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doi:10.2204/iodp.proc.303306.107.2006 Site U13071Expedition 303 Scientists2Background and objectivesIntegrated Ocean Drilling Program Site U1307 is located 53 km (28 nmi) northwest of Site U1306 (Fig. F1 in the “Site 1306” chapter) in 2575 m water depth, 300 m deeper than the water depth at Site U1306. The objective at Site U1307 was to recover the older part of the section on the Eirik Drift below the Quaternary sections recovered at Sites U1305 and U1306. Along seismic Line 25 (Fig. F1) of the multichannel seismic (MCS) network obtained over the Eirik Drift during Knorr cruise KN166-14, the Pliocene section is accessible at penetration depths less than ~300 m and is therefore recoverable with the advanced piston corer (APC). The MCS profile for Line 25 indicates a series of well-developed mudwaves with axes oriented approximately northwest–southeast (Fig. F2). The Quaternary sediment drape thins from northeast to southwest along this seismic line, bringing the Pliocene section closer to the sediment/water interface. The oldest sediment recovered at Site U1306 (294.8 meters below seafloor [mbsf]) was uppermost Pliocene (~2 Ma). In Figure F2, we estimate the section recovered at Site U1306 transposed onto Line 25 (Fig. F1). Extrapolation of the Site U1306 penetration depth along Line 19 to Line 25 yields a corresponding thickness of 230 m and a mean sedimentation rate of 11 cm/k.y., whereas the mean sedimentation rate for the recovered section at Site U1306 was 15 cm/k.y. The 162 m penetration at Site U1307 succeeded in recovering sediments as old as 3.58 Ma, at a mean sedimentation rate of 4.5 cm/k.y. Site U1307 (proposed Site LAB8C) was not a primary site for Expedition 303. It was necessary to remain in the vicinity of the Eirik Drift after termination at Site U1306 because of a severe storm along the intended transit to proposed Site IRD1A (Site U1308). Nonetheless, the two holes drilled at Site U1307 demonstrated the feasibility of recovering pre-Quaternary sediments on the Eirik Drift at penetration depths achievable with the APC. This will allow the high-resolution Quaternary climate record recovered at Site U1306 to be extended into the Pliocene. It will also contribute significantly to our understanding of the sedimentary architecture of the Eirik Drift and the formation of the mudwaves by providing detailed stratigraphic records of the pre-Quaternary sequence that can be correlated throughout the MCS network obtained during cruise KN166-14. The preservation of microfossils and the pristine magnetostratigraphic records at Sites U1305–U1307 indicate that the sediments have the required attributes for precise stratigraphic age control in the framework of a paleointensity-assisted chronology. The effort to recover the pre-Quaternary sequence, and hence pursue the pre-Quaternary sedimentation history of the Eirik Drift and extend the high-resolution environmental record into the Pliocene, is intended for continuation during Expedition 306. |
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