Collaborative Research: Illuminating the Cenozoic Alkenone pCO2 Record

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). This study will improve the foundation of a widely used geologic proxy for atmospheric carbon dioxide. To correctly predict the magnitude of future global warming from rising greenhouse gas (GHG) concentrations, it is essential to understand the relationship between GHGs – particularly carbon dioxide (CO2) – and Earth’s temperature. One way to investigate this relationship is to examine how Earth’s temperature and CO2 levels have varied together in the past. Direct measurements of atmospheric CO2 extend back only to 1958, while measurements from bubbles of atmosphere trapped in ice cores extend back to about 1 million years (Ma) ago. Any estimate of CO2 older than about 1 Ma requires the use of a proxy for CO2 concentrations. One such proxy is based upon how ‘picky’ marine algae are about the two different isotopes of carbon. When algae grow with high CO2 levels they mostly take up the isotope carbon-12, and use less of the isotope carbon-13. When they grow with low CO2 levels they take up relatively more carbon-13. However, recent findings show that marine algae may also change how ‘picky’ they are due to other factors besides CO2 levels. The primary goal of this project is to investigate how these other factors affect the algal CO2 proxy through controlled laboratory experiments and analysis of modern ocean sediments. These findings can then be applied to ancient ocean sediments, in order to improve interpretations of reconstructed CO2 levels in the past. This project will train a post-doctoral investigator, a Ph.D. student, and two summer undergraduate interns.Accurate reconstructions of past atmospheric pCO2 levels can improve predictions of future warming by providing geologic tests during Earth conditions that were different from the short observational record. An important proxy for CO2 concentrations during the past 55 million years is the carbon isotopic composition of long-chain unsaturated ketones (alkenones) from the Noelaerhabdaceae family of algae. The crucial assumptions of the existing proxy are that: (i) carbon isotope fractionation (Ep) by these algae is set by the rate of diffusive supply of CO2 relative to the rate of carbon use, and (ii) the fractionation is primarily governed by the carbon-fixing enzyme RuBisCO, with an assumed isotope effect of roughly 25 per mil. Recent work challenges these assumptions, suggesting that non-diffusive supply of CO2 is ubiquitous, that kinetic RuBisCO fractionation in these algae may be as small as about 11 per mil and that irradiance independently influences carbon isotope fractionation. Additional information is needed to incorporate these new factors into quantitative reconstructions of pCO2, including re-interpreting Cenozoic alkenone Ep values. Project objectives include:i. Measure the response of alkenone Ep to variations in irradiance, cell size, and pCO2 in laboratory chemostat and dilute batch cultures,ii. Revise the quantitative framework for alkenone paleobarometry, developing practical equations to use for paleo-pCO2 reconstruction, andiii. Test this revised laboratory-based framework with new sediment analyses from the modern and Pleistocene ocean.The project goal is to provide a framework, based in mechanistic rather than empirical evidence, for understanding alkenone Ep variations during the Cenozoic. This will help develop an equation relating Ep to CO2 that uses variables that can be measured or easily estimated, and that could be adopted readily by the paleoclimate community.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.