DOUGHNUT-LIKE FRAGMENTATION OF BACKBARRIER SALTMARSHES ALONG THE US GEORGIA BIGHT IS AIDED BY RECYCLING OF MARSH-EDGE AND LAGOON SEDIMENTS

The ability of saltmarshes to survive accelerated sea-level rise remains highly debated, ranging from long-term survival to near-future platform drowning. Commonly, such studies rely on regional to global compilations of accretion rates, an approach which fails to consider small-scale spatial variability in sedimentation and in-situ organic-matter production.

Here we use a combination of gamma geochronology and inorganic (grain size and bulk density) and organic (stable and radioisotopes) proxies of sediment provenance to quantify rates of vertical accretion of, and the source of inorganic sediment contributions to, backbarrier marshes located near the margins of saltmarsh platforms behind four barrier islands along the Georgia Bight. Four sediment cores and 8–10 samples of 1–4 cm thick marsh-surface deposits associated with Hurricane Irma were collected from each of these sites. In addition, surface grab samples of likely source endmembers for those storm deposits (fluvial, marsh, lagoon, and marine) were collected from two of the barrier systems.

We find that marshes at nearly all our platform-edge sampling sites exceed the rate of relative sea-level rise, often by more than a factor of 1.5. Comparison of these new accretion rates with 80 additional measurements compiled across the Georgia Bight reveals that marshes situated closer to inlets and large bays—yet still inboard of any marsh levees—generally accrete faster than those adjacent to small creeks or within platform interiors. We attribute this to the higher flux of allochthonous sediment reaching high-exposure marsh platform edges. Stable-isotope mixing models reveal this sediment likely originates from the adjacent lagoon floor, sediments which themselves are a mixture of offshore sediments and sediment eroded from the adjacent marsh edge. Together, these results demonstrate the importance of sediment recycling—from eroding marsh edge, to lagoon floor, and to the marsh surface—thereby allowing saltmarshes to keep pace with sea-level rise. However, our findings also suggest that backbarrier marshes are trending towards rapid, doughnut-like fragmentation: marsh interiors are near a tipping point towards drowning, whereas storm-driven sediment recycling allows enhanced local resilience along well-exposed, platform-edge marshes.