A multi-pronged approach to understanding moisture dynamics in North America since the Last Interglacial using proxy records and climate model output

Abstract

This dissertation aims to enhance our understanding of moisture patterns across North America during and since the Last Interglacial (LIG: ~129,000-116,000 years BP) using established and novel paleo-moisture proxies in cave mineral deposits (speleothems) and by comparing North American proxy networks with climate model output. I assess broad-scale hydroclimate changes across North America by comparing networks of moisture-sensitive proxy records from the literature with climate model output during the two most recent periods when global temperatures were warmer than the pre-Industrial – the LIG and the mid-Holocene (MH: ~6,000 years BP). I assess the degree of agreement between proxy records and the latest generation of Paleoclimate Modeling Intercomparison Project 4 (PMIP4) simulations and probe the drivers of rainfall patterns under different climate states. Agreement between proxies and models is higher for the LIG than for the MH, likely because the orbital forcing is greater during the LIG, and an ensemble subset of three individual models maximizes LIG agreement overall. An assessment of atmospheric dynamics in this subset indicates that LIG precipitation anomalies in North America were driven by relatively weaker winter North Pacific pressure gradients and steeper summer North Pacific and Atlantic gradients. Additionally, I present the first speleothem calcium isotope record for North America and generate semi-quantitative rainfall estimates for the early and middle Holocene in coastal California. I then directly compare these paleo-precipitation estimates with extreme rainfall variability in modern California. I also further the development of the new calcium isotope paleo-rainfall proxy by collecting modern calcium isotope data from three North American cave systems. I compare these data with established moisture-sensitive proxies (carbon isotope ratios, trace element ratios) to investigate how calcium isotope variability in modern cave systems responds to varying rainfall rates and inform how this proxy can be best applied in paleoclimate studies. In sum, this dissertation leverages multiple types of environmental data to provide valuable paleoclimate context for the hydrologic changes that are ongoing across North America.