Feldspar Marker Horizons Research Articles

Living root contributions dominate vertical accretion, but not carbon burial, in two SE Australian tidal wetlands

Organic matter derived from allochthonous and autochthonous sources makes an important contribution to the accumulation and burial of “blue carbon” in tidal wetlands. Organic matter accretion is also a mechanism by which tidal marshes and mangroves adjust vertically to the pressure of sea-level rise, through feedbacks between marsh autotrophic productivity and hydroperiod. However, the separate contributions of inorganic matter, detrital organic matter and living roots to marsh accretion have rarely been documented. We used a network of Surface Elevation Table-Marker Horizon (SET-MH) stations SE Australia as a benchmark against which the accumulation of inorganic, organic and living root material was measured. Established in 2000–2002, the SET-MH stations allowed for sampling accretion and elevation gain in mangrove and saltmarsh in fluvial and marine sand geomorphic settings. We found living root material to be the dominant contributor to the volume of accretion above feldspar marker horizons and elevation gain over the 20-year experimental period. However, detrital organic matter, being of lower bulk density than living roots, contributed most to carbon burial. Higher rates of accretion and elevation gain were associated with higher contributions of pore volume, and lower contributions of inorganic matter. The rate of subsidence increased with accretion and was lower where inorganic matter contributions to mass were higher. The results highlight the importance of biological and hydrological contributions to accretion and elevation gain in a region subject to riverine minerogenic inputs.

Atmospheric River Storm Flooding Influences Tidal Marsh Elevation Building Processes

AbstractDisturbances are a key component of ecological processes in coastal ecosystems. Investigating factors that affect tidal marsh accretion and elevation change is important, largely due to accelerating sea‐level rise and the ecological and economic value of wetlands. Sediment accumulation rates, elevation change, and flooding were examined at five marshes along a riverine‐tidal gradient in the northern San Francisco Bay‐Delta, California, USA during an Atmospheric River storm event in 2017 using Surface Elevation Tables (SETs), feldspar marker horizons (MH), and continuous water‐level sensors. Our results showed that localized marsh flooding increased during the storm event, but not evenly across sites. Marsh surface elevation increased the most at the tidal freshwater marsh site in response to the storms, with an average surface elevation gain of 45.6 ± 13.1 mm, and the least at a tidal saline marsh with an average surface elevation gain of 4.0 ± 1.2 mm. A marsh located on the large embayment did not exhibit an immediate response to the storm but had a surface elevation gain of 21.5 ± 13.7 mm 6 months after the storm. During the storm period, marsh distance to the bay was the strongest predictor of elevation change, followed by SET‐MH elevations. Conversely, during non‐storm periods, SET‐MH elevation was a relatively strong predictor of elevation change. Atmospheric Rivers appear to be a major factor affecting short‐term spatial and temporal variability in flooding and sedimentation rates in tidal marsh systems. Incorporating information about storms into monitoring could increase our understanding of how episodic storms can impact marshes.

Sea level rise and marsh surface elevation change in the Meadowlands of New Jersey

The marshlands of the Meadowlands of New Jersey are valuable wetland ecosystems in a highly developed urban area and provide a natural habitat to more than 285 species of birds, a great variety of fishes, and many other living organisms. It is not clear if these ecosystems and their associated ecological services will persist under conditions of accelerated sea level rise (SLR), in geography where space for a landward retreat of marshlands is limited. In this study, we used the deep rod surface elevation table method and feldspar marker horizons to measure surface elevation change and vertical accretion rate in five marshland sites over 11 years. The controlling parameters of the accretion rate were explored. The results showed that sediments were not limited for vertical accretion. About 16% of the total suspended solids reaching the marsh via the tide was trapped by the marsh surface. Hydraulic duty alone cannot explain differences in deposition rates between low and high marsh. Precipitation, snow accumulation, and sea surge from storms were the main drivers influencing subsidence. The overall subsidence rate was 1.5 ± 1.3 mm/year. All sites combined showed increases in surface elevation of 4.0 ± 0.7 mm/year. This rate of increase is not enough to keep up with the 8 mm/year SLR prediction. There is a 50% chance that in 80 years, 7% of current marshlands will be underwater or will convert to unvegetated mudflats, and most high marsh habitats will disappear.

