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doi:10.2204/iodp.proc.318.105.2011

Operations

Transit to Site U1357

Site U1357 was the southernmost of the shelf sites, all of which are prone to ice coverage. Shortly after departing Site U1356 at 0615 h on 2 February 2010, we began to encounter floating ice. The vessel speed was frequently adjusted to match visibility and ice conditions as we proceeded on a southeast heading toward the site. We were able to negotiate through large concentrations of icebergs and growlers with the benefit of a clear and sunny day providing excellent visibility. The 218 nmi transit to Site U1357 took 25.7 h at an average speed of 8.5 kt. We were positioned over Site U1357 at 0800 h on 3 February. After a short wait to evaluate the movement of proximal ice, we began assembling the advanced piston coring (APC)/extended core barrel (XCB) drilling assembly. All times in this section are given in local ship time, which was Universal Time Coordinated + 11 h.

Site U1357

Hole U1357A

Coring in Hole U1357A began at 1445 h on 3 February, and the first core established the seafloor depth at 1025.9 meters below rig floor (mbrf) (1014.8 meters below sea level [mbsl]). However, the extremely diffuse boundary between bottom water and seafloor sediment precludes accurate definition of water depth using a standard shipboard sonar system; most coring systems (including the APC) typically do not recover this boundary. Cores 318-U1357A-1H through 21X penetrated to 186.6 m and recovered 183.87 m (99%) (Table T1). All APC cores at this site were obtained with nonmagnetic core barrels unless otherwise noted. The last APC core (Core 318-U1357A-20H) was an incomplete stroke. We then took a single XCB core from 185.6 to 186.6 mbsf, and it bottomed out in diamict. Coring was interrupted for 4 h when an iceberg approached close to the ship. Prior to pulling out, the hole was filled with 79 bbl of 10.5 ppg mud. The bit cleared the seafloor at 1120 h on 4 February.

Hole U1357B

After offsetting the ship 50 m to the east, we started coring in Hole U1357B at 1345 h on 4 February. The seafloor was at 1028.0 mbrf (1017.0 mbsl). APC Cores 318-U1357B-1H through 19H penetrated to 170.7 mbsf and recovered 172.44 m (101%) (Table T1). Prior to pulling out, the hole was displaced with 72 bbl of 10.5 ppg mud. The bit cleared the seafloor at 0340 h on 5 February.

Hole U1357C

Hole U1357C, offset 25 m west of Hole U1357A, was started with the APC at 0645 h on 5 February. APC Cores 318-U1357C-1H through 11H penetrated to 103.8 mbsf and recovered 110.7 m (107%) (Table T1). Once again, coring had to be suspended because of the proximity of several icebergs. This time, however, the ice passed close enough that we had to abandon the hole so we could offset the ship 2.8 km to the southeast to allow the ice to pass through the drilling area. The bit cleared the seafloor at 1358 h on 5 February and we had to wait until the next morning to retrieve the beacon.

At this time, the weather deteriorated to gale force winds with gusts measured up to 58 kt, and the combined sea and swell reached 30 ft. During heavy snow squalls the visibility dropped to 0.5 nmi. To ensure the safety of the vessel, lookouts were posted to observe for growlers and icebergs as the vessel adopted a controlled drift back toward location. As the storm faded, the sea quickly flattened enough for the beacon to be recovered at 0815 h on 5 February.

The plan was to reoccupy Site U1357 to finish coring the lowermost ~80 m of the section a third time to help provide a more complete composite section and to obtain downhole logs. The weather forecast, however, predicted winds gradually increasing to 45–50 kt by the evening of 7 February, with occasional gusts in excess of 60 kt and poor visibility in heavy snow squalls. Because of the high concentration of icebergs of all sizes in the Site U1357 area, safety concerns dictated that the most prudent course of action was to depart the region. We departed Site U1357 at 0900 h on 6 February. The total time at Site U1357 was 63.4 h (2.6 days) of which 24.25 h (38% of total) were due to delays caused by ice.

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