Congratulations to Rich Fiorella who successfully defended his dissertation on April 11, 2016.

Advisor: Chris Poulsen

Abstract: Stable water isotopes represent a powerful tracer of the thermodynamic history of water in an air parcel, and are also thought to be preserved in a wide array of geologic proxy materials. As a result, the isotopic compositions of proxy materials in continental environments are commonly used to estimate past climates, elevations, or environments.

However, the modern controls on the spatiotemporal distribution of stable water isotopes are poorly understood in many environments. This dissertation addresses uncertainties in modern water cycling in high elevation regions and in variable forest canopies, with an eye toward understanding how these processes may impact our interpretation of proxy material isotopic compositions. Chapters 2 and 3 present multiyear records of the isotopic composition of precipitation and surface waters in the central Andes, respectively. Chapter 2 elucidates a strong connection between the isotopic composition of precipitation and continental-scale climate dynamics. These measurements affirm elevation as a primary control on isotopic composition, but this relationship varies substantially in space and time. Chapter 3 demonstrates that precipitation isotopes reflect surface water isotopes along the eastern Andean flank, but are evaporatively modified on the high elevation plateau, where the majority of paleoelevation proxies have been recovered. Coupled with paleoclimate simulations indicate the Andes were likely more evaporative when they were lower, these results suggest that prior interpretations of Neogene elevation change from stable isotope proxies are likely overestimated. Chapter 4 presents seasonal and interannual records of the isotopic composition of near-surface water vapor in a deep mountain valley, and demonstrates the influences that local and remote processes have on near-surface vapor. Chapter 5 investigates how an intermediate canopy disturbance influences water cycling in a northern Michigan forest be investigating the vertical structure of the isotopic composition of near-surface vapor. These observations highlight the utility of stable water isotopes to constrain a wide array of process-level information unavailable from measurements of water mass fluxes alone. Further, they illustrate the potential for long-term isotope monitoring to detect local-to-regional changes in atmospheric moisture transport and ecohydrology and validate predictions of land-atmosphere water exchange fluxes in land surface and Earth system models.