A high resolution (every 4-5 kyr) benthic foraminiferal oxygen and carbon isotope record was constructed for an approximately 3 million year long section of pelagic sediment spanning the Oligocene/Miocene boundary (22.7-25.6 Ma) at Site 929 (4358 m water depth), on Ceara Rise in the western equatorial Atlantic. Key findings include (1) detailed characterization of a large-scale transient glaciation at the O/M boundary (Mi-1 event of Miller et al., 1992), (2) concentrations of variance in climate and ocean chemistry at periods of 19-23 kyr and 41 kyr throughout the interval, and (3) a gradual increase in the amplitude of low frequency (440 kyr) oscillations in carbon isotopic composition that peaks at Mi-1.To evaluate the nature of the low frequency cycles in oxygen and carbon isotopic ratios we constructed lower resolution (every 20 kyr) records from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise, Site 926 (3598 m water depth). The Site 926 benthic and planktonic records show a similar range of variability in oxygen isotope values suggesting that the higher frequency oscillations recorded prior to and during the Mi-1 glacial event at Site 929 might also reflect variations in ice-volume. Moreover, parallel trends in the carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low frequency cycles and long-term drift towards heavier carbon isotope values represent changes in the global carbon isotope budget.