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doi:10.2204/iodp.proc.346.106.2015

Operations

Site U1425 (proposed Site Y-1) was occupied at two different times during Expedition 346, first following operations at Site U1424 on 28 August 2013 and then ~3 weeks later after revisiting IODP Site U1427 on 24 September (see below) to acquire additional material for postcruise studies. The return to Sites U1425 and 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 these two important sites drilled earlier in the expedition. We drilled Hole U1425E during the revisit.

Four holes were cored at Site U1425 using the full and half advanced piston corer (APC) and the extended core barrel (XCB) (Table T1; see also Fig. F2 in the “Expedition 346 summary” chapter [Tada et al., 2015a]). Hole U1425A was cored to 9.5 m CSF-A, Hole U1425B was cored to 407.2 m CSF-A, Hole U1425C was cored to 25 m CSF-A, Hole U1425D was cored to 427 m CSF-A, and Hole U1425E was drilled 3 weeks later to 113.1 m CSF-A. A total of 148 cores obtained 955.5 m of sediment (97% recovery). The advanced piston corer temperature tool (APCT-3) was deployed four times.

Transit from Site U1424

The 180 nmi distance between Sites U1424 and U1425 was covered in 17.6 h at an average speed of 10.2 kt. The sea passage ended at 0230 h on 28 August 2013. The vessel was maneuvered over the location coordinates, thrusters were lowered into position, and the vessel was turned over to dynamic positioning control. By 0300 h, the vessel was stabilized for rig floor operations to begin.

Hole U1425A

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 1911.4 meters below rig floor (mbrf) (6 m above the observed seafloor depth on the ship’s precision depth recorder), an APC core barrel was deployed. The first core barrel was recovered empty. The bit was repositioned 10 m lower at 1901.4 mbrf for the second attempt. Hole U1425A was started at 0905 h on 28 August 2013. This time, however, the core barrel was recovered full, once again missing a reliable seafloor determination.

Hole U1425B

The ship was offset 15 m north, a new APC core barrel was deployed, and the bit was positioned at 1918.4 mbrf. Hole U1425B was spudded at 1005 h on 28 August 2013, establishing a seafloor depth of 1919.1 mbrf. APC/XCB coring continued to a final depth of 407.2 m CSF-A, the scientific target depth for this site. Coring operations in Hole U1425B consisted of 28 full APC cores, 23 half APC cores, and 10 XCB cores. We recovered 397.25 m of core (98%). Three successful temperature measurements were taken using the APCT-3 temperature shoe on Cores 346-U1425B-4H, 7H, and 10H (37.3, 65.8, and 94.3 m CSF-A, respectively). The hole was circulated clean, the logging tools were moved forward to the rig floor, the top drive was set back, and the drill string was pulled to a logging depth of 80 meters below seafloor (mbsf). At 1115 h on 30 August, preparations for wireline logging were initiated. The paleo combo tool string was deployed to the bottom of the hole to 403.2 m WSF (see the “Methods” chapter [Tada et al., 2015b]) and recorded spectral gamma ray, caliper, magnetic susceptibility, resistivity, and lithologic density logs. The second string consisted of the Formation MicroScanner (FMS)-sonic tool string, which recorded resistivity images of the borehole, caliper, sonic velocities, and natural gamma radiation (NGR) data. The FMS-sonic string reached 396.5 m WSF, 10.5 m off the bottom of the hole (407 m CSF-A). The logging tools were rigged down, and the drill string was pulled clear of the seafloor at 0200 h on 31 August.

Hole U1425C

After offsetting the ship 15 m south of Hole U1425A, Hole U1425C was spudded at 0355 h on 31 August 2013. Core 346-U1425C-1H recovered 5.95 m, establishing a seafloor depth of 1918.9 mbrf. This core was followed by two additional full-length APC cores to 25 m CSF-A. Because these cores were for optically stimulated luminescence sampling, they were handled per the protocols established at Site U1424, taking care to shield the core from any natural or artificial light as much as possible. The drill string was pulled clear of the seafloor at 0540 h on 31 August, ending Hole U1425C operations.

