Oxygen isotope fractionations among silicates, carbonates, and sulfate from Nakhla and Lafayette can be used to resolve how multiple oxygen isotope reservoirs formed and evolved on Mars and to gain insight into the environment and processes that led to the formation of the SNC carbonates and other secondary minerals. Carbonates and sulfate from Nakhla and Lafayette carry an imprint of atmospheric chemistry analogous to that documented on Earth but reflect an inhospitable environment where ozone, hydrogen peroxide, and odd oxygen compounds transferred their signature of atmospheric oxygen isotope fractionations to water, ice, and minerals that formed in the regolith at low temperature. Such an environment would not be conducive to the preservation of easily oxidized organic compounds. Its existence provides an abiotic, isotopic benchmark. Biotic processes should produce isotopic fractionations that lie on a coherent mass fractionation line and not the mass-independent fractionation relationships observed in the present study. ¿ 2000 American Geophysical Union