Sediment Accretion Research Articles

Economic valuation of suspended sediment and phosphorus filtration services by four different wetland types: A preliminary assessment for southern Ontario, Canada

AbstractWetlands are known for their water filtration (or purification) function. Although different wetland types differ in their filtration capacity, they are usually aggregated together in economic valuation studies. Here, we explicitly separate the valuation of the suspended sediment and phosphorus (P) filtration services of the four major wetland types—bogs, fens, marshes and swamps—found in southern Ontario, Canada. The areal extents of the four wetland types are derived from the Canadian Wetland Inventory (CWI) progress map, while the sediment accretion rate is used as the key variable regulating the suspended sediment and P filtration functions. Based on available literature data, we assess the relationship of the sediment accretion rate to wetland size. Because only weak positive correlations are found, we assign a mean (average) sediment accretion rate to each wetland type. The sediment accretion rates are combined with mean soil P concentrations to estimate Pretention rates by the wetlands. The replacement cost method is then applied to valuate the sediment and P filtration services. The unit values for both sediment and P retention decrease in the order: marshes > bogs ≈ swamps > fens. The total value of sediment plus phosphorus removal by all wetlands in southern Ontario amounts to $4.2 ± 2.9 billion per year, of which about 80% is accounted for by swamps. We further assess the costs of different options to offset the additional P loading generated in a hypothetical scenario whereby all wetlands are converted to agriculture. The results demonstrate that replacing the P filtration function of existing wetlands with conventional land management and water treatment solutions is not cost‐effective, hence reinforcing the importance of protecting existing wetlands.

Four-Dimensional Investigation of Gravel Beach Ridge Accretion and 50 Years of Beach Recharge at Dungeness, UK, Using Historic Images, GPR and Lidar (HIGL)

Dungeness is a cuspate foreland on the south coast of England that is the largest shingle feature in Europe and includes hundreds of beach ridges. It is also the location of two nuclear power stations that were constructed in the 1960s. The dominant southwest waves cause longshore drift from west to east, eroding the southwest side of Dungeness, accompanied by accretion on the east side. A record of this eastward movement and sediment accretion is preserved by the shingle beach ridges. The power stations are located on the eroding southwestern side of the ness, and a system of beach recharge has been used to move shingle from the downdrift, east-facing shore to the updrift, southwest-facing shore to protect the power stations from coastal erosion. We use a novel combination of historic images, ground-penetrating radar (GPR), and Lidar (HIGL) to investigate accretion and beach ridges at Dungeness during the past 80 years. We report changes in accretion along the coast and use GPR to determine the thickness of beach gravels. The amount of accretion, represented by the width of the backshore, decreases downdrift from south to north. The number of beach ridges preserved also decreases from south to north. By combining the shingle thickness from GPR with elevation data from Lidar surveys and records of beach accretion measured from aerial images, we estimate the volume and mass of gravel that has accumulated at Dungeness. Historic rates of beach accretion are similar to recent rates, suggesting that the 55 years of beach recharge have had little impact on the longer-term accretion downdrift.

Coastal morphodynamic analysis in Buleleng Regency, Bali—Indonesia

Sediment as erosion product can affect shorelines, making sediment transport a key process to consider in coastal and shoreline management. Field surveys and secondary data can identify where suspended matters are distributed and deposited to analyze sediment uniformity factors: beach morphology and materials. This research set out to determine the Buleleng Regency's morphodynamic aspects based on the coastal landscape's physical characteristics and the processes acting upon each sediment cell. Field observations were conducted at five stations, from Tukad Gerokgak to Tukad Saba estuary. Jaelani's spectral transformation has been applied to analyze Total Suspended Solids using Sentinel 2A imagery. The laboratory test results of grain-size samples were processed on GRADISTAT, then the depositional environment and sediment transport direction were determined from average grain size, standard deviation, skewness, and kurtosis. Shoreline change, an indicator of coastal morphodynamics, was mapped from Landsat images in 2000, 2008, and 2019 using the Digital Shoreline Analysis System. Statistical analysis on GRADISTAT provided details on depositional environment and sediment transport and deposition based on grain-size distribution. Results indicate poorly sorted medium grain size: gravel (stone) to coarse sand, making up the sediment population from Tukad Gerokgak to Tukad Saba. Generally, sediment is deposited toward coarse, even very coarse, grain on a strongly sloping beach, and there is a high likeliness of sediment accretion. Identified morphodynamic characteristics suggest that the coastal landscape needs structural mitigation to overcome the accelerating impact of human activities and physical processes.

