Detailed analysis of tropical climate dynamics is lacking for the Early to Middle Miocene, even though this time interval bears important analogies for future climates. Based on high-resolution proxy reconstructions of sea (sub)surface temperature, export productivity and dust supply at Ocean Drilling Program Site 959, we investigate orbital forcing of the West African monsoon in the eastern equatorial Atlantic across the prelude, onset, and continuation of the Miocene Climatic Optimum (MCO; 18-15 Ma). Along with previously identified eccentricity periodicities of ~400 kyr and ~100 kyr, our records show that climate varied on ~27-17 kyr, ~41 kyr, and ~60-50 kyr timescales, which we attribute to precession, obliquity, and their combination tones, respectively. Three intervals with distinct variability were recognized: (1) strong eccentricity, obliquity, and precession variability prior to the MCO (18.2-17.7 Ma), (2) strong influence of obliquity just after the onset of the MCO (16.9-16.3 Ma) concurring with a node in the 2.4 Myr eccentricity cycle, and (3) dominant eccentricity and precession variability during the MCO between 16.3 and 15.0 Ma. Sedimentation at Site 959 was influenced by orbitally-paced variations in upwelling intensity and North African aridity related to West African monsoon dynamics. Continuously present patterns of precession imply low-latitude forcing, while asymmetric eccentricity and obliquity imprints and strong obliquity influence suggest that Site 959 was also affected by high-latitude, glacial-interglacial dynamics.
