The evolution of land plant root systems occurred stepwise throughout the Devonian, with the first evidence of complex root systems appearing in the mid-Givetian. This biological innovation provided an enhanced pathway for the transfer of terrestrial phosphorus (P) to the marine system via weathering and erosion. This enhancement is consistent with paleosol records and has led to hypotheses about the causes of marine eutrophication and mass extinctions during the Devonian. To gain insight into the transport of P between terrestrial and marine domains, we report geochemical records from a survey of Middle and Late Devonian lacustrine and near-lacustrine sequences that span some of these key marine extinction intervals. Root innovation is hypothesized to have enhanced P delivery, and results from multiple Devonian sequences from Euramerica show evidence of a net loss of P from terrestrial sources coincident with the appearance of early progymnosperms. Evidence from multiple Middle to Late Devonian sites in Greenland and northern Scotland/Orkney reveal a near-identical net loss of P. Additionally, all sites are temporally proximal to one or more Devonian extinction events, including precise correlation with the Kačák extinction event and the two pulses associated with the Frasnian/Famennian mass extinction. For all sites, weathering, climate, and redox proxy data, coupled with nutrient input variability, reveal similar geochemical responses as seen in extant lacustrine systems. Orbitally forced climatic cyclicity appears to be the catalyst for all significant terrestrial nutrient pulses, which suggests that expansion of terrestrial plants may be tied to variations in regional and global climate.

The evolution of land plants in terrestrial environments brought about one of the most dramatic shifts in the history of the Earth system — the birth of modern soils — and likely stimulated massive changes in marine biogeochemistry and climate. In particular, multiple marine mass extinctions characterized by widespread anoxia, including the Late Devonian mass extinction around 375 million years ago (Ma), may have been linked to terrestrial nutrient release driven by newly-rooted landscapes. Here, we use recently generated constraints from Earth’s lacustrine rock record as variable inputs in an Earth system model of the coupled C-N-P-O2-S biogeochemical cycles in order to evaluate whether recorded changes to phosphorus fluxes would be adequate to sustain Devonian marine biogeochemical perturbations and extinction dynamics. Results show that globally scaled riverine phosphorus export during the Late Devonian mass extinction generates widespread marine anoxia and produces carbon isotope, temperature, oxygen, and carbon dioxide perturbations generally consistent with the geologic record. Similar results for a competing extinction mechanism, large scale volcanism, suggest the Late Devonian mass extinction was likely multifaceted with both land plants and volcanism as contributing factors.

<p>Lake cycle geochemical correlations.</p>

<p>Lake cycle geochemical correlations.</p>

<p>Lake cycle geochemical correlations.</p>