Seasonal variations in sedimentation and benthic mineralization of organic carbon (OC) were investigated in a Faroese fjord. Deposited particulate organic carbon (POC) was mainly of marine origin, with terrestrial material only accounting for < 1%. On an annual basis the POC export from the euphotic zone amounted to 10.2 mol C m^− 2 yr^− 1 equating to 37% of the net primary production, and maximum sedimentation rates were associated to the spring bloom. The dynamics in the benthic solute exchange were governed by stratification that isolated the bottom water during summer and intensified sediment resuspension during winter. The POC export from the euphotic zone could not sustain the benthic mineralization rate (10.8 mol C m^− 2 yr^− 1) and the calculated burial rate (9.8 mol C m^− 2 yr^− 1) of organic material in the central basin. This indicated considerable focusing of material in the central part of the fjord. This was supported by the fact that the measured benthic mineralization rate – in contrast to most investigations – actually increased with increasing water depth. In August, when mineralization was at its maximum, the dissolved inorganic carbon (DIC) release from the sediment increased by 2.2 mmol m^− 2 d^− 1 for every m increase in water depth at 30–60 m depth. Due to sediment focusing, the OC burial in the deepest part of the fjord was 9.8 mol C m^− 2 yr^− 1. This was 2.4 times higher than the average OC burial in the fjord, estimated from the total sedimentation, and benthic mineralization accounting for the water depth related changes in activity. The study in Kaldbaksfjørður underscore that fjords are important sites for long time OC burial, but emphasize the need for accounting for spatial variations when extrapolating results from a single or few stations to the scale of the entire fjord.

Journal of Marine Systems. 177, s. 53-61 9 s. DOI:10.1016/j.jmarsys.2016.09.005