Plant cuticle as a proxy for paleoecology and paleoatmospheric composition : modern calibration and application to the early Paleocene of the San Juan Basin, New Mexico.
Human alteration of land cover and combustion of fossil fuels has elevated atmospheric CO2, causing the Earth to warm. The geological record provides an archive of how organisms and ecosystems respond to changes in climate and atmospheric CO2 allowing us to assess the implications of a warmer planet. This dissertation focuses on using fossil plant cuticle to reconstruct ancient ecosystems and atmospheric conditions. In particular, I develop, test, and apply leaf cuticle proxies to (1) reconstruct canopy structure and (2) atmospheric CO2 of the early Paleocene from the San Juan Basin (SJB) New Mexico. I reconstructed canopy structure using single species and multi-species approaches. I used light experiments to quantify the response of cell size and undulation and carbon isotopes of modern Platanus occidentalis. I then developed and applied a proxy based on cell wall undulation for daily light integral (DLI) to fossil Platanites leaves from early Paleocene floras in the SJB. The distribution of high DLI values from fossil leaves indicate that either (1) most of the canopy mass is within the upper portion of the crown or (2) leaves exposed to more sunlight are preferentially preserved. I then used a novel proxy to reconstruct leaf area index (LAI) based on the cell wall undulation and aspect ratio of multiple species using dispersed plant cuticle from seven localities in the SJB. I find that reconstructed LAI is comparable to modern broadleaf forests and indicates a stable canopy in the early Paleocene. Changes in LAI were only weakly associated with changes in climate, instead, likely linked to changes in the SJB fluvial system evolution. I then evaluate early Paleocene CO2 by applying a leaf gas-exchange model to Platanites leaves of four early Paleocene localities from the SJB. I first calibrate the model using P. occidentalis and P. acerifolia, in which the model accurately predicts present-day CO2. Applying the model to the early Paleocene, I find CO2 fluctuates between ~660 to 1140 ppm between 65.66 and 64.59 million years ago. These elevated levels of atmospheric CO2 during the early Paleocene are aligned with the elevated temperature during this period.