High-resolution speleothem record of climate variability during the late Pleistocene from Spring Valley Caverns, Minnesota A THESIS SUBMITTED TO THE FACULTY OF THE GRADUATE SCHOOL OF THE UNIVERSITY OF MINNESOTA BY Julia Anne Nissen IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE R. LAWRENCE EDWARDS February, 2018 © Julia Anne Nissen 2018 ALL RIGHTS RESERVED Acknowledgements I would like to thank those who have made this research possible. First, to my advisor Larry Edwards, for his guidance and support throughout my time at the University of Minnesota. I would also like to thank my committee members Calvin Alexander and Josh Feinberg, as well as prior committee members Andy Wickert and Katsumi Matsumoto, for their men- torship. I would like to acknowledge John Ackerman for his generosity in allowing me to work in Spring Valley Caverns and thank him for his continued support for scientific research at the University of Minnesota. This work would not have been possible without his contributions. Additionally, thanks to Martin Larsen and other members of the Minnesota Caving Club for their support during caving trips. Their dedication to caving ensures the preservation of these amazing sites. This research was made possible through the work of staff at the Characterization Facility at the University of Minnesota, the University Imaging Centers, and the stable isotope laboratory at Xi’an University. A big thanks to Javier Garcia Barriocanal and Guillermo Marques for their help and patience. I cannot thank the current and former individuals at the Isotope Lab enough for their assistance and camaraderie during my time at the University of Minnesota. To Yanbin Lu and Julie Retrum for their help in introducing me to laboratory procedures. To Mellissa Warner for her impressive work in Python, which inspired me to find new ways of improving data processing using programming. To Pu Zhang, who works tirelessly to ensure the lab is running smoothly and is never too busy to answer questions. To Zongyi Wang for his help in writing the Python programs. To Mathieu Pythoud for being the best office mate, and to Akemi Berry, I cannot imagine these past few years without you. i Dedication This work is dedicated to those who supported me unconditionally throughout its devel- opment. To my amazing parents, to my brother and sister-in-law, and most of all, to Laura. ii Abstract Understanding how the Earth’s oceanic and atmospheric systems responded to abrupt cli- matic forcings in the past is crucial in determining potential effects of anthropogenic climate change. This is of particular importance in the mid-continental United States, an agricultural hub that produces much of the world’s corn and soybeans. High resolution paleorecords in the region remain sparse, restricting the predictability of global climate models and limiting our knowledge of atmospheric teleconnections across North America. This study analyses the growth and stable isotopic concentration of late Pleistocene speleothem samples from Spring Valley Caverns, located in SE Minnesota. Timing of growth for six samples was determined using 230Th dating and confocal microscopy, recording ages be- tween 114 - 29 ky BP. This places all sample growth within the last glacial period. Sample growth largely correlates with warm conditions in the North Atlantic, indicating elevated temperature and moisture availability. Growth during MIS4 may be due to enhanced anti- cyclonic activity over the Laurentide Ice Sheet, resulting in anomalous southerly moisture transport. High resolution stable isotope analysis was completed for two samples, showing both long term trends and short term variability. Samples show a steady decrease in δ13C values from 104 - 29 ky BP, suggesting a transition from C4 to C3 dominated ecosystems consistent with global cooling. However, δ18O values climb steadily throughout this period. This is indicative of drier conditions and an increase in Gulf of Mexico sourced moisture in the region. Short term variability from 64 - 44 ky BP shows impressive correlation to global paleorecords. North Atlantic cold events are shown as an increase in δ18O values, and North Atlantic warm events as a decrease δ18O values. As δ18O and δ13C generally covary during this period, elevated δ18O values suggest regional aridity. The transport of Gulf of Mexico summer moisture into the region appears significantly susceptible to changes in North Atlantic climate, most likely through large scale atmospheric systems such as the North Atlantic Oscillation and Pacific-North America teleconnection. The results of these records agree with a previous study of Spring Valley Caverns from the Holocene (Dasgupta, 2008). This signifies that large scale dynamics present during the last