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doi:10.2204/iodp.pr.318.2010

Operations

During Expedition 318 we conducted operations in 14 holes at 7 sites. We cored 3071 m and recovered 1972 m of core (Table T1). Downhole logs were collected in two holes. Here we describe the coring and logging operations at each of these sites. The overall time distribution included 5.85 days in port, 22.7 days in transit, and 35.6 days on site. The on site time includes 4.9 days in which we had to cease operations because of the severe Antarctic weather and ice conditions. The transit time also includes >4 days spent trying to reach some of the shelf sites only to be turned back by ice conditions.

Wellington port call

The expedition began when the first line was secured to Aotea Quay, Centre Port, Berth 5 in Wellington, New Zealand, at 1420 h on 3 January 2010. All times in the operations section are given in local ship time unless otherwise noted. Initially this was Universal Time Coordinated (UTC) + 13 h. During transit to the first site, the ship's clocks were set back 1 h at 1200 on 12 January to UTC + 12 h and then another 1 h at 2400 on 13 January to UTC + 11 h, where it remained for the duration of the expedition.

As soon as routine customs and immigration formalities were concluded, off-loading of surface freight was initiated. The expedition United States Implementing Organization (USIO) staff and Chief Scientists boarded the ship on 4 January. In addition to routine replenishment of expendables and off-loading of the previous expedition's cores and returning freight, we loaded 740 MT of marine gasoil, repaired one of the thrusters and a propulsion motor, conducted laboratory/ship tours for guests and visiting dignitaries, mounted the wind wall on the core receiving platform, and installed a new touch-screen rig instrumentation monitor in the drillers console.

Because this was a high-latitude expedition that operated in the challenging environment off the coast of Wilkes Land, Antarctica, two critical specialists were added to the shipboard team. A senior weather forecaster with previous IODP expedition experience joined the vessel to provide timely analysis of the complex weather environment of the Southern Ocean. A seasoned ice observer with extensive familiarity with both Arctic and Antarctic regions also joined the expedition. The combined skill set provided by these participants enhanced the safety of the vessel and optimized the ability of the crew to maximize on-site operational flexibility.

Transit to Site U1355

The vessel departed on the 1847 nmi journey to the first site when the last line was released from the dock at 1045 h on 9 January 2010. The vessel sailed at full speed, except for a short time during which the brushes on a propulsion motor (16A) were replaced. By midnight on 9 January, the ship had sailed 116 nmi at an average speed of 9.3 kt.

The ship transited south-southwest off the east coast of the New Zealand's South Island. Preliminary analysis of ice coverage appeared to be promising for our first site (U1355) with temperatures in that region of 0.2°C, winds from the east-southeast increasing to 35–40 kt, and cloudy conditions with occasional snow or sleet.

The vessel passed within 30 nmi of Auckland Island on the morning of 12 January. Once the vessel moved south of the protection of Auckland Island, it was exposed to the open expanse of the Southern Ocean and the powerful low-pressure systems that populate this region. The vessel negotiated through intense winds and sea conditions from 12 January until the morning of 15 January. Frequent course changes and reductions in speed were made to avoid the most intense parts of a large and severe low-pressure system that was crossing the projected ship track. Even though the vessel remained on the outer edge of the system, the combined sea and swell reached 40 ft and the winds were clocked at as high as 60 kt. Vessel motion ranged from moderate to heavy and on one occasion the ship experienced a 13° roll. The ship's clocks were set back 1 h at 1200 on 12 January to UTC + 12 h and then another 1 h at 2400 on 13 January to UTC + 11 h.

The vessel began to ride more smoothly in improving sea conditions by the afternoon of 15 January and was proceeding at nearly 10 kt to Site U1360. During the evening, the first penguins were observed, hinting that we might be approaching ice-strewn waters. At 0345 h on 16 January, the first iceberg was spotted on radar at 9 nmi and then visually observed at 5 nmi through the fog and mist as it passed on the port side. It was pinnacled and rigged and estimated to be ~300 ft long and 50 ft high. There were also a few growlers in the vicinity. During the day, the vessel speed was reduced when fog and mist reduced the visibility to <1/8 nmi. At 0440 h on the morning of 17 January, ship's radar picked up a large iceberg at 17 nmi with a calculated closest point of approach of 7 nmi to starboard.

