LAUSR

Abstract. Fjord sediments are recognized as hotspots for the burial and storage of organic carbon, yet little is known about what drives the formation of these coastal carbon stores and how this has altered over time. Here we show that fjords can act as sustained hotspots for carbon burial and storage over Holocene timescales. Further we investigate the role of North Atlantic climate and humans in the evolution of a coastal carbon store using sediment records from a temperate Scottish fjord. Our findings indicate that climate and anthropogenic activity have independently driven increases in terrestrial carbon to the marine environment. When both these drivers were coupled, the terrestrial response was pronounced and the relative proportion of terrestrial OC in the marine sediments increases from 5 % up to 70 %. We hypothesize that sustained human disturbance through the late Holocene sensitized the catchment to abrupt climate reorganizations. The results highlight the importance of fjords for carbon burial and the significance of terrestrial carbon subsidy to the long-term carbon store.