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Expedition 320 was the first of two expeditions that comprise the PEAT science program. Expedition 320 began in Honolulu, Hawaii, and drilled 16 holes at 6 sites (Sites U1331–U1336; Table T1) before returning to Honolulu.
Unless otherwise noted, times in this operations section are given in local ship time, which was Hawaii Standard Time (UTC – 10 h) for Sites U1331–U1333. During the transit to Site U1334, shipboard clocks were advanced 1 h, resetting local ship time to UTC – 9 h. Shipboard clocks were returned to Hawaii Standard Time (UTC – 10 h) during the transit from our last site (U1336) back to Honolulu.
Expedition 320 officially began when the first line was passed ashore to Pier 29 of Honolulu harbor at 1042 on 5 March 2009. Following the routine U.S. customs and immigration formalities, port call activities were initiated with the changing and crossover of Overseas Drilling Limited (ODL) and IODP personnel. Expedition scientists boarded the ship on 6 March.
During the ~5 day port call, the usual food stocks and provisions were loaded and off-going freight disembarked. Items of note that were loaded in Honolulu included ~707 MT of marine gasoil, 147 short tons of bulk attapulgite, 7 joints of 20 inch casing, 16 joints of 16 inch casing, and 46 joints of 10were two boxes of core liners and two reentry cone assemblies.
In accordance with the port call plan, customer service representatives came on the vessel to perform maintenance and repair on the ship's elevator and the HVAC system. A Siemens engineer also performed upgrades to the throttle system program.
A number of shipboard tours were conducted for the IODP Science and Technology Panel (STP), several classes from the University of Hawaii (USA), and National Science Foundation (NSF) visitors. Of the unplanned events, a replacement refrigerated food container had to be leased at the last moment because the 10 ft auxiliary unit located on the accommodation roof could not be repaired in the time available. The U.S. Coast Guard interrupted the loading of compressed gas cylinders on 9 March for nearly an hour to address concerns relating to the handling of these items at this facility.
Following the release of the last line from the pier at 1500 h on 10 March, the JOIDES Resolution exited the harbor. Outside the harbor, a small vessel came alongside to deliver a radioactive source and two neutron tools required for the logging effort. The vessel then departed at full speed to the first site (U1331) with an expected arrival the evening of 14 March.
The transit to the first site of Expedition 320 was made through generally moderate seas and swell with predominantly overcast skies and occasional showers signifying that we were under the influence of the ITCZ. We began positioning over Site U1331 at 2330 h on 14 March 2009. The 1081 nmi voyage from Honolulu was accomplished at an average speed of 10.4 kt.
The 4 day voyage was almost without incident. The zero-speed module which monitors shaft speed and direction failed 1 h out of port, and the breaker supplying power to starboard shaft Motor 17A and port shaft Motor 14B tripped offline 2 h before arriving on the location.
Once the vessel was positioning over Site U1331 (PEAT-1C), an operational test of the dynamic positioning (DP) system was undertaken. Concurrent with the DP testing, the making up of the bottom-hole assembly and spacing out of the core barrel and colletted delivery system was accomplished. As the drill string was deployed, each joint of pipe was measured and the internal clearance of each tubular inspected in accordance with routine practices.
Because the precision depth recorder (PDR) was inoperative, the driller carefully extended the bit and tagged the seafloor. Using the passive heave compensator, the bit was pulled back 10 m from this depth. Hole U1331A was spudded with the APC at 0750 h on 16 March, 2009. The depth calculated from the recovery of the first core was established at 5127.3 m DRF (5116.2 meters below sealevel [mbsl]). The offset of the rig floor was 11.1 m, so the water depth as determined by drill string length was 5116.2 m. Prior to spud-in, the drill string was flushed and cleaned of rust particles by pumping down a clean-out plug and two round trips of an XCB core barrel. Depths reported in this operations section are based on the measurements using the length of the drill string and are either drilling depth below rig floor (DRF) or DSF.
