STRANGELOVE OCEAN REVISITED: SOLUTION TO THE PRODUCTIVITY-ISOTOPE PARADOX (Invited Presentation)

The Cretaceous-Paleogene mass extinction event was accompanied by an apparent collapse of the oceanic vertical carbon isotope gradient, the magnitude of which is canonically assumed to indicate the vigor of the marine biological pump, that is, the flux of particulate organic matter (POM) from the pelagic to the abyssal zone of the ocean (export production). To the extent that export production is supported by primary production in the surface ocean, the biological pump strength is considered a measure of the “health” of the ocean. In the modern ocean, though, the magnitude of this surface-to-deep carbon isotope gradient does not reflect the spatial pattern of export production. It instead reflects the efficiency (what fraction of the upwelling nutrients are assimilated by organisms and exported to the deep with settling particles), rather than the strength of the biological pump. Upwelling zones have strong but inefficient biological pumps and relatively small carbon isotope gradients. Thus it is possible that the collapse of the isotope gradient could indicate an increase in export production. Imposing a shallowing of the depth of remineralization, reflecting a reduction in fecal pellet packaging and/or carbonate ballasting of POM, on an intermediate complexity ocean model leads to the nearly complete collapse of the vertical carbon isotope gradient, but an increase in both primary and export production, including an increase in the flux of organic matter to the seafloor. The reason is that nutrients are regenerated close to the surface, such that the upwelling flux of nutrients to the photic zone is enhanced. Thus, the collapse of the carbon isotope gradient does not indicate a reduction in the food source to the benthos, as is commonly thought. This resolves a longstanding paradox of the response to K/Pg mass extinction, i.e., the lack of extinction of benthic foraminifera despite apparent loss of their food source.