EESS Winter Seminar Series, Daniel Stolper, California Institute of Technology
EESS Winter Seminar Series, Daniel Stolper, California Institute of Technology, "Incorporating the nitrogen cycle into the modern O2 cycle using isotopes", Abstract: Molecular oxygen (O2) in the atmosphere is created almost entirely from the photosynthetic fixation of CO2into organic carbon coupled to the oxidation of water to O2. O2has built up in the atmosphere over geological time due to the subsequent burial and storage of a small proportion of that organic carbon into sediments. The biological fluxes of O2into and out of the atmosphere represent the largest fluxes of O2on the Earth and represent how productive the biosphere is. Multiple sources and sinks of O2exist (e.g. terrestrial vs. marine) and must be understood both mechanistically and in terms of their relatives sizes to have a quantitative understanding of the modern oxygen cycle in the present and inform models of the oxygen cycle in the past and future. A critical constraint for models of the modern oxygen cycle is the isotopic composition of atmospheric O2, which is set by the isotopic composition of the photosynthetic fluxes into and respiratory sinks out of the atmosphere. Any successful model of the atmospheric oxygen cycle must independently predict the isotopic composition of atmospheric O2. Current models that use isotopes as a constraint treat respiration as coupled exclusively to organic carbon oxidation. However, the oxidation of ammonia by microorganisms with O2 represents another major sink for O2. Ammonia oxidation is potentially responsible for up to ~10% of global respiration and thus should be included in models of the oxygen cycle. However, the isotopic discrimination of O2 by ammonium oxidizing organisms is completely unknown, making it impossible at present to include its effects on various models of the oxygen cycle that are constrained by isotopes. I will present a framework for understanding the oxygen cycle using oxygen isotopes and a set of experiments to measure the isotopic discrimination of various ammonia-oxidizing organisms.