Investigating Mexican paleoclimate with precisely dated speleothems

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2020.Speleothems, or sedimentary rocks formed in caves, act as valuable archives of past climate change due to their suitability for U-series dating and high-resolution proxy analysis. These records can provide insights into water availability and controls on hydrology prior to the instrumental record. In this thesis, I present three records from newly-analyzed Mexican stalagmites using stable isotope (oxygen and carbon) and trace element to calcium (Mg/Ca and Sr/Ca) ratios as proxies for changing hydroclimate. Chapter 2 presents a precisely dated, mid-Holocene record of high rainfall and limited precipitation variability in the Yucatan Peninsula, Mexico. Chapters 3 and 4 present novel climate records from northeastern Mexico, an understudied region of North America. Both records come from cave sites within the Mexican arid zone, which is simultaneously experiencing increased water scarcity and a rapidly growing population. In Chapter 3, I examine a speleothem from the first millennium of the Common Era, which showed that there is a precipitation dipole between northern and southern Mexico. Chapter 4 highlights, for the first time at decadal resolution, the northeast Mexican response to the 8.2 ka event and the Younger Dryas. These chapters show that the San Luis Potosí region is vulnerable to droughts under multiple climate mean states, and is subject to drying as Atlantic Meridional Overturning Circulation weakens due to anthropogenic climate change. The climate records detailed in this thesis improve our understanding of controls on Mexican hydroclimate and can serve as benchmarks for climate models.This work was funded by US National Science Foundation (NSF) grants AGS-1702848 (M. Medina-Elizalde), AGS–1502877 (S. Burns), AGS-1804512 and AGS-1806090 (K. Johnson and D. McGee). I was also supported by the NSF Graduate Research Fellowship and the MIT School of Science Dean’s Fellowship. Fieldwork and analysis were funded by the WHOI Ocean Ventures Fund, the MIT EAPS Student Research Fund, and the MIT International Science and Technology Initiatives (MISTI) Mexico program. Initial work for this project was also supported by UC MEXUS-CONACYT Collaborative Grant from the University of California Institute for Mexico and the United States (UC MEXUS CN-16-120). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation