Dr. Sierra Petersen is an Associate Professor at the Department of Earth and Environmental Science and has recently joined the Museum of Paleontology with a Research Scientist appointment. Sierra is a stable isotope geochemist who specializes in mollusk shells and the geochemical signatures preserved in them to interpret past climates and marine environmental conditions. Her work has greatly contributed to the development and use of carbonate clumped isotope thermometry for quantifying past global temperature. She has used the mollusk fossil record to quantify local temperatures during past hothouse climates and transitional periods, such as the Cretaceous and Paleocene Thermal Maxima and Eocene-Oligocene transition, and to address temperature changes through the end-Cretaceous extinction event. Most recently, her work has shifted to include understanding how species behavior and biology affects the isotopic signatures preserved in shells.
Sierra obtained a BS from the California Institute of Technology, and she has a MS and PhD degree from Harvard University. She initially joined the University of Michigan as a Postdoctoral Fellow in the Department of Earth and Environmental Sciences and was later appointed as an Assistant Professor. Sierra was also recently promoted to Associate Professor in the Earth Department. She kindly shared with us some insights into her passion for research and profound interest on the marine fossil record.
How were you drawn to work with fossils?
I first came to work with fossils as a window to past climate. My primary training is as an isotope geochemist, paleoclimatologist, and paleoceanographer. I study the chemistry of fossil shells to reconstruct past climate. Most of this work is on marine mollusks, reconstructing past ocean temperatures over the Cretaceous and Cenozoic.
Coming from this angle, I initially considered fossils to be a tool with which I could reconstruct past climate. I treated a shell fossil like a black box, faithfully recording ancient ocean conditions in its chemistry, and ignored the fact that the shell was made by an organism. As I’ve spent more time looking at fossil mollusks, collecting them myself from the field, and thinking about how and when and why a mollusk grows its shell, my thinking has evolved.
Fossil shells are actually IMPERFECT recorders of climate because they are made by organisms with preferences for when and how they like to grow their shells. Mollusks grow shell at different rates depending on their age, the season, the water temperature, etc. and sometimes stop growing altogether if it gets too hot/cold. They aren’t found everywhere I’d like to be able to reconstruct past conditions. Instead of these facts ruining my ability to perfectly reconstruct ancient ocean conditions, I now view these as a whole new source of information about the organisms themselves. This has made me love fossils even more!
What is the piece of research you have been most proud of?
It’s hard to pick just one! My current students are pursuing exciting work in both the Cretaceous Western Interior Seaway and the Plio-Pleistocene Western Atlantic. In the later project, we are trying to explain a regional molluscan mass extinction event. We are bringing a combination of morphometric and geochemical techniques to bear to understand the drivers of extinction and impacts on molluscan species. Stay tuned for some splashy results of this project in the years to come.
I am also very proud of work out of my group redefining vital effects in mollusks (Curley et al., 2023; Biological Reviews). We have shown that geochemical differences between the inner and outer shell layer in bivalves (we call these biologically derived isotopic fractionations or BioDIFs) can tell you something about shell precipitation mechanisms and processes going on inside the (no longer preserved) soft body of the mollusk. Most excitingly, these BioDIFs show the characteristics of a trait – they are the same within a single species even under different environmental conditions. We have proposed that this could be used to study the evolution of shell precipitation mechanisms and can be used to test phylogenies created using independent methods (e.g. morphometrics, genetics). This new way of thinking has opened up many new research questions/directions, which is always the sign you’re onto something good!
In your own words, why is it important to study the fossil record?
It’s important to study the fossil record for the same reasons it’s important to study past climate. The brief snapshot of climate and life experienced directly over the past few hundred or thousand years of human history represents such a narrow slice of the range of conditions Earth has experienced over its existence. There is no way we can accurately understand slow processes like evolution or long-term climate feedbacks without taking a wider view.
This becomes increasingly important when we look to our (much hotter) future. There are no analogs for such a warm future in the past few hundred years – we need to go much farther back to find an Earth that was as warm and to see how life responded to warmth like that. This is why I love studying time periods like the Cretaceous Thermal Maximum – life and climate were so hot and foreign then, it’s hard to even imagine that world!
I also think studying the fossil record gives me a healthy perspective on where we as humans fit into the bigger picture of life on Earth. Just another mammal! If Earth history was one year, we are a blip before midnight on New Year’s Eve. Lots of other forms of life were at this party before us.
