Spatiotemporal controls on organic matter sourcing to minerogenic salt marshes

Abstract Salt marshes are important 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 sequestered in minerogenic 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 USA high marshes and used a mixing model to elucidate differences in organic matter sources across estuaries and through time. Autochthonous biomass production consistently accounted for only half of overall organic matter accumulation, and the remainder was allochthonous, originating from a combination of estuarine and terrestrial sources. Salt marshes with high sediment loads buried more terrestrial organic matter, whereas those with low loads and substantial subtidal habitat buried more estuarine‐derived organic matter. When assessed through depth/time, stable isotope trends indicated that decomposition is not the primary control. Instead, organic matter became increasingly more terrestrial in recent decades, especially in salt marshes with low fluvial loads. We hypothesize that salt marshes with lower loads took longer to regain lost elevation following coseismic subsidence of the 1700 ce Cascadia earthquake. 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 organic matter, though spatiotemporal complexities obscured any potentially significant trends. While the term “blue” typically excludes terrestrial carbon, minerogenic salt marshes still perform the important ecosystem function of burying organic matter.