Geological and physical setting

With an area of 2.29×106 km2 and a volume of 3.75×106 km3, the Bering Sea is the third largest marginal sea in the world, surpassed only by the Mediterranean and South China seas (Hood, 1983). Approximately one-half of the Bering Sea is a shallow (0–200 m) neritic environment, with the majority of the continental shelf spanning the eastern side of the basin off Alaska from Bristol Bay to the Bering Strait (Fig. F1). The northern continental shelf is seasonally ice covered, but little ice forms over the deep southwest areas. In addition to the shelf regions, two significant topographic highs have better CaCO3 preservation than the deep basins: Shirshov Ridge, which extends south of the Koryak Range in eastern Siberia along 170°E and separates the southwestern part of the Bering Sea into two basins, Komandorski (to the west) and Aleutian (to the east); and Bowers Ridge, which extends 300 km north from the Aleutian Island arc (Fig. F1). The Aleutian Basin is a vast plain 3800–3900 m deep with occasional gradually sloping depressions as deep as 4151 m (Hood, 1983).

Three major rivers flow into the Bering Sea: the Kuskokwim and Yukon rivers drain central Alaska and the Anadyr River drains eastern Siberia (Fig. F1). The Yukon is the longest of the three rivers and supplies the largest discharge into the Bering Sea. Its discharge peaks in August because of meltwater and is about equal to that of the Mississippi. It has a mean annual flow of 5×103 m3/s, which is about two-thirds the annual flow of the Columbia River (Hood, 1983).

Today, a substantial amount of water is transported in and out of the Bering Sea across the Aleutian Island arc and the Bering Strait through passes (Figs. F1, F2, F3). Water mass exchange with the Pacific through the Aleutian Islands, such as through the Kamchatka Strait, is significant, linking Bering Sea conditions to the Pacific climate. The Alaskan Stream, an extension of the Alaskan Current, flows westward along the Aleutian Islands and enters the Bering Sea mainly through the Amchitka Strait and to some extent through the Near Strait west of Attu Island in the eastern Aleutian Islands (Fig. F1). A part of the Subarctic Current also joins the Alaskan Stream, resulting in a combined volume transport of 11 Sv (Ohtani, 1965).

Bottom and intermediate depth water in the Bering Sea originates from the North Pacific. After flowing into the Bering Sea it is slightly modified by the mixing of relatively fresh, warm water with very small amounts of bottom water formed within the Bering Sea today (Warner and Roden, 1995). Nutrient concentrations of North Pacific origin are high compared to all other regions in the global oceans; this explains the very low oxygen concentrations in the Bering Sea today (Fig. F4). The oxygen and nutrient composition of these waters is further modified by denitrification (Lehmann et al., 2005) and respiration of organic matter in the water column (Nedashkovskiy and Sapozhnikov, 1999). Respiration and the development of an oxygen minimum zone (OMZ) is particularly intense at water depths of ~1000 m (Fig. F4).

Much of the Pacific water entering the Bering Sea is matched by outflow through the Aleutian Islands. The most significant outflow is through the Kamchatka Strait, which has a maximum depth of 4420 m (Stabeno et al., 1999) (Figs. F1, F3). If some component of North Pacific Intermediate Water (NPIW) or deep water formed in the Bering Sea in past times, particularly when sea level was lower, it would have flowed out through the Kamchatka Strait or a secondary outlet near the Commander–Near Strait at 2000 m (Fig. F1).

The unidirectional northward transport of water mass (0.8 Sv) from the Bering Sea through the Bering Strait to the Arctic Ocean contributes to the salinity and biogeochemical contrast between the Pacific and the Atlantic. The Bering Strait region is one of the most biologically productive regions in the world (324 g C/m2/y over a wide area [2.12×104 km2]) (Sambrotto et al., 1984). Much of this biologically produced organic matter and the associated nutrients flow into the Arctic Ocean because of the northward current direction. This may profoundly influence the present dominance of carbonate production in the Atlantic versus opal production in the Pacific, as described by models of basin-to-basin fractionation (Berger, 1970) and "carbonate ocean vs. silica ocean" (Honjo, 1990). Flow through the Bering Strait, which is ~50 m deep today (fig. 5 in Takahashi, 2005), was certainly different at times of lower sea level or enhanced perennial sea ice cover. The closing of this gateway and the accompanying changes in ocean and river flow through time could have caused changes in global patterns of circulation or in nutrient and salinity distributions.

