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Rock varnishes are ubiquitous in arid regions on Earth, and are believed to be commonly present on Mars. Here we report high water-soluble sulfate and nitrate contents in desert varnishes from the Death Valley region of southwestern U.S.A., and that sulfate in varnishes possess delta O-17/delta O-18 ratios that do not fall on terrestrial mass-dependent fractionation line. Sulfate from wet and dry atmospheric deposition is probably the source of the delta O-17 anomalies. The anomalies are only moderately lower than that of aerosol sulfates collected from the greater Los Angeles area, indicating probably more than half of the sulfate in desert varnish is supplied by atmospheric deposition. This finding suggests that Earth surface environments are constantly accumulating delta O-17-anomalous sulfate from the atmosphere: arid and stable conditions facilitate the preservation of these atmospheric signatures. This finding also indicates that different delta O-17/delta O-18 ratios found in different components in the Martian meteorites may result from atmospheric chemical processes. Copyright (C) 2001 Elsevier Science Ltd.