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doi:10.2204/iodp.proc.320321.107.2010

Operations

Times for Site U1335 are given in local ship time, which was universal time coordinated (UTC) – 9 h.

Transit to Site U1335

The 375 nmi voyage from Site U1334 to Site U1335 took 42.3 h at an average speed of 8.9 kt. Shortly after departing Site U1334, propulsion Motor 16A on the port propeller shaft had to be taken offline because of defective field coils. This was the second propulsion motor to exhibit the same problem during Expedition 320. Propulsion Motor 15A on the starboard shaft had to be removed from the grid shortly after leaving Honolulu. It is expected that these units will be repaired during the port call in Honolulu. The rest of the voyage was made with reduced revolutions on both shafts (120 rpm instead of 140 rpm). Although capable of higher shaft revolutions, the lower shaft power was necessary to keep the ambient temperature in the propulsion room from moving into a critically high range.

Site U1335

Hole U1335A

The ship slowed and was in dynamic positioning (DP) mode over Hole U1335A at 1630 h on 15 April 2009. We assembled the drill string, lowered it to the seafloor, and started coring Hole U1335A at 0355 h on 16 April. The estimated water depth based on the recovery of the first core was 4339.0 m DRF (4327.5 mbsl) (Table T1).

Cores 1H through 36H were taken from 0 to 341.4 m DSF, and we recovered 354.7 m (104%) (Table T1). APC piston coring to this depth is an operational highlight for this site. Nonmagnetic core barrels were used for Cores 1H through 16H, and steel barrels were used for all other cores. Nine core barrels required drilling over to release them from the formation (Cores 19H through 22H, 26H, 27H, and 34H through 36H) and Cores 16H and 36H only partially stroked.

We then switched to XCB coring and took Cores 37X through 45X from 341.4 to 421.1 m DSF and recovered 67.9 m (85%). We retrieved Core 45X after taking ~1 h to advance only 3 m. When retrieving this core, it apparently became stuck in the drill string just above the bottom-hole assembly. We attempted to free the stuck core barrel with the wireline jars by alternating the tension on the coring line for ~1 h when the overshot shear pin parted. Once we recovered the sinker bars and wireline jars, we pumped another core barrel down on top of it and this succeeded in dislodging the stuck core, which dropped back down to the bit. When Core 45X was recovered, it contained hard limestone and the XCB bit was very worn down, so we decided it was best to stop coring in this hole. The basement depth is deeper than anticipated and suggests higher seismic velocities in the sediment column from Site U1335 than those from the nearest site (574). Once Hole U1335A was completed, the bit was pulled free of the seafloor at 1725 h on 18 April and the vessel offset 25 m west.

Hole U1335B

We started coring Hole U1335B at 1955 h on 18 April with the bit 5 m deeper than at Hole U1335A. The seafloor depth based on the recovery of the first core was 4339.6 m DRF (4328.1 mbsl).

APC Cores 1H through 41H were taken from 0 to 378.2 m DSF, and we recovered 392.7 m (104%). At the time, this was the second deepest APC penetration in the history of ocean drilling. Nonmagnetic core barrels were used on Cores 1H through 19H, and steel barrels were used on all remaining cores. Formation temperature measurements were made with the APCT-3 at 22.3, 41.3, 60.3, 79.3, and 98.3 m DSF (Cores 3H, 5H, 7H, 9H, and 11H, respectively). Twelve core barrels had to be drilled over to release them from the formation (Cores 19H, 20H, and 35H through 41H). Cores 37H through 41H did not achieve a full stroke and were advanced by recovery.

After switching to XCB coring, we took Cores 42X through 46X from 378.2 to 417.5 m DSF and recovered 36.0 m (92%). Coring was terminated when basement was reached.

Originally we had planned three holes at Site U1335. However, after recovering a continuous Miocene section with nearly complete recovery and overlap between the two holes, we decided to not core a third hole at Site U1335. The fact that the cores contained frequent turbidites and a very weak magnetic signal in the interval with the distinctly colored (diagenetically altered) sediment also contributed to this decision. Instead we decided to use our remaining time to core at Site U1336 to provide the second PEAT expedition more information for optimizing their operations plan.

We pulled the drill string out of the hole, and the bit cleared the seafloor at 1215 h on 21 April. Before the drill string was recovered, we spent 1 h to slip and cut 115 ft of drill line. Once drilling line maintenance was completed, the drill string was recovered, the beacon retrieved, and the drilling equipment secured for transit. The vessel departed for Site U1336 at 2145 h on 21 April.

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