Quantifying the Biogeochemical Impact of Land Plant Expansion in the Mid Devonian and Implications in Marine Anoxic Events

Abstract

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, presented here are 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 (from Greenland and northern Scotland/Orkney), reveal a near-identical net loss of P. Nitrogen and Carbon isotopes from a subset of these lakes confirm elevated input of terrestrial plant material concurrent with P perturbations. Terrestrial P input appears to be episodic in nature, suggesting land plant expansion was driven by an external catalyst in the study region. All sites analyzed are temporally proximal to significant marine extinctions, including precise correlation with the Kačák extinction event and the two pulses associated with the Frasnian-Famennian (F/F) mass extinction. The episodic expansion of terrestrial plants appears to be tied to variations in regional and global climate, and in the case of the F/F extinction, also to atmospheric changes associated with large scale volcanism. Using P data presented here as an input into an Earth system model of the coupled C-N-P-O2-S biogeochemical cycles shows that globally scaled riverine phosphorus export during the Frasnian-Famennian mass extinction generates widespread marine anoxia consistent with the geologic record. While timing precludes land plants as an initiating mechanism in the F/F extinction, these results suggest they are implicated in every marine extinction event in the Mid to Late Devonian.