By midnight on 16 January, the vessel had traveled 1505 nmi at an average speed of 8.3 kt. The estimated time of arrival on site was mid-morning on 18 January. Satellite analysis of ice coverage in the region indicated the drill site area was mostly free of large icebergs and floes. The winds at our first site were forecasted to pick up and peak at ~40 kt the evening of 18 January and then decrease. Temperatures were expected to be ~0°C. The new long-range outlook through 22 January indicated good weather conditions on site.

Throughout the week, the drilling crew continued preparations for drilling operations in the severe Antarctic weather conditions. A wind wall was constructed around the drill floor, exposed piping (heating, air, drain, and water) was insulated, electrical lines were run for heaters in the mud pump room and behind the drawworks, and external space heaters were tested.

As we approached Site U1360 at the reduced speed of 8 kt, the captain received an updated satellite image from the National Ice Center that indicated a large tongue of ice in the direct path to Site U1360. This was obscured in previous imagery by cloud cover. This necessitated adding a dogleg in the track to Site U1360 so that we could steer around the leading edge of the tongue.

As the vessel altered course and speed in heavy mist and fog, large icebergs began to appear on the radar and a few growlers were spotted visually. When the fog suddenly cleared, icebergs of various shapes and sizes were observed around the vessel. The vessel proceeded at slow speed attempting to approach Site U1360 amid the ice when the weather deteriorated during the afternoon of 18 January. With the winds gusting to 35 kt and accompanied by snow and reduced visibility, the effort to reach Site U1360 was suspended 19 nmi short of the objective until ice conditions were more favorable. The captain altered course to Site U1355 located ~167 nmi to the north-northwest. The most recent satellite pictures indicated that Site U1355 appeared to be free of floating ice.

Site U1355

The vessel exited the hostile ice conditions and proceeded at an average speed of 6.4 kt to Site U1355, arriving on site at 1045 h on 19 January 2010. The circuitous voyage from Wellington to the first site covered 2021 nmi at an average speed of 8.5 kt. During the voyage from the area of Site U1360 to Site U1355, no icebergs were spotted on the radar or visually.

An APC/XCB bottom-hole assembly (BHA) was made up with an 11 7/16 inch bit and lowered to the seafloor. The initial mudline attempt was made at 0705 h on 20 January with the bit at 3735 meters below rig floor (mbrf; 5.7 m shallower than the corrected depth of 3740.7 mbrf inferred from the precision depth recorder [PDR]). When the core barrel was recovered, we found that the bottom of it had broken off at the connection joining the two 4.5 m nonmagnetic sections. To test the nature of the seafloor, the driller gently tagged seafloor with the bit at ~3740 mbrf. The bit was slowly rotated and a mild increase in torque was observed, indicating that the seafloor was firm but apparently not unusually hard.

Another mudline core was attempted using the more durable standard steel core barrels with the bit positioned at 3735 mbrf. The pressure bleed-off indicated a full stroke but the core winch operator was unable to recover the APC core barrel with the coring line. For nearly 1.5 h, the core winch operator and driller tried various techniques to recover the corer with the coring line without success. The only recourse was to recover the drill string. The bit was back on the rig floor at 2225 h on 20 January. This second APC core barrel was also broken off. Once the remains of the core barrel were cleared from the bit throat, it was decided to attempt a spud with the RCB coring system.

An RCB BHA with a new C-4 9 7/8 inch bit and mechanical bit release were made up and deployed. The seafloor depth was again confirmed at 3740.0 mbrf (3729.0 mbsl) by the driller gently tagging the bit on the seafloor. Coring in Hole U1355A was finally started at 0905 h on 21 January. The first core advanced 3.5 m but recovered 5.3 m of core. This could have occurred because of uncertainty in the precise seafloor depth and/or by the drill bit repeatedly tagging the seafloor as a result of ship heave. RCB Cores 318-U1355A-1R through 4R penetrated from 0 to 31.7 mbsf and recovered 14.95 m (47%). The cores recovered unconsolidated coarse sands and gravel, so the recovery was quite remarkable. However, coring had to be terminated because of unstable hole conditions caused by the coarse and unconsolidated sediments.

We decided to abandon this site and find an area less hostile to coring. We considered making another attempt to return to the shelf sites, but based on remote sensing data and information from a research vessel in that region it was felt that ice conditions in the area had not significantly improved. Instead, we decided to move to Site U1356, located 82 nmi west-northwest of Site U1355. After a total time of 2.8 days on site, we departed Site U1355 at 0600 h on 22 January.

