Sites 1039-1043 of Ocean Drilling Program (ODP) Leg 170 were drilled along the active Pacific margin of Costa Rica in the western equatorial Pacific (Figs. F1, F2). The primary objectives of the leg were to better characterize the structural, sedimentological, and geochemical parameters within this dynamic margin (Kimura, Silver, Blum, et al., 1997); the age of sediments is a crucial factor in determining the rates of these and related processes. Neogene diatom biostratigraphy is very well defined for the equatorial Pacific region and has been developed over the years by a number of researchers. The diatom biostratigraphic zonation for the equatorial Pacific used during Leg 170 is that of Baldauf and Iwai (1995). This scheme was developed during ODP Leg 138 and is a revision of the scheme proposed by Barron (1985a, 1985b) and Baldauf (1985), which is partly based on the direct calibration of diatom datums to magnetostratigraphic datums by Burckle (1972, 1977, 1978). This paper adheres to the most recent geomagnetic polarity time scale of Berggren et al. (1995a, 1995b). The calibration of the diatom datums follows that of Shackleton et al. (1995) as listed in Table T1 (Shipboard Scientific Party, 1997).