The vessel was offset 7.0 nmi to Site 1235 (proposed Site SEPAC-14A) with the drill string deployed to 375 m and using the dynamic positioning system. The transit took 5.5 hr at an average speed of 1.3 kt. The 3.5-kHz precision depth recorder (PDR) was monitored throughout the transit and was used as a final check of site characteristics by comparison to precruise survey data. The vessel was on location at 0412 hr on 17 April 2002.
Hole 1235A was initiated with the advanced piston corer (APC) at 0603 hr on 17 April, with the bit at 494.0 meters below rig floor (mbrf). Core 1H recovered the mudline, indicating a seafloor depth of 500.7 mbrf, 1.7 m shallower than the corrected PDR depth estimate. Piston coring deepened the hole until Core 20H failed to achieve a full stroke at 181.3 meters below seafloor (mbsf). The recovery was 182.8 m (Table T1). Cores were oriented starting with Core 4H. Downhole temperature measurements were taken with the APC temperature (APCT) tool (Table T1). Most cores were afflicted with voids from gas that expanded during the retrieval process. Holes were drilled into the core liners to relieve some of the gas pressure before cores were processed on the catwalk. No extended core barrel (XCB) cores were attempted at this site based on the experience at Site 1234 and because of the coring disturbance associated with gas expansion and ship heave. The bit was pulled free of the seafloor at 1955 hr on 17 April.
The vessel was offset 10 m east relative to Hole 1235A. A bottom water temperature measurement was first obtained with the APCT tool and the bit at 497 mbrf. This was also the bit depth to accomplish the desired stratigraphic overlap with the first hole. Hole 1235B was initiated with the APC at 2100 hr. The mudline was again recovered, indicating a seafloor depth of 499.5 mbrf. Piston coring advanced to the APC refusal depth of 176.2 mbsf. The last core (Core 19H) sustained liner damage resulting from deceleration forces incurred when the formation prevented a full stroke of the corer. The cores were oriented starting with Core 3H. Of the 176.2 m of sediment cored, 178.5 m was recovered (recovery = 101%) (Table T1). Downhole temperature measurements were taken with Cores 4H and 7H (35.5 and 64.0 mbsf, respectively). The bit was pulled free of the seafloor at 1000 hr on 18 April.
When the liner of Core 16H was being cut into sections on the catwalk, it suddenly fragmented into countless pieces and the sediment exploded as a result of gas pressure. The core was completely destroyed. Following this incident, the marine laboratory specialists wore plastic face shields and used Kevlar blankets while carrying the core to the core receiving platform as a precaution against possible injury.
The vessel was offset 10 m east of Hole 1235B. A second bottom water temperature measurement was taken before we cored Hole 1235C. Combined with all temperature measurements in the previous holes, a thermal gradient of ~3.8°C/100 m was determined for this site. The bit was positioned at a depth of 500 mbrf (0.5 mbsf in Hole 1223B) in order to cover the remaining stratigraphic gaps. Hole 1235C was initiated with the APC at 1110 hr on 18 April. Because this hole was started with the bit below the mudline, the inferred seafloor depth from the previous hole was assumed for Hole 1235C (499.5 mbrf). Piston coring had advanced to 152.5 mbsf when operations concluded at the site. A total of 152.0 m was cored and 151.8 m was recovered (recovery = 99.9%) (Table T1). The cores were oriented starting with Core 3H.
The nonmagnetic (monel) core barrel was deployed for the first time during Leg 202 (Cores 3H, 5H, 7H, 9H, and 11H) after an excessive magnetic overprint was discovered in connection with APCT tool deployments. Cores retrieved with the monel barrel turned out to have less overprint than those retrieved in the regular steel barrels.
This site was under the influence of a strong southerly wind that gradually built through the duration of the afternoon of 17 April and continued into the next day. The combined sea and swell eventually grew to >25 ft by the afternoon of 18 April and was responsible for an average vessel heave of 4 m. The combination of heave and gas expansion compromised the quality of the cores at this site.
The vessel was secured for transit and left location at 2300 hr on 18 April for the 4-day transit to Site 1236, the first site in Peruvian waters.