THE IMPACT OF THE DEVONIAN BIOGEOCHEMICAL TRANSITION ON MARINE FAUNAL DIVERSITY

The transition to the modern biogeochemical relationship between land and sea during the Middle and Late Devonian is recorded in the geochemistry of nutrient dynamics and the pattern of marine faunal diversity loss. The expanding areal extent of Devonian lowland forests, facilitated by periodic transgressive events, generated an increasing outwelling flux of reactive nitrogen from rivers and estuaries as well as higher rates of wet and dry atmospheric reactive N deposition. Conversely, progressively deeper and more mature forest soils retarded the weathering flux of phosphorus. High marine productivity was facilitated by remobilization of P from anoxic sediments and an increase in the residence time of P in the photic zone. While episodes of global oceanic anoxia, such as the Upper Kellwasser interval, may have precipitated discrete episodes of widespread extinction, two-thirds of Devonian diversity loss was a function of diminished rates of origination. This pattern of diversity loss is best explained by the frequently observed unimodal relationship between diversity and productivity. The abundance of trophic resources in Middle and Late Devonian seas diminished spatial heterogeneity causing some animal populations to increase. Large populations suppressed origination rates because large populations are inherently more resistant to evolutionary innovation and consequent speciation.