I am a Ph.D. candidate in Earth & Planetary Science, and I work with Prof. Paul Renne and Dr. Warren Sharp at the Earth & Planetary Science Department and Berkeley Geochronology Center. My research focuses on elucidating the timing and tempo of human biological and cultural evolution and understanding the environments in which past humans subsisted. We collaborate with professors in the departments of Integrative Biology and Anthropology to address more integrative questions focused on paleoecology and human evolution.
Evidence of human evolution roughly spans a paltry 0.1% of Earth’s history, but the earliest life on Earth is still being understood from more than 3 billion years ago. And yet, we humans have utilized natural resources for every aspect of our daily lives in ways no other creature on Earth has before us. Exactly when in our history, and under what environmental conditions we made evolutionary and creative leaps, is the focus of my research.
The Earth is a dynamic system and understanding its past requires a myriad tool kit, one of which is the rock record. Preservation of the rock record occurs in the three dimensions of space, but to correlate events over vast areas where the rock record can be discontinuous, we need to know the fourth dimension of time. Geochronology puts a time axis on the rock record by utilizing the radioactive elements trapped in rock-forming minerals to tell time. Like radiocarbon dating, the methods I use are based on the physical mechanisms of radioactive decay, where certain isotopes of an element decay at a known rate to produce daughter elements. The geochronologic technique (or “geochronometer”) one uses will depend on the kinds of questions being asked.
In my case, I want to refine our understanding of what environmental conditions were like when humans were evolving, developing stone tools and agriculture, and expanding their geographic range within and out of Africa. For these questions, I need geochronometers capable of producing precise and accurate ages ranging from the nearly modern day back to ~6 million years ago, when our human ancestors, or hominids, first evolved in Africa. I apply 40Ar/39Ar and Uranium series (or U-series) techniques; this allows me to sample a wider variety of environments and rock formations for dating rather than using only one technique. 40Ar/39Ar geochronology derives from the traditional K-Ar technique and can be applied to timescales between ~103 and ~108 years; in human-occupied landscapes, we mostly apply this technique to volcanic ashes produced from explosive eruptions. U-series geochronology is most useful at timescales of ~0 to 750 thousand years, and I sample carbonate rocks that can be found in soils, caves, and as biologically-formed minerals, such as corals and eggshells, often found in association with human-occupied sites.
Measuring the age of a rock requires careful preparation and the use of sensitive instruments called mass spectrometers. We first have to isolate the component of a rock sample of interest, usually a particular mineral, and this can require many steps including crushing, sieving, dissolution, and density, magnetic, and/or chemical separation. We can then measure the isolated rock fraction on a mass spectrometer: this relies on the fundamental relationship between magnetic and electric fields to measure the precise concentration of different masses of ionized isotopes when run through a controlled magnetic field. We know the half-lives of radioactive parent isotopes such as 40K and 238U fairly well, so if we can measure the parent and daughter products precisely, we can determine how long it has been since the daughter products have grown in a rock sample as a result of radioactive decay. Because geologic processes forming minerals and rocks can be complex, slow, or discontinuous, the age of a rock is not
always the simple product of calculating an age based on these measurements. Geochronologists must also understand the relationships between different geologic, physical, and chemical processes, and how this will impact a calculated age. This often requires fieldwork and other laboratory techniques to corroborate our results and interpretations.
I think one of the best parts of geology is fieldwork, and this sets it apart from many other STEM fields. Fieldwork is a respite from the business of modern life – no phone service, no internet, no screens, (also no running water or much electricity), just nature and a time to imagine what the world was like thousands to billions of years ago. One of my projects includes fieldwork in the East African Rift Valley, where among the earliest human origins derive. Here, we sleep in tents for a number of weeks to survey and collect fossils, archaeology, and volcanic ashes for dating. On a typical day, I will get up before the sun rises, eat lots of carbs, prepare my pack with water, a rock hammer, trowel, maps, field notebook, GPS, and other amenities, and then we all head out in a caravan with the professors and the field crew. The geology crew (myself included) surveys the area with paleontologists and archaeologists to see what the relevant finds are in the area, and then we walk out the vertical extent of the sediments by hiking all over the landscape to describe a stratigraphic section encompassing the finds. Here, I am looking for volcanic ashes because they contain minerals that can be dated with 40Ar/39Ar technique. On any given day, we hike 5-10 miles, we see various wildlife (my favorites are ostriches and baboons), and we encounter local people herding animals.
Rocks are like time capsules, chapters in Earth’s history book. But, you have to be able to read the rock record to understand what it is telling you about the past. I am grateful to have been able to return to school for my Ph.D., but it took a lot of personal exploration to get here. In those years between undergraduate and graduate school, I discovered the intersection of my interests and my talents; I also learned what I could do every day with satisfaction, and, just as important, what I could absolutely NOT do every day without going crazy! My favorite part of what I do is the constant learning. I am never bored, and I am always being challenged.