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Transit to Site U1381

After a 11.4 nmi transit to Site U1381 from Site U1380, the vessel stabilized over Site U1381 at 2133 h (Universal Time Coordinated – 6 h) on 8 April 2011. The position reference was a combination of GPS signals and a single acoustic beacon. The positioning beacon was deployed at 2152 h on 8 April and recovered at 1527 h on 12 April.

Site U1381

Two holes were cored at this site (Table T1). Hole U1381A was cored to 164.1 mbsf with a 9⅞ inch rotary core barrel (RCB) core bit and a 171.13 m long bottom-hole assembly (BHA). Coring continued through Core 334-U1381A-26R to 164.1 meters below seafloor (mbsf). Coring was terminated at 0045h on 12 April 2011. Coring in Hole U1381B had two major objectives. The first objective was to establish a temperature gradient for the sediment section above oceanic crust. The second objective was to take samples for geochemistry and microbiology in the uppermost 30 m of sediment. Sediment Temperature Tool (SET) temperature measurements were taken at ~30, 50, 70, and 90 mbsf. Overall core recovery for Site U1381 was 46.5%. A total of 29 cores were recovered after coring 193.1 m. The total length of core recovered at this site was 89.84 m. Hole U1381B was terminated after drilled interval 334-U1380B-43 after the last temperature measurement was made at ~90 mbsf.

Hole U1381A

Rig floor operations commenced at 2133 h on 8 April. The RCB BHA was made up with a Rock Bit International C-4 RCB bit and mechanical bit release. The trip to the seafloor was uneventful. The top drive was picked up, the drill string was spaced out, and the hole was spudded at 0330 h on 9 April. Coring continued for 69.25 h and was terminated after Core 334-U1381A-26R to move to Hole U1381B.

Total cored interval in Hole U1381A was 164.1 m with 73.86 m of core recovered for an RCB system recovery of 45.0%. Last core on deck for Hole U1381A was at 0045 h on 12 April. The hole was displaced with 50 bbl of 10.5 ppg mud, and the drill string was pulled back to just above the seafloor with the top drive in place. The seafloor was cleared at 0220 h, ending Hole U1381A.

Hole U1381B

After offsetting the vessel 20 m west, Hole U1381B was spudded at 0325 h on 12 April and advanced with the RCB coring system to 29.0 mbsf before running the SET. Three RCB cores were taken in Hole U1381B with a 15.98 m recovery. The cores were taken without circulation and were essentially punch cores. At 29.0 mbsf, the SET was run on wireline and the first measurement was taken. The hole was then advanced without recovery to 50 mbsf. The core barrel was pulled and the SET was again lowered by wireline to take the second measurement. The procedure was repeated for measurements at 71 and 91 mbsf. After the last temperature measurement was taken, the hole was displaced with 10.5 ppg heavy mud and the drill string was pulled back to surface. The rotary bit cleared the seafloor at 1440 h and cleared the rotary table at 1835 h. The beacon was recovered at 1527 h and the rig was secured for transit at 1857 h, ending Hole U1381B and Site U1381 on 12 April. The vessel left Site U1381 on a 100.6 nmi transit to Puntarenas, Costa Rica, after completing operations during Expedition 334.