Secular Changes to the Biological Carbon Pump and its Interrelation with Past Extinction Events

The efficiency of the biological carbon pump depends not only on the rate of carbon fixation and export out of the surface layer, but also (primarily) on the depth at which organic matter is remineralized. Changes to this depth impact the vertical redox structure of the ocean and have been hypothesized to account for long-term shifts in marine ecosystem structure. I use a 3-dimensional Earth system model of intermediate complexity to quantitatively examine the effects of secular changes to the biological pump for the end-Permian ocean. As Paleozoic marine animals were of smaller body sizes than today, which reduced the efficiency of OM transfer to the deeper ocean, I investigate the influence of shallower remineralization depths on the vertical redox structure of the ocean and toxic hydrogen sulfide (H$_2$S) emissions to the atmosphere. I find that the locus of OM remineralization in the water-column in combination with modest changes in nutrient load strongly effects the spread of ocean anoxia and euxinia. Small remineralization depth changes significantly alter oxygen and H$_2$S concentrations on the continental shelf and slope – benthic areas specifically important as they were occupied by diverse and ecologically complex faunal communities. Furthermore, the remineralization depth has large effects on H$_2$S emissions to the atmosphere, thus leading to potentially toxic surface conditions at least locally. Overall, these model simulations suggest that secular changes in strength and efficiency of the biological carbon pump define occurrences of anoxia/euxinia and thus may help explain important changes to marine ecosystem structure during past extinction events.