Factors influencing blue carbon accumulation across a 32‐year chronosequence of created coastal marshes

AbstractSaline coastal marshes are blue carbon ecosystems with relatively high soil carbon (C) stocks and high rates of soil C accumulation. Loss of saline wetlands due to relative sea‐level rise, land‐use change, and hydrologic alterations liberates previously stored C and reduces the capacity for future C sequestration. Widespread wetland loss has prompted marsh restoration and creation projects around the world; however, little is known about the timescale and capacity for created marshes to function as blue C sinks and the role of environmental conditions in mediating soil C accumulation in restoration sites. Using a chronosequence of five created saline marshes ranging in age from 5 to 32 yr and two adjacent natural reference marshes in southwest Louisiana, USA, short‐ and longer‐term C accumulation rates (SCAR and LCAR, respectively) were determined using feldspar marker horizons and peat depth in cores at six locations in each marsh. Created marshes ranged in elevation from −12 to 41 cm (NAVD88) and supported assorted plant community compositions driven by local environmental conditions. SCAR ranged from 75 to 430 g C·m−2·yr−1, which were comparable in the two youngest and two oldest marshes. Longer‐term CAR ranged from 18 to 99 g C·m−2·yr−1 but did not significantly differ among marshes of different ages. Our findings indicate that LCAR in these created marshes were influenced by site‐specific environmental conditions (i.e., stem density and mineral sediment) rather than marsh age. Results suggest that conditions appropriate for the establishment of vegetation with high stem densities, such as Distichlis spicata and Spartina patens, may facilitate higher LCAR in created marshes, which may be useful for restoration project planning and mitigation of climate change.

The Potential Resiliency of a Created Tidal Marsh to Sea Level Rise

Abstract. The purpose of this study was to determine the elevation dynamics of a created tidal marsh on the North Carolina coast. Deep rod surface elevation tables (RSET) and feldspar marker horizons (MH) were installed in plots to measure net surface elevation changes and to quantify contributing processes. Twelve total plots were placed on four elevation gradient transects (three transects within the created marsh and one within a reference marsh), with three plots along each transect. Elevation gradient transects included a low marsh plot dominated by , a middle marsh plot dominated by , and a high marsh plot dominated by . RSET and MH were measured in December 2012, January 2014, April 2017, and March 2018. Elevation change ranged from 1.0 to 4.0 mm year-1 within the created marsh and from -0.4 to 2.0 mm year-1 within the reference marsh. When compared to the long-term linear trend in local relative sea level rise (RSLR) of 3.10 ±0.35 mm year-1, the middle marsh plots within the created marsh trended toward survival, with an observed elevation increase of 3.1 ±0.2 mm year-1. Alternatively, the low and high marsh plots within the created marsh trended toward submergence, with observed elevation increases of 2.1 ±0.2 and 1.3 ±0.2 mm year-1, respectively. These results indicate that a created marsh can display elevation dynamics similar to a natural marsh and can be resilient to current rates of RSLR if constructed with a high elevation capital. Surface elevation changes were observed over a short time period and in a relatively young marsh, so it is uncertain if these trends will continue or how the long-term relation with RSLR will develop. While this study provided initial data on the ability of created tidal marshes to respond to observed sea level rise, subsequent observations are needed to evaluate the long-term elevation dynamics. Keywords: Resiliency, Sea level rise, Surface elevation tables, Tidal marsh, Vertical accretion.