The Role of Sediment Accretion and Buoyancy on Subduction Dynamics and Geometry

AbstractSubducted sediments are thought to lubricate the subduction interface and promote faster plate speeds. However, global observations are not clear‐cut on the relationship between the amount of sediments and plate motion. Sediments are also thought to influence slab dip, but variations in subduction geometry depend on multiple factors. Here we use 2D thermomechanical models to explore how sediments can influence subduction dynamics and geometry. We find that thick sediments can lead to slower subduction due to an increase of the megathrust shear stress as the accretionary wedge gets wider, and a decrease in slab pull as buoyant sediments are subducted. Our results also show that larger slab buoyancy and megathrust stress due to thick sediments increase the slab bending radius. This offers a new perspective on the role of sediments, suggesting that sediment buoyancy and wedge geometry also play an important role on large‐scale subduction dynamics.

Ecological succession and fine sediment accretion influence local patch dynamics of a pioneer riparian species (Typha minimaHoppe)

AbstractChanges in environmental variables induced by ecological succession and hydrogeomorphological processes affects riparian vegetation dynamics along rivers. The biotic and abiotic factors driving local patch dynamics of riparian species is little known for early successional stages.To fill this gap, we examined local patch dynamics and frequency of occurrence of a protected riparian pioneer species (Typha minimaHoppe) and related it to several biotic and abiotic variables along the Isère River (France) from 2016 to 2019.We monitored biotic and abiotic factors related to ecological succession and sediment accretion. We implemented Linear Mixed and Generalized Mixed Models to analyze how environmental factors affected patch boundary dynamics ofT. minima. Finally, we built a Structural Equation Model to describe potential relationships betweenT. minimafrequency, height above mean water level, vegetation cover and composition.We found that a thick layer of silt and sand deposits as well as a moderate herbaceous and low shrub cover layer characterizedT. minimapatch progression close to the mean water level.Typha minimastands did not seem to expand on coarser substrates with lower herbaceous cover. Its maintenance was associated with large silt and sand deposits and high cover of herbaceous plants and shrubs well above the mean water level. The variables characterizing patch regression were not clearly identified. Finally, the interaction between bar height, vegetation cover and succession stage did not explain variations inT. minimafrequency.Our results provide knowledge about the environmental variables associated withT. minimapatch dynamics, which are useful for future conservation and restoration projects. Habitat rejuvenation through restoration of natural river flow regimes could sustain localT. minimatransplantation and large population turnover essential to the conservation of this species.

Seasonality and Characterization Mapping of Restored Tidal Marsh by NDVI Imageries Coupling UAVs and Multispectral Camera

Salt marsh evolution is strongly affected by tidal processes and ecology, which regulate sediment accretion and erosional rates. A balance between marsh erosion and deposition in a restored tidal wetland is crucial for analyzing restoration strategies to adopt in a natural context. Here, we present an integrated approach monitoring salt marsh seasonal changes over several months in a microtidal restored salt marsh of the Paul S. Sarbanes Ecosystem Restoration Project at Poplar Island (MD, USA). The project is undertaken at a restoration site where sediment dredged from the shipping channels in the upper Chesapeake Bay is being used to restore a tidal marsh habitat in mid-Chesapeake Bay. We flew an Unmanned Aerial Vehicle (UAV) with an RGB and a multispectral camera to obtain a high-resolution map of the planimetric position of vegetation and to monitor the health of the marsh vegetation in diverse seasons. Due to its extension of 400 m by 400 m, a total of four flight plans were necessary to cover the entire marsh flying at a 40 m altitude obtaining a 2 cm Ground Sample Distance (GSD). This technique provides reliable results at a very low cost, enabling an accurate assessment of the marsh platforms to be conducted over time, due to both the very high spatial resolution and the precise georeferencing of the images for the comparisons. Our results show seasonal variability in the two dominant species colonizing the low marsh, Spartina alterniflora, and high marsh, Sporobolus pumilus. While the lower marshes showed a higher variability along seasons, the up-land vegetation showed persistent green foliage during cold seasons. Detecting salt marsh evolution and seasonality coupled with field measurements can help to improve the accuracy of hydrodynamic and sediment transport models. Understanding the drivers of salt marsh evolution is vital for informing restoration practices and designs, in order to improve coastal resilience, and develop and coastal management strategies.

