Unexpectedly low organic carbon burial efficiency in anoxic sediments is linked to the absence of physical protection: lessons from the Western Gotland Basin (Batic Sea)

Burial of organic carbon (OC) in marine sediments controls atmospheric CO₂ and O₂ concentrations and is a key process in the global carbon cycle. Marine sediments bury ~160 Tg C yr-1 globally, of which ~90% in continental shelf sediments, which makes these sites a burial hotspot. It is generally assumed that OC is buried more efficiently in sediments underlying anoxic bottom waters. Recently, however, sediments of the Gotland Basin (Baltic Sea) have been shown to have unusually low OC burial efficiencies (~10%) considering their sediment accumulation rates, despite being overlaid by anoxic bottom waters.  

To investigate the reason for this lower-than-expected OC burial efficiency, we sampled five sites along a transect (including two sites for Fe-OM assessment) across the Western Gotland Basin. Sulphate reduction rate measurements showed that the OC reactivity was much higher than was expected for the age of the OC. Subsequent analysis of iron-organic matter associations and OC-to-surface area ratios showed that the potential for physical protection via either coprecipitation with iron minerals or mineral adsorption was very low (below the detection limit in one core and a maximum of 7% of OC linked to Fe in the other, which is low compared to the global average of ~20%). Our results suggest that in the Western Gotland Basin, the absence of physical protection seems to allow for unusually high mineralisation rates (low OC burial efficiency) despite anoxic conditions. Consequently, the link between oxygen exposure and OC mineralisation rate is likely not as universal as currently assumed.