Piston coring advanced the hole to 138.2 m DSF just above the chert layer suggested by the seismic record. To avoid damaging the APC, operations were then conservatively switched to the XCB corer. We cored to 138.2 m DSF with APC Cores 320-U1331A-1H through 15H and recovered 141.3 m of core (102%). Cores 1H through 11H were oriented with the FlexIt tools.
Cores 12H through 15H were not oriented because the FlexIt tool was not deployed in anticipation of having to drill over stuck core barrels. The pullout force to recover the core barrels from the sediment was gradually increasing with depth and then decreased; it was anticipated that the drillover technique to free stuck core barrels might have to be employed. We switched to the standard, more durable steel core barrels starting with Core 12H instead of the nonmagnetic core barrels.
XCB coring advanced the hole from 138.2 m DSF to basement at ~190.6 m DSF. We cored to 52.4 m with Cores 15X to 21X and recovered 13.6 m (26%). From Core 18X to 22X (157.3–190.6 m DSF), recovery was negatively impacted by the presence of chert. We did manage to recover a short (0.42 m) section of the basal carbonate section in the last core, and bottomed out with a small sample of basalt. The total average recovery of APC/XCB coring in Hole U1331A was 81%.
Upon completion of the coring program, we conducted downhole logging. Downhole logging completed one run to a total depth of the borehole with the "paleo"-combo tool string (NGR, GRA density, and magnetic susceptibility). The data were used to guide the coring strategy in subsequent holes at this site. The planned second run with the FMS-sonic tool was aborted because of problems with the logging winch.
After the bit was pulled above the seafloor at 0820 h on 18 March 2009, the vessel was offset 20 m to the west of Hole U1331A. Hole U1331B was spudded with the APC, and the recovery of the first core was used to calculate the seafloor depth at 5127.4 m DRF (5116.3 mbsl). The vertical offset to Hole 1331A was 5 m deeper to cover gaps between cores in the first hole. APC Cores 320-U1331B-1H through 17H advanced from 0 to 156.6 m DSF; we cored 156.5 m and recovered 163.13 m (104%). Five formation temperature measurements were made with the APCT-3 in Cores 320-U1331B-2H, 3H, 5H, 7H, and 10H (19.6, 29.1, 48.1, 67.1, and 95.6 m DSF, respectively).
We switched to XCB coring for Core 18X (156.6–166.3 m DSF) but did not recover any core. Based on the drill, log, and core data from the first hole, we decided to drill with a center bit (without coring) through the chert interval (166.3–177.0 m DSF) and then attempt a single APC core in the sediment section below. APC Core 20H was attempted below the chert in the early Eocene basal carbonate section, but the corer was not able to penetrate the formation. We drilled ahead again without coring to 179.0 m DSF and one more XCB core was taken from 179.0–188.5 m DSF, but it did not recover any material. From 156.6 to 188.5 m DSF, we XCB cored 19.2 m but did not recover any material. An additional 12.7 m was drilled without coring. The bit was pulled free of the seafloor at 2220 h on 19 March, and the vessel was offset 20 m west of Hole U1331B.
The program for the third hole of the site was to spot core and fill in gaps in the sedimentary record from the first two holes and to obtain any core from below the chert. Hole U1331C was spudded with the APC at 0155 h on 20 March 2009. The recovery of the first core was used to calculate the seafloor depth of 5128.0 m DRF (5116.9 mbsl). Cores 320-U1331C-1H through 4H penetrated to 38.0 m DSF, and we recovered 39.2 m (103%). The interval from 38.0 to 59.0 m DSF was then drilled without coring, and Core 6H was taken from 59.0 to 68.5 m DSF (9.99 m recovered; 105%). We then drilled without coring from 68.5 to 92.5 m DSF and took APC Core 8H (92.5–102.0 m DSF; recovery = 9.21 m, 97%).
