The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene
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The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene Climate Variability in the High-Latitude North Atlantic by Ursula Quillmann B.A., University of Colorado, 2003 M.Sc., University of Colorado, 2006 A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Doctor of Philosophy Department of Geological Sciences 2014 This thesis entitled: The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene Climate Variability in the High-Latitude North Atlantic written by Ursula Quillmann has been approved for the Department of Geological Sciences _____________________________ Dr. Thomas M. Marchitto _____________________________ Dr. Anne E. Jennings Date: _________________ The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. In Memory of Danielle 11.07.1988-11.22.2012 I love you forever iv Quillmann, Ursula (Ph.D., Geological Sciences) The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene Climate Variability in the High-Latitude North Atlantic Thesis directed by Dr. Anne E. Jennings and Dr. Thomas M. Marchitto Holocene paleoclimatic reconstructions provide longer time series than instrumental records, allowing us to examine climate variability under more extreme boundary 18 conditions. I reconstructed temperature and Osw using paired measurements of 18 Mg/Ca ratios and Ocalcite of planktonic and benthic foraminifera at sites in the subpolar North Atlantic that are sensitive to Subpolar Gyre (SPG) dynamics today. Glacial freshwater from the final stages of the decay of the Laurentide ice sheet influenced SPG dynamics and was routed via the SPG throughout the subpolar basin. Near Iceland we recorded the 8.2 ka cooling event and the freshwater spike from the catastrophic 18 outburst flood of proglacial lakes Agassiz and Ojibway using paired Mg/Ca and Ocalcite of the benthic foraminifer Cibicides lobatulus. This evidence for a brief cooling and freshening is supported by a coeval increase in arctic benthic foraminifera and a 18 decrease in biogenic carbonate. Through the Holocene, the SPG warmed and the Osw became heavier (saltier), suggesting a progression towards a contracted gyre. This is in sharp contrast to the established view of the rest of the North Atlantic, which was warmest during the early-mid Holocene. We attribute the SPG warming trend to a weakening of the NAO-like circulation in response to increasing winter insolation through the Holocene. Overlying this general trend were abrupt shifts that cannot be v explained by changes in insolation. Between 10,000 and 8,000 cal yr BP, my study sites 18 recorded cold temperature and light Osw values, suggesting an extended SPG. Between 8,000 and 6,000 cal yr BP, the SPG changed shape but was still extended. This change coincided with the onset of Labrador Sea water formation, which today is associated with intensified SPG circulation. Between ~6,000 and 3,800 cal yr BP, I see a strong influence of Atlantic water on the SW Iceland shelf, consistent with a contracted SPG. After 4,000 cal yr BP, freshwater from the Arctic Ocean appeared on the SW Iceland shelf. The water column as recorded in benthic and planktonic foraminiferal assemblages went from well mixed in the early Holocene to stratified in the late Holocene. Overall my records show that the SPG played an important and, in some ways, surprising role in the Holocene climate evolution of the North Atlantic. vi Acknowledgments First and foremost, I would like to thank my advisors, Tom Marchitto and Anne Jennings, for their continuous support and guidance throughout the course of my PhD research. Tom had his door always open and was ready to discuss my data. He was very supportive when the tephra contamination in my samples frustrated me beyond measure. Anne had an open door policy as well. She loves forams at least as much as I do and we spent countless hours discussing forams and their meanings. Tom and Anne are both excellent writers and I learned a lot from both of them. I would also like to thank John Andrews. He was always involved in my research and always available to help me. Thanks for helping with the statistical analysis of my data. I am especially grateful to him for making a research cruise possible for me. The month I spent on the CCGS Hudson is unforgettable. Not only did I start to appreciate all the work and passion that goes into retrieving sediment cores from the ocean floor, but I am also left with many memories of incredible icebergs and landscapes very few people are lucky enough to see. Seeing a polar bear on an ice floe was spectacular. The experience of being at sea made me a