glacial period continue to affect regional climate, and thus these results may prove useful not only in improving our understanding of abrupt climate events during the late Pleistocene iii but in predicting impacts on the region from anthropogenic warming. iv Executive Summary Speleothem samples from Spring Valley Caverns were studied in order to determine the primary drivers behind long and short term climate variability in SE Minnesota during the last glacial period. While a study looking at Holocene climate in Spring Valley indicated that the site experienced effects of abrupt North Atlantic events, preliminary site data from the last glacial period were inconclusive (Dasgupta, 2008, MacKinney et al., 2013, Shapiro, 2007). In Chapter 1, we introduce specifics of long and short term climate variability during the late Pleistocene and discuss advantages and primary methods of speleothem research. While glacial-interglacial cycles are largely paced by orbital forcing, frequent changes in thermo- haline circulation resulted in abrupt climate change throughout the last glacial period. The effects of these events are present in paleorecords globally, but the methods of transmission remain unknown in certain regions. For instance, while late Pleistocene sites in western North America are largely controlled by variation in Pacific-sourced winter precipitation and sites along the Gulf Coast are dominated by changes in Atlantic-sourced summer pre- cipitation, records in the mid-continent are both sparse and inconclusive. Speleothems are invaluable as paleorecords as they are able to be dated at high resolution using 230Th and confocal microscopy. Additionally, they retain signatures of oxygen and carbon isotopic variability inherited from changes in hydrological and environmental conditions, and thus can be used to make paleoclimate interpretations. Chapter 2 discusses the modern climate in SE Minnesota, in order to better understand how conditions may have changed during the last glacial period. Southern Minnesota cur- rently receives much of its precipitation during the summer months, and moisture is largely derived from Gulf of Mexico (D. Baker et al., 1978, Simpkins, 1995). When compared to large scale sea surface temperature patterns in the Atlantic and Pacific basins, higher than average temperatures in the Atlantic appear to contribute to a regional decrease in annual precipitation, while temperature anomalies in the Pacific do not appear to have a clear correlation to precipitation amount. Stronger correlations occur between regional precipitation amount and large scale atmospheric patterns. A damping of the ridge-trough system across North America during the winter months, characteristic of a negative Pacific- North American (PNA) teleconnection index, is associated with a strengthening of the Great Plains Low Level Jet and an increase in southerly summer moisture flow and regional precipitation (Higgins et al., 1997). Additionally a weakening of pressure systems across the North Atlantic, characteristic of a negative North Atlantic Oscillation (NAO) index, is also associated with higher than average summer precipitation. Modern data from the Twin v Cities shows that the oxygen isotopic signature of precipitation is positively correlated to temperature, as is modern calcite at Spring Valley Caverns (Mohr, 2000, Dasgupta, 2008). However, a calcite Holocene records indicates that temperature is likely not the primary driver behind oxygen isotopic variability over time (Dasgupta, 2008). Original research is presented in Chapter 3. Six samples were dated by 230Th and confocal microscopy, and two samples were analyzed for stable isotopic variation. Dating and stable isotope sampling resolution was significantly improved on samples SVC06 and SVC05, and a method of automatic band counting resulted in improved estimates for the duration of SVC05 growth phases. Growth phases largely correlate to Greenland interstadial events, indicating an increase in regional temperature and moisture availability. Additionally, four samples experienced the start of a growth phase in the middle of MIS4. This is most likely due to an increase in anti-cyclonic activity over a heightened Laurentide ice sheet in the NE. This activity may have strengthened southerly summer flow, increasing available summer moisture to the region. Stable isotope values show an increase in δ18O and a decrease in δ13C values throughout the last glacial period. In order to achieve higher δ18O values at a lower annual temperature compared to modern, SE Minnesota would either need to experience an increase in fraction of moisture remaining during moisture trajectory or an increase in proportion of Gulf of Mexico moisture. Based off models and proxy records, it seems