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doi:10.2204/iodp.proc.346.108.2015

Operations

Site U1427 (proposed Site YB-1) was occupied at two different times during Expedition 346, first following operations at Site U1426 (6 September 2013) and then ~2 weeks later after completing operations at IODP Site U1430 (22 September) to acquire additional material for postcruise studies, as described below. A return to Site U1427 was possible because all scientific and operations objectives were met ~5 days before the end of the expedition; therefore, the remaining time available was spent reoccupying two sites drilled earlier in the expedition (Sites U1427 and U1425). Preliminary shipboard scientific results from Holes U1427A and U1427B indicated that this site exhibits a continuous Late Pleistocene sedimentary sequence with high sedimentation rates and excellent carbonate preservation, both of which are ideal for high-resolution paleoceanographic studies. Therefore, we drilled Hole U1427C during the revisit.

Three holes were cored at Site U1427 using the full and half advanced piston corer (APC) and the extended core barrel (XCB). Hole U1427A was cored to 548.6 m core depth below seafloor, Method A (CSF-A) (see the “Methods” chapter [Tada et al., 2015b]) (81 cores) with a recovery of 542.59 m (98.9%). Hole U1427B was cored to 405.6 m CSF-A (61 cores) with a recovery of 422.41 m (105%). Hole U1427C was cored to 351.1 m (52 cores) with a recovery of 367.77 m (105%) (Table T1; see also Fig. F2 in the “Expedition 338 summary” chapter [Tada et al., 2015a]). Four formation temperature measurements were performed, and downhole wireline log data were obtained to 548.5 m WSF (see the “Methods” chapter [Tada et al., 2015b]) in Hole U1427A.

An exciting aspect of operations at Site U1427 included the acquisition of the deepest continuously recovered APC-cored sequence in Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP)/IODP history, in Hole U1427A, as well as the deepest piston core (346-U1427A-81H) from 490.4 m CSF-A.

Transit from Site U1426

The 66 nmi distance to Site U1427 was covered in 6 h at an average speed of 11.1 kt. Sea passage ended at 0542 h on 6 September 2013. The vessel was maneuvered over the location coordinates, a wide-angle positioning beacon was deployed, thrusters were lowered into position, and the vessel was turned over to dynamic positioning control. By 0615 h, the vessel was stabilized, and rig floor operations began.

Hole U1427A

A three-stand APC/XCB bottom-hole assembly (BHA) was assembled, and the drill string was run to the bottom. After picking up the top drive and spacing out the drill string to a depth of 329.4 meters below rig floor (mbrf) (6.0 m shallower than precision depth recorder depth), an APC core barrel was deployed. The first core barrel recovered 1.79 m of core, establishing a seafloor depth of 337.1 mbrf. Oriented APC coring continued using full-length nonmagnetic core barrels through Core 346-U1427A-25H to 228.7 m CSF-A. The base of Core 25H recovered reworked volcanic sand, and the next core contained volcanic ash. At that point, we switched to the half APC system and continued coring through Core 81H to a depth of 490.4 m CSF-A, making this, as mentioned above, the deepest continuously cored sequence ever recovered with the APC coring system in DSDP/ODP/IODP history. An additional 6 XCB cores were cut to a final total depth of 548.6 m CSF-A. The hole was circulated and displaced with 10.5 ppg weighted mud, the top drive was set back, and the pipe was pulled up to logging depth (80 m WSF). Rig-up for wireline logging began at 0630 h on 8 September. Both the paleo combo and Formation MicroScanner (FMS)-sonic tool strings were deployed to 548.5 m WSF, and all wireline logging runs were successful. By 1715 h that same day, the rig floor was rigged down from logging. The pipe was lowered (without the top drive) to 519 m CSF-A, and a 10 bbl (24 m long) 14.5 ppg cement plug was displaced using a circulating head. The drill string was pulled clear of the seafloor at 2020 h on 8 September 2013, ending Hole U1427A and beginning Hole U1427B. Total recovery for this hole was 542.6 m (99%). A total of 56 half APC cores were recovered. Four formation temperature measurements were taken using the advanced piston corer temperature tool (APCT-3) temperature shoe at 30.3, 58.8, 87.3, and 115.8 m CSF-A.

