Exceptionally high respiration rates in the reactive surface layer of sediments underlying oxygen-deficient bottom waters

Organic carbon (OC) burial efficiency, which relates the OC burial rate to respiration in the seafloor, is a critical parameter in the reconstruction of past marine primary productivities. The current accepted theory is that sediments underlying oxygen-deficient (anoxic) bottom waters have low respiration rates and high OC burial efficiencies. By combining novel in situ measurements in anoxic basins with reaction-transport modelling, we demonstrate that sediments underlying anoxic bottom waters have much higher respiration rates than commonly assumed. A major proportion of the carbon respiration is concentrated in the top millimeter-the so-called 'reactive surface layer'-which is likely a feature in approximately 15% of the coastal seafloor. When re-evaluating previously published data in light of our results, we conclude that the impact of bottom-water anoxia on OC burial efficiencies in marine sediments is small. Consequently, reconstructions of past marine primary productivity in a predominantly anoxic ocean based on OC burial rates might be underestimated by up to an order of magnitude.

Automated summary

Organic carbon (OC) burial efficiency is a critical parameter for reconstructing past marine primary productivities. Previous belief that sediments underlying oxygen-deficient (anoxic) bottom waters have low respiration rates and high OC burial efficiencies is challenged by novel in situ measurements and reaction-transport modelling. It is found that sediments underlying anoxic bottom waters actually have much higher respiration rates, which indicates that reconstructions of past marine primary productivity might be greatly underestimated in a predominantly anoxic ocean based on OC burial rates.