediments revealed reductive redox reaction progression without complications from transport and biogenic microenvironments: Mo(VI) dominated initially, followed by increases in ΣMo (dMo/dt ~7nM/h) and production of Mo(V) under low sulfide conditions (ΣH2S: 25–100μM; Mo(V) as high as 160nM). Mo(V) was subsequently lost rapidly from solution (dMo(V)/dt ~−5nM/h) and residual ΣMo, presumably a mixture of Mo(VI) and a small percentage of Mo(IV), was gradually reestablished under highly sulfidic conditions (ΣH2S >100μM). Mo(V) is clearly produced as a transient dissolved intermediate during reductive redox reaction succession. Mo(V) may react with particles or disproportionate in the presence of polysulfides into Mo(IV), which likely rapidly adsorbs–precipitates as pyritic Mo–Fe–S, sulfidized organic complexes, or perhaps MoS2. Mo(VI), which remains, at least temporarily in solution as thiomolybdate is removed more slowly. In contrast to reductive reactions, reoxidation of reduced sediment results in rapid release of Mo dominated by Mo(VI). Dynamic diagenetic cycling and the existence of Mo(V) as a dissolved reaction intermediate must be accounted for in models of Mo fixation and associated isotopic fractionation in sulfidic deposits. [Copyright &y& Elsevier]

Abstract: Responses in feeding ecology of a benthic forage fish, mummichogs (Fundulus heteroclitus), to altered prey resources were investigated in chronically polluted salt marshes (the Arthur Kill–AK, New York, USA). The diet niche breadth of the AK populations of mummichogs was significantly lower than that of the reference population, reflecting reduced benthic macroinfaunal species diversity. Most of the AK populations also had 2–3 times less food in their gut than the reference population. This disparity in gut fullness among the populations appeared to be partly due to ingested prey size shifts; some of the AK populations ingested fewer large prey than the reference population. Furthermore, benthic assemblages were strongly associated with sediment-associated mercury; gut fullness of the AK populations also significantly decreased with increasing mercury body burdens. These results indicate that chronic pollution may have directly (chemical bioaccumulation) and indirectly (reduced prey availability) altered the feeding ecology of mummichogs. [Copyright &y& Elsevier]

Abstract: Salt marshes are potentially threatened by sea level rise if sediment supply is unable to balance the rising sea. A rapid sea level rise is one of the pronounced effects of global warming and global sea level is at present rising at an elevated rate of about 3.4mm y−1 on average. This increasing rate of sea level rise should make it possible to study the effect of rapidly rising sea level on salt marsh accumulation. However, such an understanding is generally hampered by lack of available data with sufficient precision. Here we present a high-precision dataset based on detailed radiometric measurements of 137Cs in 10 sediment cores retrieved at a natural and unmanaged micro tidal salt marsh. Two distinct 137Cs-peaks were found in all cores, one peak corresponding to the 1963-maximum caused by testing of nuclear weapons in the atmosphere and the other to the Chernobyl accident in 1986. Salt marsh accretion has generally kept pace with sea level rise since 1963 but comparison of the accumulation rates of minerogenic material in the period 1963–1986 and 1986–2003 revealed a slight decrease in accumulation with time in spite of an observed increase in inundation frequency. The observed decrease in sediment deposition is significant and gives reason for concern as it may be the first sign of a sedimentation deficiency which could be threatening this and other salt marshes in the case of a rapidly rising sea level. Our work demonstrates that the assumption of a constant relationship between salt marsh inundation and sediment deposition is not necessarily valid, even for a salt marsh that receives most of its allocthonous sediment from the adjacent sea. The apparent decrease in sediment deposition indicates that the basic assumption of sufficient sediment supply used in contemporary models dealing with salt marsh accretion is most probably not valid in the present case study and it may well be that this is also the case for many other salt marshes, especially if sea level continues to rise rapidly as indicated by some climate change scenarios. [Copyright &y& Elsevier]

Abstract: Existing geotechnical approaches that describe volumetric changes in intertidal sediments in response to applied vertical effective stresses are limited by a lack of empirical research into their one-dimensional compression behaviour. In this paper we address this deficiency by presenting the results of an investigation into the compression behaviour of minerogenic low marsh and tidal flat sediments. We have tested samples of these sediment types obtained from Greatham Creek (Cowpen Marsh, Tees Estuary, UK). Analysis of physical properties and oedometer compression tests demonstrates that, contrary to the implicit assumptions of existing models, the surface sediments studied are overconsolidated. Structural variability between samples arises due to sedimentological factors, notably variations in organic content. We attribute overconsolidation to tidal exposure and falls in groundwater level that permit desiccation and cause capillary suction stresses. Greater rates of compression with respect to effective stress occur in sediments with higher initial voids ratios and more open, unstable initial structures. Variability in structure decreases with application of higher effective stresses due to the destructuration of the sediments, which also creates increased homogeneity of compression behaviour under higher effective stresses. We subsequently develop a new conceptual framework to describe compression behaviour in minerogenic intertidal sediments that incorporates overconsolidation. We advocate a statistical approach that accounts for structural variability and variations in compression behaviour at effective stresses less than and greater than the yield stress. We argue that our conceptual framework is broadly applicable to minerogenic intertidal sediments at different locations and burial depths within Holocene stratigraphic sequences providing site-specific compression data are collected. Inter-site transfer and application of measured material properties should not be undertaken due to local variations in compression behaviour resulting from varying ecological, sedimentological, geochemical, climatic, geomorphic and hydrographic conditions. The individual characteristics of different field locations should be carefully considered before the suggested framework is routinely applied. [Copyright &y& Elsevier]