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Abstract Coastal saltmarshes keep pace with sea-level rise through in-situ production of organic material and incorporation of allochthonous inorganic sediment. Here we report rates of vertical accretion of 16 new sediment cores collected proximal to platform edges within saltmarshes located behind four barrier islands along the southeast United States coast. All but two of these exceed the contemporaneous rate of relative sea-level rise, often by a factor of 1.5 or more. Comparison with 80 additional measurements compiled across the Georgia Bight reveals that marshes situated closer to inlets and large bays generally accrete faster than those adjacent to small creeks or within platform interiors. These results demonstrate a spatial dichotomy in the resilience of backbarrier saltmarshes: marsh interiors are near a tipping point, but allochthonous mineral sediment fluxes allow enhanced local resilience along well-exposed and platform-edge marshes. Together, this suggests that backbarrier marshes are trending towards rapid, doughnut-like fragmentation. 

Abstract Landward migration of coastal ecosystems in response to sea-level rise is altering coastal carbon dynamics. Although such landscapes rapidly accumulate soil carbon, barrier-island migration jeopardizes long-term storage through burial and exposure of organic-rich backbarrier deposits along the lower beach and shoreface. Here, we quantify the carbon flux associated with the seaside erosion of backbarrier lagoon and peat deposits along the Virginia Atlantic Coast. Barrier transgression leads to the release of approximately 26.1 Gg of organic carbon annually. Recent (1994–2017 C.E.) erosion rates exceed annual soil carbon accumulation rates (1984–2020) in adjacent backbarrier ecosystems by approximately 30%. Additionally, shoreface erosion of thick lagoon sediments accounts for >80% of total carbon losses, despite containing lower carbon densities than overlying salt marsh peat. Together, these results emphasize the impermanence of carbon stored in coastal environments and suggest that existing landscape-scale carbon budgets may overstate the magnitude of the coastal carbon sink. 

Abstract Expansion of drainage networks through the headward erosion of tidal creeks is an eco‐geomorphologic response of salt marshes to accelerated sea‐level rise (SLR). This response can counter the negative impacts of an elevation deficit by increasing drainage and encouraging plant health, thereby reducing potential for submergence and marsh platform loss. In the wetlands of Cape Romain, SC, intense bioturbation near creek heads by the common marsh crabSesarma reticulatumhas been found to facilitate sediment erosion and rapid creek growth. This keystone grazer has been recently observed to have increasing influence on landscape evolution throughout the southeast US coast. Here, we compare measurements taken at Sapelo Island, GA, with those previously collected at Cape Romain, to confirm that eco‐geomorphic feedbacks facilitating creek growth at each location are similar, and to compare these processes under differing background conditions. We use sediment cores, precise elevation measurements and historical imagery to compare substrate properties, elevation within the tidal frame, creek growth rates and drainage morphology at both sites. Our results show identical processes; however, the higher elevation of the marsh at Sapelo Island leads to shallower and shorter periods of tidal inundation, explaining the greater soil strength and lower belowground biomass compared with the marsh at Cape Romain. The smaller tidal range at the site in Cape Romain compared with Sapelo Island translates to a proportionally shallower depth of tidal creeks, which therefore requires less erosion to produce headward creek extension. These effects are likely to have contributed to slower growth rates of tidal creeks at Sapelo Island during the past several decades of SLR. Our findings highlight the similarities in process but differences in rates in how marshes are responding to climate‐related stress. 

