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doi:10.2204/iodp.proc.323.201.2013

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 323, a long sediment record was recovered from Site U1341 on the Bowers Ridge (Fig. F1). This location, situated in a south-central position in the Bering Sea, is presently under the influence of North Pacific surface waters entering the Bering Sea through channels between the Aleutian Islands and was proximal to the entry of the glacial Alaskan Stream (Stabeno et al., 1999; Takahashi, 2005). It is therefore a key position to link calcareous nannofossil records from the Central and North Pacific with recently available records recovered during Expedition 323.

Coccolithophores and calcareous nannofossils are widely distributed in the world’s oceans since the Mesozoic and are a useful tool for paleoenvironmental analyses and age determination. In the Bering Sea, calcareous nannofossils were studied by Bukry (1973) and Worsley (1973) on materials recovered during Deep Sea Drilling Project (DSDP) Leg 19 in 1971, followed by studies by Beaufort and Ólafsson (1995), Ólafsson and Beaufort (1995), and Sato et al. (2002) on records from DSDP Leg 145 just south of the Aleutian Islands. More recently, coccolithophores have been studied on sediment traps recovered in the area (Takahashi et al., 2012). All these studies have shown that assemblages preserved in sediment records of the area are characterized by low diversities and very low numbers. Moreover, information on calcareous nannofossil assemblages in sediments older than the last glacial cycle in the Bering Sea is scarce and discontinuous because drilling procedures during DSDP Leg 19 prevented the recovery of continuous sediment records (Creager, Scholl, et al., 1973).

In this study, we analyzed the calcareous nannofossil record from Site U1341 preserved in the sediments corresponding to the late Pliocene–early Pleistocene transition (~3.5–2 Ma). Our goal was to observe the responses of different calcareous nannofossil taxa to the onset of Northern Hemisphere glaciations and correlate this record to others in the North Pacific to perform a paleoenvironmental study in this oceanic area and extend calcareous nannofossil biostratigraphic schemes to higher latitudes in the North Pacific. Regarding this last objective, the application of standard zonation schemes such as Martini (1971) to Bering Sea materials is problematic because the known biogeographical distribution of marker taxa such as discoasterids does not cover high latitudes in the Pacific Ocean (Sato et al., 2002); therefore, in the selected time interval we focused on locating the high-latitude dominance reversal noted by Sato and Kameo (1996) and Sato et al. (2002) between Dictyococcites spp. (small) and Coccolithus pelagicus, which was dated by Sato et al. (2002) to ~2.75 Ma and is considered roughly synchronous to the closure of the Isthmus of Panama and the onset of Northern Hemisphere glaciation.

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