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doi:10.2204/iodp.proc.318.108.2011

Operations

Transit to Site U1360

We departed for Site U1360 at 2345 h on 11 February 2010 after coring Hole U1359C. The voyage began in gale force winds and rough seas, but these slowly abated during the day. As we moved south, we began to observe numerous icebergs and had to adjust speed and heading to adapt to conditions. Approximately 15 nmi from location we encountered an ice tongue comprised of pack ice and icebergs that blocked direct access to Site U1360. We had to detour ~30 nmi to the west to find a clear passage around the ice. The outside temperature of –5°C combined with the 35 kt winds created a wind chill of about –20°C and ice began forming on the deck and the outside bulkheads. All times in this section are given in local ship time, which was Universal Time Coordinated + 11 h.

Site U1360

We arrived at Site U1360 at 1800 h on 12 February 2010. However, we had to wait 5.25 h in dynamic positioning mode for two large icebergs in the immediate vicinity to move away. We were finally on site and started deploying the drill sting at 2315 h on 13 February (Table T1). We decided to use a shorter, two-stand bottom-hole assembly so we could reduce the time the rigid drill collars were above the seafloor. We did this to reduce the chances of repeating the drill collar pin failure that occurred in Hole U1358B.

After the driller tagged seafloor with the bit at 506.0 meters below rig floor, we started RCB coring in Hole U1360A at 0425 h on 13 February. Cores 318-U1360A-1R through 7R penetrated to 70.8 mbsf and recovered 7.04 m (10%). Because of the relatively hard seafloor, rotary coring proceeded slowly and carefully. Coring had to stop when large icebergs forced an early termination of the hole. We started to prepare a free-fall funnel so that we could attempt to return to the same hole after the next severe storm. Unfortunately, the hole had to be abandoned because of the approaching iceberg before it could be deployed.

We departed Site U1360 at 0615 h on 14 February. We planned on returning to this site, but moved to deeper water to avoid riding out the next intense low pressure system among the ice in shallow water. This system was predicted to arrive on the evening of 14 February with predictions for 50 kt winds (gusts up to 60 kt) and poor visibility in snow flurries on Monday.

We arrived at a position in deeper water that was 50 nmi north of the ice pack. The 104 nmi transit to this location took 10.2 h at 10.2 kt. We then went into dynamic positioning mode to safely ride out the storm. The storm system arrived as predicted late Sunday evening and by Monday was upon us with sustained winds of 50 kt and gusts as high as 62 kt as well as heavy seas and poor visibility during periods of heavy snow. The sea and swell continued to build during most of 15 February with the vessel motion reflecting the intensity of a storm that increased to just short of hurricane force. The barometer bottomed out at 972 mbar and remained there for the rest of the afternoon. At 1800 h on 15 February, we had to offset the ship because of an approaching iceberg. The storm began to abate during the late evening and we began the transit back to Site U1360 early the next morning (1000 h on 16 February).

During the transit, we adjusted speed and course so that we would not overtake the remnants of the storm that was still south of us and so that we would arrive at first light the next morning. During the daylight hours of 17 February, we made four attempts to penetrate the northern edge of the extensive pack ice and iceberg barrier that existed between the vessel and Site U1360. However, each time we were blocked by the ice and had to reverse course. So that we could avoid having to maneuver the vessel amidst the ice during the darkness, we moved to a position ~100 nmi northwest of Site U1360 and “parked” in dynamic positioning mode throughout the evening of 17 February.

The next morning we decided to attempt to reach Site U1360 from the south. We began the transit at 0545 h on 18 February. We sailed around the western edge of the growing ice tongue, then moved east-southeast past the Antarctic Circle at 1340 h and attempted to approach Site U1360 from the southwest. This time we were able to get within 8 nmi of Site U1360 before encountering a heavy concentration of pack ice and icebergs blocking access to the site. We reluctantly reversed course and began retracing our course westward in order to move back around to the northern edge of the ice tongue during the evening of February 18. At this point, we decided to attempt to get to Site U1358 or other alternate sites on the outer shelf on the northern edge of the ice tongue.

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