Eutrophication by decoupling of the marine biogeochemical cycles of C, N, and P: A mechanism for the Late Devonian mass extinction

The Late Devonian mass extinction was unusually protracted and ecologically selective, with preferential diversity losses among reef-building organism s and tropical, shallow-water faunas in general, We have investigated the L ink between the extinction's unique characteristics and changes in biogeoch emical cycling through analyses of the delta(13)C and C:N:P atomic ratios o f organic matter buried across the Kellwasser Horizons in western New York State, Each horizon is characterized by (1) a long-term, +4 parts per thous and-5 parts per thousand excursion in delta(13)C, similar to 3 parts per th ousand of which occurs within the horizon, and (2) a dramatic increase in t he burial ratios of C:N:P, from values of similar to 100:15:1 to an average of similar to 5000:170:1. On the basis of these results, we propose that ( 1) increased efficiency of biolimiting nutrient recycling, resulting from c yclic water column stratification and mixing, promoted eutrophication durin g Kellwasser deposition in New York, and (2) the isotope excursions represe nt the composite effect of long-term, global organic C burial, and local ch anges in photosynthetic C isotope fractionation related to nutrient availab ility. 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.