Previous   |   Next

doi:10.2204/iodp.proc.323.111.2011

Site explanations and seismic profiles in the Bering Sea

Ten Bering Sea drilling sites were proposed: seven "highest priority" sites (SHR-1B, BOW-12B, BOW-14B, BOW15A, GAT-3C, GAT-4C, and UMK-4D), two "high-priority alternate" sites (SHR-1B and UMK-3B), and one "second priority" site (KST-1B) (Table T1; Fig. F1). Prioritization was provided in response to a request from IODP-SAS-SSEP. Four relatively deep sites were proposed (triple-APC holes to refusal and a single extended core barrel [XCB] hole to 700 meters below seafloor [mbsf]), two on Bowers Ridge (Sites BOW-12B and BOW-14B) and two Bering gateway sites (Sites GAT-4B and GAT-3B). At each of the other sites, triple holes employed APC to refusal (~200 mbsf). To achieve all scientific objectives at Bowers Ridge, Site BOW-15A was proposed during the late stage.

The basic drilling strategy was to recover the paleoclimatic, paleoenvironmental, and paleoceanographic record archived in Bering Sea sediment, as well as to achieve good horizontal coverage about surface and deepwater conditions needed to reconstruct water column gradients. Proposed drilling sites were located between 837 and 3891 meters below sea level (mbsl) with a range of depth coverage. Based on the results of DSDP Site 188 (2649 mbsf) on Bowers Ridge (Echols, 1973) and site survey Cruise KH99-3 piston cores (Yasuda and Sagawa, 2003; Asahi and Takahashi, 2003), good carbonate and foraminifer assemblage preservation was expected.

Bowers Ridge

Bowers Ridge is topographically and structurally linked to the Aleutian Island arc. Bowers Ridge extends northward from the ridge of the Aleutian Island arc to curve westward and subparallel to the trend of the Aleutian Ridge. The summit of Bowers Ridge is generally flat-topped and deeper than 1000 m. Sediment cores from Bowers Ridge were intended to provide records from the shallowest part of our vertical depth transect to slightly deeper depth, and, when compared to Umnak Plateau and Beringian margin sites, would be used to evaluate vertical water mass gradients. Sediment cores from Bowers Ridge would also serve to represent the southernmost part of the Bering Sea when compared to other, more northerly, proposed sites. Good foraminifer preservation, as found at DSDP Site 188 on Bowers Ridge at 2649 m depth (Echols, 1973), was expected at Bowers Ridge sites. Based on the results of Leg 19, it was anticipated that drilling to a subsurface depth of ~700 m would recover diatomaceous sediments for the entire Pleistocene and Pliocene and perhaps mudstone from the late Miocene (Scholl and Creager, 1973).

The site survey data at Bowers Ridge were obtained in 1999 on Cruise KH99-3 of the Hakuhou-maru (Fig. F3). In their last communication, EPSP requested (1) annotation of the seismic data to reflect crossings and inclusion of shotpoints on the track lines shown on the bathymetric map; (2) relocation of the site to the line crossing; and (3) provision of a map of multibeam bathymetry used to construct the bathymetric map. To comply, we reprocessed the original site survey data of Cruise KH99-03. We updated and corrected a series of sets of navigation data, swath map bathymetry, 3.5 kHz subbottom profiles, and MCS reflection profiles. We used updated shotpoint numbers for seismic profiles corrected against rechecked navigation data. Shotpoint numbers assigned in the descriptions below were systematically corrected from the initial submitted version of data and profiles. Water depth at each of the sites was estimated using multibeam bathymetry (SeaBeam 2120HR). Seismic velocity analysis was also performed to assess the presence of gas hydrate.

Site BOW-12B (U1340)

The location of Site BOW-12B (Figs. F4, F5, F6, F7; Table T1) was positioned at time 0602 h, Shotpoint 5863, on Line Stk6-1 (Figs. F8, F9) and time 0942 h, Shotpoint 1537, on Line Stk6-3 (Figs. F10, F11). For the initial proposed site, we tried to locate the site on the seismic crossing point, responding to the recommendation of EPSP to do so, where it seemed to present no problem for drilling on swath bathymetry (Fig. F5) and subbottom profiles (Fig. F6). However, on seismic Line Stk6-1, the crossing point was located at a small mound on the seafloor (which appears to be a mud volcano), and the upper sediments between 1.75 and 2.0 two-way traveltime (TWT) seemed to be of condensed stratification, slightly disturbed in the eastern part. For these reasons, we moved the site slightly from the crossing point to Shotpoint 910 on Line Stk6-1, where sediments appear to be well stratified in the deeper sedimentary section below the proposed site (Figs. F8, F9).

