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Reinterpretation of the Elatina Rhythmite Fold Structures: Evidence for
a Seasonal Slushball Earth and Giant Impact Lunar Formation ~ 4.4 Ga

Adviser: Adam C. Maloof

Zachary M. Morse


“I also was motivated by challenging what other people had concluded ...”


I was definitely a bit of a procrastinator in college and chose my thesis topic late for a science major. It was after fall break of senior year. I had shopped around for a topic and adviser a little over the summer but also worked full time and didn’t complete any research. Back on campus, I spoke with a few professors and was told that I should have done research over the summer and that I couldn’t say anything significant by doing research only during the school year. Then I spoke to Professor Adam Maloof. We knew each other first from a freshman seminar I had taken on a whim (mostly because I wanted to go to California over fall break with the class) called “Active Geological Processes” and then from working together on my second junior paper. Despite not having experience in most of Professor Maloof’s areas of research and not wanting to continue researching the topic of my junior paper, I knew I wanted to work with him again.

My first nugget of thesis advice is to think about your choice of adviser as much as your choice of topic. I wanted to work with Professor Maloof because he was incredibly enthusiastic about every topic we discussed and was going to be a great adviser. I knew he would push me to pursue new avenues of research and to try to answer as many questions as I could with my research. I also knew that he had high standards for academic work and so would make me work as hard as I could. Professor Maloof and, by extension, his graduate student, Catherine Rose, were also willing to share research and knowledge with me throughout my thesis. They taught me lab techniques, gave me rocks to use for research, shared field photographs with me, and discussed any questions I had. These were important qualities for my adviser to have, and I think everyone should think about what is important to them in an adviser. It would also be wise to speak with potential advisers before the end of junior year.

Before I left for fall break, Professor Maloof explained the significance of the Elatina Formation, the rocks I would be studying for my thesis, and the type of research I could likely complete on it during that school year. During fall break, I spent a week in Tunisia and read about a dozen papers on tidal bed forms and the Elatina Formation as background, and I was hooked on the topic. It was significant and challenging but not esoteric, and I was definitely breaking into new territory. My previous research and coursework had focused on geochemistry, oceanography, and geomorphology. I was able to change gears by taking relevant coursework, reading as much background as I could get my hands on, and talking to Professor Maloof and Catherine Rose. Professor Maloof’s course “Sedimentology” was critical in shaping my understanding of the physical processes relevant to my topic. My second nugget of thesis advice is to not be afraid to step out of your comfort zone and research a new topic. In addition to giving you a strong foundation in the fundamentals of a discipline, I think a big part of undergraduate education is to teach you to think like a geoscientist, economist, sociologist, etc. Trying a new topic is not better than sticking with a topic you already know a lot about, but there is something to be said for stretching your ability to think like a researcher in your discipline.

Another aspect of my thesis that I loved was that I was able to contribute to knowledge in my field. I examined Australian sedimentary rock of the Neoproterozoic age (~650 million years ago) known as the Elatina Formation. This point in Earth’s history is critical because it is about 100 million years before the Cambrian explosion, which is the first appearance of complex animal fossils in our records, and any major geological events during this period could have affected the course of evolution. There is evidence of at least two long-term glacial events during the Neoproterozoic age on every continent except Antarctica. These data indicate that every continent on Earth was covered in glaciers at the same time. The two logical hypotheses explaining this phenomenon are that every continent existed at high latitude or that the entire Earth was covered in ice. In 1992, paleomagnetic data from the Elatina Formation indicated that glaciers existed at the equator, which led geologists to the theory that global glaciations had occurred. The theory is called Snowball Earth.

The Snowball Earth hypothesis comes in two main forms, icehouse Earth and slushball Earth. The icehouse Earth theory says that Earth was fully encased in ice year-round, even the equatorial ocean. The slushball Earth theory says that there was an open equatorial ocean and active hydrological cycle during Snowball Earth. The extent of ocean ice during Snowball Earth is a key question because early life developed in the ocean. A fully capped ocean would have killed everything. Open ocean at the equator would have allowed the survival of specific organisms. Computer simulations of Earth’s ocean-atmosphere system during Snowball Earth have been used to test the probability of both scenarios and indicate that a slushball scenario was more likely, but rock-based evidence for pointing to one or the other of the two scenarios has been slower in coming to light. My study of the Elatina Formation cast doubt on the existence of glaciers at the equator during Snowball Earth and answered the question of the extent of sea ice during Snowball Earth using evidence taken directly from sedimentary rocks.