A short interval of 0.5 m was drilled to 102.5 m DSF, and Core 10H was taken from 102.5 to 112.0 m DSF; we recovered 8.48 m (89%). The interval from 112.0 to 129.0 m DSF was drilled without coring, and Cores 2H through 14H were obtained from 129.0 to 157.5 m DSF (29.1 m recovered; 102%). Following a drilled interval of 19.5 m through a chert sequence (157.5–177.0 m DSF), a third attempt to core the short interval below the chert and above the top of basaltic oceanic crust was attempted.
This early Eocene section was one of the highest priority sections at this site and was poorly recovered in the previous two holes. This time the drillers successfully obtained this interval in APC Cores 17H and 18H (177.0–189.0 m DSF), coring 12.0 m and recovering 14.0 m (117%). Judging by the deformation of the cutting shoe and a bent 15 ft section of core barrel, it was surmised that Core 18H had more than vigorously encountered basement.
During the spot coring in Hole U1331C, all piston cores up to and including Core 14H employed nonmagnetic core barrels and were oriented with the FlexIt tool. Standard steel barrels were used to obtain Cores 16H and 17H and did not use the orientation tool to avoid damaging it. We recovered 109.5 m (102%) in the cored interval of 107.0 m. An additional 82.0 m was drilled without coring. With the recovery of Core 17H, the decision was made to end coring on this site and to proceed to Site U1332.
After the drill string was pulled free of the seafloor at 1300 h on 21 March, the pipe trip was temporarily suspended for 1.5 h to accomplish the slipping and cutting of 115 ft of drilling line. Once drilling line maintenance was completed, tripping was resumed. The beacon was retrieved at 1500 h while the drill string was being recovered. At 0500 h on 22 March the drilling equipment was secured and the vessel departed for the second site of Expedition 320.
Following completion of Site U1331, we started heading east to Site U1332. The vessel made slow progress into a 20 kt wind and against a strong current with moderate pitching and rolling into a 6–8 ft swell with spray occasionally over the bow. These elements conspired to reduce the average speed of the 66.1 nmi voyage to Site U1332 to a lethargic 7.1 kt.
After the 9.25 h transit, we began positioning over the site at 1445 h on 22 March 2009. We assembled the bottom-hole assembly, and spaceout of the colletted delivery system was verified. Because the PDR was not working, it was necessary for the driller to carefully lower the bit and tag the seafloor to verify the exact depth. As the driller was preparing to spud the hole, the display that indicates coring line position relative to the rig floor failed. Because it is imperative for the core winch operator to know where the coring line is at all times, operations had to be suspended for 3 h while the defective unit was replaced.
Hole U1332A was spudded with the APC at 1050 h on 23 March. The water depth calculated from the recovery of the first core was established at 4935.1 m DRF (4923.9 mbsl). APC Cores 320-U1332A-1H through 14H penetrated from 0 to 125.9 m DSF, and we recovered 131.9 m (104%). All piston cores were oriented with the FlexIt tool. Because of the potential presence of chert horizons, no downhole temperature measurements were attempted in the initial hole of this site. We switched to XCB coring after Core 14H, which required 70,000 lb of overpull to extract the core barrel from the sediment.
We recovered 13.8 m (51%) in XCB Cores 15X through 18X (125.9 to 152.4 m DSF). Coring was terminated when Core 18X was recovered with a piece of basaltic basement. Hole U1332A was cored to 152.4 m, and we recovered 145.6 m (96%).
After coring was finished, we prepared the hole for logging by flushing it with 65 bbl of attapulgite mud and then dropping a go-devil to open the lockable float valve (LFV). We then displaced the hole with 80 bbl of attapulgite mud and raised the bit to 78 m DSF.
We then deployed a tool string consisting of the magnetic susceptibility, GRA density, and NGR tools. This tool string acquired good logs over the entire open hole interval. Unfortunately, the tool string parted from the logging wireline when attempting to recover the tool and the tool string was lost in the hole.
We spent ~ 18 h conducting three unsuccessful coring line fishing attempts to recover the logging tool string. After acknowledging that spending more time fishing for the tool string would not be productive, the decision was made to seal Hole U1332A with 15 bbl of cement above the lost logging tool, extending from 125 to 90 m DSF. Deploying the cement had to be delayed for 4 h while the cement pumps were repaired.