Water depth was estimated at 1313 m by multibeam bathymetry (Fig. F5). A penetration depth of 700 meters below seafloor (mbsf) was proposed and approved by EPSP at this site, and we expected recovery of a continuous sedimentary section accumulating since the late Miocene. However, because wavy and weak seismic reflections were recognized in the upper sequence of the processed seismic reflection horizons at Site BOW-12B (Figs. F9, F11), it seemed likely that a continuous Quaternary section might be difficult to recover. No obvious bottom-simulating reflector (BSR) was recognized at this site (Figs. F8, F10). Seismic velocity analysis around the site indicated that P-wave velocity increases gradually with depth, implying the absence of methane gas hydrate (Fig. F12).

Site BOW-14B (U1341)

The location of Site BOW-14B (Figs. F13, F14, F15; Table T1) was positioned on the crosssing point of the seismic survey lines at time 1045 h, Shotpoint 650, of Line Stk5-1 (Fig. F16) and time 1825 h, Shotpoint 2488, on Line Stk5-5 (Fig. F17). The sedimentary section appeared to be well stratified at the crossing point (Figs. F18, F19, F20). A penetration depth of 700 mbsf was proposed at this site, expecting recovery of a continuous sedimentary section building since the late Miocene at a deeper water depth (2166 mbsl) than at Site BOW-12B (1313 mbsl). Prominent BSRs are observed on Line Stk5-5 at ~3.5–3.8 s TWT, which was estimated to be at 750–850 mbsf (Figs. F18, F19, F20, F21). A low-velocity zone was detected immediately beneath the BSR (Fig. F12), suggesting the presence of free gas immediately below interstitial deposits of methane gas hydrate. For this reason, penetration depth at this site was modified by EPSP to 600 mbsf.

Site BOW-15A (U1342)

Site BOW-15A (Figs. F22, F23, F24; Table T1) was located at the well-stratified location at Shotpoint 410 on Line Stk4-1 (Fig. F25) because of weak and wavy seismic reflections between Shotpoints 450 and 550 on Line Stk4-1 (Fig. F25), which is north of seismic crossing point at time 1620 h, Shotpoint 505, on Line Stk4-1 and time 1921 h, Shotpoint 1203, on Line Stk4-3 (Fig. F26). Based on seismic data, Site BOW-15A was added in order to achieve the proposed science objectives of Bowers Ridge drilling to recover a continuous and complete Quaternary record because wavy and weak seismic reflections were recognized in the upper sequence of the reprocessed seismic reflection at both Sites BOW-12B and BOW-14B. A penetration depth of 165 mbsf at Site BOW-15A by multi-APC coring was approved by EPSP, which was deemed necessary to recover the Quaternary sedimentary sequence covering the much older basement core of Bowers Ridge.

Bering/Arctic gateway

The continental slope or Beringian margin bordering the north side of the Aleutian Basin represents a gateway to the Arctic Ocean through the Bering Strait farther to the north. Sediments needed to reconstruct the evolution of gateway oceanography and climate can be readily recovered along the Beringian margin. Stratigraphic changes in sediment CaCO₃ and opal content ought to reveal the exchange history of Pacific-Atlantic water masses. Pollen analysis will help decipher the temporal variability of continental environments to the west and east. Studies of ice-rafted lithic debris as well as ice algae will disclose the history of glaciation as well as sea ice formation. Sediment trap data collected close to Beringian gateway sites provide ground-truthing information needed to reconstruct paleoceanographic 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). The late Miocene/Pliocene boundary was placed at ~400 mbsf at DSDP Site 190 (water depth = 3875 mbsl) in the abyssal floor of the western Aleutian Basin (Koizumi, 1973) and the bottom age of the hole at 627 mbsf in the middle Miocene. Thus, we can reasonably anticipate that by drilling to 700 mbsf along the gateway margin much of the Pliocene–Pleistocene–Holocene sequence, if not some of the upper Miocene, would be recovered. Owing to scanty recovery by DSDP using the bit rotation technology available in the early 1970s, we do not know whether hiatuses existed at Site 190, although it is probable that we have a continuous section here of mixed turbiditic and diatomaceous deposits. After the 1999 site survey Cruise KH99-3 of the Hakuhou-maru and discussion with EPSP, gateway margin drilling sites (GAT-3C and GAT-4C) were selected in areas topographically protected from turbidite deposition (Fig. F27). We also prioritized a relatively shallow site (GAT-3C) to optimize the chances for good carbonate preservation and another (Site GAT-4C) at deeper depths for water chemistry comparison.

