Modeling Ocean Dynamics and Vegetation-Climate Interactions Under Evolving CO2 During the Late Paleozoic Ice Age

Abstract

Icehouses, such as the current glacial state and Late Paleozoic Ice Age (LPIA; ~340 to 290 million years ago), make up less than 25% of the past billion years of Earth’s history and are associated with major environmental change. Thus, exploring climate dynamics during the LPIA expands our view of the climate system and improves our confidence in future climate projections. Despite major advancement in our understanding of CO2-induced changes in supercontinental glaciation during the LPIA, far less is known about how the superocean and biosphere factored into the climate system. This dissertation presents new understanding of regional and global ocean dynamics and tropical vegetation-climate feedbacks under evolving atmospheric pCO2 during the LPIA by simulating these processes in an Earth system model framework. The chapters in this dissertation integrate novel Earth system model simulations with environmental proxy data to better understand how climate dynamics drove marine and terrestrial change during the LPIA. Chapters 2 and 3 investigate glacial-interglacial changes in ocean circulation and temperature from the global to regional scale to better constrain the nature and role of the superocean in the LPIA. Chapter 2 explores the effects of changing CO2, sea level, and high-latitude ice extent on global ocean circulation, salinity, and temperature patterns. Results from this chapter represent the first global estimates of surface currents and thermohaline circulation for the LPIA and connect insights from coastal proxy records to large-scale ocean dynamics. Chapter 3 refines the spatial scale of interest by investigating the patterns of seawater oxygen isotopic composition, temperature, and circulation in the North American Midcontinent Sea. Results from this chapter demonstrate that local variations in runoff contribute to much of the high spatial variability observed in oxygen isotopic records from the sea. This chapter demonstrates how isotope-enabled Earth system models can be used to constrain local processes in semi-restricted ancient inland seas, and thus improve proxy-based interpretations of seawater temperature and chemistry related to large-scale paleoceanographic events. Chapter 4 shifts from the marine to terrestrial realm and explores vegetation-climate interactions across low-latitude Pangaea to better understand the role of paleo-plant physiology in paleotropical climate change. This chapter presents the first methodology for translating Pennsylvanian fossil leaf characteristics and insights from process-based ecosystem modeling into extinct plant types in an Earth system model. Using novel paleo-plant types that represent key Pennsylvanian tropical plants and their modern plant analogs, this chapter explores tropical vegetation-climate feedbacks under different levels of CO2 as well as wetland and dryland tropical forests. Results from this chapter show that paleo-plants transpire more water overall and respond differently to elevated CO2 compared to modern plants, leading to regional variations in low-latitude precipitation and soil water that better capture the tropical moisture gradient inferred from terrestrial proxy records. This chapter highlights the important role that age-specific paleo-plant physiology plays in vegetation-climate interactions in deep time. In sum, findings from this dissertation demonstrate the importance of atmospheric pCO2 in driving past icehouse climate dynamics and the utility of integrating Earth system models and proxy records in deep time.