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doi:10.2204/iodp.sp.323.2009

Coring and drilling strategy

The coring program prioritizes seven sites (Plan A) (Table T2), along with four alternate sites (Table T4), in the Bering Sea (Figs. F1, F2). However, the final operations plan and number of sites to be cored is contingent upon changes to the JOIDES Resolution operations schedule, operational risks (see below), and the outcome of a request for permission to occupy three sites in Russian territorial waters (Sites SHR-1B, SHR-3B, and KST-1B). In the event that such approval is not forthcoming, a back-up plan (Plan B) (Table T3) has been devised that substitutes Sites SHR-3B and KST-1B for high-priority Site GAT-3C (and/or Site NAV-1B).

The drilling strategy will consist of advance piston coring in three holes at each site to refusal, with the possible exception of proposed Site BOW-15A, which has shallower depth objectives. An additional hole will be cored to a depth of 25 mbsf at most sites. The sedimentary material recovered from these additional holes will be dedicated to microbiological sampling to satisfy the requirements for achieving the scientific objectives of IODP APL #739. The "drillover" technique will be employed to maximize APC penetration where desirable. For planning purposes, the APC refusal depth for most sites is estimated at 200 meters below seafloor (mbsf). The depth objective for Site BOW-15A is 165 mbsf. Triple-coring at each site will ensure continuity and undisturbed recovery of the stratigraphic section, but additional short APC holes may be drilled as required to achieve complete stratigraphic coverage (i.e., to eliminate all coring gaps and disturbed intervals in a composite section) in the APC interval. In accordance with routine drilling procedures, downhole temperature measurements will be obtained with the advanced piston corer temperature (APCT-3) tool and, where warranted, with the Davis-Villinger Temperature Probe (DVTP). Temperature measurements will allow reconstruction of the thermal gradient at each site. This information will help constrain the history of burial diagenesis of the sediments encountered.

The target depth at three proposed sites (BOW-12B, BOW-14B, and GAT-4C) is greater than the APC refusal depth; the deeper sections will be advanced with the extended core barrel (XCB) and may be cored with the rotary core barrel (RCB) to the final depth objective of 700 mbsf for Sites BOW-12B and GAT-4C and 555 mbsf for Site BOW-14B. XCB drilling will allow penetration through a significant portion of the Pliocene and possibly into the upper Miocene. Time estimates assume that the rotary system will be deployed in the two deeper sites. Should the drilling and coring of the Russian sites not be approved, Site GAT-3C will be cored with the APC to refusal and then with the XCB to 400 mbsf. Site NAV-1B will be cored with the APC system to 150 mbsf.

Wireline logging with the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings is planned for four sites (UMK-4D, BOW-12B, GAT-4C and SHR-3B) and for four sites in the back-up plan (UMK-4D, BOW-12B, GAT-4C and GAT-3C). Logging will be used for correlating the sediment column to the sediment physical properties and the seismic reflection profiles (see "Wireline logging").

Operational risks

Weather and the nature of the sediments in the Bering Sea may present operational risks that could negatively impact drilling/coring operations, core recovery, core quality, and rate of penetration. Expedition 323 has been scheduled to take place during the summer, the optimum weather season; however, severe weather and fog may still occur and could adversely impact operational efficiency and transit speed. Additionally, the possible presence of ice rafted debris, chert layers, and glacial deposits could lead to bent core barrels and damaged APC shoes and XCB bits.

Drill sites

Age estimates of targeted drilling holes

Our age estimates of the deepest sediments at the bottom of each hole (Table T1) are mostly based on the thickness of Holocene sections in nearby locations and the ages of sediments recovered during DSDP Leg 19 based on biostratigraphic data. However, given the known variations in sedimentation rates in the region, actual sedimentation rates, particularly during cold periods and in the older part of the geologic record, could be quite variable. Using a range of possible sedimentation rates, we expect that the proposed triple-APC drilling will penetrate through a significant part of the Pleistocene at least, and that deeper drilling will penetrate into the Pliocene and hopefully at least to the lower Pliocene and/or the upper Miocene at the Bowers Ridge (BOW-12B and BOW-14B) and Bering/Arctic Gateway sites (GAT-4C and GAT-3C).

Umnak Plateau (primary Site UMK-4D and alternate Site UMK-3B)

The first drill site will be UMK-4D on the Umnak Plateau, located off Bristol Bay (Figs. F1, F2). Parts of the Alaskan Stream flow into the Bering Sea through Unimak Pass and Amukta Pass today; thus the sediments on the Umnak Plateau can be studied to monitor the exchange of Pacific and Bering Sea waters. These passes are fairly shallow (~50 m and 430 m) so that no intermediate or deep water flows out to the Pacific in this region. According to Scholl and Creager (1973), there are Pleistocene diatomaceous sediments with ash layers in the uppermost 120 m at DSDP Sites 184 and 185, followed by Pliocene diatomaceous sediments below. Sedimentation rates are ~67 m/m.y. They also indicated that the diatomaceous sediments have neritic components, probably influenced from the Bristol Bay region. Thus, we anticipate encountering Pleistocene and Pliocene diatomaceous sediments in the 200 m of APC cores to be drilled here. Sediment samples from these sites will be used to monitor the surface water exchange from the Pacific into the Bering Sea, as well as surface conditions in the easternmost part of the Bering Sea. Site UMK-3B is an alternate.

Bowers Ridge (primary Sites BOW-12B, BOW-14B, and BOW-15A)

Three high-priority drilling locations are located on the Bowers Ridge, which is topographically linked and perpendicular to the Aleutian Island arc, extending northward from the Aleutian Island arc with the top of the ridge generally shallower than 1000 m (Figs. F1, F2). The three primary sites are BOW-15A (near the top of the ridge, 837 m water depth), BOW-12B (1313 m), and BOW-14B (on the western slope of the Bowers Ridge at 2166 m), where we expect to encounter foraminiferal preservation as good as that found at DSDP Site 188 (Echols, 1973), based on piston core data. Based on results from DSDP Leg 19, we anticipate that drilling at one site (BOW-15A) to 165 m and at two sites (BOW-12B and BOW-14B) to APC refusal will allow for the recovery of diatomaceous sediments from the entire Pleistocene and into the middle Pliocene; extending penetration deeper using the RCB (at Sites BOW-12B and BOW-14B) will allow for recovery of lower Pliocene diatomaceous sediments and upper Miocene mudstones (Scholl and Creager, 1973).

Samples from these sites will provide records from the shallowest part of our vertical depth transect to a slightly deeper depth at 2166 m and will be used to evaluate vertical water mass gradients when compared to other sites. They will also be used to represent surface water and climate conditions in the southernmost part of the Bering Sea when compared to other drilling sites.

Bering/Arctic Gateway (primary Site GAT-4C and alternate Sites GAT-3C and NAV-1B)

This northern Aleutian Basin region represents a gateway to the Arctic Ocean through the Bering Strait. High-priority Site GAT-4C (1975 m water depth) and alternate Site GAT-3C (3209 m) are located in areas that are topographically protected from possible turbidites (Figs. F1, F2). The upper Miocene/Pliocene boundary was placed at ~400 mbsf at DSDP Site 190 (water depth = 3875 m) in the Aleutian Basin (Koizumi, 1973), and the bottom age of the hole at 627 mbsf was in the middle Miocene. Thus, we can be reasonably certain that by drilling to 700 mbsf we will at least have the entire Pliocene–Pleistocene–Holocene sequence, if not the upper Miocene, at this site. Because of the scanty nature of the previous DSDP coring effort, we do not know whether hiatuses existed at Site 190, although it is probable that we have a continuous section here. Based on our 1999 Hakuhou-Maru site survey Cruise KH99-3, we have prioritized a relatively shallow site (GAT-4C) to optimize the chances of good carbonate preservation and the nearby alternate site (GAT-3C) at deeper depths for comparison.

Alternate Site NAV-1B is situated near the location of piston core HLY-02-02-3JPC (60°07.6738´N, 179°26.5078´W) at 1132 m water depth. Core 3JPC yielded high sedimentation rates (0.14 m/k.y. during the Holocene, 2.42 m/k.y. during the deglaciation, and 0.91 m/k.y. during the Last Glacial Maximum) and intermittently laminated sediments (Cook et al., 2005). This site is well situated to record changes in surface ocean conditions and NPIW at a resolution that may allow comparison with other high-resolution sites such as the Santa Barbara and Cariaco Basins and the GISP2 record.

Influx of sediments from the Bering/Arctic Gateway region can be readily captured here to reconstruct the gateway's evolution. Change in CaCO₃ and opal contents in sedimentary records may tell us Pacific–Atlantic water mass exchange history. Pollen analysis may play an important role here to decipher the temporal variability of the continental environments to the west and east. Ice-rafted pebbles as well as ice algae analyses will tell us the history of glaciation and sea ice formation. Sediment trap data close to these sites will provide groundtruthing for paleoceanographic reconstruction of surface water conditions near the Bering Strait based on diatoms and coccolithophores (e.g., Coccolithus pelagicus, a cold-water taxon that occurs in high abundances at the trap stations, and its ancestors).

Shirshov Ridge (primary Site SHR-3B and alternate Site SHR-1B)

The Shirshov Ridge extends south from Siberia, forming a shallow topographic ridge that separates the Aleutian and Komandorski Basins (Figs. F1, F2). Samples from the highest priority Site SHR-3B (2232 m water depth) will be used to monitor deep water mass flowing out of the Bering Sea to the Pacific and surface conditions in the eastern Bering Sea. Alternate Site SHR-1B (963 m) could expand the vertical extent of the reconstruction of subsurface water mass conditions. Based on known sedimentation rates in this region, drilling to 200 mbsf will penetrate through the Pleistocene and possibly into the upper Pliocene.

Kamchatka Strait (primary Site KST-1B)

Proposed primary Site KST-1B is inside of the Kamchatka Strait (sometimes called the Komandorski Strait) and located at 3435 m water depth. This represents a strategic location for a deep water mass exiting from the Kamchatka Basin (sill depth through Kamchatka Strait is >4000 m) (Figs. F1, F3). Mammerick (1985) discussed a possibility of a bottom thermohaline circulation as a cause of the Meiji sediment tongue (Ewing et al., 1968), which has features similar to the North Atlantic drifts, especially in the general shape, length, and thickness of the various sediment bodies (Scholl et al., 1977, 2003). Drilling here will help to test the Mammerick hypothesis of the thermohaline circulation. Sediments in the Meiji sediment tongue may be supplied from the Bering Sea through the Kamchatka Strait (Scholl et al., 2003). This site represents one of the deepest sites of our vertical depth transect and is the site closest to the outflow of water from the Bering Sea into the North Pacific Ocean.

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