Professor of Earth and Planetary Sciences & Co-Director of Graduate Studies
He/Him/His Primary research interests: Isotope geochemistry and historical geobiology; re-animating ancient ecosystems and ocean chemistry using stable isotope systems, chemical speciation techniques, modern microbial experiments (for calibration) and theoretical considerations.
I'm interested in connections between the geologic and biospheric carbon cycles. Specifically, my work aims to understand how processes occurring in river basins transfer carbon between these two cycles in order to regulate atmospheric CO2 concentrations over geologic timescales. To do so, I combine a suite of isotope geochemistry techniques (including compound-specific isotope measurements and novel reaction monitoring methods) with inverse models, satellite products, and geospatial analysis. My current projects include analysis of multi-year time-series samples from the Ganges-Brahmaputra and Congo Rivers, high-frequency samples from mountainous rivers in Taiwan, isotope analysis of bacteriohopanepolyols in continental shelf sediments, and development of the Ramped PyrOx radiocarbon instrument. I'm additionally working on reconstructing the mechanisms that control Cenozoic CO2 variability using inverse modeling methods.
I combine approaches from the bio- and geo-sciences to address big-picture questions about the history of life on Earth and the potential for life elsewhere. This research is motivated by a desire to understand how life and the planet have changed together through time to reach the state that they’re at today and how that might be different on other planets where the environmental context for life or evolutionary contingency differ. My primary research goal is to understand the origin and evolutionary history of major metabolic pathways that have defined the primary productivity of the biosphere, such as photosynthesis, methanogenesis, and nitrogen fixation. These metabolisms have fueled life on Earth for most of its history, but were not all present at the origin of life. Instead, evolutionary innovations have accumulated through time, gradually increasing the productivity of the biosphere to what it is today. Understanding the origin of these metabolisms can help us to understand how and when life on Earth became productive and began to drive geochemical cycles, and will help us to predict how life may evolve on other planets.
The intimate interaction between microbial life and earth leaves specific chemical signatures that record information on microbial activity, and paleo-environmental conditions. As a geobiologist, my goal is to interpret these signatures and reconstruct an accurate picture of paleo-environments and microbial communities, and the nature of their co-evolution through time. To date, I have focused on the sedimentary biogeochemical sulfur cycle and its main metabolic pathways: microbial sulfate reduction and microbial sulfur disproportionation. These run the reductive and oxidative branches of the sedimentary sulfur biogeochemical cycle, respectively. As such, they respond to and track the evolution of Earth’s surface redox conditions. Deconstructing the net preserved isotopic signature into its individual metabolic components requires a thorough understanding of the biochemistry of each, and of the environmental information enclosed in their specific sulfur isotopic signatures. For this, I apply a wide range of research tools. These include stable isotope geochemistry, environmental observations, pure culture microbial experiments (using both wild-type and deletion mutant strains), modeling of intracellular dynamics, and construction of three-dimensional models of crucial proteins in these key metabolic pathways.
For more details on current projects and a list of publications, please visit my website!
Ana Gonzalez Valdes received her B.A. in Earth Sciences from Columbia University in 2016. At Columbia and Caltech she worked to understand how microbes in unique environments cycle nutrients like phosphorus and sulfur. At Harvard Ana will be spanning the Johnston and Pearson labs and hopes to dig deeper into microbial nutrient cycling and how it can be measured isotopically.
She/Her/Hers Katherine received her BA in Environmental Chemistry from Columbia University in 2014. Since then, she has worked at the US Geological Survey on Quaternary paleoceanogprahic and hydroclimate reconstruction in the Arctic. At Harvard, she will be working in the Johnston and Schrag labs.
Frasier got his BS in Earth Science from Rice University in 2013. While he was at Rice, he did research in biogeochemistry focusing broadly on trying to quantify global carbon fluxes and budgets. Here in the Johnston group Frasier hopes to expand his knowledge of global sulfur cycling and how the global cycles are expressed locally.
Haley received a BA in Geology from Carleton College in 2017. After graduating, she worked as a TA for a geology field camp in New Zealand and as a lab assistant in the Johnston lab. As a graduate student she plans to merge her interests in field geology, sedimentology, and stable isotope geochemistry. She will focus on using minor oxygen isotopes as a proxy to reconstruct past climates.
She/Her/Hers Anna completed a BSc in Chemistry and in Geophysical Sciences at the University of Chicago in 2014. Since 2013 she has worked in a stable isotope ratio laboratory there, measuring oxygen isotope ratios in phosphates contained in small shelly fossils and tooth enamel. Anna’s interests pertain to reconstructing chemical cycling in ancient environments through a combination of field work and laboratory techniques.
Alyssa is a senior concentrating in Chemistry and Earth and Planetary Sciences, with a secondary field in the Comparative Study of Religion. She joined the Johnston group in Summer 2014. Her thesis research involves evaluation of organic molecules as potential proxies for atmospheric oxygen isotopes.
Ben is an isotope geochemist with diverse interests in the Earth sciences. In the Johnston Group, he is currently exploring questions surrounding the evolution of the atmosphere and biosphere over the most recent billion years of Earth history using a triple-oxygen isotope approach.
He received his Ph.D. from the University of Calgary in 2013 for developing a novel stable isotope approach for characterizing oil sands reservoir fluids, and was awarded a Vanier Canada Graduate Scholarship for his work. From 2002 to 2008 he attended McMaster University in Hamilton, Canada where he earned B.Sc. and M.Sc. degrees in Earth and Environmental Sciences. His research during this time focused on compound-specific isotope analysis of lipid biomarkers with application to environmental remediation and monitoring. From 2006 to 2010 Ben was part of the Pavilion Lake Research Project team, a NASA Exploration Analog site, and an international collaborative research project. The project provides insight into the earliest life on Earth, and aims to change the way humans explore outer space.
My work mainly focuses on establishing novel mineral proxies to track sulfur cycle dynamics in deep time, notably carbonate-associated sulfate (CAS). This system, alongside the robust yet infrequent barite record, potentially represents a powerful way to track sulfur and oxygen isotope behavior in order to constrain marine chemistry in the past. Additionally, I have started some culture work with the sulfate-reducing bacterium Desulfovibrio vulgaris, and am currently investigating the evolution of its isotope-fractionating metabolism.