Causes and Impacts of Rainfall Variability in Central Mexico on Multiple Timescales

Causes and Impacts of Rainfall Variability In Central Mexico on Multiple Timescales By Tripti Bhattacharya A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geography in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Anthony R. Byrne, chair Professor John C. Chiang Professor Bonnye L. Ingram Professor Richard S. Dodd Professor Cindy Looy Spring 2016 ! Abstract Causes and Impacts of Rainfall Variability In Central Mexico on Multiple Timescales by Tripti Bhattacharya Doctor of Philosophy in Geography University of California, Berkeley Professor Roger Byrne, Chair The eastern sector of Mexico’s Trans-Mexican Volcanic Belt is a semi-arid region, where interannual rainfall variability is significantly stresses regional water resources and the livelihood of millions. The region is linked to a broader summertime rainfall regime known as the North American Monsoon (NAM). My dissertation uses multiple lines of evidence, from geochemistry to climate model output, to understand the causes of long-term droughts in this region. My first dissertation chapter uses instrumental data to diagnose the causes of El Niño-induced droughts in Mexico. This work explores the mechanisms responsible for rainfall changes over Central Mexico during the developing versus decaying phase of an El Niño event. This study was the first to demonstrate the importance of moisture transport anomalies in reducing rainfall in highland Mexico during the decay phase of an El Niño. My second dissertation chapter uses the oxygen stable isotope ratios (δ18O) of lacustrine carbonates as well as elemental geochemistry to reconstruct late Holocene drought in Central Mexico. This sub-centennially resolved record is the first to identify a significant dry interval in central Mexico from 1300-1100 cal yr. B.P., which may be temporally coherent with increased drought frequencies recorded on the Yucatan Peninsula. These results also hint at a role for climate change in regional prehistoric cultural changes at the nearby site of Cantona. My third dissertation chapter explores impacts of late Holocene droughts on the terrestrial ecosystems in Central Mexico. I reconstructed past vegetation and fire dynamics from pollen and microscopic charcoal, and compared these data to our stable- isotope based climate reconstruction and regional archaeological records. My fourth chapter is an exploration of the causes of centennial-scale across Mexico and Central America in the late Holocene. This work identifies the spatiotemporal patterns of late Holocene drought by synthesizing Mesoamerican proxy records. It presents a new hypothesis pointing to the role of changes in Atlantic circulation in causing droughts in Mesoamerica. Finally, I synthesize the knowledge in each of the dissertation chapters and point to avenues of future research. ! ! 1! Table of Contents Acknowledgments ii Introduction 1 Chapter One: Spatial variability and mechanisms underlying El Niño-induced droughts in Mexico 6 Chapter Two: Cultural implications of Late Holocene Climate Change In the Cuenca Oriental, Mexico 35 Chapter Three: Late Holocene anthropogenic and climatic influences on the regional vegetation of Mexico’s Cuenca Oriental 57 Chapter Four: Mechanisms underlying pre- Medieval Droughts in Mesoamerica 86 Conclusions and Future Directions 115 ! i! Acknowledgments This work was made possibly by the support of advisers, collaborators, friends, family, and general well-wishers. I would first like to thank the committee members with whom I worked most closely, my chair Roger Byrne, and John Chiang. Both supported my interdisciplinary research, providing both solid guidance and ample freedom as I tackled my dissertation research and sought to define my own research program. I would also like to acknowledge my committee members and collaborators, and departmental colleagues who have consistently been available to provide their expertise. Stable isotope measurements were made possible through the guidance and technical support of Lynn Ingram and Wenbo Yang. Tim Teague provided training in XRF methods. Additional lab facilities and measurements were provided by Christiane Hassel at IU Bloomington, and Susan Zimmerman and Tom Guilderson at the Lawrence Livermore National Laboratory. Finally, my research in Mexico would not have been possible without the previous research and collaboration of Harald Boehnel and his student Kurt Wogau at UNAM Juriquilla, and Ulrike Kienel at the GFZ in Potsdam. Richard Dodd and Cindy Looy were important sources of advice and guidance along my journey in graduate school. Undergraduate assistants Ellie Broadman, Allison Schwartz, and Jeemin Rhim were important parts of my research efforts. I owe much to the staff, faculty, and graduate students in the Department of Geography. I would especially like to credit my labmates; Alicia Cowart, Dyuti Sengupta, Liam Reidy, and David Wahl; as well as the broader community of Roger Byrne’s academic ‘children’ and ‘grandchildren.’ The support staff, Delores, Nat, Josh, Marjorie, Dan, Eron, Mike, and Deborah helped make being a graduate student at a large public university quite painless by helping with the logistics of stipends, lab facilities, computers, and the trans-Atlantic shipment of ancient sediments. The faculty and students have been incredibly supportive. Norman Miller’s ‘Climates of the World’ course piqued my interest in climate dynamics. In addition, I owe a special debt to Andrew Friedman and Yuwei Liu, former members of the Chiang group, for initiating me into the world of Matlab, NCL, and climate diagnostics. Conversations with Geography faculty, especially Laurel Larsen, have consistently challenged me to think about the importance of interdisciplinary research. The current and former students, especially Sarah Knuth, Mary Whelan, Jon Markle, Alicia Cowart, Mollie Van Gordon, Jeff Martin, and Alex Tarr, taught me much about being an academic and a human. I am deeply thankful for my family and friends who have taught me the meaning of unconditional support. From the time when they helped me collect fossils and interesting leaves as a child, my parents have never wavered in their belief in scientific curiosity and that I could do anything I set my mind to, given enough hard work. I also continue to draw support from my friends, old and new. Finally, I owe a deep debt of gratitude to Heather Bryant, Andrew Harmon, and the sangha at the Berkeley Shambhala Center, who have taught me the importance of mindfulness, discernment, and courage in all aspects of life. ! ii! Introduction Motivation and Approach Anthropogenic climate change poses key challenges for ecosystems and human sustainability in semi-arid regions. This is true especially in the Cuenca Oriental, a basin to the East of Mexico city, where the modern rainfall climatology indicates that the area is already marginal for maize agriculture (Heisey and Edmeades, 1999). The region features a monsoonal climate, which is coupled to the broader North American, or Mexican, Monsoon (Higgins et al., 2007). This monsoon, featuring a reversal in surface winds and a summertime peak in convective activity, is distinct from other monsoonal systems. Observational studies show that this circulation is shallower than other monsoonal systems, and that tropical wave disturbances as well as mid-latitude circulation patterns play a key role in initiating the surges of convection characteristic of the monsoon (Nie et al., 2010; Bordoni and Stevens, 2006). While the traditional NAM region is generally thought of as encompassing western Mexico and the American Southwest, the heating of the land surface and northward sweep of rainfall characteristic of the monsoon includes much of Central Mexico as well (Tucker, 1999). The overall goal of this dissertation is to clarify patterns of past climate changes in Mexico’s Cuenca Oriental, and to use this data to understand the climatic processes influencing this important monsoonal system. To this end, I use a unique approach by combining proxy-based records of past environmental change with diagnostic analyses of observational data and general circulation model (GCM) output. Paleoclimatic records can expand our knowledge of climate variations beyond the limited instrumental record, and shed light on the way regional climates respond to fundamental changes in Earth’s boundary conditions (e.g. changes in greenhouse gases, insolation, and other factors). The insights from paleoclimatic records can be synthesized with our understanding of climate physics gleaned from model output and observational data to generate novel hypotheses about the causes of past climate changes. Working at the intersection of these two fields therefore offers a unique opportunity to advance our fundamental understanding of the physics of the climate system. Understanding the causes of past climate changes in Central Mexico and Mesoamerica has important implications for our understanding of human history. Much research on past Mesoamerican droughts has focused on linking the archaeological record with past droughts to identify the potential influence of climate on past societies (e.g. Hodell et al., 2005; Medina-Elizalde et al., 2010; Lachniet et al., 2012; Kennett et al., 2012). Central Mexico is an especially fascinating setting in which to study past human- environment relations, since it was host to a rich cultural milieu in the pre-Columbian period, and is close to the hypothesized

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