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

AbstractIntertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

Interactions between biotic and abiotic processes determine biogeomorphology in Yangtze Estuary coastal marshes: Observation with a modeling approach

Coastal salt marshes are biogeomorphic ecosystems with coupled vegetation and hydro-sedimentary processes. A significant biogeomorphic feature with a positive loop between vegetation colonization, sedimentary accretion, and edge scarp progradation can be found in coastal marshes with a highly depositional environment, such as the Yangtze Estuary. Together with field surveys, we used a process-based salt marsh model to characterize the spatiotemporal dynamics of vegetation and geomorphology in a tidal wetland in the Yangtze Estuary from 2014 to 2019. This model incorporated interactions between biotic processes (plant growth, colonization, and hydrological resilience) and abiotic processes (tidal transport of propagules, sediment inputs, and habitat-specific sediment deposition). Comparisons with field measurements showed that the model satisfactorily described the interannual variations in vegetation growth and lateral expansion, sedimentary dynamics and elevation change, and formation of marsh-edge scarps at two investigation sites. In particular, both observation and simulation revealed that the degree of sediment supply regulated the rates of vegetation behavior and geomorphic changes. The rates of plant establishment and growth were higher at the site with high sediment supply, and a higher rate of vegetation expansion and sedimentary accretion drove a faster seaward progradation of marsh-edge scarps, compared to the site with low sediment supply. Our results indicate that the interactions between biotic and abiotic processes, together with sediment supply, strongly determine the heterogeneity of marsh biogeomorphology in a highly depositional system. Thus, there is a vast potential for application of process-based models in formulating tidal wetland management and assessing future system resilience.

Evaluating adaptation options to sea level rise and benefits to agriculture: The Ebro Delta showcase

Sea level rise (SLR) is threatening low-lying coastal areas such as river deltas. The Ebro river Delta (Spain) is representative of coastal systems particularly vulnerable to SLR due to significant sediment retention behind upstream dams (up to 99%), thereby dramatically reducing the capacity for deltaic sediment accretion. Rice production is the main economic activity, covering 66% of the delta area, and is negatively affected by SLR because of flooding and soil salinization. Therefore, appropriate adaptation measures are needed to preserve rice production. We combined Geographic Information Systems and Generalized Linear Models to identify zones prone to flooding and increasing soil salinity, and to calculate the so-called sediment deficit, that is the amount of sediment needed to raise the land to compensate flooding and soil salinization. We modelled SLR scenarios predicted by the IPCC Fifth Assessment Report, and analysed the economic feasibility (not the technical feasibility) of reintroducing fluvial sediments retained in the upstream river dam reservoirs into the delta plain, which can contribute to maintaining land elevation and rice production with SLR. To do this, the costs of the sediment reintroduction measures and their benefits in terms of avoided loss of rice production income were evaluated with an approximate economic cost-benefit analysis. Results predicted that between 35 and 90% of the rice field area will be flooded in the best and worst SLR scenarios considered (SLR = 0.5 m and 1.8 m by 2100, respectively), with a sediment deficit of 130 and 442 million tonnes, with an associated cost of sediment reintroduction of 13 and 226 million €. The net benefit of rice production maintenance was 24.6 and 328 €/ha. The proposed adaptation measure has a positive effect on rice production and can be considered as an innovative management option for maintaining deltaic areas under SLR.

