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doi:10.2204/iodp.proc.323.111.2011

Site survey and data processing

Progress of collecting site survey data sets

Initially, the bulk of the seismic profiles in the Bering Sea presented in IODP Proposal 477 were collected by U.S. expeditions (USNS Bartlett in 1970 and R/V Lee in 1980) (Fig. F2). In August 1999, new site survey data were specifically collected for proposed sites on Bowers Ridge (BOW), over the continental slope (Beringian margin) of the Aleutian Basin (GAT; gateway sites to the Arctic Ocean), and Umnak Plateau (UMK) during Cruise KH99-3 Leg 3 of the Hakuhou-maru (formerly operated by Ocean Research Institute, the University of Tokyo and now by the Japan Agency for Marine-Earth Science and Technology [JAMSTEC]) (Fig. F2).

Cruise KH99-3 Leg 3

Cruise KH99-3 Leg 3 was conducted from 29 July to 25 August 1999 (Takahashi, 2005). During the cruise, high-resolution swath bathymetry and MCS data and sediment cores were collected in support of IODP Proposal 447.

High-resolution swath mapping

High-resolution swath mapping was conducted using the SeaBeam 2120HR system and Integrated Global Positioning System (GPS)/Inertial Navigation systems installed on the Hakuhou-maru. The accuracy of the depth measurement is ~0.1% of the water depth. A positioning and orientation system for marine vessels (POS/MV) integrated navigation system and orientation system was used to monitor the ship motion in all three axes. GPS positions were acquired based on the World Geodetic 1984 (WGS84) system. A 3.5 kHz, high-resolution profile was also collected. Swath mapping was conducted over BOW, GAT, and UMK sites.

Multichannel seismic survey

MCS profiles were recorded as described in Table T2. Two air guns were deployed, a 3000 cm³ gun on the port side and an 1800 cm³ system to starboard. See SURVEY in "Supplementary material" for multichannel seismic data in SEG-Y format.

Sediment coring

Sea-bottom sediments were collected using a piston and multiple corers provided by Ocean Research Institute (ORI), University of Tokyo.

Seismic velocity and target depth estimation

In order to estimate target drilling depths for proposed sites, we used velocity measurement on sediment cores recovered during Deep Sea Drilling Project (DSDP) Leg 19 (Creager, Scholl, et al., 1973). In addition, we applied stacking MCS velocity data from United States Geological Society (USGS) Cruise L6-80 of the R/V Lee and UMK-3A Line 6. Velocity controls and estimated penetration depths are included in the seismic profiles shown for each proposed site. For GAT and BOW sites, velocity analyses were applied during the reprocessing of reflection data.

Age estimates of the proposed drill sites

Estimates of sedimentation rates for proposed sites were based mostly on the thickness of Holocene sediment recovered in surface cores from nearby locations (Table T1) and were unlikely to accurately represent mean sedimentation rates over long periods of time and, in particular, during cold, glacial periods. Using a range of possible sedimentation rates we expected that the proposed triple–advanced piston coring (APC) would at least penetrate through a significant part of the Pleistocene and that deeper drilling would possibly penetrate to upper Miocene beds at Bowers Ridge (Sites BOW-12B and BOW-14B) and gateway (Sites GAT-4B and GAT-3B) sites on the Beringian margin.

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