Changes in organic carbon source and storage with sea level rise-induced transgression in a Chesapeake Bay marsh

Organic matter (OM) accumulation in marsh soils affects marsh survival under rapid sea-level rise (SLR). This work describes the changing organic geochemistry of a salt marsh located in the Blackwater National Wildlife Refuge on the eastern shore of Chesapeake Bay that has transgressed inland with SLR over the past 35–75 years. Marsh soils and vegetation were sampled along an elevation gradient from the intertidal zone to the adjacent forest, representing a space-for-time substitution of the process of marsh transgression. Stable carbon isotope analysis of bulk OM gives evidence for a transition from C3 upland-sourced OM to C4-dominated marsh vegetation over time. The vegetative source of the OM changes along a marsh-upland mixing line from herbaceous angiosperm-sourced lignin in the lower elevation marsh to a woody gymnosperm signature at the upper border of the marsh. The results of stable isotope and lignin analyses illustrate that landward encroachment of marsh grasses results in deposition of herbaceous tissues exhibiting relatively little decay. This presents a possible mechanism for OM stabilization as marshes migrate inland.