Site U1356

We departed from Site U1355 at 0600 h on 22 January 2010. The short voyage to Site Site U1356 was without incident except for spotting a lone iceberg both on radar and visually. The vessel was positioning at Site U1356 at 1345 h on 22 January. The 82 nmi journey was made at an average speed of 10.7 kt.

Because of the problems starting a hole with APC coring at the previous site, we decided to start coring with the RCB coring system. The drill string was lowered to the seafloor and the RCB bit tagged it at 4003 mbrf. This was 21.5 m deeper than the corrected PDR depth of 3981.5 mbrf.

RCB coring in Hole U1356A started at 0025 h on 23 January. RCB Cores 318-U1356A-1R through 106R penetrated to 1006.4 mbsf and recovered 350.13 m of core (35%). The presence of glacial erratics below the first two cores adversely affected recovery. Recovery varied considerably from 0% to 99%, but it improved with depth. For example, Cores 318-U1356A-66R through 92R penetrated 254.3 m of formation (622.3–876.6 mbsf) and recovered 140.58 m (55%).

No significant hole problems were encountered. We started circulating 20 bbl of mud frequently beginning at 296.7 mbsf. This was increased to every other core starting with Core 318-U1356A-69R (660.6 mbsf). A single 40 bbl sweep was circulated at 631.9 mbsf and then 20 bbl mud sweeps were circulated at 814, 834, 852, 872, and 877 mbsf. Below this depth we circulated 20 bbl of mud after each core.

Minor disruptions to operations occurred for 1 h on 27 January when the WKM ball valve and remote actuator required servicing and when we had to make an extra wireline run to recover Core 318-U1356A-97R after shear pin parted during the initial retrieval attempt.

The total target depth of the hole was revised as we progressed to ensure full characterization of the greenhouse to icehouse transition across the inferred Eocene/Oligocene boundary reflectors. Ultimately, coring reached 1006.4 mbsf. As soon as coring was finished, we started to prepare the hole for logging by sweeping the hole with 50 bbl of sepiolite and then raising the drill bit back up to 103 mbsf. A tight spot was encountered at 409 mbsf. While lowering the bit back down into the hole to 476 mbsf, a ledge was encountered at 400 mbsf. At this point, the weather had deteriorated (up to 20 ft swells; 35 kt winds) so that we had to abandon the hole. Based on weather forecasts, we also thought conditions would not improve substantially for long enough to allow a reasonable chance to log the hole. We retrieved the drill string and by the time the drill string was recovered, the weather system had passed and the sea conditions were relatively mild. After the beacon was recovered, we began the transit to Site U1357 at 0615 h on 2 February.

The only ice seen during operations at Site U1356 occurred on 29 January when a pinnacle-shaped iceberg was detected 12 nmi to the southeast. It was estimated to be 40 m high, 110 m long, and moving haphazardly at ~0.5 kt. Two growlers were also observed visually in the general area of the iceberg. The iceberg came no closer than 10 nmi, did not impact operations, and drifted off radar at 0430 h on 30 January.

Site U1357

Site U1357 was the southernmost of the shelf sites, all of which are prone to ice coverage. Shortly after departing for Site U1357 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 APC/XCB drilling assembly.

We started coring in Hole U1357A at 1445 h on 3 February, and the first core established the seafloor depth at 1025.9 mbrf (1014.8 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%). All APC cores at this site were obtained with the 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.

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%). 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, 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%). 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 with 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.

Site U1358

The 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′ 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 of 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.

We were positioning over Site U1358 at 1730 h on 6 February. The 93 nmi transit was accomplished at an average speed of 10.9 kt. 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 mbrf 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 had penetrated only 2.0 m and recovered 1.10 m (55%). 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%).

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 forecasted 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

After failed attempts to return to Site U1360, we decided to try to return to Site U1358. During 19 February 2010, 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 the 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.

Site U1359

We began transit to Site U1359 at 1530 h on 7 February 2010. While departing the shelf, we had to negotiate around and through an assortment of pack ice and grounded ice bergs; these became less concentrated and finally disappeared as we moved off the shelf. As we arrived in the vicinity of Site U1359 at midnight, we experienced near–gale force winds, rough seas, and visibility down to 4 nmi in freezing rain. We lowered the thrusters and stabilized near the site using the Global Positioning System but waited on the weather to improve until the next morning. The 73 nmi transit to Site U1359 was accomplished in 8.5 h at 8.6 kt.

