The Oligocene and Eocene epochs are critical for understanding the Earth climate system because they correspond to a transitional phase between the early Paleogene greenhouse and the Neogene icehouse. During this time, associated with the opening of the Tasmanian Seaway and Drake Passage, the Antarctic Ice Sheet markedly increased in size and the formation of cold deep water began in the Southern Ocean (e.g., Kennett, 1977; Lawver and Gahagan, 2003). Many proxy-based paleoceanographic studies in the Southern Ocean (foraminiferal stable oxygen isotopes and the occurrence of ice-rafted debris) have clarified that the formation of Southern Component Water (SCW) is closely related to Antarctic cooling during the late Eocene–Oligocene (Zachos et al., 1993, 1996; Zachos et al., 2001).
In contrast, studies of the Oligocene paleoenvironment of the abyssal Pacific Ocean are relatively rare because researchers have focused on upper to middle bathyal deposits, which are sufficiently above the carbonate compensation depth to guarantee carbonate preservation. However, it is important to investigate paleoenvironments in various water depths and regions to understand the behavior and influence of SCW. For example, paleoceanographic information from various paleodepths is necessary in reconstructing the properties and distribution of deepwater masses in the Indian Ocean (Nomura et al., 1997).
For this study, we investigated Oligocene–early Miocene abyssal benthic foraminiferal assemblages at Sites 1218 (4826 m water depth) and 1219 (5063 m water depth) of Ocean Drilling Program (ODP) Leg 199 (Fig. F1) in the eastern equatorial Pacific. Because of good age constraints and good preservation of microfossils, the Oligocene at these sites is ideal for study of the equatorial Pacific abyssal environment. We report the preliminary results of the abyssal benthic foraminiferal assemblages in understanding properties and distribution of the deepest water masses of the eastern equatorial Pacific region during the Oligocene.