Organic carbon recycling in Baltic Sea sediments – An integrated estimate on the system scale based on in situ measurements

In situ measured benthic fluxes of dissolved inorganic carbon (DIC), a proxy for organic carbon (OC) oxidation or recycling rates, are used together with burial rates based on measured sediment accumulation rates (SAR) and vertical distribution of OC in the sediment solid phase to construct a benthic OC budget for the Baltic Sea system.

The large variability in recycling rates (4.3 ± 0.87–33 ± 17 mmol C m−2 d−1) and burial rates (1.2 ± 0.8–5.9 ± 1.8 mmol C m−2 d−1) between different sub-basins and between different depositional areas within the basins is accounted for in the budget. Our results indicate that sediments in the Baltic Sea have much higher recycling rates and lower burial rates of OC than previously found.

The sediment budget calculations show that 22 ± 7.8 Tg C yr−1 of OC is recycled to the water column due to organic matter oxidation, while long term burial amounts to 1.0 ± 0.3 Tg C yr−1. For the Baltic Sea as a whole, 96% of the particulate OC (POC) deposited on the sea floor (23 ± 7.8 Tg C yr−1; the sum of recycling and burial) is recycled back to the water column.

However, the burial efficiency (i.e. the fraction buried of the total deposition) shows large variability between the different basins (2.5–16%). The total benthic POC deposition is approximately 20% higher than the estimated POC source originating from primary production in the water column and riverine input.

This difference is likely within the uncertainty range of our budget calculations, however it indicates that the POC sources might be underestimated. The results from this study enhance the understanding of OC delivery, deposition and cycling in the Baltic Sea, and help improving existing Baltic OC budgets.