After 7 h, the weather conditions improved enough so that we could assemble the drill string to the seafloor. We started APC coring in Hole U1359A at 1800 h on 8 February with the bit at 3012 mbrf. Based on recovery of the first core, the water depth was 3020.9 mbrf, 9.1 m deeper than the corrected depth from the PDR. Cores 318-U1359A-1H through 17H penetrated to 145.4 mbsf and recovered 124.27 m (86%). Temperature measurements were made while taking Cores 318-U1359A-4H, 7H, 10H, and 13H (29.1, 57.6, 86.1, and 114.6 mbsf, respectively). Nonmagnetic core barrels were used for all piston cores after Core 318-U1359A-1H but were not oriented. After APC refusal, we deepened the hole with XCB Cores 318-U1359A-18X through 22X from 145.4 to 193.5 mbsf and recovered 29.81 m (62%). Total recovery for Hole U1359A was 80%. Rather than continuing to deepen the hole with the XCB, we decided to stop so that we could core two more APC holes to provide a more complete section.

The bit cleared the seafloor at 1700 h on 9 February, and we offset the vessel 25 m west-southwest. We started coring Hole U1359B at 1815 h with the bit at 3017 mbrf. Seafloor was established at 3018.8 mbrf. APC Cores 318-U1359B-1H through 23H penetrated to 209.0 mbsf and recovered 183.59 m (88%). Nonmagnetic core barrels were used for all APC cores. We then deepened the hole with the XCB for Cores 318-U1359B-24X through 28X from 209.0 to 252.0 mbsf and recovered 15.33 m (36%). The total recovery for Hole U1359B was 79%. The bit was pulled clear of the seafloor at 2240 h on 10 February.

The vessel was offset 25 m west-southwest of Hole U1359B. We started coring in Hole U1359C at 2340 h on 11 February with the bit at 3020 mbrf. Seafloor depth was established at 3022.3 mbrf. APC Cores 318-U1359C-1H through 18H penetrated to 168.7 m and recovered 150.73 m (89%). We planned to stop coring at this time to be able to depart for high-priority shelf sites so that we could take advantage of a forecasted period of good weather. We planned on returning to this site, so we did not retrieve the seafloor beacon at this time. The bit cleared the seafloor at 1620 h and was back onboard at 2340 h on 11 February. We departed for Site U1360 at 2345 h on 11 February. The total time on Site U1359 was 89.25 h.

Return to Site U1359

After operations at Site U1360 and multiple attempts to return to that site and other shelf sites, we returned to Site U1359 at 2200 h on 19 February. We assembled an RCB BHA, lowered it to the seafloor, tagged the seafloor with the bit at 3023.0 mbrf, and started drilling Hole U1359D at 0530 h on 20 February 2010. We drilled without coring to 152.2 mbsf and then started RCB coring at that depth to overlap with previous APC/XCB coring that had penetrated to 252 mbsf. RCB Cores 318-U1359D-2R through 48R penetrated from 152.2 to 602.2 mbsf and recovered 269.7 m (60%). The last core was recovered on deck at 1145 h on 23 February.

In preparation for downhole logging, we flushed the hole with a 50 bbl mud sweep and made a wiper trip up to 82.6 mbsf and then back down to 602.2 mbsf. After another 50 bbl sepiolite mud sweep, we released the bit at the bottom of the hole, displaced the hole with 191 bbl of 10.5 ppg mud, and raised the end of the pipe to 96.9 mbsf for logging.

We were able to conduct two very successful logging runs in excellent hole conditions. The triple combo and FMS-sonic tool strings were able to log the entire hole from 602 mbsf up to the end of the pipe. The FMS-sonic tool string was back on deck at 2000 h on 23 February. In accordance with our Marine Mammal Protocol, we waited until daylight the next day before conducting the check shot log utilizing the Schlumberger VSI.