After the cementing operations were completed, the bit was pulled free of the seafloor at 0800 h on 26 March and the vessel was offset 20 m north of Hole U1332A. Before coring could resume, the drill string was flushed with seawater to remove any cement from the tubulars and bit nozzles.
Hole U1332B was spudded at 1230 h on 26 March 2009. We started coring Hole U1332B with the bit offset 5 m deeper than the seafloor depth established for Hole U1332A but only penetrated to 2.1 m CSF-A below the mudline. We recovered 118.4 m (107%) in APC Cores 320-U1332B-1H through 13H (0–110.1 m DSF). In an attempt to maintain an offset with the first hole, there were short advances with Core 3H (8.0 m) and 11H (5.0 m). The APCT-3 was deployed while taking cores at six different depths: 11.6, 19.6, 38.6, 57.6, 76.6, and 100.6 m DSF (Cores 2H, 3H, 5H, 7H, 9H, and 12H, respectively). Nonmagnetic core barrels were used on all cores except 13H.
We then switched to the XCB and took Cores 14X to 18X from 110.1 to 148.6 m DSF and recovered 23.4 m (61%). Coring was terminated when we recovered ~2.4 m of dark brown sediment above several small pieces of oceanic basalt in Core 18X.
At Hole U1332B we cored a total of 148.6 m and recovered 141.8 m (95%). The drill string was pulled out of the hole and the bit cleared the seafloor at 2230 h on 27 March.
Hole U1331C was designed to provide stratigraphic overlap and confirm stratigraphic correlations with Holes U1332A and U1332B. After the vessel was offset 30 m north of Hole U1332B, Hole U1332C was spudded at 0105 h on 28 March 2009. The seafloor depth calculated from the recovery of the first core was 4934.0 m DRF (4922.8 mbsl). Piston coring then routinely proceeded to 85.0 m DSF, during which the advances of Cores 320-U1333C-6H (4.0 m advance) and 8H (7.0 m advance) were adjusted to maintain overlap with previous holes. At ~1330 h on 28 March, while retrieving Core 10H, an electrical transient attributed to the rotating condenser caused two of the three main generators to trip off the main bus and resulted in a load shedding sequence to various systems on the vessel, which included loss of control voltage to all Thyrig bays for ~10 min. The consequence of the loss of Thyrig control voltage was a short-term loss of power to thrusters, propulsion, and drilling motors. During this short event, the DP 3% watch circle (percentage of water depth or ~150 m off the hole) was not exceeded. The main breakers quickly reset and power was restored to all main systems by 1341 h. We think the switching circuit for removing the rotating condenser from the main bus may be defective.
Because of the power loss, the coring line parted while attempting to recover Core 10H, and we had to make two fishing trips with the coring line to recover the sinker bars and the full core barrel. Unfortunately, this APC core was near the Eocene/Oligocene boundary and was very disturbed. APC coring continued to 113.5 m DSF where coring was switched to the XCB. All the APC cores were obtained with nonmagnetic core barrels and with the FlexIt core orientation tool except Core 13H, for which we used standard steel core barrels. APCT-3 formation temperature measurements were made at 36.0 m DSF (Core 4H) and 75.5 m DSF (Core 9H). APC Cores 1H through 13H extended from 0 to 113.5 m DSF, and we recovered 122.04 m (108%). XCB Cores 14X through 18X extended from 113.5 to 155.5 m DSF, where basement was encountered, and we recovered 26.02 m (62%). The total core interval with both coring systems was 155.5 m with 148.1 m recovered (95%).
Once the final core was on deck, we started recovering the drill string. The seafloor beacon was successfully recovered on deck at 1202 h on 29 March. At 1930 h on 29 March, the drilling equipment had been secured and we departed for Site U1333.
The 176 nmi voyage to Site U1333 took 18.3 h and was accomplished at an average speed of 9.6 kt. The journey required a little more than the allotted time because the speed was adversely affected by having to sail into a 22–26 kt wind and a strong current.
