Placing barrier‐island transgression in a blue‐carbon context

Abstract

Backbarrier saltmarshes are considered carbon sinks; however, barrier island transgression and the associated processes of erosion and overwash are typically not included in coastal carbon budgets. Here, we present a carbon‐budget model for transgressive barrier islands that includes a dynamic carbon‐storage term, driven by backbarrier‐marsh width, and a carbon‐export term, driven by ocean and backbarrier shoreline erosion. To examine the impacts of storms, human disturbances and the backbarrier setting of a transgressive barrier island on carbon budgets and reservoirs, the model was applied to sites at Core Banks and Onslow Beach, NC, USA. Results show that shoreline erosion and burial of backbarrier marsh from washover deposition and dredge‐spoil disposal temporarily transitioned each site into a net exporter (source) of carbon. The magnitude of the carbon reservoir was linked to the backbarrier setting of an island. Carbon reservoirs of study sites separated from the mainland by only backbarrier marsh (no lagoon) decreased for over a decade because carbon storage could not keep pace with erosion. With progressive narrowing of the backbarrier marsh, these barriers will begin to function more persistently as carbon sources until the reservoir is depleted at the point where the barrier welds with the mainland. Undeveloped barrier islands with wide lagoons are carbon sources briefly during erosive periods; however, at century time scales are net carbon importers (sinks) because new marsh habitat can form during barrier rollover. Human development on backbarrier saltmarsh serves to reduce the carbon storage capacity and can hasten the transition of an island from a sink to a source.

Modeling organic carbon loss from a rapidly eroding freshwater coastal wetland

“…Furthermore, the role of geomorphic changes, such as erosion and overwash, in the freshwater wetland carbon cycle is poorly understood. These geomorphic changes are critical for assessing the value and vulnerability of these landscapes because they can alter the area of the wetland as well as export previously stored carbon 3 , 4 .…”

“…Shoreline erosion and wetland burial by overwash can reduce the carbon stock, which is the total amount of organic carbon stored within the wetland soil, in coastal wetlands by exporting organic carbon-rich sediment from wetlands and reducing wetland area 3 – 5 . Shoreline erosion exports carbon from the wetland stock and overwash deposition narrows the wetland by burying it with sand, which reduces the capacity of a wetland to actively sequester carbon 4 , 6 . The influence of these geomorphic processes on wetland carbon must be considered in order to fully evaluate coastal wetland carbon budgets.…”

Relative sea‐level change regulates organic carbon accumulation in coastal habitats

“…C org accumulation rates from 1,100 to 800 Cal year BP were higher at F25 than at the inner sites. Because F25 was located close to the barrier spits of the lagoon mouth, the abundant sediment supply from outside would have contributed to the high rate of C org accumulation (Theuerkauf & Rodriguez, ). In contrast, C org accumulation was inhibited from 800 to 200 Cal year BP in Furen Lagoon.…”

“…In particular, the enhancement of C org accumulation rates by raising of RSL in subtidal zones (Figure 7) indicates that this process has a negative feedback on climate change. Previous studies have reported that supratidal and intertidal vegetated habitats and lagoons can potentially maintain their elevations in response to enhanced sea-level rise (McKee et al, 2007;Mudd et al, 2009;Theuerkauf & Rodriguez, 2017). However, if the sediment load or root growth is insufficient to maintain elevation above sea level, these habitats can be eroded and submerged (Cahoon et al, 2003;Kirwan & Mudd, 2012;McLeod et al, 2011;Mudd et al, 2009).…”

Representing the function and sensitivity of coastal interfaces in Earth system models

“…Internal deterioration of salt marshes, through salinity intrusion, herbivory, eutrophication, or other chronic factors has also been linked with sediment export 82 . Both lateral erosion and internal deterioration can be considered as net neutral processes from a budgetary perspective if landward migration corridors are available 26,83 . However, given the rapid nature of salt marsh loss and extensive coastal development, it is likely that salt marsh loss is a net contributor of material across the coastal interface.…”