Organic Carbon Fluxes on Seasonal to Decennial Timescales in Patagonia's Largest River‐Fjord System

Abstract Fjord sediments are global sinks of organic carbon (OC), contributing to the long‐term storage of atmospheric CO 2 . Despite this recognition, the transfer and burial of OC in fjord sediments are still poorly quantified and suffer from a sampling bias toward distal environments where marine OC is dominant. Here we present organic geochemical data obtained on sediment samples (suspended river sediments, fjord sediment trap, surface fjord sediments, sediment core) collected in the Baker river‐fjord system, with a particular focus on the Baker River submarine delta, which is fed by Chile's largest river. We measured total OC contents and OC stable isotope composition to quantify the amount and type of OC (marine or terrestrial) stored in the fjord submarine delta. We find that OC fluxes are twice higher in summer (106 ± 6 g OC/m 2 /yr) than in winter (53 ± 3 g OC/m 2 /yr) due to higher sediment discharge from meltwater. Sediment trap OC fluxes are on the same order of magnitude than those in the nearby sediment core (103 ± 15 g OC/m 2 /yr) during the last 35 years, suggesting rapid OC burial in sediments. Carbon isotopes suggest that the OC stored in the fjord submarine delta is predominantly of terrestrial origin. We calculate that the Baker submarine delta buries 3.8 ± 0.6 kt OC/yr, which corresponds to 26 ± 11% of the estimated OC annual flux delivered by the Baker River (14.4 ± 5.5 kt OC/yr). Fjord deltas should thus be considered in fjord OC budgets as they could significantly raise global estimates of terrestrial OC burial in marine sediments.