Long-Term Changes in the Abundance of Benthic Foraging Birds in a Restored Wetland

Estuaries have historically been subject to considerable habitat loss, and continue to be subjected to such in areas where the natural landward migration of intertidal habitats is constrained by hard coastal defences. Thus, in estuaries where direct (e.g., port development) or indirect (e.g., sea level rise) processes are predicted to threaten intertidal habitats and associated waterbird species, there is a regulatory requirement to produce compensatory intertidal habitats. Managed realignment (MR) is a shoreline management practise that is undertaken to build sustainable coastal defences and create intertidal habitats in estuaries. This nature-based solution brings multiple benefits in the form of carbon storage, increased resilience to flooding, and, potentially, the formation of new habitats, which is the topic of this study. A 75-ha site at the Paull Holme Strays (Humber Estuary, United Kingdom) was monitored over a 10-year period following MR to examine the change in the abundance of waterbirds in the chosen site in response to the physical processes occurring there. Using digital terrain models (DTMs) collected via light detection and ranging (LiDAR), we examined how four compensatory target species responded to changes in elevation after the creation of the site. It was shown that the very rapid accretion of estuarine sediment occurred in the first decade of the new re-created intertidal, which, over time, led to changes in the numbers of benthic foraging birds supported. Furthermore, elevation change was also driven by this sediment accretion, the rate of which depended on the initial bed elevation of the sectors within the site. Ten years after the recreation of the habitat, the spatial heterogeneity in the bed elevation remained high; however, the sectors with the lowest elevations accreted the most over the 10-year period. The foraging number of the four waterbird species that colonised the MR site significantly declined above a certain elevation, with this effect being most pronounced for the Eurasian curlew (Numenius arquata). The number of common shelducks (Tadorna tadorna), dunlins (Calidris alpina), and common redshanks (Tringa totanus) declined significantly after initial peaks 5–7 years after the creation of the site, reflecting the ongoing elevation changes. Thus, this study highlighted the need for long-term studies to understand how species respond to large-scale habitat construction. It can also aid in predicting the suitability of an MR site for waterbirds in the medium and long term.

Tidal marsh restoration enhances sediment accretion and carbon accumulation in the Stillaguamish River estuary, Washington.

Tidal marshes have been recognized globally for their ability to sequester “blue carbon” but there is still a need for studies investigating the marsh response to restoration, particularly in the Pacific Northwest United States. Here we report carbon stocks and accumulation rates for restored and natural tidal marshes in the Stillaguamish River estuary in Puget Sound, Washington, where a 60-hectare marsh was reintroduced to the tidal regime from its previous use as diked and drained farmland. We found that the restoration not only maximized carbon accumulation but also enhanced resilience to rising sea levels. Four years after restoration, mean sediment carbon stocks in the upper 30 cm within the restored marsh (4.43 kg C m-2) were slightly lower than those measured in the adjacent natural marshes (5.95 kg C m-2). Mean carbon accumulation rates, however, were nearly twice as high in the restored marsh (230.49 g C m-2 yr-1) compared to the natural marshes (123.00 g C m-2 yr-1) due to high rates of accretion in the restored marsh (1.57 cm yr-1). Mean elevation change rates were nearly twice that of corresponding 210Pb accretion rates, but all were greater than the current rate of sea level rise.

The stabilizing effect of high pore-fluid pressure along subduction megathrust faults: Evidence from friction experiments on accretionary sediments from the Nankai Trough

Pore-fluid pressure is an important parameter in controlling fault mechanics as it lowers the effective normal stress, allowing fault slip at lower shear stress. It is also thought to influence the nature of fault slip, particularly in subduction zones where areas of slow slip have been linked to regions of elevated pore-fluid pressure. Despite the importance of pore-fluid pressure on fault mechanics, its role on controlling fault stability, which is determined by the friction rate parameter (a−b), is poorly constrained, particularly for fault materials from subduction zones. In the winter of 2018-19 the accretionary complex overlying the Nankai Trough subduction zone (SW Japan) was drilled as part of Integrated Ocean Drilling Program (IODP) Expedition 358. Here we test the frictional stability of the accretionary sediments recovered during the expedition by performing a series of velocity-stepping experiments on powdered samples (to simulate fault gouge) while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges, despite only containing 25% phyllosilicates, are strongly rate-strengthening and exhibit negative values for the rate-and-state parameter b. We find that for experiments where the effective normal stress is held constant and the pore-fluid pressure is increased the Nankai gouges become more rate-strengthening, and thus more stable (an increase in (a−b) of ∼6 × 10−5 MPa−1 with increasing pore-pressure). In contrast, when the pore-fluid pressure is held constant and the effective normal stress is varied, there is minimal effect on the frictional stability of the gouge. The increase in frictional stability of the gouge at elevated pore-fluid pressure is caused by an evolution in the rate-and-state parameter b, which becomes more negative at high pore-fluid pressure. These results have important implications for understanding the nature of slip in subduction zones and suggest the stabilizing effect of pore-fluid pressure could promote slow slip or aseismic creep on areas of the subduction interface that might otherwise experience earthquake rupture.