At 1400 h on 30 March, the vessel began positioning over the site in DP mode. Once the drilling assembly was deployed, the driller carefully lowered the bit and tagged the seafloor at ~4875 m DRF. The bit was raised 10 m, and Hole U1333A was spudded at 1225 h on 31 March 2009. The seafloor depth calculated from the recovery of the first core was 4865.0 m DRF (4853.7 mbsl); however, this depth measurement is questionable because the recovery was a full core (10.05 m).
APC Cores 320-U1333C-1H through 10H were taken from 0 to 95.0 m DSF, and we recovered 104 m (109%). We switched to XCB coring after the core barrel for Core 10H had to be drilled over to free the fully stroked core barrel from the sediment. All APC cores were obtained with nonmagnetic core barrels; the FlexIt orientation tool was not deployed since there were questions about its data reliability.
XCB Cores 11X through 22X penetrated from 95.0 to 184.1 m DSF, and we recovered 77.0 m (87%). Coring ended when we recovered basalt in Core 22X. The drill string was pulled out of the hole and cleared the seafloor at 1155 h on 1 April.
The vessel was offset 25 m east of Hole U1333A, and APC coring in Hole U1333B started at 1305 h on 1 April 2009 with the bit 5 m shallower than the first hole. We recovered 7.7 m in Core 320-U1333B-1H, so the seafloor was established at 4861.8 mbsf (4850.5 mbsl).
Based on the drilling and coring data from the first hole, the drillers were confident that by drilling over stuck core barrels, the APC could safely be pushed deeper than the 95.0 m penetration that was realized in the first hole. This confidence was justified when APC Cores 1H through 18H were advanced to 162.7 m DSF and we recovered 166.4 m (102%). Nonmagnetic core barrels were used on Cores 1H through 9H, and standard steel core barrels were deployed on all subsequent piston cores. Formation temperature measurements were made with the APCT-3 tool at 26.7, 45.7, 64.7, and 83.7 m DSF (Cores 3H, 5H, 7H, and 9H, respectively). The FlexIt orientation tool was not used. Three stuck core barrels (Cores 13H, 16H, and 17H) required drilling over when they could not be released from the sediment despite 70,000 lb of overpull. Core 18H failed to achieve a full stroke of the core barrel, and no further APC coring was conducted.
Two XCB cores advanced the hole from 162.7 to the final depth of 180.3 m DSF, and we recovered 13.44 m (76%). Coring ended when we recovered basalt in Core 20X. The drill string was pulled out of the hole and cleared the seafloor at 1845 h on 2 April.
Hole U1333C was started 25 m east of and 2.75 m shallower than Hole U1333B. Core 320-U1333C-1H was shot at 2000 h on 2 April 2009, and we recovered 1.65 m, so the water depth for this hole is 4865.1 m DRF (4853.8 mbsl).
Cores 1H through 21H penetrated from 0 to 163.2 m DSF, and we recovered 176.1 m (108%). Seven core barrels (Cores 13H through 19H) required drilling over to release the barrels from the sediment when the application of 70,000 lb of overpull was not successful. Nonmagnetic core barrels were used on Cores 1H through 13H. Standard steel core barrels were used on all subsequent APC cores. The advancement of four cores was adjusted to maintain the overlap with previous holes: Cores 11H (6.0 m advance), 12H (5.0 m advance), 16H (4.5 m advance), and 19H (4.0 m advance). All other cores advanced the full 9.5 m except for Core 21H, which did not penetrate into the formation at all—so we switched to XCB coring.
XCB Cores 22X through 24X penetrated from 163.0 to 177.0 m DSF, and we recovered 0.87 m (6%). Coring in this hole ended when we recovered basaltic basement in the last core.
Once the seafloor beacon was recovered on board, the drill string recovered on the rig floor, the thrusters raised, and the drilling equipment secured, the vessel departed for the Site U1334 at 1815 h on 4 April.