Numerical modeling of salt marsh morphological change induced by Hurricane Sandy

The salt marshes of Jamaica Bay serve as a recreational outlet for New York City residents, mitigate wave impacts during coastal storms, and provide habitat for critical wildlife species. Hurricanes have been recognized as one of the critical drivers of coastal wetland morphology due to their effects on hydrodynamics and sediment transport, deposition, and erosion processes. In this study, the Delft3D modeling suite was utilized to examine the effects of Hurricane Sandy (2012) on salt marsh morphology in Jamaica Bay. Observed marsh elevation change and accretion from rod Surface Elevation Tables and feldspar Marker Horizons (SET-MH) and hydrodynamic measurements during Hurricane Sandy were used to calibrate and validate the wind-waves-surge-sediment transport-morphology coupled model. The model results agreed well with in situ field measurements. The validated model was then used to detect salt marsh morphological change due to Sandy across Jamaica Bay. Model results indicate that the island-wide morphological changes in the bay's salt marshes due to Sandy were in the range of −30 mm (erosion) to +15 mm (deposition), and spatially complex and heterogeneous. The storm generated paired deposition and erosion patches at local scales. Salt marshes inside the west section of the bay showed erosion overall while marshes inside the east section showed deposition from Sandy. The net sediment amount that Sandy brought into the bay is only about 1% of the total amount of reworked sediment within the bay during the storm. Numerical experiments show that waves and vegetation played a critical role in sediment transport and associated wetland morphological change in Jamaica Bay. Furthermore, without the protection of vegetation, the marsh islands of Jamaica Bay would experience both more erosion and less accretion in coastal storms.

Understanding land subsidence in salt marshes of the Venice Lagoon from SAR Interferometry and ground-based investigations

The existence of salt marshes and tidal morphologies is strictly connected to their elevation with respect to the mean sea level. Quantifying land subsidence of these high-valued transitional environments is therefore crucial to investigate their long-term possible survival, also in view of the expected climate changes. However, monitoring with a certain accuracy their movements has been challenging until now due to the peculiar features of these morphological forms: they are difficult to access, made of largely unconsolidated deposits, without anthropogenic structures, relatively far from anthropogenic facilities, and become submerged by the sea water twice a day. For these reasons, they have never be linked to traditional levelling and GPS networks, and also standard Interferometric SAR applications returned very poor results in terms of spatial and temporal coverage. An advanced Persistent Scatterer Interferometry (PSI) technique on a 5-year long stack of X-bandwidth SAR acquisitions of the Venice Lagoon is here presented. The regularity of the acquisitions, the short satellite revisiting time (11days), the high image resolution (~3×3m), and the strategies used in the PSI application have allowed us to detect thousands of measurable persistent targets (PTs) in the Venice Lagoon salt marshes. The measured displacements range from small uplifts to subsidence rates of more than 20mm/yr. The analyses of the observed displacements point out that land subsidence is much larger on man-made than natural salt marshes, with a significant negative correlation with the marsh age. In addition, land subsidence with the presence of halophytic vegetation species is generally smaller than on unvegetated marshes. Finally, at a few selected sites, the integration of the PSI outcome with local ground-based measurements, such as multi-depth benchmarks, feldspar marker horizons and surface elevation tables, has allowed quantifying the displacement variability versus depth and therefore developing a first conceptual model of the salt marsh consolidation and accretion processes.

The Role of Sediment Dynamics for Inorganic Accretion Patterns in Southern California’s Mediterranean-Climate Salt Marshes

