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The development of late middle Miocene through recent sedimentation rates was determined for Integrated Ocean Drilling Program Site U1338 in the eastern equatorial Pacific using available geomagnetic chron boundaries and biostratigraphic biohorizons provided by calcareous nannofossils and diatoms. These data were set in a revised depth splice of the three holes at Site U1338 and provide a lower resolution image of the sedimentation rate history in the region of Site U1338 through relatively long linear interpolations between 11 chosen age-depth control points encompassing 18 My. The resulting sedimentation rates depict the movement of Site U1338 from just south of the Equator with modest sedimentation rates starting at 11 m/My to tripling of these rates near 13 Ma, when the site moved onto the Equator and into the equatorial high-productivity belt. High sedimentation rates continued into the earliest Pliocene (5 Ma) except for a 50% decrease during the so-called "carbonate crash" centered around 9.6 Ma ± 0.4 My. The site's movement north, away from the equatorial high-productivity belt, began between 5.0 and 4.2 Ma, when the site was located ~75-90 nmi north of the Equator, and resulted in a series of successively decreasing sedimentation rates to the present rate of 11 m/My, identical to the rate when sedimentation began at Site U1338 at nearly 18 Ma, when the site was located ~100 nmi south of the Equator.

Assemblages of upper lower through upper Miocene Discoaster spp. have been quantified from Integrated Ocean Drilling Program (IODP) Site U1338 in the eastern equatorial Pacific Ocean. These assemblages can be grouped into five broad morphological categories: six-rayed with bifurcated ray tips, six-rayed with large central areas, six-rayed with pointed ray tips, five-rayed with bifurcated ray tips and five-rayed with pointed ray tips. Discoaster deflandrei dominates the assemblages prior to 15.8 Ma. The decline in abundance of D. deflandrei close to the early-middle Miocene boundary occurs together with the evolution of the D. variabilis group, including D. signus and D. exilis. Six-rayed discoasters having large central areas become a prominent member of the assemblages for a 400 ka interval in the late middle Miocene. Five-and six-rayed forms having pointed tips become prominent in the early late Miocene and show a strong antiphasing relationship with the D. variabilis group. Discoaster bellus completely dominates the Discoaster assemblages for a 400 ka interval in the middle late Miocene. Abundances of all discoasters, or discoasters at the species level, show only (surprisingly) weak correlations to carbonate contents or oxygen and carbon isotopes of bulk sediment when calculated over the entire sample interval.

Abstract. Examination of Upper Miocene–Lower Pliocene sediments at IODP Site U1338, in the Eastern Equatorial Pacific, provided new data on the distribution range of the calcareous nannofossil genus Catinaster. In addition to the the well-known occurrence of Catinaster coalitus and Catinaster calyculus in the early Late Miocene, we document Catinaster mexicanus in both the mid-late Miocene and the Early Pliocene. We confirm its taxonomic validity, rejecting previous interpretations of Pliocene C. mexicanus specimens as the result of dissolution of Discoaster altus. Instead, the Pliocene appearance of C. mexicanus seems to originate from the D. altus lineage. The short interval of occurrence (c. 50 ka) in the Late Miocene may document a preliminary evolutionary emergence of C. mexicanus that lacks any relationship with the other Catinaster species. Clear ancestor species to validate its independent origin from Discoaster are, however, missing. In the stratigraphic intervals where Catinaster species are found, their co-occurrence with Discoaster species bearing a prominent star-shaped boss on one side is noteworthy. This suggests that Catinaster and Discoaster at times developed a common morphological feature (a stellate structure, with or without hexaradiate symmetry), possibly under recurrent changes in climatic/environmental conditions. The data presented on C. mexicanus suggest a wider geographical distribution than previously thought, extending from the tropical Pacific to the Gulf of Mexico, equatorial Atlantic and tropical Indian oceans.