Devonian Organic Matter Variability within the Appalachian Basin
Delaney Leigh McCarthy ’21, Minneapolis, Minnesota
The Devonian Plant Explosion, when plants increased in diversity and abundance during the Middle to Late Devonian, is known to have had a wide range of effects on the Earth System. Terrestrial floral ecosystems went from rock-covering moss to multi-storied forests with trees reaching up to 30 meters high. Previous studies have proposed a relationship between the newly evolving forest ecosystems, soil formation, and nutrient runoff to oceans with ocean anoxia, black shale deposition, glaciation, and mass extinction during the Middle to Late Devonian. This shift, which increased nutrient flux into nearby oceans, resulted in a global increase in black shale deposition.
Two hypotheses related to the expansion of forests were tested using weight percent organic carbon and organic carbon isotopic analysis of samples from a 553km transect, consisting of three localities in the Appalachian Basin from the Middle-Late Devonian (Givetian-Frasnian). The first being that as proximity to open-ocean settings increases, terrestrial biomass decreases. The second is that terrestrial organic matter input to marine settings will increase through time as forests expand. Carbon isotope results show that terrestrial biomass does indeed decrease as proximity to open-ocean settings increases. Terrestrial organic matter through time appears to be increasing, although the pattern is overprinted by both Devonian sediment supply changes and modern carbon contamination of outcrop samples.
Humans today are drastically changing the carbon cycle and are thought to have initiated the current mass extinction. The results of this study provide a baseline for understanding how an abrupt carbon cycle change affects the lithosphere, atmosphere, and biosphere. Furthermore, by comparing the rates and magnitudes of carbon cycle changes in the past to those occurring today, we can better predict the future effects of anthropogenic (human impact on the environment) climate change.