Buried hurricane legacies: increased nutrient limitation and decreased root biomass in coastal wetlands

AbstractPlant identity and cover in coastal wetlands is changing in worldwide, and many subtropical salt marshes dominated by low‐stature herbaceous species are becoming woody mangroves. Yet, how changes affect coastal soil biogeochemical processes and belowground biomass before and after storms is uncertain. We experimentally manipulated the percent mangrove cover (Avicennia germinans) in 3 × 3 m cells embedded in 10 plots (24 × 42 m) comprising a gradient of marsh (e.g., Spartina alterniflora, Batis maritima) and mangrove cover in Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on surface sediment accretion, soil chemistry (carbon, C; nitrogen, N; phosphorus, P; and sulfur, S), and root biomass. Data were collected before (2013 and 2016), one‐month after (2017), and one‐year after (2018) Hurricane Harvey crossed the area, allowing us to measure stocks before and after the hurricane. The accretion depth was higher in fringe compared with interior cells of plots, more variable in cells dominated by marsh than mangrove, and declined with increasing plot‐scale mangrove cover. The concentrations of P and δ34S in storm‐driven accreted surface sediments, and the concentrations of N, P, S, and δ34S in underlying soils (0–30 cm), decreased post‐hurricane, whereas the C concentrations in both compartments were unchanged. Root biomass in both marsh and mangrove cells was reduced by 80% in 2017 compared with previous dates and remained reduced in 2018. Post‐hurricane loss of root biomass in plots correlated with enhanced nutrient limitation. Total sulfide accumulation as indicated by δ34S, increased nutrient limitation, and decreased root biomass of both marshes and mangroves after hurricanes may affect ecosystem function and increase vulnerability in coastal wetlands to subsequent disturbances. Understanding how changes in plant composition in coastal ecosystems affects responses to hurricane disturbances is needed to assess coastal vulnerability.

On the use of large-scale biodegradable artificial reefs for intertidal foreshore stabilization

Combining foreshore ecosystems like saltmarshes and mangroves with traditional hard engineering structures may offer a more sustainable solution to coastal protection than engineering structures alone. However, foreshore ecosystems, are rapidly degrading on a global scale due to human activities and climate change. Marsh-edges could be protected by using connected ecosystems, such as shellfish reefs and seagrass beds, which can trap and stabilize sediments, thereby reducing hydrodynamics loads on the saltmarsh edge. In our study, we aimed to test the effect of large-scale biodegradable artificial reefs on tidal flat accretion and/or stabilization. We hypothesized that the structures would attenuate waves and trap sediment. For this, a large-scale experiment was conducted on the tidal flats of the Dutch Wadden Sea, by installing biodegradable artificial reefs along 630 m. Waves, sediment dynamics and sediment properties around the structures were monitored over three years. Our results demonstrate that intact structures attenuated circa 30% of the wave height with water levels below 0.5 m. Variability in wave-attenuation increased when the wind direction was parallel to the structures/foreshore. Sediment dynamics were variable due to the exposed nature of the location and environmental heterogeneity because of the landscape-scale set-up. We observed local sediment accretion up to 11 cm, however the effect did not expand beyond 10 m from the landward edge of the structures and up to 10 cm scouring was also found. Additionally, near sediment properties were not affected by the presence of the artificial reefs. Long-term effects could not be assessed due to the degradation of the structures during the experimental period. In general, we conclude that artificial reefs have the potential to attenuate waves and trap sediment on tidal flats. However, to benefit connected foreshore ecosystems like salt marshes, an even larger implementation scale and the use of more resistant structures in exposed sites is needed to affect long-term tidal flat morphology.

