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

Abstract

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.

Burial of Organic Carbon in Swedish Fjord Sediments: Highlighting the Importance of Sediment Accumulation Rate in Relation to Fjord Redox Conditions

“…This explains the general pattern of OC burial efficiency decreasing with increasing oxygen exposure time (Hartnett et al., 1998). However, the role of redox on early diagenesis is historically a contentious topic and consequently the effect of oxygen on OC burial is poorly constrained (Arndt et al., 2013; Cowie & Hedges, 1992a, 1992b; Jessen et al., 2017; Van De Velde et al., 2023; Vandewiele et al., 2009). High OC burial rates in fjords are, in part, attributed to high OC terr loads and low oxygen bottom waters.…”