Salt marsh resilience to sea-level rise depends on marsh plain elevation, tidal range, subsurface processes, as well as surface accretion, of which suspended-sediment concentration (SSC) is a critical component. However, spatial and temporal patterns of inorganic sedimentation are poorly quantified within and across Salicornia pacifica (pickleweed)-dominated marshes. We compared vertical accretion rates and re-examined previously published suspended-sediment patterns during dry-weather periods at Seal Beach Wetlands, which is characterized by a mix of Spartina foliosa (cordgrass) and pickleweed, and for Mugu Lagoon, where cordgrass is rare. Mugu Lagoon occurs higher in the tidal frame and receives terrigenous sediment from an adjacent creek. Feldspar marker horizons were established in winter 2013–2014 to measure accretion. Accretion rates at Seal Beach Wetlands and Mugu Lagoon were 6 ± 0.5 mm/year (mean ± SE) and 2 ± 0.3 mm/year. Also, the estimated sediment flux (g/year) across the random feldspar plots was 3.5 times higher at Seal Beach Wetlands. At Mugu Lagoon, accretion was higher near creeks, although not statistically significant. Dry-weather SSC showed similar concentrations at transect locations across sites. During wet weather, however, SSC at Mugu Lagoon increased at all locations, with concentrations decaying farther than 8 m from tidal creek edge. Based on these results from Mugu Lagoon, we conclude accretion patterns are set by infrequent large flooding events in systems where there is a watershed sediment source. Higher accretion rates at Seal Beach Wetlands may be linked to lower-marsh elevations, and thus more frequent inundation, compared with Mugu Lagoon.

Surface elevation change and susceptibility of coastal wetlands to sea level rise in Liaohe Delta, China

The Liaohe Delta in China is an ecologically and commercially important wetland system under threat from sea level rise and marsh subsidence. Sediments deposited in coastal marshes could offer wetlands a potentially important means for adjusting surface elevation with rising sea level, yet coastal wetland stability in Liaohe Delta is not well understood due to limited data from long-term experiments. In this study, wetland surface elevation and vertical accretion were measured from 2011 to 2015 using a surface elevation table (SET) and feldspar marker horizons in two Phragmites and two Suaeda marshes receiving Liaohe River water. The analysis shows that the Phragmites marshes exhibited higher rates of marsh accretion and elevation change than the Suaeda marshes. The two Phragmites marsh sites had average surface elevation change rates at 8.8 and 9.3 mm yr−1, vertical accretion at 17.4 and 17.6 mm yr−1, and shallow subsidence at 8.6 and 8.3 mm yr−1. The average rates of elevation change, vertical accretion, and shallow subsidence at two Suaeda marsh sites were 5.8 and 6.3 mm yr−1, 13.6 and 14.8 mm yr−1, and 7.8 and 8.5 mm yr−1, respectively. The trends suggest that coastal marshes in Liaohe Delta are experiencing changes in average soil elevation that range from a net increase of 0.3 mm y−1 to 6.9 mm y−1 relative to averaged sea level rise in Bohai Sea reported by the 2016 State Oceanic Administration People's Republic of China projection (2.4–5.5 mm y−1), which indicated that the four wetland sites would adjust to the sea level rise and even continue to gain elevation, especially for the Phragmites sites. Nevertheless, the vulnerability of coastal wetlands in Liaohe Delta need further assessment considering the accelerated sea level rise, the high rate of subsidence, and the declining sediment delivery owing to anthropogenic activities such as dam constructions in the river basin.

Allochthonous and autochthonous contributions to carbon accumulation and carbon store in southeastern Australian coastal wetlands

Estimates of carbon store and carbon accumulation rate in mangrove and saltmarsh are beset by issues of scale and provenance. Estimates at a site do not allow scaling to regional estimates if the drivers of variability are not known. Also, carbon accumulation within soils provides a net offset only if carbon is derived in-situ, or would not otherwise be sequestered. We use a network of observation sites extending across 2000 km of southeastern Australian coastline to determine the influence of geomorphic setting and coastal wetland vegetation type on rates of carbon accumulation, carbon store and probable sources. Carbon accumulation above feldspar marker horizons over a 10-year period was driven primarily by tidal range and position in the tidal frame, and was higher for mangrove and saltmarsh dominated by Juncus kraussii than for other saltmarsh communities. The rate of carbon loss with depth varied between geomorphic settings and was the primary determinant of carbon store. A down-core enrichment in δ13C was consistent with an increased relative contribution of mangrove root material to soil carbon, as mangrove roots were found to be consistently enriched compared to leaves. We conclude that while surface carbon accumulation is driven primarily by tidal transport of allocthonous sediment, in-situ carbon sequestration is the dominant source of recalcitrant carbon, and that mangrove and saltmarsh carbon accumulation and store is high in temperate settings, particularly in mesotidal and fluvial geomorphic settings.