On backflow associated with oceanic and continental subduction

SUMMARY A popular idea is that accretion of sediment at a subduction zone commonly leads to the formation of a subduction channel, which is envisioned as a narrow zone located above a subducting plate and filled with vigorously circulating accreted sediment and exotic blocks. The circulation can be viewed as a forced convection, with downward flow in the lower part of the channel due to entrainment by the subducting plate, and a ‘backflow’ in the upper part of the channel. The backflow is often cited as an explanation for the exhumation of high-pressure/low-temperature metamorphic rocks from depths of 30 to 50 km. Previous analyses of this problem have mainly focused on the restricted case where the walls bounding the flow are artificially held fixed and rigid. A key question is if this configuration can be sustained on a geologically relevant timescale. We address this question using a coupled pair of corner flows. The pro-corner accounts for accretion and deformation directly above the subducting plate, and the retro-corner corresponds to a deformable region in the overlying plate. The two corners share a medial boundary, which is fully coupled but is otherwise free to rotate and deform. Our results indicate that the maintenance of a stable circulating flow in a narrow pro-corner (<15°) requires an unusually large viscosity ratio, μretro/μpro > 103. For lower viscosity ratios, the medial boundary would rotate rearwards, converting the initially narrow pro-corner into an obtuse geometry. For a stable narrow corner, we show that the backflow within the corner is caused by downward convergence of the incoming flow and an associated downward increase in dynamic pressure, which reaches a maximum at the corner point. The total pressure is thus expected to be much greater than predicted using a lithostatic gradient, which means that estimates of depth from metamorphic pressure would have to be adjusted accordingly. In addition, we show that the velocity fields associated with a forced corner flow and a buoyancy-assisted channel flow are nearly identical. As such, structural geology studies are not sufficient to distinguish between these two processes.

The evolution of slate microfabrics during progressive accretion of foreland basin sediments

Here, we study slate microfabrics from the exhumed accretionary wedge of the central European Alps and focus on the development of foliation. High-resolution micrographs from novel BIB-SEM imaging and Synchrotron X-ray Fluorescence Microscopy are analysed with 2D auto-correlation functions to quantify the geometry and spacing of slate microfabrics along a metamorphic gradient covering the outer and inner wedge (200–330 °C). The sedimentary layering primarily controls the morphology of the slate microfabrics. However, from outer to inner wedge, a fabric evolution is observed where diagenetic foliations gradually transform to secondary continuous and spaced foliations. With increasing metamorphic grade, the amount of recrystallized phyllosilicate grains and their interconnectivity increase, as does clast/microlithon elongation (aspect ratios up to 11), while foliation spacing decreases to <20 μm. This foliation evolution under non-coaxial deformation involves a combination of mechanical rotation of phyllosilicates, fracturing, and fluid-assisted pressure-dissolution-precipitation creep. The latter is the dominant deformation mechanism at T > 230 °C and accommodates background strain in the inner wedge. The evolving microstructural anisotropy is interpreted to lead to strain weakening by structural softening and may provide preferential fluid pathways parallel to the foliation, enabling the dehydration of large rock volumes in accretionary sediment wedges undergoing prograde metamorphism.

Recent Carbon Storage and Burial Exceed Historic Rates in the San Juan Bay Estuary Peri-Urban Mangrove Forests (Puerto Rico, United States).

Mangroves sequester significant quantities of organic carbon (C) because of high rates of burial in the soil and storage in biomass. We estimated mangrove forest C storage and accumulation rates in aboveground and belowground components among five sites along an urbanization gradient in the San Juan Bay Estuary, Puerto Rico. Sites included the highly urbanized and clogged Caño Martin Peña in the western half of the estuary, a series of lagoons in the center of the estuary, and a tropical forest reserve (Piñones) in the easternmost part. Radiometrically dated cores were used to determine sediment accretion and soil C storage and burial rates. Measurements of tree dendrometers coupled with allometric equations were used to estimate aboveground biomass. Estuary-wide mangrove forest C storage and accumulation rates were estimated using interpolation methods and coastal vegetation cover data. In recent decades (1970–2016), the highly urbanized Martin Peña East (MPE) site with low flushing had the highest C storage and burial rates among sites. The MPE soil carbon burial rate was over twice as great as global estimates. Mangrove forest C burial rates in recent decades were significantly greater than historic decades (1930–1970) at Cañno Martin Peña and Piñones. Although MPE and Piñones had similarly low flushing, the landscape settings (clogged canal vs forest reserve) and urbanization (high vs low) were different. Apparently, not only urbanization, but site-specific flushing patterns, landscape setting, and soil fertility affected soil C storage and burial rates. There was no difference in C burial rates between historic and recent decades at the San José and La Torrecilla lagoons. Mangrove forests had soil C burial rates ranging from 88 g m−2 y−1 at the San José lagoon to 469 g m−2 y−1 at the MPE in recent decades. Watershed anthropogenic CO2 emissions (1.56 million Mg C y−1) far exceeded the annual mangrove forest C storage rates (aboveground biomass plus soils: 17,713 Mg C y−1). A combination of maintaining healthy mangrove forests and reducing anthropogenic emissions might be necessary to mitigate greenhouse gas emissions in urban, tropical areas.