Site GAT-3C (U1344)

Site GAT-3C (Figs. F28, F29, F30; Table T1) was placed at Shotpoint 2860 on seismic Line Stk3-7 (Figs. F31, F32) near the crossing points of seismic Line Stk3-7 and Line Stk3-5 (Figs. F33, F34). A penetration depth of 700 mbsf at Site GAT-3C was approved by EPSP.

Based on detailed navigation information and seismic profiles (Figs. F28, F32, F34), a prominent BSR on Line Stk3-5 was not recognized below Site GAT-3C. A low-velocity zone was not detected beneath the interval of increased velocity (Fig. F35), suggesting the absence of methane gas hydrate at Site GAT-3C.

Site GAT-4C (U1343)

Site GAT-4C (Figs. F36, F37, F38; Table T1) was positioned at Shotpoint 351 on Line Stk1 (Figs. F39, F40) near the crossing point with seismic Line Stk1 and on Line Stk2 (Figs. F41, F42). The selection of the approximate crosssing point was made because at the crossing point irregular reflections were recognized around the BSRs. A penetration depth of 700 mbsf was approved by EPSP.

Obvious BSRs were recognized on Lines Stk1 and Stk2 at a subsurface depth of 1.0 s TWT (Fig. F43). A low-velocity zone was not detected immediately beneath the BSRs, suggesting the absence of interstitial gas around Site GAT-4C (Fig. F35).

Umnak Plateau

Umnak Plateau is located north of Umnak Island in the eastern corner of the deepwater Bering Sea Basin (Fig. F1). Parts of the Alaskan Stream flow into the Bering Sea through Umnak and Amukta Passes and over the plateau. Sediments on Umnak Plateau can be studied to monitor the exchange of Pacific and Bering Sea waters. These passes are fairly shallow (~50 and 430 mbsl), so intermediate or deep water cannot flow out through them to the Pacific. On Umnak Plateau at DSDP Sites 184 and 185, Scholl and Creager (1973) reported Pleistocene diatomaceous sediments with ash layers in the top 120 mbsf overlying Pliocene diatomaceous sediments. Sedimentation rates are ~67 m/m.y. The ash layers also indicate that diatomaceous sediments have neritic components, probably influenced from the Bristol Bay region immediately to the east. Thus, we anticipated recovering Pleistocene and Pliocene diatomaceous sediments in the 200 m of APC cores proposed to be recovered at Umnak Plateau to monitor surface water exchange from the Pacific into the Bering Sea as well as surface conditions in the eastern most region of the Bering Sea Basin.

MCS site survey data were provided by USGS Cruise L6-80 of the Lee (1980), later augmented in 1999 by multibeam bathymetry and 3.5 kHz subbottom profile data collected on Cruise KH99-3 of the Hakuhou-maru.

Sites UMK-4D (U1339) and UMK-3B

Site UMK-4D, a highest priority site, and Site UMK-3B (not drilled during Expedition 323), an alternate site, were positioned by combining site surveys of two cruises (Table T1). The sites were originally proposed to be positioned along MCS seismic Profiles 2, 5, and 6 of USGS Cruise L6-80 of the Lee (Fig. F44), which were the original digital data recorded along these lines but only digitized seismic images of paper prints of velocity processed and stacked 24-channel data. Navigation and bathymetry maps for USGS Cruise L6-80 of the Lee and Cruise KH99-03 of the Hakuhou-maru were created with Global Mapping Tool (GMT) software (Wessel and Smith, 1995) using Global Topo Data of the Geographic Information Network of Alaska (GINA) (www.gina.alaska.edu/); the maps are shown, respectively, in Figures F44 and F45. A single line of swath bathymetric data and 3.5 kHz subbottom profile was collected during Cruise KH99-03 and used to estimate 1885 mbsl water depth at Site UMK-4D and 1898 mbsl water depth at Site UMK-3B (Figs. F46, F47).

Site UMK-4D is at the point of seismic Lines 2 (Shotpoint 3275) near the crossing point of Lines 2 (Shotpoint 3312) and 5 (Shotpoint 625) of USGS Cruise L6-80 of the Lee (Figs. F44, F48, F49, F50; Table T1). Site UMK-3B is at the crossing point of Lines 2 (Shotpoint 2617) and 6 (Shotpoint 721) of the same cruise of the Lee (Figs. F44, F48, F51, F52; Table T1).