Retention of Riverine Sediment and Nutrient Loads by Coastal Plain Floodplains

Despite the frequent citation of wetlands as effective regulators of water quality, few quantitative estimates exist for their cumulative retention of the annual river loads of nutrients or sediments. Here we report measurements of sediment accretion and associated carbon, nitrogen, and phosphorus accumulation as sedimentation over feldspar marker horizons placed on floodplains of the non-tidal, freshwater Coastal Plain reaches of seven rivers in the Chesapeake Bay watershed, USA. We then scale these accumulation rates to the entire extent of non-tidal floodplain in the Coastal Plain of each river, defined as riparian area extending from the Fall Line to the upper limit of tidal influence, and compare them to annual river loads. Floodplains accumulated a very large amount of material compared to their annual river loads of sediment (median among rivers = 119%), nitrogen (24%), and phosphorus (59%). Systems with larger floodplain areas and longer floodplain inundation retained greater proportions of riverine loads of nitrogen and phosphorus, but systems with larger riverine loads retained a smaller proportion of that load on floodplains. Although the source and long-term fate of deposited sediment and associated nutrients are uncertain, these fluxes represent the interception of large amounts of material that otherwise could have been exported downstream. Coastal Plain floodplain ecosystems are important regulators of sediment, carbon, and nutrient transport in watersheds of the Chesapeake Bay.

Effect of River Sediment on Phosphorus Chemistry of Similarly Aged Natural and Created Wetlands in the Atchafalaya Delta, Louisiana, USA

The goal of wetland creation is to produce an artificial wetland that functions as a natural wetland. Studies comparing created wetlands to similarly aged natural wetlands provide important information about creation techniques and their improvement so as to attain that goal. We hypothesized that differences in sediment phosphorus accretion, deposition, and chemistry between created and natural wetlands in the Atchafalaya Delta, Louisiana, USA were a function of creation technique and natural river processes. Sediment deposition was determined with feldspar marker horizons located in created and natural wetlands belonging to three age classes (<3, 5-10, and 15-20 yr old). Phosphorus fractions were measured in these deposited sediments and in suspended and bedload sediment from the Atchafalaya River. Bedload sediment had significantly lower iron- and aluminum-bound, reductant-soluble, and total phosphorus than suspended sediment due to its high sand percentage. This result indicates that wetlands artificially created in the Atchafalaya Delta using bedload sediment will initially differ from natural wetlands of the same age. Even so, similarities between the mudflat stratum of the <1- to 3-yr-old created wetland and the mudflat stratum of the 15- to 20-yr-old natural wetland support the contention that created wetlands in the Atchafalaya Delta can develop natural characteristics through the deposition of river suspended sediment. Differences between three created wetland strata, the 15- to 20-yr-old willow stratum and the <1- to 3-yr-old willow and mixed marsh strata, and their natural counterparts were linked to design elements of the created wetlands that prevented the direct deposition of the river's suspended sediment.

Wetland surface elevation, vertical accretion, and subsidence at three Louisiana Estuaries receiving diverted Mississippi River water