Primary succession in an Atlantic salt marsh: From intertidal flats to mid‐marsh platform in 35 years

Abstract Although salt marsh is a classic example of primary succession, the underlying mechanisms and their time‐scales are poorly understood. As salt marsh succession depends on sediment accretion, the amelioration of abiotic conditions associated with increasing elevation suggests potential explanatory roles for facilitation, competition and the stress‐gradient hypothesis. We present a 35‐year longitudinal study of salt marsh development from intertidal flat to a mid‐marsh platform at Odiel Marshes in south‐western Iberia. Using permanent plots, this work chronicles changes in elevation and marsh morphology, their evolving effects on sediment redox potential and salinity and the colonisation and changing patterns of dominance of halophytic species. Sporadically colonising clumps of the low‐marsh species Spartina maritima trapped sediment to form raised tussocks, which increased in elevation and area. Reduced tidal inundation and locally improved drainage promoted higher redox potentials and allowed colonisation by a sequence of species less tolerant of reducing conditions: Sarcocornia perennis, its hybrid with high‐marsh S. fruticosa, and Atriplex portulacoides. Unlike its centrifugally colonising predecessors, A. portulacoides invaded from the tussock edges. Transplant experiments designed to investigate its late establishment on tussocks showed that seedling survival depended on elevational differences as small as 4 cm. After increasing in elevation by c. 1 m (c. 29 mm/year), coalescence of the tussocks formed a marsh platform at a level corresponding to mean high tides. This supports a theoretical punctuated transition from ‘submergence marsh’ to ‘emergence marsh’, previously postulated for this tidal elevation. Synthesis. The unexpected rapidity of this primary succession highlights the central role of facilitation. Vertical sediment accretion, locally engineered by colonising species, progressively alleviates abiotic stress and allows colonisation by species that are less tolerant of chemically reducing conditions but are ultimately better competitors.

High-resolution sedimentological and geochemical records of three marshes in San Francisco Bay, California

The San Francisco Bay has the largest concentration of salt marshes in the state of California. In the last 170 years, the vast majority of the historic tidal wetlands in the Bay have been significantly altered or destroyed due to diking, filling and other processes. Many of the remaining marshes have been impacted by changing sedimentation regimes and related loadings of pollutants such as heavy metals (Sr, Al, Fe, Ti, Cu, Pb, Ni, Zn) over this period, making ecological trajectories and resilience to disturbance uncertain. Here we examine changing sediment accretion rates, coupled with the longest and highest resolution heavy metal accumulation data from three San Francisco Bay marshes, to examine European impacts in the context of earlier Holocene variability. The results are consistent with larger-scale analyses and indicate that European alterations of landscapes and shorelines had geographically variable impacts on marsh sediment accumulation. Despite differential impacts on net sediment accretion, initial European impacts appear to have decreased the proportion of organic material in the marsh sediments, likely due to the delivery of eroded inorganic sediment from landscapes and shorelines. The results confirm significant European impacts on the geochemistry of marshes in the San Francisco Bay over the last ~150–200 years. Post-European arrival levels of Pb are unprecedented throughout the earlier Holocene. However, these values have declined in recent years. Concentrations of Sr, Ti, Cu, Ni, and Zn also increased following European arrival. Our results show that post-European concentrations are not so far removed compared to pre-European maximum concentrations. As sedimentation regimes and emissions of Pb and delivery of other metals have decreased, and as organic productivity increased on the marshes, environmental trajectories are shifting back towards their immediate pre-European conditions.

Relationship Between Subduction Erosion and the Up‐Dip Limit of the 2014 Mw 8.1 Iquique Earthquake

AbstractThe aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the updip limit of coseismic rupture propagation. Aftershocks updip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate.