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doi:10.2204/iodp.sp.323.2009

Background

Geological and physical setting

With an area of 2.29 x 10⁶ km² and a volume of 3.75 x 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: the 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 (western part) and Aleutian (eastern part); and the Bowers Ridge, which extends 300 km north from the Aleutian Island arc (Figs. F1, F2). 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 drain central Alaska and the Anadyr 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 the Mississippi. It has a mean annual flow of 5 x 10³ m³/s, about two-thirds the annual flow of the Columbia River (Hood, 1983).

A substantial amount of water is transported in and out of the Bering Sea across the Aleutian Island arc and the Bering Strait today, through passes illustrated in Figure F1 and by a bathymetric cross section in Figure F3. The water mass exchange with the Pacific through Aleutian Island passes such as the Kamchatka Strait is significant, which links 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). 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 (Fig. F3). If some component of 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. F3).

Furthermore, the unidirectional northerly transport of water mass (0.8 Sv) from the Bering Sea through the Bering Strait to the Arctic Ocean contributes to the biogeochemical contrast between the Pacific and the Atlantic. The Bering Strait region is one of the most highly biologically productive regions in the world, 324 g C/m²/y over a wide area (2.12 x 10⁴ km²; Sambrotto et al., 1984). Much of the biologically produced organic matter and 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 of the "carbonate ocean vs. silica ocean" (Honjo, 1990). Flow through the Bering Strait, which is ~50 m deep today, was certainly different at times with a lower sea level or enhanced perennial sea ice cover. Closing of this gateway and accompanying changes in ocean and river flow through time could have caused changes in global patterns of circulation or in nutrient and salinity distributions (Takahashi, 1998, 1999).

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

During DSDP Leg 19 in 1971, six sites were drilled in the Bering Sea and four 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 are targeted by the planned IODP Bering Sea expedition. The current drilling plan will provide the first deployment of APC technology in the region and thus the first continuous high-resolution records of the past 5 m.y. or longer from the Bering Sea. Specifically, drilling of DSDP Sites 184 and 185 on the 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 the 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 the Shirshov Ridge and the Kamchatka Strait, they are located in the deep basins around the Shirshov Ridge (water deeper than 3800 m), and sediments recovered were mainly turbidite sequences with reworked microfossils, making paleoceanographic interpretations difficult. This IODP expedition to the Bering Sea will not include drilling in the deep basins; rather, it targets topographic highs above the basin floors where deposition of reworked sediments will be minimal.

In 1992, several important sites adjacent to the Bering Sea were explored during ODP Leg 145: Sites 881, 882, 883, and 884 (Rea et al., 1995; Rea, Basov, Janecek, Palmer-Julson, et al., 1993; Rea, Basov, Scholl, Allen, et al., 1995). Maslin et al. (1996) observed a dramatic increase in 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 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 should be able to 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 much higher resolution sections, relative to those available in the pelagic realm, that we anticipate.

IODP Expedition 302 (Arctic Coring Expedition [ACEX]) to the 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 terms of the acquisition of sediments spanning from the Holocene to the Cretaceous (Backman, Moran, McInroy, Mayer, et al., 2006; Moran et al., 2006), it has been difficult to advance the understanding of the significance of the Bering Strait Gateway on global or regional climate change without being able to compare new Arctic records to those on the Pacific side of the Bering Strait. Thus, the present plan for drilling in the Bering Sea is 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 and higher frequency climate oscillations, can only be assessed by comparing results from Bering Sea drilling together 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 deepwater 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 records from the Bering Basin tend 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, such as those in the Bering Sea (Cook et al., 2005), the Okhotsk Sea (Sakamoto et al., 2005; Ono et al., 2005), the California margin (e.g., Behl and Kennett, 1996), and the Japan Sea (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 Sea source(s) of NPIW could reach the California margin and influence the depth or strength of the oxygen minimum zone (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 with changes in North Atlantic climate (e.g., Behl and Kennett, 1996; Tada et al., 1999) indicates that processes linking Atlantic and Pacific climates could play an important role in global climate change.

There are a number of possible theories that explain paleoceanographic data from the North Pacific marginal seas by implicating changes in NPIW formation (e.g., like that used by Behl and Kennett [1996] to explain observations in the Santa Barbara Basin), 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). Tests of these theories would greatly benefit from documentation of surface and deep water conditions from the North Pacific marginal seas. Only drilling will allow for the reconstruction of the climate cycles and for the evaluation of whether patterns observed in the last glacial cycle are characteristic of all glacial–interglacial cycles. Only recovery of continuous Pliocene–Pleistocene sediments will allow us to evaluate if the mean state of the climate system (warm versus cold) determines the higher frequency sensitivity, behavior, and climatic impact of these marginal seas. Finally, comparing 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 will provide insight into whether the generation of these oscillations is related to NPIW ventilation and/or to ice sheet size and dynamics.

Glacial–interglacial climate change

There was enhanced dense water formation in the last glacial cycle, probably from the Okhotsk and the 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 been fluctuating during the cold and warm periods, implying changes in the configuration of Pacific Ocean circulation (Keigwin, 1995; Matsumoto et al., 2002). However, there are some contradictory results depending on the nutrient proxy used (δ¹³C versus Cd/Ca in benthic foraminifers). 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 have been made in glacial records from the Bering Sea and just outside the Bering Sea on the Detroit Seamount in the North Pacific, suggesting a source of ventilated intermediate water coming from the Bering Sea and/or Detroit Seamount region (Gorbarenko, 1996).

Two examples of prospective reconstructions of Bering Sea paleoceanography made possible by Bering drilling can be viewed from recent studies performed on piston cores collected during the 1999 R/V Hakuhou-Maru site survey cruise (Takahashi et al., 2005). Sea ice distribution during the glacial period was modulated by the surface water circulation partially governed by the topography resulting from the sea level drop (Fig. F4). It is noteworthy that the distribution of sea ice and water masses is significantly different in the two basins west and east of the Bowers Ridge, depending on water circulation. Past sources of NPIW formation during four different time slices have been reconstructed based on the intermediate water–dwelling radiolarian species Cycladophora davisiana (Fig. F5). The role of the Bering Sea in NPIW formation is visible during the cold intervals.

Despite evidence that Pacific circulation was different during the last glacial cycle, little is known about what caused circulation to change or what role the Pacific played in determining extreme climate conditions. From extensive studies of the North Atlantic, it is clear that ice sheet dynamics and changes in thermohaline circulation in the Atlantic can readily influence climate, yet there is no widely accepted paradigm that explains how the North Pacific participates in and possibly dominates 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

In the warm Pliocene (~4.5–3.0 Ma) there is compelling evidence that North Pacific mid-depth water (~2500 m) had much lower nutrient concentrations than today, indicating that it was more strongly ventilated (Kwiek and Ravelo, 1999; Ravelo and Andreasen, 2000). Although increased subsurface ventilation in the cold Last Glacial Maximum (LGM) and the warm Pliocene could be interpreted in a number of different ways and are likely not explained by the same processes, only data that directly reflect conditions in the Bering Sea (and the Okhotsk Sea) can help to 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. Records of ice-rafted debris (IRD), benthic foraminiferal δ¹⁸O and δ¹³C, and other paleoceanographic proxies from DSDP and ODP sites indicated that the 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 ice-rafted debris is found along the Aleutian Islands (DSDP Site 192) than farther north in the Bering Sea (Sites 186 and 191) because of 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 Northern Hemisphere glaciation 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, perhaps the onset of Northern Hemisphere glaciation 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 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 aim of this planned drilling is to recover better records of the oceanographic evolution related to the Miocene/Pliocene gateway history.

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 because of 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 to resolve whether major changes in Pacific/Atlantic partitioning of oceanographic properties were related to changes in flow through the Bering Sea. Recent findings of Neodenticula seminae, a dominant extant subarctic Pacific diatom, in Atlantic waters may be in response to global warming, with the Arctic Ocean providing a passage for this species from the Pacific to the Atlantic. This species has been extinct in the Atlantic since 0.8 Ma (Baldauf, 1986); thus, the recent reemergence in the Atlantic appears to be a significant indication that climate change in the Arctic influenced the distribution of this species.

Seismic studies and site survey data

Seismic and acoustic profiles of the sediments to be drilled at the Bering/Arctic Gateway (GAT), Bowers Ridge (BOW), and Umnak Plateau (UMK) sites were primarily surveyed during the Hakuhou-Maru cruise (Seattle–Tokyo) in August 1999, and their details are incorporated in this Scientific Prospectus. Together with the geophysical survey effort, piston coring was performed during the same cruise in order to verify that promising paleoceanographic studies can be accomplished in the Bering Sea (Takahashi et al., 2005). In addition to the Hakuhou-Maru site survey, seismic profiles obtained by U.S. expeditions—U.S. Naval Service Bartlett Cruise 02 (1970) and U.S. Geological Survey Lee Cruise L6-80-BS (1980)—were utilized to facilitate drilling approval at the Umnak Plateau, Shirshov Ridge (SHR), and Kamchatka Strait (KST) sites and also have been incorporated here (Fig. F6). Crossing seismic lines that were used to target alternate Site NAV-1B include Farnella-2-86 SCS Line 4A and Discoverer 4-80 Line 18. Supporting site survey data for this expedition are archived at the IODP-MI Site Survey Data Bank (ssdb.iodp.org).

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