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An Oligocene to recent sedimentary sequence was recovered in Hole U1490A in the northern portion of the Eauripik Rise (western Pacific Ocean) during International Ocean Discovery Program Expedition 363. High-resolution sampling and moderate to good calcareous nannofossil preservation allowed us to adjust the depths for nannofossil events reported shipboard. This study identified 22 zonal boundary markers and 30 secondary calcareous nannofossil events. Because of better preservation of the calcareous nannofossils in the Pliocene–Pleistocene units, more calcareous nannofossil events were observed in that interval than in the older units. In the Miocene units, the discoasters and helicosphaerids (which are important zonal boundary markers) are poorly preserved (fragmented and recrystallized), making it difficult to identify nannofossil zones. In lieu of zonal boundary markers, secondary calcareous nannofossil events were used to refine the biostratigraphy in the Miocene interval. Introduction International Ocean Discovery Program (IODP) Site U1490 is located in the northern portion of the Eauripik Rise, which is a north–south trending tectonically stable elevated ridge separating the East and West Caroline Basins. The site is situated at 05°48.95ʹN, 142°39.27ʹE at a water depth of 2341 m (Figure F1). Three holes were drilled at Site U1490: Holes U1490A, U1490B, and U1490C. Hole U1490A recovered ~380 m of upper Oligocene to recent sediments. Compared to the sediments recovered from other sites cored during Expedition 363, Site U1490 has a relatively low sedimentation rate. The sedimentation rates varied through the recovered intervals and ranged from 0.9 cm/ky in the early to middle Miocene to 5 cm/ky in the late Oligocene, whereas the average sedimentation rate for the whole core was ~1.5 cm/ky. This rate is lower than the rates at nearby sites, including Sites U1488 (~2.5 cm/ky) and U1489 (~2 cm/ky). The sediments from Site U1490 contain calcareous microfossils (nannofossils and foraminifers), siliceous microfossils (radiolarians and diatoms), sponge spicules, clay minerals, and ash. The hole was divided into three subunits (IA, IB, and IC) during shipboard description based on the lithology and composition (i.e., kind of biogenic material, type of clay minerals, and the presence or absence of volcanic ash) (Rosenthal et al., 2018b). A generalized lithologic log for Site U1490 is shown in Figures F2 and F3. Subunit IA is ~185 m thick and is composed of early Miocene to recent calcareous nannofossil and foraminifer ooze and low amounts of clay minerals and volcanic ash (Figure F2). Subunit IB is ~78 m thick and ranges in age from late early to early late Miocene (Figure F3). Compared to the previous unit, a significant increase in clay minerals and biosiliceous material was observed in Subunit IB. Subunit IC is a 124 m thick early Oligocene to middle Miocene succession. The siliceous particles that dominate this subunit have distinct dark gray to black indurated layers and nodules and chert fragments (Figure F3) (Rosenthal et al., 2018a). Shipboard calcareous nannofossil biostratigraphic study of core catcher samples (~10 m interval), supplemented in some parts by samples collected from selected sections, recognized 30 calcareous nannofossil events; 15 of these events were zonal boundary markers, and the other 15 were supplementary events. The Pleistocene interval was constrained by 7 calcareous nannofossil events, the Pliocene was constrained by 4 events, the Miocene was constrained by 17 events, and the Oligocene was constrained by 2 events. The top common occurrence of Cyclicargolithus abisectus, which has an estimated age of ~24 Ma (Rio et al., 1990; Backman et al., 2012), was found near the base of the hole (Rosenthal et al., 2018a). This study refines the shipboard biostratigraphy of Hole U1490A by examining sediments collected at a higher resolution sampling interval (~1.5 m) of one sample per section. Y.I.L. Doyongan and A.G.S. Fernando Data report: refinement of calcareous nannofossil biostratigraphy IODP Proceedings 2 Volume 363 Figure F1. Location of Site U1490.

Data report: refinement of calcareous nannofossil biostratigraphy from the late Oligocene to the Pleistocene, IODP Expedition 363 Hole U1490A