Holocene Climate Change in the Subtropical Eastern North Atlantic
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Holocene Climate Change in the Subtropical Eastern North Atlantic: Integrating High-resolution Sclerochronology and Shell Midden Archaeology in the Canary Islands, Spain A dissertation submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY in the Department of Geology of the College of Arts and Sciences by Wesley George Parker B.A., Geological Sciences, Ohio University, 2015 B.A., Global Studies – Latin America, Ohio University, 2015 Dissertation Committee Dr. Yurena Yanes, Chair Dr. Aaron Diefendorf Dr. Henry Spitz Dr. Donna Surge Dr. Amy Townsend-Small Dissertation Abstract High-resolution paleoclimate data are necessary to investigate climate mechanisms that operate at seasonal scale, like the North Atlantic Oscillation (NAO). The oxygen isotope (δ18O) composition of mollusk shells are a powerful paleoclimate proxy that records the highest (submonthly) resolution because: (1) shells are abundant and often well preserved in the geological and archeological record, (2) many shells grow almost continuously year-round, and 18 (3) the δ Oshell values of many mollusks record sea surface temperature (SST). The application of δ18O paleothermometry in new species and localities requires calibration studies to ensure that selected taxa reliably record SSTs. Additionally, anthropogenic and diagenetic alteration must be assessed to ensure the proper acquisition of paleotemperature data. Shells middens of the Canary Islands, located in the subtropical eastern Atlantic Ocean, provide a unique opportunity to generate novel high-resolution paleoclimate data over the last two millennia useful to assess variations in the NAO and its influence on coastal ecosystems and civilizations. Here I present four research projects that address the calibration and application of δ18O paleothermometry using two species of intertidal gastropods, Patella candei and Phorcus atratus. The first section (Chapter 1) presents an introduction and justification of the scientific work. In the first project (Chapter 2), I conduct a calibration study of both gastropod species 18 18 using the modern δ Oshell, δ Owater, and in-situ SSTs collected throughout a year at 15-day intervals. Results indicate that both gastropods are credible repositories of paleotemperature data, however P. candei displays a consistent +1.3‰ isotopic deviation from expected values at equilibrium. In the second study (Chapter 3) I assessed body size change through time of P. candei shells from archaeological sites in the western islands to investigate possible influences of climate change and overfishing pressures. Results suggest that known rapid climate events did not correlate with shellfish body sizes, however comparison of archaeological and modern shells ii demonstrated a ~27% reduction in size across modern specimens, indicating an increase in anthropogenic harvesting pressures over time. The third study (Chapter 4) utilizes carbonate- target radiocarbon dating to investigate the scale of time averaging (age mixing) in archaeological assemblages. Results reveal that, in some sites, the span of shell ages in a single stratum exceeds the range of analytical uncertainty, indicating that a single date per stratum may be insufficient to adequately constrain the age of all material in the stratum and when possible, each shell must be individually dated. The final study (Chapter 5) applies δ18O paleothermometry to 18 radiocarbon-dated shells of P. atratus to address the impact of the Medieval Climate Anomaly (MCA; AD 900-1300). Results demonstrated that increased upwelling in the NW African Upwelling Zone during the MCA likely induced SSTs that were ~2.5 °C cooler than the modern, and that this likely indicated a persistently positive mode of the NAO during the MCA. This dissertation sets the foundations of how to properly use several species of gastropods as paleoclimate proxies and has generated new high-resolution SST data for a region (NW Africa) and time interval (MCA) still debated by the scientific community. iii iv Acknowledgements It takes a village to raise a child, and it takes an even larger village to raise a PhD student. I have been blessed with a large and supportive group of family, friends, colleagues, and teachers, and I would not be who I am today without the guidance and love you all provide me. First and foremost, I would like to thank Dr. Yurena Yanes, my advisor, mentor, confidante, and advocate. When you took me on as your first PhD student we knew that we would have to learn how to navigate this journey together, and it was certainly an adventure for both of us. Along the way we found much common ground, mutual support, and respect for one another. I am forever grateful that you saw the potential in me and encouraged me to grow into the scientist I have become, and I am excited to continue to share this journey as friends and collaborators in the years to come. Additionally, I would like to thank my committee, Dr. Aaron Diefendorf, Dr. Henry Spitz, Dr. Donna Surge, and Dr. Amy Townsend-Small for your constructive feedback, personal support, and thoughtful contributions throughout my graduate school career, and the Geology Department at the University of Cincinnati for being my second home for 4.5 years and for facilitating my professional and personal growth as a scientist, student, and scholar. My family has also always played a pivotal role in supporting my aspirations. My mother, Deborah, encouraged me to read, explore, and be unabashedly curious about the world. My father, Steven, encouraged me to be resourceful, goal-oriented, and analytical. My brother, Jason, encouraged me to be empathetic and adventurous, and was always willing to take my phone calls (sometimes at 2:00 am) when I needed to hear a friendly voice. And my sister-in- law, Danielle, encouraged me to be both self-assured and self-reflective, especially when I felt v lost or confused. I would also like to thank my extended family, who make the time to ask me about my life, work, future and dreams. I have also been blessed with an incredible group of friends including Kit John, Amanda Piet, Chelsea Zwayer Irvin, Mackenzie Glasgow, Chris Sheehan, Rusty Lockett, Clair Kronk, Steven Wade, Adriane Lam, Sarah Sheffield, Jen Bauer, and many, many others who have provided me with endless positivity, food, laughter, and companionship. My laboratory group has also been full of incredible young scientists, including Elizabeth Bullard, Nora Soto, Evan New, Abbey Padgett, Catherine Nield, Reed Sanchez, and Zeke King Phillips, all of whom have become friends, mentors, and supporters throughout my time in Cincinnati. In addition to these, I would like to specifically thank Ron Attreau for welcoming me to Cincinnati and standing by my side through the majority of my graduate school career. The support you provided me is a gift that I cherish, and I wish you the best of luck as our paths take us in new and exciting directions. Finally, my life has been full of dedicated educators and mentors who always encouraged me to be inquisitive, make mistakes, ask for help, and work tirelessly toward the fulfillment of my dreams. Some of these educators include Judy Mason Bannon, Jaimie Thomas, Denise Pierson Gambill, Sarah Zettler, Alycia Stigall, Dan Hembree, and Cecil Walters. You saw the potential in me and encouraged me to follow my dreams to become the scientist, scholar, and community member I am today. Thank you. vi Table of Contents DISSERTATION ABSTRACT .................................................................................................................................. II ACKNOWLEDGEMENTS ........................................................................................................................................ V TABLE OF CONTENTS ......................................................................................................................................... VII CHAPTER 1 – INTRODUCTION ............................................................................................................................. 1 REFERENCES ............................................................................................................................................................ 10 CHAPTER 2 - CALIBRATION OF THE OXYGEN ISOTOPE RATIOS OF THE GASTROPODS PATELLA CANDEI CRENATA AND PHORCUS ATRATUS AS HIGH-RESOLUTION PALEOTHERMOMETERS FROM THE SUBTROPICAL EASTERN ATLANTIC OCEAN ....................... 17 ABSTRACT ................................................................................................................................................................ 17 1. INTRODUCTION ..................................................................................................................................................... 18 2. MATERIALS AND METHODS .................................................................................................................................. 20 2.1. Geographical, climatic and geological setting ............................................................................................ 20 2.2. Ecology and morphology of Patella candei crenata ................................................................................... 21 2.3. Ecology and morphology of Phorcus atratus .............................................................................................. 23 3. FIELD SAMPLING METHODS .................................................................................................................................