Sources of organic matter to salt marshes: Insights from Oregon, USA

Salt marshes are important blue carbon sinks; however, identification of the drivers of carbon burial rates is challenging because estuaries sit at terrestrial and marine interfaces. Here, we address the questions: what are the sources of organic matter (OM) sequestered in Oregon salt marshes, and how do sources change through time? We characterized down-core sediment biogeochemistry (C:N, δ13C, δ15N) for the last century in seven Oregon high marshes and used a mixing model to elucidate differences in OM sources across estuaries and through time. Autochthonous biomass production consistently accounted for half of overall OM accumulation, and the remainder was allochthonous, originating from a combination of estuarine and terrestrial sources. Salt marshes with high sediment loads buried more terrestrial OM, whereas those with low loads and substantial subtidal habitat buried more estuarine-derived OM. When assessed through depth/time, stable isotope trends indicated that decomposition is not the major control. Instead, OM became increasingly more terrestrial in recent decades, especially in salt marshes with low fluvial loads. We hypothesize that salt marshes with low loads took longer to regain lost elevation following coseismic subsidence of the 1700 Cascadia earthquake than salt marshes with higher loads. This result magnifies the role of river floods in sediment and carbon accumulation on the marsh surface. Ultimately, the highest carbon burial rates coincided with highest fractions of terrestrially-sourced OM, though spatiotemporal complexities obscured any potentially significant trends. While the term “blue” typically excludes allochthonous carbon, salt marshes still perform the important ecosystem function of burying OM.