We rigged up the VSI and started lowering it downhole at 0745 h on 24 February. The VSI was able to reach within 5 m of bottom of the hole, but we had to postpone starting the check shots because a few whales had entered within the mammal exclusion zone. Once we were able to start, we discovered that the VSI caliper arm could not be extended to clamp the tool against the borehole wall. However, we were able to set the tool on the bottom of the hole and get good enough coupling with the formation to collect data at that depth. We could have retrieved the VSI tool for repair/replacement, but we decided conclude logging so we could attempt to reach one of our high-priority shelf sites. After we recovered the VSI at 1330 h on 24 February, we retrieved the drill string, with the end of the pipe clearing the seafloor at 1440 h and arriving back on the rig floor at 2050 h. We departed for the last attempt at occupation of one of the high-priority shelf sites at 2245 h on 24 February.

Site U1360

We departed for Site U1360 at 2345 h on 11 February 2010. 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.

We arrived at Site U1360 at 1800 h on 12 February. 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. We decided to use a shorter, two-stand BHA 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 happened in Hole U1358B.

After the driller tagged seafloor with the bit at 506.0 mbrf, 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 (FFF) 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. This 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 Mb 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.

Site U1361

After completing operations at Hole U1359D and attempting to reach our shelf sites (proposed Site WLSHE-12A, Site U1358, or Site U1360), we decided to conduct our final operations at Site U1361. We positioned over Site U1361 at 1945 h on 25 February 2010. The total transit distance from Hole U1359D to near-shelf proposed Site WLSHE-12A and finally to Site U1361 covered 149 nmi at an average speed of 7.1 kt. We lowered the drill string and started coring Hole U1361A at 0710 h on 26 February. Based on the recovery of the first core, the seafloor depth was 3465.5 mbrf (3454.3 mbsl), or 5.3 m shallower than the corrected PDR depth. APC coring advanced to 151.5 mbsf and recovered 150.9 m (100%). The first core was obtained with a standard steel core barrel, but all subsequent APC cores were taken with nonmagnetic core barrels. Temperature measurements were made while taking Cores 318-U1361A-4H, 7H, 10H, and 13H at 37.5, 66.0, 94.5, and 123.0 mbsf, respectively.

We switched to XCB coring and deepened the hole from 151.5 to 388.0 mbsf. Cores 318-U1361A-17X through 41X recovered 188.0 m (79%). After Core 318-U1361A-41X was recovered at 0120 h on 28 February, we had to suspend coring to deploy a FFF because an iceberg was moving directly toward the site.

We raised the bit to 90 mbsf and deployed a FFF at 0700 h on 28 February. At that time, the iceberg was 5.5 nmi away and continuing to move toward the ship. We waited until ~0900 h to see if it would change course, but it was still heading toward us. At 0945 h, we pulled the bit out of the hole and started moving the ship to the north and out of the iceberg's path.

The ~500 ft wide iceberg passed directly over the hole at 1245 h. Once the iceberg had cleared the site and was no longer a threat, we moved back over the hole by 1500 h and the reentry camera/sonar system was deployed. At 1555 h, the video failed when the system was 200 m below the ship. When we recovered the camera system we found that water had entered the cable head assembly on the main umbilical cable. As soon as it was repaired we lowered it to the end of the drill string.

We reentered Hole U1361A at 2137 h on 28 February after 15 min of searching and positioning the drill bit over the FFF. We lowered the drill string into the hole as the camera system was retrieved, and by 0130 h on 1 March the bit had reached the bottom of the hole at 388.0 mbsf without having experienced any significant drag while running into the hole. Because of the little time remaining before we had to depart for Hobart, Australia, we decided not to continue coring and prepare the hole for downhole logging.

The hole was flushed with 50 bbl of sepiolite mud and then displaced with 179 bbl of 10.5 ppg heavy mud. The drill string was raised to 88.6 mbsf for logging. We conducted two successful logging runs with the triple combo and the FMS-sonic tool strings, with both reaching the bottom of the hole. Once logging was concluded, the bit was pulled free of the seafloor and the vessel offset 50 m north of Hole U1361A.

APC coring in Hole U1361B started at 0840 h on 2 March and established the water depth at 3466.9 mbrf. We only had time for two piston cores before the operational time for the expedition ran out, in part as a result of a delay caused by ice buildup on the APC piston rod. APC Cores 318-U1361B-1H and 2H penetrated to 12.1 mbsf and recovered 12.0 m (100%).

The drill string was recovered and the BHA disassembled into component parts by 2400 on 2 March. We started our transit to Hobart at 0030 h on 3 March. The last line ashore in Hobart was at 1500 h on 8 March.

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