Linking the Terrestrial and Marine Biogeochemical Cycles of Nitrogen and Phosphorus in the Late Frasnian Appalachian Basin

The Late Devonian witnessed the emergence of a new relationship between the terrestrial and marine biogeochemical cycles of the typically limiting nutrients nitrogen and phosphorus. The development of extensive lowland forest ecosystems augmented the flux of fixed N to coastal marine ecosystems while the development of deep soils retarded the flux of P, increasing the relative abundance of N to P in coastal seas. The accumulation of terrestrially-derived NO₃- would have been particularly pronounced in epicontinental seas such as the Appalachian Basin which enjoyed limited communication with the open ocean. Nitrogen and carbon stable isotope analyses of sediments at or near the Frasnian/Famennian boundary within the Appalachian Basin indicate an underutilization of NO₃- coupled with high productivity corresponding to the globally correlated Upper and Lower Kellwasser horizons. High C_org:P values from similarly-aged sediments reveal that the supply of P necessary to support high productivity was remobilized from organic matter within oxygen-depleted sediments. Previous models that have implicated “nutrients” in black shale deposition and diversity loss in the Late Devonian should be reevaluated taking into consideration the significant differences in the biogeochemical cycling of N and P on the land and in the sea.