The Late Devonian mass extinction was unusually protracted and ecologically selective, with preferential diversity losses among reef-building organisms and tropical, shallow-water faunas in general. We have investigated the link between the extinction's unique characteristics and changes in biogeochemical cycling through analyses of the δ13C and C:N:P atomic ratios of organic matter buried across the Kellwasser Horizons in western New York State. Each horizon is characterized by (1) a long-term, +4‰–5‰ excursion in δ13C, ∼3‰ of which occurs within the horizon, and (2) a dramatic increase in the burial ratios of C:N:P, from values of ∼100:15:1 to an average of ∼5000:170:1. On the basis of these results, we propose that (1) increased efficiency of biolimiting nutrient recycling, resulting from cyclic water column stratification and mixing, promoted eutrophication during Kellwasser deposition in New York, and (2) the isotope excursions represent the composite effect of long-term, global organic C burial, and local changes in photosynthetic C isotope fractionation related to nutrient availability. This eutrophication model forges a mechanistic link between proposed Late Devonian climatic cooling and the selective demise of taxa likely to have been narrowly adapted to oligotrophic conditions.