Decoding Devonian Mass Extinctions: New Evidence Linking Land Plant Expansion to Marine Anoxia

The Devonian Period was fraught with tremendous biological, ecological and atmospheric change. The evolutionary advancement of fully developed root structures by land plants in the later Devonian revolutionized the Earth's surface, constituting the emergence of the first Critical Zone. This biological innovation within the terrestrial biosphere initiated a pseudo-modern process of soil formation dynamics, and in turn significantly impacted nutrient and carbon cycling within the global oceans. It has been theorized that this extraordinary transformation upon Earth's surface vastly increased nutrient flux to paleo oceans, resulting in widespread anoxia and potentially driving multiple marine mass extinctions. To date however, definitive evidence linking these events has remained elusive. In order to answer this long standing scientific question, land-based measures of P weathering and mobilization during this time were developed to provide a dynamic and quantitative basis for assessing the global role of plant and soil evolution. Here we present results from multiple Devonian lacustrine sequences from both Euramerica and Gondwana which show evidence of a net loss of P during root development coincident with the appearance of early trees such as the progymnosperms Archaeopteris and Svalbardia. Critically, two Mid Devonian study sites, one in northern Scotland and the other in eastern Greenland, both of which formed in extensional basins surrounding the Caledonian Mountains, reveal a near identical net loss of P despite their geographic separation. The presence of identical nutrient pulses in evolutionarily similar, yet geographically different regions provides critical support to the hypothesis that the expansion and diversification of land plants released unprecedented levels of nutrients driving global eutrophication and anoxia.