What I did first was to reexamine structures within the Elatina Formation previously interpreted as soft sediment folds and used for paleomagnetic fold tests. These tests determine the latitude at which a rock hardened but only work if the fold in the rock happened after hardening. I concluded that the folds are ladder ripples, which are formed by current and wave action during the deposition of material on the sea floor before it turns into rock. Therefore the paleomagnetic data derived from these ripples is not conclusive evidence of low-latitude glaciation during Snowball Earth and must be reexamined in the context of ripple formation rather than folding. I concluded that the case for global, low-latitude glaciation as it has been presented previously in the literature must be considered without the paleomagnetic evidence from the Elatina Formation, and the extent of low-latitude glaciation is certainly in question.

What I did next was to polish samples from the Elatina Formation using a lapidary wheel, which is basically a massive power sander for rocks. I scanned digital images of the polished rock faces so that I could count and measure individual layers within the rock. The thicknesses of the layers are representative of the size of the outgoing tide on a day during the Neoproterozoic age because each layer of the rock was created by one outgoing tide depositing sand and silt on the sea floor. I found that the thicknesses of the layers increased and decreased with the same frequency as the size of the tide changes due to the alignment of the sun and moon. I expected to find this when I started the project but was still in awe when it worked. The rock was recording the frequency of the relative movement of the Earth, moon, and sun! The most interesting period within the thickness of the rock layers was very similar in length to the period of a modern tide that is caused by changes in atmospheric pressure and thermal expansion of seawater. These changes are tiny and the tidal range, the difference between high and low tide, from the modern atmospheric-thermal tide is less than 1 centimeter. Because this tide showed up in my data, it is clear that there was open water during Snowball Earth. The sun could not have heated the ocean under thick sea ice nor could atmospheric pressure move thick sea ice enough to change the amount of deposition. My adviser disagrees with this conclusion concerning Snowball Earth, but he always encouraged me to use my observations and to draw my own conclusions. His skepticism and questions were very much appreciated because they forced me to defend my observations and search for support in existing literature, thereby improving the quality of my argument.

What I did after that was to use my ancient tidal data to calculate an approximate time at which the moon might have formed by the impact of a Mars-sized celestial body with the Earth. My adviser and I didn’t expect that I would have time to complete this research but as my project developed we found that the ripple structures in my samples provided an opportunity to make it happen. This process was complicated but incredibly satisfying.

My third nugget of thesis advice is to pick a thesis topic that will consume you and motivate you. I loved what I worked on, and every time I figured out the answer to a question it made me want to learn more. I also was motivated by challenging what other people had concluded and adding to knowledge in my field. I was able to offer a new perspective on rocks that have been the linchpin in a prominent theory concerning Earth’s history and the evolution of complex life. My thesis presentation also was an incredible experience because I got to teach people about a topic that had consumed my attention for seven months. If I had not been so interested in and motivated by my topic, I don’t think the process would have been nearly as enlightening and enjoyable.

Whatever department you decide to join on your winding road through Princeton, I sincerely hope your thesis experience is as challenging and satisfying as mine.

Reinterpretation of the Elatina Rhythmite Fold Structures: Evidence for
a Seasonal Slushball Earth and Giant Impact Lunar Formation ~ 4.4 Ga

Zachary M. Morse

Adam C. Maloof

Assistant Professor of Geosciences

“That is one of the most important things the Princeton senior thesis teaches students—to be creative and independent.”

When I took Zach Morse on as a thesis advisee, I was a bit worried. I feel very strongly that one of the most important skills to have before starting a thesis is the ability to collect and analyze data. By data collection, I mean anything from conducting a well thought-out survey on a street corner, or designing a rigorous tree-diameter measuring campaign on campus, to mining and digitizing data from published sources. And by data analysis, I mean pairing quantitative interrogation using the basic tools of statistics, image processing, spectral analysis, etc., with creative interpretation. However, in four years at Princeton, I have found that these are the rarest skills among seniors. Even when interpreting a political poll, a day on the stock market, or a report about climate change, students should have a basic knowledge of statistics and models, yet many students start their thesis research without these skills. For this reason, data collection and analysis are the central themes in all my undergraduate courses, whether the courses are designed for freshmen or upperclassmen.

