Supplementary methods, figures and tables from 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 <i>in situ</i> 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.

有机碳（Organic carbon, OC）埋藏效率是将有机碳埋藏速率与海底呼吸作用相关联的关键参数，亦是重建远古海洋初级生产力的核心指标。目前学界公认的理论认为，缺氧（anoxic）底层水体下方的沉积物呼吸速率较低，有机碳埋藏效率较高。本研究通过将缺氧盆地（anoxic basins）中的新型原位（in situ）观测与反应-输运建模相结合，证实缺氧底层水体下方的沉积物呼吸速率远高于此前的普遍认知。碳呼吸的主要部分集中在表层毫米级区域——即所谓的“反应性表层（reactive surface layer）”——这一特征大概率存在于约15%的沿海海底区域。结合本研究结果重新评估已发表的相关数据后，我们得出结论：底层水体缺氧对海洋沉积物有机碳埋藏效率的影响微乎其微。因此，若基于有机碳埋藏速率重建以缺氧环境为主的远古海洋初级生产力，所得结果可能被低估达一个数量级。