EPSP requested that the annotated BSR on these seismic lines should be confirmed as either a methane gas hydrate or silica transition BSR. The depth of the BSR was estimated at 550–650 mbsf (Fig. F53). A low-velocity zone was detected beneath the BSR at proposed Site UMK-3B on Line 6 (Fig. F54). DSDP drilling at Umnak Plateau found a prominent silica transition BSR at both Sites 184 and 185. A prominent velocity increase, from ~1.65 km/s above to 2.2 km/s below the BSR was also measured on recovered cores (Fig. F55). The proposed depth of penetration of the sites was only 200 mbsf, well above the BSR at 550–650 mbsf.

Shirshov Ridge

Shirshov Ridge extends south from the Siberian continent, forming a shallow topographic ridge separating the Aleutian and Kamchatka Basins, the latter commonly referred to as the Komandorsky Basins.

Sites SHR-3B and SHR-1B

The site survey data for Sites SHR-3B and SHR-1B (not drilled during Expedition 323) were collected in 1970 during Cruise 02 of the USNS Bartlett (Fig. F56). Although the proposed sites were picked along seismic lines, calculation of the their positions was based on hand-written navigation information. Swath bathymetry and high-resolution subbottom profiles are not available for these sites. Navigation maps were produced using digital trackline data archived and maintained on line by the USGS (Figs. F56, F57, F58). Survey lines and bathymetry shown on these figures were created using GMT and GINA. We corrected the site position (latitude and longitude) by linear interpolation between navigation times on the seismic line because shot numbers tied to position coordinates are not included in the archived information. The water depth of each site was estimated based on gridded bathymetric data.

The highest priority Site SHR-3B (2232 mbsl; Table T1) was proposed to monitor deepwater masses flowing southward out of the Bering Sea to the Pacific and also surface conditions in the eastern Bering Sea (Figs. F57, F59, F60). Alternate Site SHR-1B (963 mbsl; Table T1) would serve to expand the vertical extent of the subsurface water masses (Figs. F58, F61, F62). Based on known sedimentation rates (8.0 cm/k.y.) in this region (Table T1), drilling to 200 mbsf at both sites should penetrate through the Pleistocene and possibly into the late Pliocene. Time-depth conversion used for the two sites is based on the MCS data gathered by the USGS during Cruise L6-80 of the Lee (Figs. F59, F61). Seismic profiles collected by the USNS Bartlett, Cruise 02, during Legs 54 and 55 document that the upper 200 m of the sedimentary sequences at both of the proposed sites would be well-stratified and safe locations for drilling (Figs. F60, F62).

Kamchatka Strait

The Kamchatka Strait site was located to monitor deepwater masses exiting southward from Kamchatka Basin through Kamchatka Strait or Kamchatka Pass to the Pacific Basin. The sill depth of the pass is ~4200 m. Mammerickx (1985) presented the possibility that exiting deep waters driven by thermohaline circulation within the Bering Sea Basin deposited the Meiji sediment tongue that overlies the northwest corner of the Pacific Basin (Ewing et al., 1968). This large sediment body is similar to North Atlantic Drifts, especially in general shape, length, stratigraphic fabric, and thickness (Scholl et al., 1977, 2003). Drilling at the Kamchatka Strait would help test the Mammerickx hypothesis of thermohaline-driven circulation within the Bering Sea. Much of the sediments in the Meiji Drift body have been supplied from the Bering Sea through the Kamchatka Strait (Scholl et al., 2003; VanLaningham et al., 2009). The proposed site represents one of the deepest of our vertical depth transect and the site closest to the outflow of water from the Bering Sea into the North Pacific.

Site KST-1B

Only a single-channel seismic profile for proposed Site KST-1B (not drilled during Expedition 323) was collected in 1970 during Cruise 02 of the USNS Bartlett (Fig. F63). Survey lines and bathymetry shown on these figures were created using GMT and GINA because swath bathymetry and high-resolution subbottom profiles are not available for this site. The site position (latitude and longitude) was corrected in the same manner for proposed Sites SHR-3B and SHR-1B (see above).

Site KST-1B (alternate site) is located inside Kamchatka Strait at 3435 mbsl at time 0703 h on the Bartlett seismic line (Fig. F64). The shallow penetration depth of 200 mbsf should not provide a safety concern (Fig. F65).

Top of page   |   Previous   |   Next