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doi:10.2204/iodp.proc.346.105.2015

Operations

Three holes were drilled at Site U1424 (proposed Site JB-1) (Table T1; see also Fig. F2 in the “Expedition 346 summary” chapter [Tada et al., 2015a]). Hole U1424A was cored using the advanced piston corer (APC) to 158.8 m core depth below seafloor (CSF-A) (see the “Methods” chapter [Tada et al., 2015b]). Similarly, Hole U1424B was cored with the APC to 154.7 m CSF-A, and Hole U1424C was cored to only 63.9 m CSF-A. A total of 41 cores obtained 380.68 m of sediment (101% recovery).

Transit from Site U1423

The transit to Site U1424 was very short. The 98.5 nmi distance was covered in 10 h at an average speed of 9.9 kt. The sea passage ended at 0738 h on 25 August 2013. The vessel was maneuvered over the location coordinates and turned over to dynamic positioning control. By 0815 h the vessel was stabilized for rig floor operations to begin at Site U1424.

Hole U1424A

The drill string was tripped to the seafloor, and the bit was positioned at 2806 meters below rig floor (mbrf) for the mudline core. However, this core barrel was recovered empty. The bit was repositioned at 2816 mbrf for the second attempt, and Hole U1424A was spudded at 1525 h on 25 August 2013. Based upon recovery from the first core, a seafloor depth of 2818.7 mbrf was established. APC coring continued with 17 cores recovered to 158.8 m CSF-A. With the top drive left in place, the drill string was pulled clear of the seafloor at 0645 h on 22 August, ending Hole U1424A. Coring recovered 161 m of sediment (101%). One temperature measurement was taken using the advanced piston corer temperature tool (APCT-3) temperature shoe on Core 346-U1423A-4H at 35.3 m CSF-A. This measurement fell exactly on the temperature gradient established during Leg 127 when this site was first cored (Site 794). Therefore, no additional temperature measurements were made at this site.

Hole U1424B

The ship was offset 15 m north of Hole U1424A, and Hole U1424B was started at 0740 h on 26 August 2013. Recovery from Core 346-U1423B-1H established a seafloor depth of 2819.8 mbrf. APC coring continued uneventfully with 17 cores recovered to 154.7 m CSF-A. The bit cleared the seafloor at 2035 h on 26 August, ending Hole U1424B. Coring recovered 155.3 m of sediment (100%).

Hole U1424C

The ship was offset 15 m south of Hole U1424A, and Hole U1424C was spudded at 2125 h on 26 August 2013. The operations plan for this hole was amended to conserve valuable operating time. Cores 346-U1424C-1H through 3H were taken for postexpedition optically stimulated luminescence (OSL) dating, and Cores 4H through 6H were taken for high-resolution geochemical analyses. Hole U1424C was started with the bit positioned at the same depth as it was for Hole U1424A (2416 mbrf). APC coring extended from the seafloor to 63.9 m CSF-A.

OSL sampling involved special core handling procedures, and Hole U1424C was intentionally planned to be cored during the night to minimize core exposure to light. Cores 1H through 3H were cut in sequence into 1.5 m whole-round sections directly on the drill floor sequentially as the core was pulled out of the core barrel and placed into opaque aluminum-lined pouches, labeled, sealed, and stored in refrigerated core storage. The drill string was pulled out of the hole, clearing the seafloor at 0320 h on 27 August, and was back on the rig floor at 0815 h on 27 August. The rig floor was secured for transit, thrusters were raised, and the sea passage to IODP Site U1425 began at 0854 h on 27 August.

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