Wetland surface elevation and vertical accretion were measured from 1996 to 1999-2000 using a sediment elevation table (SET) and feldspar marker horizons in nine paired wetlands receiving Mississippi River water from the Caernarvon, West Pointe a la Hache (WPH), and Violet river diversions. The Caernarvon study sites had wetland surface elevation change rates ranging from 0.1660.31 to 0.4260.21 cm y 21 . Vertical accretion ranged from 0.7560.04 to 1.5760.05 cm y 21 ,a nd shallow subsidence ranged from 0.59 to 1.21 cm y 21 . Wetland surface elevation at the WPH study sites initially increased 2.3 to 3.3 cm during the first seven months of the study and then steadily decreased over the following year. The overall rate of elevation change ranged from 0.2760.09 to 0.7060.11 cm y 21 . Vertical accretion and shallow subsidence ranged from 1.2460.08 to 1.8460.07 cm y 21 and 0.54 to 1.27 cm y 21 , respectively. The Violet sites lost elevation and had the highest subsidence rates in this study, most likely due to a combination of hydrologic alteration and low diversion discharge. Wetland elevation decreased throughout the study, with rates ranging from 21.1060.24 to 22.3460.41 cm y 21 . Vertical accretion and shallow subsidence rates at the Violet-Near and Far sites were 0.4460.10 and 0.4460.11 cm y 21 and 2.78 to 1.54 cm y 21 , respectively. The Violet-Mid site wetland was burned in Winter 1999, leading to more than 4.0 cm decrease in material measured over the marker horizon and contributing to the lowest accretion rate measured in this study of 0.3460.05 cm y 21 . Analysis of regional relative sea-level rise (RSLR) indicates that all Caernarvon sites and the WPH-Near and Mid sites are keeping pace with RSLR. This study indicates that the use of river diversions can be an effective coastal restoration tool, with efficiency related to the proximity to riverine source and degree of hydrologic alteration, quantity of river water released, and land uses of the receiving wetland basin. Landscape modifications such as spoil banks associated with oil and gas access canals negate the benefits of river water introduction by limiting wetland-water interaction and should be removed in conjunction with river diversion implementation for effective wetland restoration.

Vegetation change and surface elevation dynamics in estuarine wetlands of southeast Australia

Vertical accretion and surface elevation trends were studied in mangrove and saltmarsh wetlands in southeast Australia. A total of 69 surface elevation tables, each associated with three feldspar marker horizons, was deployed in 10 wetlands across 7 estuaries, and monitored for three years. Saltmarsh and mangrove vegetation distributions were mapped for the same estuaries, and elevation characteristics of the wetlands were modelled. Rates of vertical accretion were found to correlate with tidal range. No relationship was found between rates of vertical accretion and surface elevation increase. A positive relationship was demonstrated between contemporary rates of saltmarsh surface elevation change and longer-term rates of mangrove encroachment into saltmarsh. We conclude that landward mangrove encroachment may be facilitated by local factors contributing to saltmarsh compaction during drought conditions.

Surface Elevation Dynamics in a Regenerating Mangrove Forest at Homebush Bay, Australia

Following the dieback of an interior portion of a mangrove forest at Homebush Bay, Australia, surface elevation tables and feldspar marker horizons were installed in the impacted, intermediate and control forest to measure vertical accretion, elevation change, and shallow subsidence. The objectives of the study were to determine current vertical accretion and elevation change rates as a guide to understanding mangrove dieback, ascertain the factors controlling surface elevation change, and investigate the sustainability of the mangrove forest under estimated sea-level rise conditions. The study demonstrates that the influences on surface dynamics are more complex than soil accretion and soil autocompaction alone. During strong vegetative regrowth in the impacted forest, surface elevation increase exceeded vertical accretion apparently as a result of belowground biomass production. In addition, surface elevation in all forest zones was correlated with total monthly rainfall during a severe El Nino event, highlighting the importance of rainfall to groundwater recharge and surface elevation. Surface elevation increase for all zones exceeded the 85-year sea level trend for Sydney Harbour. Since mean sea-level also decreased during the El Nino event, the decrease in surface elevation did not translate to an increase in inundation frequency or influence the sustainability of the mangrove forest. These findings indicate that subsurface soil processes such as organic matter accumulation and groundwater flux can significantly influence mangrove surface elevation, and contribute to the long-term sustainability of mangrove systems under a scenario of rising sea levels.