Relationship to previous drilling in the Bering Sea, the subarctic North Pacific, and the Arctic Ocean

During DSDP Leg 19 in 1971, six sites were drilled in the Bering Sea and four were drilled just south of the Aleutian Islands in order to generally characterize the sedimentary units and tectonic and structural evolution of the Bering Sea (Scholl and Creager, 1973). Although much of the sediment section was washed away and not cored, DSDP Leg 19 provided basic information on the types and ages of sediments in two of the regions (Umnak Plateau and Bowers Ridge) that were targeted by Expedition 323. Expedition 323 marks the first deployment of advanced piston corer (APC) technology in the region and thus the first collection of continuous high-resolution records of the past 5 m.y. from the Bering Sea. Specifically, drilling of DSDP Sites 184 and 185 on Umnak Plateau revealed a Pleistocene to upper Miocene clay-rich diatomaceous ooze (Unit A) above clayey siltstone with sparse fossils (Unit B). At DSDP Site 188 on Bowers Ridge, sedimentary units similar in lithology and age to those found at the Umnak Plateau sites were found. Although Sites 190 and 191 were drilled close to Shirshov Ridge and the Kamchatka Strait, they were located in the deep basins around Shirshov Ridge (water deeper than 3800 m) and sediments recovered were mainly turbidite sequences with reworked microfossils, making paleoceanographic interpretations difficult. Expedition 323 to the Bering Sea did not include drilling in the deep basins; rather, it targeted topographic highs above the basin floors where deposition of reworked sediments was expected to be minimal.

In 1992, several important sites adjacent to the Bering Sea were explored during Ocean Drilling Program (ODP) Leg 145: Sites 881, 882, 883, and 884 (Rea et al., 1995; Rea, Basov, Janecek, Palmer-Julson, et al., 1993; Rea, Basov, Scholl, and Allen, 1995). Maslin et al. (1996) observed a dramatic increase in ice-rafted debris (IRD), a decrease in sea-surface temperature (>7.5°C) and opal mass accumulation rates (MARs) (five-fold decrease), and a decrease in both total organic carbon (TOC) and CaCO3 MARs at Site 882 (50°22'N, 167°36'E; 3244 m) at 2.75 Ma, which is coeval with the IRD change found in the Norwegian Sea and suggests that the Arctic and northeast Asia were significantly glaciated from 2.75 Ma onward. Furthermore, they suggested that the onset of Eurasian Arctic and northeast Asia glaciation occurred ~100 k.y. before the Alaskan glaciation and 200 k.y. before the glaciation of Greenland and the northeast American continent. Both McKelvey et al. (1995) and Krissek (1995) suggest that the provenance of IRD in the northwest Pacific Ocean and the Gulf of Alaska is the Bering Sea off the Kamchatka Peninsula and southeastern Alaska, respectively. By studying the Bering Sea in relation to other regions we can uncover details of the inception of glaciation in the Arctic and North Pacific regions at ~2.75 Ma. Furthermore, the reasons for differences in the timing of glaciation can be investigated in detail using the sections from the Bering Sea, which are much higher resolution than those available in the open ocean pelagic realm.

IODP Expedition 302 (Arctic Coring Expedition [ACEX]) to Lomonosov Ridge in the central Arctic Ocean took place in 2004, and the scientific community anticipated the acquisition of new information regarding the age and effects of the Bering Strait gateway to the Arctic. However, despite the expedition's success in acquiring sediments spanning the Holocene to the Cretaceous (Backman, Moran, McInroy, Mayer, et al., 2006; Moran et al., 2006), it has been difficult to advance understanding of the significance of flow changes through the Bering Strait gateway on global or regional climate change without being able to compare these new Arctic records to those on the Pacific side of the Bering Strait. Thus, the cores recovered during Expedition 323 in the Bering Sea are essential to deciphering the history of the Bering Strait gateway and its potential impact on global and regional climatic and oceanic processes. The role of the exchange of heat and chemical constituents through the Bering Strait on Arctic and North Pacific environments as well as the influence of changes in this exchange on Northern Hemisphere glaciation (NHG) and higher frequency climate oscillations can only be assessed by comparing results from Bering Sea drilling with the results from Expedition 302.

