MIT/WHOI Massachusetts Institute of Technology Woods Hole Oceanographic Institution Joint Program in Oceanography/ Applied Ocean Science and Engineering DOCTORAL DISSERTATION U-Th Dating of Lacustrine Carbonates by Christine Y. Chen February 2020 U-Th Dating of Lacustrine Carbonates by Christine Y. Chen A.B., Princeton University, 2013 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 February 2020 ©2020 Christine Y. Chen. All rights reserved. The author hereby grants to MIT and WHOI permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Author.......................................................................... Joint Program in Oceanography/Applied Ocean Science and Engineering Massachusetts Institute of Technology and Woods Hole Oceanographic Institution January 10, 2020 Certified by . David McGee Thesis Supervisor Massachusetts Institute of Technology Accepted by..................................................................... Oliver Jagoutz Chair, Joint Committee for Marine Geology and Geophysics Massachusetts Institute of Technology/ Woods Hole Oceanographic Institution 2 U-Th Dating of Lacustrine Carbonates by Christine Y. Chen Submitted to the Joint Program in Oceanography/Applied Ocean Science and Engineering Massachusetts Institute of Technology and Woods Hole Oceanographic Institution on January 10, 2020, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract Carbonates are prevalent in many modern and ancient lacustrine settings, but reconstruc- tions of past lake levels or environments from such materials have been hindered by poor chronology. Uranium-thorium (U-Th) dating has the potential to fill a gap in current geochronological tools for such archives, but past attempts have been confounded by poor understanding of the complex makeup of lacustrine carbonates, leading to misguided con- clusions on both the utility of certain geochronological tools as well as the age of these deposits. This thesis showcases strategies for the successful application of U-Th geochronol- ogy to two types of lacustrine carbonates: lake bottom sediments and tufa deposits. Chap- ter 2 presents a systematic approach to U-Th dating carbonate-rich lake sediments using the ICDP sediment core from Lake Junín, Peru. Chapters 3–5 seek to demonstrate the descriptive power of combining precise U-Th dates on tufas and other carbonates with geologic observations of their depositional context at all scales—from the outcrop to the microscale. Here, the tufas originate from a transect of closed-basin lakes in the central Andes of northern Chile. With improved sample selection and leveraging of the incontro- vertible constraints of stratigraphy and coevality, we are able to test the validity of U-Th data. Combining quality-controlled geochronological constraints with careful characteri- zation of different carbonate facies can yield new insight on the character of lakelevel changes. These case studies offer frameworks for interpreting scattered geochronologic data of any size or system. By embracing the noise in our data, we now have a richer understanding of the controls on uranium in these deposits. Of all the lessons learned, we hold the following as most important: for the determination of the age of lacustrine carbonates, geologic context—in the form of sedimentological observations, additional geo- chemical data, and paleoecological descriptions—is of equal importance to the numerical accuracy and precision of geochronological measurements. 3 Thesis Supervisor: David McGee Title: Associate Professor 4 Biography Christine Yifeng Chen (陈一枫) is the only child of Dr. Xinghao Chen (陈星浩) and Lanying Tian (田兰英). Her parents immigrated to the United States to continue their education after rules restricting the age of university students prevented them from doing so in their home country. Her father first arrived in the United States at the age of 27 bywayof New York City on January 20, 1985; he remembers the date well because it was the day of President Ronald Reagan’s second inauguration. Using most of the money he had on him, he bought a bus ticket to Newark, New Jersey, to make his way towards Rutgers University, where he had been accepted for the doctoral program in Electrical Engineering and Computer Science. Her mother joined him two years later after getting accepted to the same program. After completing a masters degree in 1990, her mother gave birth to Christine on March 12, 1991. With the help of grandparents from both sides of the family, her parents raised Christine while her father completed his dissertation in 1993. Wanting to give Christine the best childhood possible, both of her parents made the decision to remain in America, despite being far away from family in China. In 1994, the family moved