Paleoclimate constraints on terrestrial hydroclimate, silicate weathering, and the carbon cycle
Keywords: Earth System Science.

Maintenance of a habitable planet requires connections and balance among Earth's biogeochemical cycles. Further, the strength of the feedbacks and couplings determines the stability of conditions in the surface climate system necessary for the evolution of life. Records of Earth's past climate, paleoclimate records, provide constraints beyond the reach of the instrumental record on the directionality, strength and co-evolution of key Earth system cycles. This includes the geologic carbon cycle, the water cycle and the planet's energy balance. Crucially, the geography, topography and lithology of Earth's continents have two important features that are the focus of this dissertation. First, the continents provide boundary conditions that determine global circulation and hydroclimate patterns that couple Earth's water and carbon cycles (Chapters 1 and 2). Second, the land surface provides a stabilizing negative feedback in the form of silicate weathering fluxes (Chapters 3 and 4, and Appendix E), balancing the long-term carbon cycle through alkalinity and solute delivery to the oceans, and subsequent carbonate burial. In this dissertation I use data, observations and modeling to place mechanistic constraints on how interactions between Earth's surface and long-term biogeochemical cycles maintain balance and habitable conditions in our climate system conducive for the evolution of life. Fundamental to understanding our climate system is predicting the anticipated sign of terrestrial hydroclimate change during periods of climatic change. To this end I have investigated the regional response of hydroclimate change using paleoclimate records from mid-latitude lake systems. First, in Chapter 1, I have developed an inverse model to quantify how a mid-latitude lake system in Asia, the Songliao Basin, responded to a transient warming event during the Cretaceous. This model was also applied to a Holocene ostracod record from Lake Miragoane, Haiti. Second, in Chapter 2, I compile spatial distributions and size estimates of pluvial lake systems in western North America during the Pliocene-Pleistocene. By imposing mass and energy balance constraints (sensu Budyko) I forward modeled lake area distributions to demonstrate that now-arid western North America was wetter during both past colder and warmer periods during the Pliocene-Pleistocene, a result not predicted for future warming scenarios. Geologic observations primarily suggest wetter conditions globally during warmer-than-present periods. Importantly, this observation is a requirement for the operation of the stabilizing negative feedback between silicate weathering and climate. Understanding the factors that control silicate weathering rates is fundamental to constraining the evolution of Earth's carbon cycle over geologic timescales. One challenge for reconstructing past weathering rates is determining the reactivity of Earth's surface. In Chapter 3 I have quantified the reactivity of modern basalt and granite catchments using a process-based solute production model and concentration-discharge weathering relationships. This approach provides mechanisms that link runoff (i.e., terrestrial hydroclimate changes that were the focus of Chapters 1 and 2) with the distribution of global sub-aerial silicate lithologies. In Chapter 4 I utilize an emerging metal isotope system, lithium isotopes, to investigate the terrestrial weathering response to a large perturbation in the carbon cycle during the Cretaceous. We determine that the background-state of the Cretaceous weathering system was more congruent and, hence, more sensitive to perturbations in the carbon cycle than during the Neogene. Finally, in Appendix E I have quantified how step-wise geologic evolution of land plants strengthened the silicate weathering feedback over the Phanerozoic. I have developed a new reactive transport framework for evaluating the relative plant-controlled roles of hydrologic versus thermodynamic mechanisms that influence the coupling between the water cycle and silicate weathering fluxes on a continental scale. The results described in this dissertation constrain links between two connected portions of the exogenic Earth system. I have provided constraints for how the land surface records past changes in regional atmospheric circulation patterns, as well as regional water and energy balances. Further, using reactive transport modeling, I have placed new constraints on the role of plants and lithology in determining the coupling between terrestrial hydroclimate and weathering. Collectively these results suggest that the surface Earth system, our planet's fluid envelope, has been progressively tuned by the advent of continents and the evolution of life.