Millennial-scale climate changes

The Bering Sea contains sediments with high accumulation rates appropriate for the reconstruction of surface and deep water conditions and for the validation of climate/ocean hypotheses that call on these regions as a variable source of open Pacific intermediate and deep water. In addition, climate change in the Bering Basin tends to be extremely sensitive to high-frequency changes due to the semi-isolated nature of the marginal sea. Sea level drop, for example, may produce a profound effect on water mass circulation, sea ice formation, salinity, and biological productivity in the basin (e.g., Takahashi, 1999). The pelagic signals of the open Pacific do not adequately provide the high-frequency climatic history of the northwest Pacific Rim.

Changes in ventilation of subsurface water in the North Pacific may also influence climate downstream and be tied to North Atlantic climate changes on millennial timescales. Interestingly, millennial cycles in climate proxy records are apparently correlative across the North Pacific, for example in the Bering Sea (Cook et al., 2005), the Sea of Okhotsk (Sakamoto et al., 2005; Ono et al., 2005), the California margin (e.g., Behl and Kennett, 1996), and the Sea of Japan (e.g., Tada et al., 1999). Although the mechanisms for strong teleconnections between different sides of the North Pacific are not known, it has been proposed that changes in the Okhotsk (Ono et al., 2005) or Bering seas' source(s) of NPIW could reach the California margin and influence the depth or strength of the OMZ (Cannariato and Kennett, 1999; Zheng et al., 2000), thereby connecting climate/ocean changes across the Pacific Ocean. The fact that millennial-scale records from the Pacific margins also appear to correlate to changes in North Atlantic climate (e.g., Behl and Kennett, 1996; Tada et al., 1999) indicates that processes that link Atlantic and Pacific climate could play an important role in global climate change.

A number of possible theories explain paleoceanographic data from North Pacific marginal seas by implicating changes in NPIW formation due to changes in flow through the Bering Strait (Hasumi, 2002; De Boer and Nof, 2004; Shaffer and Bendtsen, 1994) and teleconnections from the tropics (Niebauer and Day, 1989; Alexander et al., 2002; Niebauer, 1998; Zhao et al., 2004; Gloersen, 1995). Our capacity to test these theories will benefit from documentation of surface and deep water conditions in the Bering Sea. Drilling has allowed us to obtain long sequences of sediments for the reconstruction of the climate cycles and for the evaluation of whether the patterns observed in the last glacial cycle are characteristic of all glacial–interglacial cycles. Long records will also be used to compare millennial climate oscillations in the Bering Sea in the Pleistocene, when there were large Northern Hemisphere ice sheets, to those in the Pliocene warm period, when there were only small Northern Hemisphere ice sheets, thereby shedding light onto whether the generation of these oscillations is related to NPIW ventilation, ice sheet size and dynamics, ocean circulation, and/or rapid reorganization of atmospheric circulation.

Glacial–Interglacial climate change

In the last glacial cycle, enhanced dense water formed, probably from the Okhotsk and Bering seas (e.g., Zahn et al., 1991; Gorbarenko, 1996). In fact, the degree of ventilation of deep and intermediate Pacific waters appears to have fluctuated during the cold and warm periods, implying changes in the configuration of Pacific Ocean circulation (Keigwin, 1995; Matsumoto et al., 2002). However, the use of different nutrient proxies (δ13C vs. Cd/Ca in benthic foraminifers) provides some contradictory results. Furthermore, the limited spatial coverage of sites in the open Pacific prevents detailed identification of the exact source of intermediate and deep water as well as the exact circulation path of subsurface water masses. Observations of glacial records from the Bering Sea and just outside the Bering Sea on the Detroit Seamount in the North Pacific suggest a source of ventilated intermediate water coming from the Bering Sea and/or the Detroit Seamount region (Gorbarenko, 1996).

Two examples of the prospective reconstructions of Bering Sea paleoceanography by Bering Sea drilling are reported in recent studies performed on the piston cores collected during the 1999 Hakuhou-Maru site survey cruise (Takahashi et al., 2005). Sea ice distribution during the glacial period was modulated by surface water circulation partially governed by the topography that resulted from the sea level drop. Note that the distribution of sea ice and water masses is significantly different in the two basins west and east of Bowers Ridge because of the pattern of surface ocean circulation (Katsuki and Takahashi, 2005). Past sources of NPIW formation during the four different time slices have been inferred from the intermediate water–dwelling radiolarian species Cycladophora davisiana (Tanaka and Takahashi, 2005). The degree of ventilation of intermediate and deep water will be fully scrutinized with more detailed information from Expedition 323.

