Carbon oxidation and bioirrigation in sediments along a Skagerrak−Kattegat−Belt Sea depth transect

Partitioning of electron acceptors and macrofaunal bioirrigation were assessed in sediments from 4 stations along a Skagerrak−Kattegat−Belt Sea depth transect. Sediment was examined for benthic fauna composition and abundance, sediment−water fluxes (O₂, dissolved inorganic carbon [DIC], NH₄ ⁺, and NO₃ ⁻), anaerobic reactions (carbon oxidation [C_ox], sulfate reduction [SR], manganese reduction [MnR], iron reduction [FeR], and ammonification [N_min]), porewater profiles (O₂, DIC, SO₄ ²⁻ and NH₄ ⁺), and solid phase profiles (organic content, Fe and Mn). Deep stations had less than half the abundance of benthic fauna than shallow stations, while the Belt Sea station was azoic due to bottom-water hypoxia. Solute fluxes and anaerobic reactions showed order-of-magnitude lower rates in sediment from deep than from shallow water. In general, anaerobic C_ox in sediments along the Skagerrak−Kattegat−Belt Sea transect is dominated by SR (>50%) in shallow water and decreases gradually when moving into deeper water and reaches 0 at ~700 m depth. FeR increases from 0 in shallow water to ~50% at ~600 m, but rapidly reaches 0 again at 700 m. MnR is close to 0 down to at least 500 m and increases to complete dominance at 700 m. The contribution of denitrification is generally below 10% at all depths. Bioirrigation-quantified as non-local exchange through diagenetic modelling-is proportional to fauna biomass and functional traits. The Br⁻ tracer approach to determine bioirrigation on newly extracted sediment onboard a ship is not recommended. It is concluded that enhanced downward translocation of O₂ into anoxic sediment through bioirrigation is the major mechanism reoxidizing subsurface metals in the deep Skagerrak and Kattegat.