The 410 nmi voyage from Site U1333 to Site U1334 took 40.8 h and was accomplished at an average speed of 10.1 kt. During the transit the ship's clocks were advanced 1 h, resetting local ship time to more closely follow the earlier rising sun as we migrated eastward to each site. Unless otherwise noted, times for Sites U1334 are given in ship local time (UTC – 9 h).
Once the vessel approached the coordinates of the new location, the captain slowed the vessel and maneuvered the vessel over the site. We were positioning over the new location by 1222 h on 6 April 2009. After assembling the drill string and lowering it to the seafloor, we attempted to spud the new site with the bit positioned 10 m shallower than the corrected depth of 4798.4 m DRF (4787.0 mbsl) from the PDR. We recovered only water, so the bit was lowered to 4798 m DRF for a second attempt. This was successful, and Hole U1334A was spudded with the APC at 0030 h on 7 April. The mudline recovered in Core 320-U1334A-1H was used to establish the seafloor depth at 4799.3 m DRF (4787.9 mbsl). This depth was within 1 m of the corrected PDR depth.
APC Cores 1H through 22H were taken from 0 to 206.9 m DSF, and we recovered 212.39 m (103%). APC coring was terminated when Core 22H did not stroke fully and required 80,000 lb to release it from the formation. Nonmagnetic core barrels were used on Cores 1H through 16H; standard steel core barrels were used on Cores 17H through 22H.
XCB Cores 23X through 32X were taken from 206.9 to 285.5 m DSF, and we recovered 77.99 m (99%). Coring was terminated when we recovered a sample of basalt in Core 32X. The total cored interval for Hole U1334A was 285.5 m, and we recovered 209.4 m (102%). The drill string was pulled out of the hole and cleared the seafloor at 2150 h on 8 April.
The ship was offset 25 m west of Hole U1334A, and we spudded Hole U1334B at 2330 h on 8 April 2009 with the bit 5 m deeper than at the first hole. APC Cores 320-U1334B-1H through 22H were taken from 3.7 to 210.7 m DSF, and we recovered 218.43 m (105%). Nonmagnetic core barrels were used on Cores 1H through 15H; standard steel barrels were used on Cores 16H through 22H. Downhole temperature measurements were obtained at 32.2, 49.2, 68.2, 87.2, and 106.2 m DSF (Cores 3H, 5H, 7H, 9H, and 11H, respectively). Core U1334B-5H was advanced 7.5 m to maintain an ~5 m vertical offset with Hole U1334A.
XCB Cores 23X through 31X were taken from 207.0 to 281.7 m DSF, and we recovered 76.84 m (103%). We stopped coring when we recovered basalt in Core 31X. We pulled the drill string out of the hole and the bit cleared the seafloor at 1840 h on 10 April.
After the ship was offset 25 m west of Hole U1334B, Hole U1334C was spudded at 2020 h on 10 April 2009 with the bit 3 m deeper than Hole U1334A. APC Cores 320-U1334C-1H through 22H penetrated from 0 to 209.0 m DSF, and we recovered 213.0 m (102%). Nonmagnetic core barrels were used on Cores 1H through 15H; Cores 16H through 22H used the standard steel barrels. We had to drill over Cores 19H, 21H, and 22H to release them from the formation. We then switched to XCB coring. Cores 23X through 33X were taken from 209.0 to 280.7 m DSF, and we recovered 72.9 m (102%). Core 26 was advanced by 6 m to maintain stratigraphic overlap with the first two holes. Coring finished when limestone including basalt clasts was recovered in the last core. Cores from Hole U1334C successfully covered the stratigraphic gaps in the first two holes.
Our final operations planned for Hole U1334C were to conduct two downhole logging runs. We circulated the hole with 50 bbl of attapulgite mud and then displaced the hole with 100 bbl of attapulgite mud. We raised the bit up to 95 m DSF and rigged up for logging. The first tool string was assembled and deployed into the pipe at 1930 h on 12 April. While lowering the logging tool string into the hole, the transmission on the logging winch failed when the tool was ~1700 m below the rig floor. We started to manually retrieve the tool string using T-bar clamps, air tuggers, and the starboard crane. After recovering ~550 m of wireline and spooling it back onto the logging winch drum, we decided to retrieve the remaining logging wireline by spooling the logging wireline onto the core winch drum. The logging tool was back on the rig floor at ~1200 h on 13 April. No more logging could be conducted during Expedition 320.
