THE IMPACT OF LAND PLANTS ON EARTH SYSTEMS: BOTTOM WATER GEOCHEMISTRY OF SILURIAN-DEVONIAN OPEN MARGIN SYSTEMS

The diversification and spread of terrestrial plants was one of the most profound changes in the history of Earth. While it is long accepted that the evolution of terrestrial plants had considerable impact on Earth systems, there are still widely varying views about the extent to which the emergence of land plants increased global productivity, drove atmospheric oxygenation, and altered the global marine redox landscape. The goals of this project are to connect and evaluate the effects of plant evolution on the marine redox record through palynology, paleobotany, and geochemistry.

Stratigraphic sections of the Ordovician-Silurian Road River Group at the Tatonduk River and the Devonian McCann Hill Chert at its type section near Hillard Peak, Alaska, were measured and sampled. These sections represent the Silurian-Devonian transition and record open continental margin sedimentation on the northwestern margin of Laurentia. Sperling et al., 2021 investigated the Road River Group along the Peel River (Yukon, Canada) where it was deposited in an intra-platformal basin, the Richardson trough. Unlike the Richardson trough which had restricted connections to the global ocean, the Hillard Peak and Tatonduk River sections represent open, unrestricted, slope settings. Restricted basins show local conditions and being restricted are more prone to stagnant, anoxic waters and local enrichments in redox sensitive trace elements (depending on water residence times). Open margins, being connected to the global system, are better for accessing global conditions. Here new iron speciation and redox sensitive trace element (Mo, U, and V) data will be presented. Preliminary data indicate that anoxic and ferruginous (non-sulfidic) bottom waters dominated in these open continental margin systems during the Ordovician and Silurian. These data point to a pulse of more dominantly euxinic (sulfidic conditions in the Pragian, consistent with data from the Richardson trough and other global proxies. On the open margin, however, euxinic conditions do not persist later in the Devonian. Future plans for palynology, carbon isotope chemostratigraphy, conodont biostratigraphy, Re-Os geochronology, and organic (biomarkers) and inorganic geochemical analyses will be discussed.