to the Greater Binghamton Area as both parents started jobs at IBM in Endicott, NY. There, they lived in a home with a backyard where Christine spent the majority of her childhood, exploring the nearby woods and creeks with her best friends down the street. Together, they wrote stories and made drawings about their adventures together. Second to spending time outside, going to school was her next favorite activity. She was always stymied by the question, “What is your favorite subject?” because all of them were fun and interesting to her in different ways. Christine attended grade school until graduating from Union Endicott High School in the spring of 2009. That fall, she enrolled at Princeton University on a full-tuition financial aid scholarship. She stepped onto campus with the intention to major in Geosciences, not only because of her interest in soil and agriculture, but also because the course catalog indicated that it was the only major to offer free field trips. Her first geology trip in October 2009 to southern California markeda major turning point in her life: scrambling down steep slopes of volcanic craters, crawling through narrow spaces of water-carved canyons, and ascending the majestic star dunes in Death Valley, she was the happiest she had ever been. The most lasting impression of the trip was made by Mono Lake and its oddly sculptured spires and knobs of tufa, exposed due to the lowering of lake levels from human demands on water. The fact that she devoted her dissertation in part to the study of tufas, many years later, may be more than a happy coincidence. 5 Acknowledgments Like all scientists, I stand on the shoulders of giants. This work would not have been feasible without the efforts of countless others before me. I have added my piece tothe edifice of human knowledge, and I hope others will find it sound, sound enough tobuild, modify, or improve on. Likewise, the fact that I am graduating with a PhD from MIT and Woods Hole was an improbability only made possible by the collective effort of all those who have invested in me at different points in my life. Where do I begin? I suppose from the start. My mother and father both came to America on student visas in the 1980s to continue their education after some delays in doing so in their home country. I am grateful to the United States of America for providing them and other immigrants the opportunity to pursue their dreams of self-betterment and a better life. As they completed their degrees at Rutgers University, they were also able to experience a slice of the American way, and they noticed that kids here seemed to have a lot of fun: they played sports, like soccer or basketball; some had hobbies playing musical instruments or making art; and kids hung out with other kids, often terrorizing the neighborhood. In short, kids seemed happy. And so, despite being in a foreign place thousands of miles away from all other family, they decided to stay in America to raise me, so that I might have a happy childhood. And I did. I am grateful to my parents for making that sacrifice. Growing up, I was lucky to live on a street with two other girls of essentially the same age as me: Debbie Miller and Laura Jurewicz. I cannot think of any two more perfect people to have grown up with. Somehow, we found each other. These are my strongest memories: crash courses on red wagons and black sleds; expeditions up the creek that felt like our secret; climbing Zarathustra and getting pine needles and sap in our hair; scavenger hunts and board games created from our imagination; riding our bikes recklessly up and down and around the steep curve of our street; and lighting things on fire that we probably shouldn’t. There are other memories, less clear, but all warm. When I think about the kind of person I want to be, it is the person who we would have dreamt of being back then. I am grateful to them for making me feel like I could be myself and still belong. I had several teachers in grade school who supported my diverse interests and long after- school stays. Sadly, some of their names now escape me. But I can remember Mrs. Simon, Mr. and Mrs. Stanko, Mr. Materese, Mrs. Golden, Ms. Henry, Mr. Friend, Ms. Trupp, Mr. Johnson, Mr. Hubert, Mr. Rinde, and my 6th grade teacher whose birthday is on June 4 and whose favorite music artist is Pink. I am grateful to them for making me feel at home. In college, I spent the majority of my time ensconced in the incomparable Guyot Hall, a timeless building with gargoyles of fossils and even a geologic feature named after it (yes, not the other way around). There, in Guyot 16, a dusty and old wood-trimmed room with models of mineral lattices cluttering the ceiling, is where I first met Adam Maloof, to whom I owe everything.