strong advocate for protecting these precious arctic environments. I have shared my passion for arctic environments with my 2,067 students I have been teaching since the fall semester of 2011 at the Colorado State University in Fort Collins. Thanks, John. After my daughter Danielle’s death, Anne, Tom, and John set up biweekly meeting with me. These meetings helped me to get back into my research. They left me space to start healing, but nudged me gently into continuing my research and were vii confident that I could finish my PhD. I could have not done it without their support and encouragement. I cannot thank them enough. I would also like to thank my other committee members, James Syvitski and David Andersen. I was James’ teaching assistant for several semesters. He was an excellent mentor on how to be a good teacher. My students at CSU are benefiting immensely from what I learned from him. Dave Andersen joined my PhD committee toward the end. He brought lots of enthusiasm and energy into the final stages of my thesis. Dave always helped me focus on the wider impact of my research. I will remember that there are ALWAYS “three things…..” I would also like to thank Hans Petter Sejrup and Walt Dean for being on my comprehensive exam committee. Special thanks goes to Patrick Cappa for helping me in the ICP-MS laboratory and to Wendy Freeman for helping me in the core processing room. Thanks to all the undergraduate researchers involved in my project: Michelle Carlson, Stephen James, Daniel Giffitts, Vivian Underhill, and Christina Sheldon. I had great technical support from Chad Stoffel and Tara Hess. Thanks for keeping me sane when my laptop acted up. Thanks to Shelly Sommer for finding the most obscure documents for me that I needed for my research. There are so many wonderful people at INSTAAR that I would like to thank, but the list would be too long. I especially want to acknowledge Jen Hall Bowman, Larry Bowlds, Shelly Sommer, and Anne Kelley for being good friends. I liked that I had so many of the student and postdoc visitors from Europe share my office space. I was sad when they left. We had lots of conversations and became viii friends, and thanks to Facebook we are still in touch. Thanks Saedis, Heidi, Steffen, Birgitte, Kirstin, Anders, Christine, and Kari. I would also like to thank the Graduate School and Dean Stevenson for making special arrangements after Danielle’s death and being supportive. Numerous travel grants came from the Graduate School and from the Department of Geological Sciences that supported attendance at international and national conferences. I would like to thank the Department of Geological Sciences, especially Lang Farmer, for supporting me through teaching assistantships, travel grants, a research grant award ($1,000), and undergraduate research help. The last years were crazy but thanks to good friends I made it through. The list is long. Thanks for all the confidence you have had in me and for cheering me on. To Lance, Chuck, Jen, Anders, and Mary: Thanks for the formatting, proofreading, and printing help. Finally, I would like to say thanks to my daughter Lena and my husband Volker. We are getting through hard times together. Lena and I encouraged each other to finish up our PhD work. Our research topics could have not been more different: German women writers in the 1800s and Holocene forams. The funding for my research came from a research grant from the National Science Foundation (Grant No. 0823525), with the PIs: Anne, Tom, and John. Thank you. NSF also supported the help from undergraduate researchers. ix Table of Contents CHAPTER 1 INTRODUCTION__ .............................................................................................1 1.1 MODERN ATMOSPHERIC AND OCEANIC CIRCULATION IN THE SUBPOLAR NORTH ATLANTIC .....1 1.2 RESEARCH QUESTIONS ........................................................................................................7 1.3 FORAMINIFERAL STABLE ISOTOPES AND MG/CA RATIOS .....................................................12 1.4 THESIS ORGANIZATION ......................................................................................................13 CHAPTER 2 COOLING AND FRESHENING AT 8.2 KA ON THE NW ICELAND SHELF RECORDED IN PAIRED 18O AND MG/CA MEASUREMENTS OF THE BENTHIC FORAMINIFER CIBICIDES LOBATULUS ...............................................................................16 2.1 ABSTRACT ........................................................................................................................16 2.2 INTRODUCTION ..................................................................................................................17