INVESTIGATING THE PROVENANCE AND TRANSPORT OF CARBON IN TIDAL INLETS: IMPLICATIONS FOR EROSION AND REDISTRIBUTION OF CARBON WITHIN THE VIRGINIA BARRIER ISLANDS, U.S.A.

The sediments of blue carbon ecosystems, including saltmarshes and lagoons, are critical sinks for organic carbon. However, these ecosystems are vulnerable to disturbance by sea-level rise and storm events, which can redistribute sediments and their associated carbon stocks. For example, barrier-island migration has recently been found to lead to beach- and shoreface exposure and erosion of high volumes of carbon formerly buried in backbarrier ecosystems. The fate of this carbon remains uncertain, as it may be redistributed through various pathways, including flux through tidal inlets, where it can be redeposited in the backbarrier. The objective of this study is to investigate the role of tidal-inlet processes in the recycling of shoreface-eroded carbon from a transgressive barrier island system. Water and sediment samples were collected from two tidal inlets along the Virginia Barrier Islands, U.S.A., during two tidal cycles to analyze sediment and carbon flux during relatively calm-water conditions. Additional endmember samples were collected from backbarrier marshes and lagoons, as well as from shoreface-outcropping peat and paleo-lagoon facies, to help discriminate among potential sediment and carbon reservoirs contributing to inlet fluxes. Sediment samples were analyzed for the bulk content and stable isotopic compositions of organic carbon and total nitrogen. Mixing models and principal component analyses were applied to assess geochemical signatures of carbon samples between ebb and flood tides to determine provenance. We find that samples collected during the ebb tide exhibit higher average δ¹³C and C:N values (-20.9, 9.27) compared to flood tide samples (-21.8, 8.23), indicating distinct differences in carbon provenance. In particular, average total organic carbon values (2.42%) of inlet samples more closely align with those from both backbarrier marsh (1.76%) and outcropping marsh (1.92%) endmembers as opposed to lagoon values (0.21% backbarrier, 0.76% outcropping), suggesting a dominant influence of marsh facies erosion on carbon dynamics. The findings of this study aim to clarify how tidal inlets contribute to potential loss of carbon from barrier systems and provide insights into the long-term dynamics of carbon storage in response to environmental changes.