Persistent organic carbon storage in river floodplains over millennia

Soils and sediments store a tremendous amount of organic carbon (OC), especially in the Arctic, but its long-term degradation rate remains poorly constrained. Most work estimate terrestrial OC loss from short-term incubation experiments spanning hours to years, even though soils and sediments develop over centuries to millennia. We quantify the changes in OC reactivity and composition across a chronosequence of floodplain deposits in discontinuous permafrost along the Koyukuk River, central Alaska. We observe minimal OC loss over ca. 6000 years, in sharp contrast to the cycling of surface biomass, which decays on decadal timescales in these environments. These results demonstrate that high-latitude permafrost floodplains are efficient carbon reservoirs that trap and preserve organic matter in transient sedimentary archives. The fate of these highly efficacious OC stores depends on the future dynamics of river migration, erosion, and sediment transport-as much as on changes to plant productivity in a warmer climate.