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doi:10.2204/iodp.proc.318.106.2011

Operations

Transit to Site U1358

Transit from Site U1357 to Site U1358 began at 0900 h on 6 February 2010. We proceeded on an easterly course along the Wilkes Land coast and briefly into the Antarctic Circle (66°33.65′S and 140°43.70′E) before turning to the northeast toward the shelf drill sites (U1358 and U1360). As we approached Site U1360, the Captain decided from visual and radar observations that there was too much floating ice in the vicinity for safe operations. This concentration of ice would likely force us to make an early departure to avoid entrapment caused by the severe weather predicted for late 7 February. We continued past Site U1360 to Site U1358, which is closer to open waters and therefore might allow us stay on location longer before the predicted storm would arrive. The 93 nmi transit was accomplished at an average speed of 10.9 kt. All times in this section are given in local ship time, which was Universal Time Coordinated + 11 h.

Site U1358

Hole U1358A

The ship was positioned over Site U1358 at 1730 h on 6 February. The drill string was lowered to the seafloor, but we had to wait 1.3 h to start coring as an iceberg moved through the drilling area. We tagged the seafloor with the bit at 510.0 meters below rig floor and started RCB coring in Hole U1358A at 2245 h on 6 February.

While cutting Core 318-U1358A-1R, an iceberg approached close enough to the vessel that we had to pull out of the seafloor at 0015 h on 7 February. We moved 300 m south-southwest of the hole to allow the iceberg to pass. Drilling Core 318-U1358A-1R penetrated only 2.0 m and recovered 1.10 m (55%) (Table T1).

Hole U1358B

After waiting 2.25 h for the iceberg to drift out of the area, we moved back over the site and started Hole U1358B at 0250 h on 7 February. RCB Cores 318-U1358B-1R through 4R penetrated to 35.6 mbsf and recovered 8.0 m (22%) (Table T1).

While attempting to cut Core 318-U1358B-5R, a drill collar connection failed and the lower stand of drill collars, mechanical bit release, assorted subs, bit, and core barrel were lost in the hole. We departed Site U1358 at 1530 h on 7 February because the weather was forecast to significantly deteriorate and we did not want to remain in an area with such high concentrations of ice during the storm. We departed for a deeper water site, Site U1359, where the concentration of icebergs was likely to be very low. Total time on Site U1358 was 22.00 h.

Attempt to return to Site U1358

Later in the expedition and after failed attempts to return to Site U1360, we decided to try to return to Site U1358. During 19 February, we sailed around the western edge of the growing ice field, turned east, and returned to reassess the ice conditions at Site U1358. When we found access to Site U1358 still blocked by ice, we diverted to an alternate shelf site (proposed Site WLSHE-12A) north of Site U1358. We were able to come within 3 nmi of this alternate site when we once more were blocked by heavy ice. At 1630 h on 19 February, we turned north and departed for Site U1359.

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