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This paper explores the role that measurements of changes in atmospheric oxygen, detected through changes in the O2/N2 ratio of air, can play in improving our understanding of the global carbon cycle. Simple conceptual models are presented in order to clarify the biological and physical controls on the exchanges of O2, CO2, N2, and Ar across the air-sea interface and in order to clarify the relationships between biologically mediated fluxes of oxygen across the air-sea interface and the cycles of organic carbon in the ocean. Predictions of large-scale seasonal variations and gradients in atmospheric oxygen are presented. A two-dimensional model is used to relate changes in the O2/N2 ratio of air to the sources of oxygen from terrestrial and marine ecosystems, the thermal ingassing and outgassing of seawater, and the burning of fossil fuel. The analysis indicates that measurements of seasonal variations in atmospheric oxygen can place new constraints on the large-scale marine biological productivity. Measurements of the north-south gradient and depletion rate of atmospheric oxygen can help determine the rates and geographical distribution of the net storage of carbon in terrestrial ecosystems.