Published benthic stable isotope data indicate long-term latest Oligocene warming and/or deglaciation was followed by a series of rapid and brief episodes of cryospheric expansion and/or global cooling during the earliest Miocene (Paul et al., 2000). Astronomically tuned high-resolution paired benthic and planktonic stable isotope records from the equatorial Atlantic (Ocean Drilling Program [ODP] Leg 154, Ceara Rise) have been used to suggest that the Mi-1 event represents a glaciation triggered by changes in orbital insolation. The lower Miocene through upper Oligocene sedimentary sequence from ODP Site 1218 presents a remarkable opportunity to study the evolution of deepwater hydrography in the equatorial Pacific Ocean. Very few data exist for the Pacific Ocean, a large region that plays a crucial role in controlling global climate. The construction of a high-resolution stable isotope stratigraphy for the Pacific Ocean and comparison with results for Ceara Rise (Paul et al., 2000) and the Southern Ocean (Billups et al., 2002) will allow us to clearly distinguish basinal climate variability over suborbital timescales across the Oligocene/Miocene transition.
Site 1218 is located in the central equatorial Pacific (8°53.378´N; 135°22.00´W), just north of the Clipperton Fracture Zone. This site was drilled on 42-Ma crust at a water depth of 4826.3 m. Three holes were drilled, and a continuous sediment section to 42 Ma was recovered. The 273-m-thick sequence is dominated by Pleistocene–middle Miocene clay and radiolarian clay, Miocene–Oligocene nannofossil ooze and chalk, and Eocene radiolarian ooze, radiolarite, and chalk. The lower Miocene through upper Oligocene composite section from Site 1218 was well-recovered during advanced piston coring (APC). Sediments retain a high-quality paleomagnetic reversal stratigraphy. Average sedimentation rates across the Oligocene/Miocene transition are 1–2 cm/k.y., relatively high for a deep Pacific setting. Lithologic and physical property data exhibit a distinct cyclicity on the centimeter to decimeter scale. An orbitally tuned age model has been developed that integrates biostratigraphic and magnetic polarity datums (Pälike et al., this volume).
Here we report the results of stable isotope analyses of benthic foraminifers from the lower Miocene through upper Oligocene sequence at Site 1218. Cores were sampled at 5-cm resolution. Benthic foraminifers range from rare to abundant throughout the interval of interest at Site 1218, and preservation of tests is very good to excellent. Benthic assemblages indicate lowermost bathyal and upper abyssal paleodepths.