Mangrove encroachment of salt marsh in Western Port Bay, Victoria: The role of sedimentation, subsidence, and sea level rise

Surface elevation tables, feldspar marker horizons, and210Pb analysis of core profiles were implemented at four sites in Western Port Bay, Victoria, Australia, to provide information on the role of sedimentation, subsidence or compaction, and enhanced sea-level rise in contributing to salt marsh decline. Photogrammetric surveys indicate that the rate of salt marsh decline that is attributable to mangrove encroachment is lower in Western Port Bay than in comparable sites in New South Wales. Differences in the rate of mangrove encroachment at Western Port Bay may be attributed to the inverse relationship found between the degree of mangrove encroachment and surface elevation increase. While sedimentation contributes to surface elevation changes, surface elevation is not solely explained by sedimentation; factors including autocompaction and changes in the water table also play a significant role in Western Port Bay. Historic sedimentation rates measured using210Pb dating techniques corresponded to contemporary sedimentation rates determined from feldspar marker horizons. Core sediment profiles show no change in sedimentation rates at three sites. A fourth site (French Island) was the only site that exhibited high rates of sedimentation, which appears to be related to local land-use changes in the area. All sites maintained their elevation with respect to sea level over the study period. Historic sedimentation exceeded sea-level rise for the past 32 yr, but it is difficult to determine the extent to which belowground processes affect surface elevation, causing deviations between surface elevation and sedimentation over longer periods.

Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests

Root systems in mangrove swamps have captured the attention of scientists for decades. Among the postulated roles of root structures include a contribution to the geomorphological stability of mangrove soils through sediment trapping and binding. In this study, we used feldspar marker horizons and sediment pins to investigate the influence of three different functional root types—prop roots in Rhizophora spp., root knees in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia alba—on vertical accretion and elevation change in three mangrove forests in the Federated States of Micronesia. Prop roots facilitated vertical accretion (11.0 mm year −1) more than pneumatophores or bare soil controls (mean, 8.3 mm year −1). Sediment elevation, on the other hand, increased at an average rate of only 1.3 mm year 1 across all root types, with rate differences by root type, ranging from −0.2 to 3.4 mm year −1, being detected within river basins. This investigation demonstrates that prop roots can assist in the settling of suspended sediments from estuarine waters, yet prop root structures are not as successful as pneumatophores in maintaining sediment elevation over 2.5 years. As root densities increase over time, an increase in turbulence-induced erosion and in shallow subsidence as organic peat layers form is expected in Micronesian mangrove forests.

Estimating the potential for submergence for two wetlands in the Mississippi River Delta

We used a combined field and modeling approach to estimate the potential for submergence for one rapidly deteriorating (Bayou Chitigue Marsh) and one apparently stable (Old Oyster Bayou Marsh) saltmarsh wetland in coastal Louisiana, given two eustatic sea level rise scenarios: the current rate (0.15 cm year−1); and the central value predicted by the Intergovernmental Panel on Climate Change (48 cm by the year 2100). We also used the model to determine what processes were most critical for maintaining and influencing salt marsh elevation including, mineral matter deposition, organic matter production, shallow subsidence (organic matter decomposition + primary sediment compaction), deep subsidence, and sediment pulsing events (e.g., hurricanes). Eight years of field measurements from feldspar marker horizons and surface elevation tables revealed that the rates of vertical accretion at the Bayou Chitigue Marsh were high (2.26 (0.09) cm yr−1 (mean ± SE)) because the marsh exists at the lower end of the tidal range. The rate of shallow subsidence was also high (2.04 (0.1) cm yr−1), resulting in little net elevation gain (0.22 (0.06) cm yr−1). In contrast, vertical accretion at the Old Oyster Bayou Marsh, which is 10 cm higher in elevation, was 0.48 (0.09) cm yr−1. However, there was a net elevation gain of 0.36 (0.08) cm yr−1 because there was no significant shallow subsidence. When these rates of elevation gain were compared to rates of relative sea level rise (deep subsidence plus eustatic sea level rise), both sites showed a net elevation deficit although the Bayou Chitigue site was subsiding at approximately twice the rate of the Old Oyster Bayou site (1.1 cm yr−1 versus 0.49 cm yr−1 respectively). These field data were used to modify, initialize, and calibrate a previously published wetland soil development model that simulates primary production and mineral matter deposition as, feedback functions of elevation. Sensitivity analyses revealed that wetland elevation was most sensitive to changes in the rates of deep subsidence, a model forcing function that is difficult to measure in the field and for which estimates in the literature vary widely. The model also revealed that, given both the current rate of sea level rise and the central value estimate, surface elevation at both sites would fall below mean sea level over the next 100 years. Although these results were in agreement with the field study, they contradicted long term observations that the Old Oyster Bayou site has been in equilibrium with sea level for at least the past 50 years. Further simulations showed that the elevation at the Old Oyster Bayou site could keep pace with current rates of sea level rise if either a lower rate for deep subsidence was used as a forcing function, or if a periodic sediment pulsing function (e.g., from hurricanes) was programmed into the model.