Hurricanes can benefit wetland accretion by augmenting the delivery of mineral sediment, an essential process allowing marshes to offset submergence during rising sea levels. Using Hurricane Gustav (2008, Louisiana) as a control, we examined eight synthetic storms with varying characteristics (track, speed, intensity, size) to evaluate sediment exchange between the inner shelf and bay and bay‐to‐marsh interfaces. All storms showed net landward sediment exchange from the inner shelf to the bay to the marsh—storms with closer proximity, higher intensity, and slower forward speed positively correlated with net sediment exchange; storm size had little impact. Except for slow‐moving storms (½ speed of Gustav), our analyses suggest that most hurricane scenarios cause net bay erosion, because more sediment is conveyed to landward wetlands than is replenished from erosion of the inner shelf. Our results suggest that the ongoing deepening of the bay will likely worsen because of rising sea levels 

Data set of measured sediment characteristics and vertical accretion rates of backbarrier marshes along the Georgia Bight. Data include bulk density (g/cm^3), water content (%), organic content (%), Pb-210 (Bq/kg), Cs-137 (Bq/kg), and calculated rates of vertical accretion (mm/yr) as measured from sediment cores collected during December 2017. Data are provided as CSV files. 

Marine transgression associated with rising sea levels causes coastal erosion, landscape transitions, and displacement of human populations globally. This process takes two general forms. Along open-ocean coasts, active transgression occurs when sediment-delivery rates are unable to keep pace with accommodation creation, leading to wave-driven erosion and/or landward translation of coastal landforms. It is highly visible, rapid, and limited to narrow portions of the coast. In contrast, passive transgression is subtler and slower, and impacts broader areas. It occurs along low-energy, inland marine margins; follows existing upland contours; and is characterized predominantly by the landward translation of coastal ecosystems. The nature and relative rates of transgression along these competing margins lead to expansion and/or contraction of the coastal zone and—particularly under the influence of anthropogenic interventions—will dictate future coastal-ecosystem response to sea-level rise, as well as attendant, often inequitable, impacts on human populations. Expected final online publication date for the Annual Review of Marine Science, Volume 16 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Abstract Salt marshes are threatened by rising sea levels and human activities, and a major mechanism of marsh loss is edge retreat or erosion. To understand and predict loss in these valuable ecosystems, studies have related erosion to marsh hydrodynamics and wave characteristics such as wave power. Across global studies, erosion is reported to be largely linearly related to wave power, with this relationship having implications for the resilience of marshes to extreme events such as storms. However, there is significant variability in this relationship across marshes because of marsh heterogeneity and the uniqueness of each physical setting. Here, we investigate the results of individual studies throughout the world that report a linear relationship and add a new dataset from the Great Marsh in Massachusetts (USA). We find that most marsh wave power and erosion data are not normally distributed and when these datasets are properly plotted to account for their distributions, the resulting relationships vary from previously published curves. Our Great Marsh data suggest that events from specific wind directions can have an outsized impact on edge erosion due to their larger fetch and wind speeds. We also find that factors other than wave attack such as edge erosion along tidal channels, can have a measurable impact on retreat rates. We show the importance of maintaining statistical assumptions when performing regressions, as well as emphasize the site-specificity of these relationships. Without calibration of a marsh erosion-wave power relationship using robust regressions for each individual marsh, such a relationship is not fully constrained, resulting in unreliable predictions of future marsh resilience and response to climate change. 

When longshore transport systems encounter tidal inlets, complex mechanisms are involved in bypassing sand to downdrift barriers. Here, this process is examined at Plum Island Sound and Essex Inlets, Massachusetts, USA. One major finding from this study is that sand is transferred along the coast—especially at tidal inlets—by parcels, in discrete steps, and over decadal-scale periods. The southerly orientation of the main-ebb channel at Plum Island Sound, coupled with the landward migration of bars from the ebb delta to the central portion of the downdrift Castle Neck barrier island, have formed a beach protuberance. During the constructional phase, sand is sequestered at the protuberance and the spit-end of the barrier becomes sediment starved, leading to shoreline retreat and a broadening of the spit platform at the mouth to Essex Bay (downdrift side of Castle Neck). Storm-induced sand transport from erosion of the spit and across the spit platform is washed into Essex Bay, filling channels and enlarging flood deltas. This study illustrates the pathways and processes of sand transfer along the shoreline of a barrier-island/tidal-inlet system and provides an important example of the processes that future hydrodynamic and sediment-transport modeling should strive to replicate.