Deposition and benthic mineralization of organic carbon: A seasonal study from Faroe Islands

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 b1%. 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 bot-
tom water during summer and intensified sediment resuspension during winter. The POC export from the eu-
photic 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 mate-
rial 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 mineraliza-
tion 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 buri-
al 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 re-
lated 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.
Regional terms: Faroe Islands, Kaldbaksfjørður