Studies of the terrestrial molecular oxygen and carbon cycles in sand dune gases and in biosphere 2

Molecular oxygen in the atmosphere is coupled tightly to the terrestrial carbon cycle by the processes of photosynthesis, respiration, and burning. This dissertation examines different aspects of this coupling in four chapters. Chapter 1 explores the feasibility of using air from sand dunes to reconstruct atmospheric O{dollar}sb2{dollar} composition centuries ago. Such a record would reveal changes in the mass of the terrestrial biosphere, after correction for known fossil fuel combustion, and constrain the fate of anthropogenic CO{dollar}sb2.{dollar} Test drilling in sand dunes shows that sand dunes do contain old air, as shown by the concentrations of chlorofluorocarbons and {dollar}sp{lcub}85{rcub}{dollar}Kr. Diffusion is shown to dominate mixing rather than advection. However, biological respiration in dunes corrupts the signal, and isotopic analysis of O{dollar}sb2{dollar} and N{dollar}sb2{dollar} shows that fractionation of the gases precludes use of sand dunes as archives. Chapter 2 further explores this fractionation, revealing a previously unknown "water vapor flux fractionation" process. A flux of water vapor out of the moist dune into the dry desert air sweeps out the other gases, forcing them to diffuse back into the dune. The heavy isotopes of N{dollar}sb2{dollar} and O{dollar}sb2{dollar} diffuse more slowly, creating a steady state depletion of heavy isotopes in the dune interior. Molecular diffusion theory and a laboratory simulation of the effect agree well with the observations. Additional fractionation of the dune air occurs via thermal diffusion and gravitational settling, and it is predicted that soil gases in general will enjoy all three effects. Chapter 3 examines the cause of a mysterious drop in O{dollar}sb2{dollar} concentrations in the closed ecosystem of Biosphere 2, located near Tucson, Arizona. The organic-rich soil manufactured for the experiment is shown to be the culprit, with CO{dollar}sb2{dollar} produced by bacterial respiration of the organic matter reacting with the extensive concrete surfaces inside. Chapter 4 examines the O{dollar}sb2{dollar}:C stoichiometry of terrestrial soil respiration and photosynthesis, in the context of using atmospheric O{dollar}sb2{dollar} measurements to constrain the size of the "missing sink" of CO{dollar}sb2{dollar}. Direct measurements of soil respiration and biomatter elemental abundance suggest a value of 1.1 {dollar}pm{dollar} 0.05 oxygen molecules per CO{dollar}sb2{dollar} molecule.