Oxygen isotope constraints on cenozoic hydroclimate and the resilience of terrestrial ecosystems
Keywords: Geological Sciences.

A central challenge in terrestrial paleoclimate research is identifying what caused the major vegetation transitions of the Cenozoic. This research has led to important advances, but fundamental questions, such as what drove the recent expansion of grass-dominated ecosystems, remain unanswered and our understanding of past climate-vegetation interactions is incomplete. While decades of work has focused causes of past vegetation transitions, much less attention has been paid to the opposite question—what causes vegetation to stay the same, even while climate changes? In my research, I analyze vegetation transitions and vegetation stasis in the Cenozoic to develop comprehensive constraints on the resilience of terrestrial ecosystems to climate. By reconstructing the resilience of ecosystems in the absence of human interference, we can better inform whether humans are degrading this resilience today, and by how much. Here, I present two case studies investigating the role of hydroclimate in past ecosystem resilience, integrating quantitative analyses and new oxygen isotope data. These case studies cover the tropical Amazon Rainforest in the late Quaternary (~30 ka to present) and the more temperate western U.S. forests of the Eocene-Miocene (~56-5 Ma). In the Amazon case, I test two hypotheses. First, I test whether Amazon rainfall in the late Quaternary is driven by the strength of the South American Monsoon. This hypothesis explains the speleothem oxygen isotope data, but it conflicts with many other proxy reconstructions. I develop a process-based model of oxygen isotopes to show that the migration of the monsoon, not changes in its strength, reconciles the isotope data with the independent proxy data. Second, I test whether the rainforest is susceptible to grassland expansion beyond a theorized aridity "tipping point". Integrating data for past vegetation, hydroclimate, and fire, I quantify the strength of feedbacks that threaten forest cover and find that they were too weak to cause forests to tip to a grassland state in the past. This result is supported by dynamic global vegetation modeling results, indicating that Amazon tree cover is highly resilient to drying, at least in the absence of widespread human-driven deforestation. In the second case study, I test the hypothesis that summer aridity in the western U.S. caused the planet's first definitive wave of grassland expansion about 26-15 million years ago. I present new oxygen isotope data from authigenic clay and carbonate minerals and compile all existing data. Paleosol clays form at wetter times of the year than carbonates, so comparing them provides insight to seasonal climate change. In the compilation of new and existing data, I identify diverging trends in the oxygen isotope ratios of clay and carbonate minerals as grasslands expand. Long-term climate changes would affect oxygen isotopes of each mineral in the same way, so these diverging trends likely point to seasonal climate change that affects one mineral more than the other. I quantify the diverging trends to show that drier winters, not drier summers, explain this shift. I further demonstrate that winter drying is likely owed to the uplift of the Cascades Range, which causes a sharp rainshadow in winter but not in summer. Grassland expansion in the western U.S. is not owed to drier summers and, instead, likely driven by the onset of the Cascades rainshadow. These two case studies emphasize the resilience of tropical and mid-latitude forests to drying. Aridity well-past a theoretical tipping point did not decimate the Amazon Rainforest, and grasslands expanded in North America only when the uplift of mountains created a strong rainshadow. Human activity is increasing the likelihood of future, catastrophic vegetation transitions by driving climate change while likely degrading the resilience of ecosystems due to land use. However, these paleoclimate results support recent work showing that some of these predicted transitions may be avoided by mitigating human activity such as uncontrolled fire ignition and deforestation