Fjords are net carbon sinks with high organic carbon (OC) burial rates; however, the key drivers of OC burial in these systems are not well constrained. To study the role of water column redox condition and OC composition on OC preservation in fjord sediments, we determined OC accumulation rates (OCAR), OC source, and OC degradation in three Swedish fjords with variable redox conditions (long-term oxic, seasonally hypoxic, and long-term anoxic). Average OCARs were variable between and within the fjords studied (2-122 g OC m-2 yr-1), but highest rates were at the mouth for each fjord. Based on a delta 13C mixing model, Swedish fjords bury predominantly marine-derived OC (similar to 83% of the total OC burial) likely because of relatively gentle slopes, low riverine discharge, and high marine inflow. Using a multi-biomarker approach (lignin, photosynthetic pigments, and total hydrolyzable amino acids) we found, terrestrially- and marine-derived OC were moderately degraded under the various redox conditions sampled, suggesting water column redox and OC source are not primary drivers of OC burial in these fjords. Rather, high sediment accumulation rates, common to fjords globally, lead to low oxygen exposure times, thus promoting efficient burial of OC regardless of its chemical composition. Fjords are net carbon sinks due in part to very high organic carbon burial rates. Despite being important in the regulation of earth's climate, the drivers of organic carbon burial in fjords are not well constrained. Here, we characterize the organic carbon buried in Swedish fjord sediments under different oxygen regimes (long-term oxic, seasonally hypoxic, and long-term anoxic) using bulk elemental and biomarker analyses. These fjords effectively bury organic carbon regardless of organic carbon source and water column oxygen conditions, likely because of high sediment accumulation rates. Fjords represent distinctive coastal systems with the capacity to retain reactive organic carbon, underscoring the importance of exploring these ecosystems in the context of global change. Swedish fjords bury primarily marine-derived organic carbon under long-term oxic, seasonally hypoxic, and long-term anoxic water columns Organic carbon burial in Swedish fjords is largely controlled by sediment accumulation rate which can limit oxygen exposure time Marine-dominated fjords are uniquely efficient sinks of labile organic carbon