Despite evidence that Pacific circulation was different in the last glacial cycle, little is known about what caused circulation to change or what role the Pacific played in determining extreme climate conditions. Extensive studies of the North Atlantic clearly show that ice-sheet dynamics and changes in meridional overturning circulation in the Atlantic can readily influence climate, yet no widely accepted paradigm explains how the North Pacific participates in and possibly impacts global climate change. Construction of long records of glacial–interglacial changes by drilling, especially under a range of boundary conditions over the Pliocene–Pleistocene, will contribute critical information needed to formulate a new North Pacific climate change paradigm.

Pliocene–Pleistocene trends

Compelling evidence shows that North Pacific mid-depth water (~2500 m) had much lower nutrient concentrations in the warm Pliocene (~4.5 to 3.0 Ma) than today, indicating that it was more strongly ventilated (Kwiek and Ravelo, 1999; Ravelo and Andreasen, 2000). Although increased subsurface ventilation in the cold LGM and the warm Pliocene could be interpreted in a number of different ways and is likely not explained by the same processes, only data that directly reflect conditions in the Bering Sea (and the Sea of Okhotsk) can help constrain interpretations.

The end of the early Pliocene warm period is characterized by the development of modern density stratification in the surface and deep North Pacific. IRD recovered at DSDP and ODP sites indicates that increased water mass stratification coincided with more extensive glaciation (Haug et al., 1999; Kwiek and Ravelo, 1999; Ravelo and Andreasen, 2000; Rea and Schrader, 1985). Furthermore, more IRD is found along the Aleutian Islands (DSDP Site 192) than farther north in the Bering Sea (Sites 186 and 191) due to more extensive ice cover in the north compared to more seasonal ice cover at the Aleutian site (McKelvey et al., 1995; Krissek, 1995). The fact that this North Pacific climate reorganization occurred synchronously with the onset of significant NHG as recorded in the Atlantic Ocean highlights the importance of studying North Pacific climate evolution as part of a comprehensive investigation of the regional expression of global climate trends.

The emergence of the Bering Land Bridge (Beringia) prior to the Neogene is not well understood. However, Pliocene climate change—and perhaps the onset of NHG, specifically—could have been affected by changes in the marine gateway connection through the Bering Strait region. The connection may have developed in the late Miocene or the early Pliocene based on the occurrences of Atlantic-type mollusks in Hokkaido, Kamchatka, and the Alaska Peninsula in the late Miocene and early Pliocene. The oldest ages for these occurrences range from 6.3–5.1 to 2.2 Ma (e.g., Uozumi et al., 1986), but a recent study documented that the age of first occurrence was 5.5–5.4 Ma (Gladenkov, 2006). One of the aims of drilling was to recover better records of oceanographic evolution related to the Miocene/Pliocene gateway history. The oldest sediment recovered was ~5 Ma, and long-term trends in Pliocene circulation will be documented to assess the possible influence of the tectonic opening of the Bering Sea if it happened as late as the Pliocene.

Flow through the Bering Strait is likely controlled mainly by sea level change and changes in the amount of perennial sea ice. Pacific to Arctic flow through the Bering Strait (~0.8 Sv) (Coachman and Aagaard, 1981) significantly influences the Pacific–Atlantic partitioning of physical and nutrient properties in the modern ocean and was possibly quite sensitive to past changes in sea level change due to its shallow sill of ~50 m (see Takahashi, 2005, for an illustration of the cross section). During glacial intervals, Atlantic Ocean biogenic sedimentation and preservation became more Pacific-like, and vice versa, and there were major changes in nutrient distributions. Drilling near the Bering Strait will help resolve whether major changes in Pacific–Atlantic partitioning of oceanographic properties were related to changes in flow through the Bering Sea. Neodenticula seminae, a dominant extant subarctic Pacific diatom, was found recently in Atlantic waters, possibly because recent warming and melting sea ice in the Arctic Ocean provided a passage from the Pacific to the Atlantic (Berard-Therriault et al., 2002; Corbyn, 2007). This species has been extinct in the Atlantic since 0.8 Ma (Baldauf, 1987), and thus its recent reemergence in the Atlantic appears to be a significant indication that climate change in the Arctic influenced the distribution of this species.