John L Loeb Associate Professor of the Natural Sciences & Co-Director of Graduate Studies
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.
Emma just moved to Boston from the land of the moose, the goose and the beaver. She received her undergraduate degree from McGill University, majoring in Biology and minoring in Geology, and recently submitted her Master’s thesis (also at McGill), which dealt with the fractionation of sulfur isotopes by a mutant strain of Desulfovibrio vulgaris incapable of reducing sulfate. Read more about Emma Bertran
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.
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.
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.
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.