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Contradictory interpretations of upper Pleistocene (120–40 ka) sedimentary deposits along the US Mid-Atlantic Coast have hindered the development of a reliable regional sea-level curve for the last glacial cycle. This study presents new and compiled sediment cores, ground-penetrating radar, topographic data, aerial imagery, and limited geochronology from geologic units emplaced along the ocean-facing side of the Virginia Eastern Shore during mid- and late-Pleistocene periods of higher-than-present relative sea level: the Accomack Member (Omar Formation), the Butlers Bluff Member (Nassawadox Formation), the Joynes Neck Sand, and the Wachapreague Formation. Minor lithologic and morphologic updates are presented for the MIS 5e/5c Butlers Bluff Member, which is interpreted as a southward-prograding spit emplaced atop penecontemporaneous shoreface sediments or older transgressive sediments which fill the Exmore Paleochannel. The Joynes Neck Sand is reinterpreted as a coastal lag deposit, correlated with the Ironshire Formation in Maryland and Delaware, likely emplaced during MIS 5c. The Wachapreague Formation is determined to be a composite unit composed of two newly mapped members—the Locustville and Upshur Neck—which differ in lithology, internal architecture, and surficial morphology. The older and western Locustville Member (MIS 5a) is characterized by progradational beach and foredune ridges built atop transgressive shoreface and backbarrier deposits, and is correlated with the Sinepuxent Formation in Maryland and Delaware. The younger and eastern Upshur Neck Member of the Wachapreague Formation (late MIS 5a) is distinguished by surficial recurved ridges and preserved washover, dune, and channel-fill structures associated with spit growth atop shoreface deposits. These findings indicate that the Wachapreague Formation was constructed during two sequential highstands: an initial phase of sea-level rise and then fall allowed for deposition of the Locustville Member as a transgressive-highstand-regressive barrier system; and, following a period of lower-than-present sea level, a later highstand resulted in partial erosion of the easternmost Locustville and growth of the Upshur Neck Member. Finally, we update earlier descriptions of an aeolian sand sheet, likely deposited during MIS 3c, that discontinuously overlies most of the east-central Virginia Eastern Shore. Together, these findings update interpretations of the depositional history of the southern Delmarva Peninsula, and allow for future refinement of the sea-level history of the last interglacial-to-glacial period along the mid-field US Mid-Atlantic coast. 

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 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 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. 

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.