A 47‐ to 60‐mm‐thick Fe‐Mn crust from Horizon Guyot (water depth 1800–1780 m), central Pacific, was used to evaluate the potential of crusts as recorders of Neogene paleoceanographic and paleoclimatic conditions. The chemical composition was determined by microprobe for 16 elements from a polished thin section. Three analyses were made per millimeter and averaged to give the composition of each millimeter. The age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the Tertiary seawater curve. The crust represents 18.5 m.y. of growth of Fe and Mn oxyhydroxides. The crust is composed of alternating botryoidal and laminated layers. The botryoidal layers formed during the same time intervals that widespread Neogene deep‐sea hiatuses were forming in bottom sediments. The botryoidal layers represent growth during times of intensified deepwater flow, whereas the laminated intervals represent more quiescent conditions. The correspondence between the botryoidal layers and the Neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust. Chemical changes in the crust take two forms. The first is represented by broad changes in the composition defined chiefly by fourth‐order polynomial fits to the chemical profiles of each element with depth in the crust. The second is high‐frequency changes in composition. The broader changes occurred primarily at about 15, 11.5, 7.4, 6.4, 5.2, and 4.6 Ma, which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles. The periods of the high‐frequency changes may reflect climatic changes that resulted from orbital forcing. These high‐frequency changes may correspond to the high‐order eccentricity periods of 3.4
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