We started pulling the drill string out of the hole and the bit cleared the seafloor at 1225 h on 13 April. After the drill string was recovered and the thrusters retracted, we began the transit for Site U1335 at 2200 h on 13 April.
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 this expedition. 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.
The ship slowed and was in 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).
Cores 320-U1335A-1H through 36H were taken from 0 to 341.4 m DSF, and we recovered 354.7 m (104%). APC piston coring to this depth is an operational highlight for this site. Nonmagnetic core barrels were used for Cores 1H through 16H; 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 (ODP 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.
We started coring Hole U1335B at 1955 h on 18 April 2009 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 320-U1335B-1H through 41H were taken from 0 to 378.2 m DSF, and we recovered 392.7 m (104%). This is 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 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 the drilling line maintenance was completed, the drill string was recovered, the beacon retrieved, and the drilling equipment secured for transit. The vessel departed for the Site U1336 at 2145 h on 21 April.
The 185 nmi transit from Site U1335 to Site U1336 took 17.8 h at an average speed of 10.4 kt. Unless otherwise noted, times for Site U1336 are given in ship local time (UTC – 9 h).
The ship slowed as it approached Hole U1336A and was in DP mode at 1545 h on 22 April 2009. Once the ship was stabilized over the new site, the drill string was deployed and the bit positioned 5 m shallower than the corrected PDR depth of 4300.4 m DRF. Hole U1336A was spudded with the APC at 0135 h on 23 April. The seafloor depth estimated from the recovery in the first core was 4296.9 m DRF (4285.3 mbsl).
APC Cores 320-U1336A-1H through 21H were taken from 0 to 184.8 m DSF, and we recovered 190.2 m (103%). Nonmagnetic core barrels were used to obtain Cores 1H through 16H; steel barrels were used for all other cores. Three cores (14H, 16H, and 21H) only partially stroked and we advanced by recovery. A hard layer at ~124 m DSF prevented Core 14H from achieving a full stroke; the layer was ~0.5 ft thick and was not recovered; based on how it drilled, we inferred it may have been chert. The last piston core (21H) required drilling over to free the barrel from the formation.
Cores 22X through 35X were taken from 184.8 to 302.9 m DSF, and we recovered 69.0 m (58%). We stopped coring before reaching the basement objective because of the decreasing rates of penetration (as low as ~4 m/h for Cores 34X and 35X), the relatively low recovery, and the possibility of obtaining a stratigraphically complete Miocene section by coring in a second hole. We pulled out of the hole and the bit cleared the seafloor at 0400 h on 25 April. We offset the ship 25 m to the south of Hole U1336A.
After positioning the bit at 4284 m DRF (5 m shallower than in Hole U1336A), we started coring in Hole U1336B at 0540 h on 25 April 2009. Based on the recovery in the first core, the seafloor depth was 4298.1 m DRF (4286.5 mbsl). Cores 320-U1336B-1H through 20H were taken from 0 to 173.9 m DSF, and we recovered 179.6 m (103%). Nonmagnetic core barrels were used for Cores 1H through 16H; steel barrels were used for the others. Core 16H only achieved a partial stroke and was advanced by recovery. Once coring was completed, the drill string was pulled out of the hole with the bit clearing the seafloor at 0400 h on 26 April. Once all the drilling equipment was secured, the vessel departed for Honolulu, Hawaii, at 1315 h on 26 April.
The 1954 nmi transit from Site U1336 to Honolulu, Hawaii, took 7.8 days at an average speed of 10.5 kt. At 2400 on 27 April, the shipboard clocks were set back 1 h so ship local time was now UTC – 10 h. Expedition 320 concluded with the first line ashore at 0700 h on 4 May 2009.