THE DEVONIAN PLANT REVOLUTION: ROOTS, WEATHERING AND THE IMPACT ON GLOBAL BIOGEOCHEMICAL CYCLING

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 started the pseudo-modern process of soil formation dynamics, and significantly impacted nutrient and carbon cycling on land and presumably to the global oceans. Marine records of this transformation indicate extreme changes in the weathering of critical nutrients such as phosphorus (P), input into the ocean, and biotic crises associated with marine anoxia. Questions remain about the relative role of enhanced external P input to the ocean, as stratification and widespread ocean anoxia occurring during this time also caused profound internal changes in the marine P cycle; however, all models require an external eutrophication pulse of P to initiate ocean anoxia. To explore this issue, and to provide an important paleoecological and nutrient weathering context to the marine records, land-based measures of P weathering and mobilization during this time will be developed to provide a dynamic and quantitative basis for assessing the global role of plant and soil evolution. Additionally, a newly developed method to measure nitrogen (N) isotopes in samples with extremely low N content (down to 5 μg N) will be utilized to measure associated changes in N dynamics during this transition. Ultimately, this project aims to confirm the hypotheses that (1) the expansion, radiation and diversification of land plants in the Devonian is directly linked to terrestrial P loss which drove extensive environmental change; (2) this phosphorus loss effected measurable changes on lacustrine nitrogen and carbon dynamics; and (3) this P pulse was not a singular event, but rather dynamic and variable, potentially explaining the multiple eutrophication pulses and subsequent biotic crises in late/end Devonian. Pilot data from the Upper Caithness Formation in the Mid Devonian collected in Northern Scotland shows evidence of a net loss of P during root development coincident with the appearance of the first lycopods (early vascular land plants) and early trees such as Archaeopteris. Preliminary N isotope analyses of a sample subset shows a dynamic N record corresponding with initial P results.