Accretion of a New England (U.S.A.) Salt Marsh in Response to Inlet Migration, Storms, and Sea-level Rise

Sediment accumulation rates were determined at several sites throughout Nauset Marsh (Massachusetts, U.S.A.), a back-barrier lagoonal system, using feldspar marker horizons to evaluate short-term rates (1 to 2 year scales) and radiometric techniques to estimate rates over longer time scales (137Cs,210Pb,14C). The barrier spit fronting theSpartina-dominated study site has a complex geomorphic history of inlet migration and overwash events. This study evaluates sediment accumulation rates in relation to inlet migration, storm events and sea-level rise. The marker horizon technique displayed strong temporal and spatial variability in response to storm events and proximity to the inlet. Sediment accumulation rates of up to 24mm year−1were recorded in the immediate vicinity of the inlet during a period that included several major coastal storms, while feldspar sites remote from the inlet had substantially lower rates (trace accumulation to 2·2mm year−1). During storm-free periods, accumulation rates did not exceed 6·7mm year−1, but remained quite variable among sites. Based on137Cs (3·8 to 4·5mm year−1) and210Pb (2·6 to 4·2mm year−1) radiometric techniques, integrating sediment accumulation over decadal time scales, the marsh appeared to be keeping pace with the relative rate of sea-level rise from 1921 to 1993 of 2·4mm year−1. At one site, the210Pb-based sedimentation rate and rate of relative sea-level rise were nearly similar and peat rhizome analysis revealed thatDistichlis spicatarecently replaced this onceS. patenssite, suggesting that this portion of Nauset Marsh may be getting wetter, thus representing an initial response to wetland submergence. Horizon markers are useful in evaluating the role of short-term events, such as storms or inlet migration, influencing marsh sedimentation processes. However, sampling methods that integrate marsh sedimentation over decadal time scales are preferable when evaluating a systems response to sea-level rise.

Estimating shallow subsidence in microtidal salt marshes of the southeastern United States: Kaye and Barghoorn revisited

Simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow (3–5 m) subsurface datum were made in selected coastal salt marshes of Louisiana, Florida, and North Carolina to quantitatively test Kaye and Barghoorn's contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate. Rates of subsidence of the upper 3–5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation-erosion table. Surface elevation change was significantly lower than vertical accretion at each site after 2 years, indicating a significant amount of shallow subsidence had occurred, ranging from 0.45 to 4.90 cm. The highest rate of shallow subsidence occurred in the Mississippi delta. Results confirm Kaye and Barghoorn's contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate, including not only compaction but also apparently shrink-swell from water storage and/or plant production-decomposition at some sites. Indeed, surface elevation change was completely decoupled from vertical accretion at the Florida site. The assumption of a 1:1 relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes. Consequently, the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits. These findings also indicate the need for greater understanding of the influence of subsurface and small-scale hydrologic processes on marsh surface elevation.