Hole U1427B

Once clear of the seafloor, the drill string was thoroughly flushed to remove any remnant cement, and an XCB was deployed to aid in flushing/cleaning the lockable flow valve. The XCB was recovered and the ship was offset 15 m north of Hole U1427A. The bit was placed at a depth of 332.4 mbrf, and an APC core barrel was deployed. Hole U1427B was spudded at 2220 h on 8 September 2013, and the 4.8 m of core recovered from the mudline core established a seafloor depth of 337.1 mbrf. APC coring with full core barrels continued through Core 346-U1427B-6H to 52.3 m CSF-A before a 1.0 m drilled interval was used to correct the core break offsets. APC coring continued through Core 12H to 100.8 m CSF-A, where a second 1 m advance was made. APC coring resumed, recovering Cores 14H through 28H to 235.8 m CSF-A. This barrel was an incomplete stroke and was very near the interval in Hole U1427A where the APC met refusal. Eight half APC cores were then recovered (Cores 29H through 36H) to a depth of 273.4 m CSF-A. Because the overpull was zero for the previous six cores, we switched back to full APC barrels beginning with Core 37H. However, this core failed to fully penetrate the formation, and coring resumed at that point using the half APC system once again. A 1.0 m advance was made to properly align the core breaks, and coring then continued recovering half APC Cores 39H through 47H to 325.4 m CSF-A. There, another 3 m “drill ahead” advance was made to realign core breaks, which had drifted close to stacking up with the earlier hole. With the exception of a fourth drill ahead of 2 m after Core 52H, half APC coring continued through Core 65H to a total depth of 405.6 m CSF-A. The hole was displaced with 190 bbl of 10.5 ppg mud, and the drill string was pulled clear of the seafloor by 0605 h on 10 September. The bit cleared the rotary table at 0735 h, the positioning beacon was recovered aboard by 0752 h, and by 0830 h the ship was under way for the next IODP drill site (U1428). Total recovery for Hole U1427B was 422.41 m (105%). Of the 61 cores recovered, 32 were half APC cores.

Return to Site U1427

Nearly 2 weeks later, after completing operations at Site U1430, the ship returned to Site U1427 to drill an additional hole and collect additional data and core material for postcruise studies. The 182 nmi between Sites U1430 and U1427 were covered in 17.2 h at an average speed of 10.6 kt. Sea passage ended at 0830 h on 22 September 2013. The vessel was maneuvered over the location coordinates, and a Falmouth Scientific positioning beacon (model BAP-547; SN1028W, 16 kHz, 206 dB) was deployed at 0958 h. Thrusters were lowered into position, and the vessel was turned over to dynamic positioning control. By 0900 h, the vessel was stabilized and rig floor operations began.

Hole U1427C

The APC/XCB BHA was quickly assembled, and the drill string was run to the bottom. For this hole, the ship was positioned 15 m south of Hole U1427A. The top drive was picked up and the pipe was spaced out, positioning the bit at a depth of 336 mbrf. Hole U1427C was spudded at 1200 h on 22 September. The first core barrel recovered 8.05 m of core, establishing a seafloor depth of 337.5 mbrf. APC coring continued using full-length nonmagnetic core barrels through Core 346-U1427C-10H to 93.5 m CSF-A. A single half APC core was then taken (Core 11H) to adjust the core break offset. APC coring then continued using full barrels through Core 25H to 224.2 m CSF-A. Coring continued from that point using the half APC through Core 52H to a total depth of 351.1 m CSF-A. Two formation temperature measurements were taken using the APCT-3 temperature shoe at 17.5 and 36.5 m CSF-A. The hole was displaced with 140 bbl of 10.5 ppg weighted mud, the top drive was set back, and the drill string was pulled clear of the seafloor at 1710 h on 23 September. The bit was clear of the rotary table by 1815 h. During the pipe trip, the positioning beacon was recovered at 1739 h. The bit and bit sub were removed, and the rig floor was secured for transit by 1845 h. Thrusters and hydrophones were pulled, and the sea voyage to return to Site U1425 was initiated at 1900 h on 23 September. Total recovery for Hole U1427C was 367.77 m (105%). Of the total 52 cores recovered, 25 were full APC cores and 27 were half APC cores. There were no cores cut using the XCB.

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