So Zach came along, and I worried about his data analysis skills. He also came to me late in the game, long after summer had passed and opportunities for extended data collection had evaporated. However, I knew Zach was creative and very smart from a freshman seminar field trip I had assisted with. I also supervised one of Zach’s junior projects (JPs), and was pleasantly surprised by Zach’s ability to learn new techniques in Geographic Information Systems (GIS) with the help of GIS Librarian Wangyal Shawa. While working together on the JP, Zach also impressed me with his writing skills—I remember being floored by how effectively he communicated the most salient problems in the field and the relevance of his work in the introduction to his JP. The JPs are very important preludes to the senior thesis and virtually unique to Princeton. On the one hand, the JPs help the student identify their strengths and weaknesses so they can design their last year of coursework to fill in the holes. And just as importantly, the JPs give the student chances to interact closely with different faculty, so that students and faculty can decide who they want to work with on the much more in-depth senior thesis.

For his senior thesis, Zach chose to study rhythmically laminated sedimentary rocks that were deposited during the final Cryogenian glaciation, approximately 650 million years ago. These rocks are crucial to our understanding of Earth’s evolving climate because they represent the best evidence in the world for equatorial glaciation at sea level. The theory goes that if you had glaciers at sea level at the equator, Earth must have been so cold that the oceans would have been sealed completely in ice—the “Snowball Earth.” But the Snowball Earth hypothesis is controversial for a number of reasons, perhaps most importantly, because organisms as complex as animals seem to have survived the climate disaster. The rhythmically laminated sediments that Zach started work on had been interpreted as tidalites in the 1970s—in other words, the thickness and coarseness of each layer, each less than 1 millimeter, was controlled by the strength of the ocean tide. So theoretically, a detailed study of these layers could tell you about the hierarchy of tidal frequencies 650 million years ago, and thus constrain the history of the Earth-moon orbit and possibly even the distribution of ocean basins and sea ice during the Cryogenian glaciation!

This ambitious project involved Zach fitting a suite of rough and broken samples into a single polished slab for time series analysis. He had to consult with my graduate student, use a range of instruments in our lapidary, and even work with Princeton’s masonry and paint shops. Zach took tremendous initiative, and when he arrived in my office one day with a precisely oriented and spectacularly polished slab containing thousands of sedimentary layers, I knew that Zach was going to piece together a very special senior thesis.

Next came the data analysis. Again, I worried that Zach, like most Princeton seniors that come to geosciences, was not prepared due to a lack of hands-on quantitative analysis in his coursework. Well, I may have been right that Zach could have benefited from some classes more focused on real data analysis, but in some ways, I was totally wrong to worry. Zach approached the problem with confidence and ingenuity. He met with Professor Frederik Simons in geosciences to adapt Frederik’s MATLAB routines for counting tree rings to the task of measuring tidal rhythmite laminae. Zach and I worked together to design a data collection and analysis strategy, but the results were a bit disappointing. The sediments were not nearly as periodically laminated as the literature would have us believe, and the results from blind spectral analysis were barely useful.

But what I love about Zach is that he was not perturbed at all by the initial setback. Instead, he came to me a few days later with a set of very insightful new observations based on detailed visual inspection of the rock slabs. It appeared that the individual laminae themselves thickened and thinned away from natural ripples in the sand formed when wave action influenced the bed of sediments. Zach found that the most complete laminae records were preserved on the lee-side of ripples. He then targeted his spectral analysis and found spectacular results. With any student, I always look forward to that moment when the student has learned enough that I can start learning from them, and not just vice versa. This moment is when the senior thesis becomes particularly rewarding (at least for me)! By this time, Zach was virtually independent. Each week, he would talk to me about a new analysis, and I was continually astonished by his creativity. In the end, Zach concluded that the anomalously strong annual forcing in his tidal record, typically caused by seasonal changes in equatorial atmospheric pressure (rather than the gravitational pull of the moon and sun), required that the ocean was not sealed in a kilometer-thick carapace of sea ice as snowball theory proposed (in other words, the ocean needed to communicate with the atmosphere). Zach determined that his data were in direct conflict with the Snowball Earth hypothesis, and spent the remainder of his thesis delving into the literature and proposing an alternative, less severe, ice age. If Zach is correct, suddenly the late Cryogenian ice age became a lot less difficult for algae and animals to survive.

Zach did not get to publish his thesis before he graduated and started his new job in New York City. However, his analysis is certainly of sufficient quality and broad interest to be published in a peer-reviewed journal. Knowing Zach, I would not be surprised if one morning I come into work to find a draft of his thesis work ready to submit for publication. That is one of the most important things the Princeton senior thesis teaches students—to be creative and independent.