CAREER: Robust Isotopic Constraints on Primary Productivity from First Principles

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The response of the global biosphere to climate change is uncertain. In particular, biosphere productivity in the present and in the past is difficult to estimate because the available tools are technically challenging to apply in the varied and often poorly understood natural environments in which life can thrive. This study will improve estimates of biosphere productivity in the present and the past through an approach that unifies theory and experiments on the biochemical reactions that drive photosynthesis and respiration. The results will be applied to determine patterns of surface-ocean productivity over the past several decades using a new compilation of existing data that will be made available to the research community. Finally, the project will support a program of annual teacher-training activities aimed at global-change science at the middle- and early-high school level, where curricula and training are currently lacking. Working with the Rice Office of STEM Engagement and the Harris County Department of Education, the investigator will host teachers for summer laboratory experiences and an annual 1-day workshop to train Houston school teachers, develop lessons on global-change science, and cultivate early connections between the geosciences and core science/math disciplines.One of the key tools for assessing past changes in biosphere productivity is stable-isotope analysis. Of this family of approaches, the isotopic composition of molecular oxygen (O2) has been applied widely to constrain the biogeochemical carbon cycle because oxygen is linked to carbon cycling at the metabolic level. Covariations between 18O/16O and 17O/16O ratios, in particular―the “triple-isotope” composition of O2―have been of great utility for determining both modern and past biosphere productivity. Even remnant signals, transferred more than a billion years ago from O2 into rocks, have been used for this purpose. However, the interpretations of such isotopic data are only justified if the biological fractionation processes are well understood. The signals on O2 are quite subtle, and the technical difficulty of triple-isotope measurements has meant that few of the foundational experiments have been repeated and validated. This project will re-evaluate the fundamental O2 fractionation factors for respiration and photosynthesis using new experiments and the tools of computational chemistry. A central hypothesis is that biological oxygen-isotope fractionation can be predicted accurately from first principles to benchmark, improve, and unify oxygen triple-isotope tracers across geoscience fields. The project will also test the sensitivity of biological O2 isotopic fractionation to environmental boundary conditions to evaluate the impacts of environmentally forced isotopic variability on the interpretation of local and global O2 budgets. As a capstone, the oceanographic triple-oxygen field data collected over the past 20 years will be compiled and re-interpreted in light of this new information.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.