Hole U1425D

The ship was offset 15 m west of Hole U1425A and Hole U1425D was spudded at 0605 h on 31 August 2013. Core 346-U1425D-1H established a seafloor depth of 1919.6 mbrf. Coring continued to a final depth of 431 m CSF-A, alternating between the full and half APC and the XCB core barrels. The coring effort consisted of 26 full APC cores, 40 half APC cores, and 4 XCB cores. Core recovery totaled 417.5 m (98%). Recovery percentages are biased in the last few cores in Hole U1425D because both the XCB and half APC systems were pushed into dolomite and siliceous claystone horizons that neither coring system was designed to handle. Despite poor recovery in the last few cores, the coring effort was scientifically successful, obtaining a dateable cored section for the last half APC core recovered (Core 72H). The top drive was set back and the drill string was recovered back aboard the ship. The positioning beacon was recovered during the pipe trip and was back aboard after lunch on 2 September. The BHA was set back in the derrick, and the vessel was secured for transit to IODP Site U1426, ending (temporarily) operations at Site U1425.

Return to Site U1425

We returned to Site U1425 after completing an additional hole at Site U1427 (3 weeks after the first visit). The 212 nmi distance between Sites U1427 and U1425 was covered in 19.5 h at an average speed of 10.9 kt. Sea passage ended at 1430 h on 24 September 2013. The vessel was maneuvered over the location coordinates, thrusters were lowered into position, and the vessel was turned over to dynamic positioning control. By 1500 h, the R/V JOIDES Resolution was stabilized and rig floor operations began again at the site.

Hole U1425E

Deployment of a positioning beacon was deemed unnecessary for this hole because the weather was good, water depth was <2000 m, and the anticipated penetration depth was only ~100 m CSF-A. Nonetheless, a Falmouth Scientific positioning beacon (model BAP-547; SN1022, 14 KHz, 208 dB) was kept on standby in case it was required.

As at Site U1427, this hole was to provide extra core material for future sampling needs and postexpedition studies. The APC/XCB BHA was assembled, and the drill string was run to the bottom. The ship was positioned 15 m east of Hole U1425A. The top drive was picked up, and the pipe was spaced out, positioning the bit at 1913.4 mbrf. The first barrel was recovered empty (water core), so the bit was lowered 3.0 m to 1916.4 mbrf, and the process was repeated. Hole U1425E was ultimately spudded at 1945 h on 24 September. The first core barrel recovered 5.69 m of core, establishing a seafloor depth of 1920.2 mbrf. APC coring continued using full-length nonmagnetic core barrels through Core 346-U1425E-13H to a total depth of 113.1 m CSF-A. Total recovery for Hole U1425E was 107.75 m (95%). A total of 13 cores were recovered in this hole, all using full-length nonmagnetic core barrels.

The top drive was set back, and the drill string was pulled clear of the seafloor at 0435 h on 25 September. All drill collars were laid out to the forward pipe rack, and the bit was clear of the rotary table by 0910 h. The bit and bit sub were removed, and the outer barrel coring system was disassembled. The rig floor was secured for transit by 0950 h. Thrusters were raised, and the sea passage to Busan was initiated at 1012 h on 25 September, successfully ending all coring operations of Expedition 346.

Transit from Site U1425 to Busan, Korea

The transit to Busan was completed without incident. The vessel arrived at the Busan pilot station at 0600 h on 27 September 2013. The Asian Monsoon Expedition 346 officially ended 24 h early with the first line ashore at 0651 h on 27 September.

It seems only fitting during this research voyage, which is the last of this phase of IODP, that we draw attention to the fact that this expedition set the record for the most amount of core recovered during any single research cruise by IODP. That the recovery of 6135 m of sediment occurred during only ~6 weeks of drilling operations makes this accomplishment even more remarkable. In combination with the record achieved of acquiring the deepest piston core (and deepest continually piston-cored sequence at Site U1427), these achievements speak of the commitment to excellence, teamwork, and, to be direct, impressively hard work of men and women dedicated to the pursuit of scientific knowledge about Planet Earth.

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