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Site U1312

Site U1312 is a reoccupation of Site 608 drilled in 1983 with a total penetration of 530 mbsf and reaching basaltic basement (capped by middle Eocene sediments) at 515 mbsf. The principal objective at Site U1312 was to recover a complete composite section of a classic upper Miocene section that became a reference section for Miocene stratigraphy through stable isotope, magnetic, and paleontological studies (Clement and Robinson, 1987; Miller et al., 1991; Gartner, 1992). Recently acquired paleomagnetic data derived from a resampling of Site 608 cores (Krijgsman and Kent, 2004) attests to the fidelity of the paleomagnetic record at this site, even after 25 y of core storage. This new sampling extended from polarity Chron 3Br to 5An and indicated a mean sedimentation rate in this part of the late Miocene of 2.5 cm/k.y. The reoccupation of Site 608 held the promise of recovery of a complete composite section, particularly for the upper Miocene and lower Pliocene. At the time of original drilling (1983) during Leg 94, composite section construction during drilling was not a viable option.

Two holes were drilled at Site U1312, however, the quality of the recovered core was very poor compared to that recovered 25 y earlier during Leg 94. Both Holes U1312A and U1312B were affected by drilling disturbance and “flow-in” such that a composite record could not be constructed for this site. The high level of drilling disturbance was attributed to excessive ship heave (>5 m), and the site was finally abandoned as weather conditions worsened. The shipboard magnetic stratigraphy was only interpretable in one hole (U1312B), and in this hole only back into the Matuyama Chron, to ~2 Ma. Shore-based magnetic studies have improved the fidelity of the magnetic record over the last ~2 m.y., and RPI proxies in this interval have been correlated to other records from the North Atlantic (Kanamatsu et al.).

In view of the high-quality magnetic data acquired from Site 608 (Clement and Robinson, 1987; Krijgsman and Kent, 2004), the lack of interpretable magnetic stratigraphy prior to ~2 Ma for the section recovered during Expedition 303/306 must be attributable to core quality, although this disturbance is not always visually apparent according to shipboard core descriptions. The lack of a complete composite section and magnetic stratigraphy beyond the last ~2 m.y. will inhibit further work on cores recovered from Site U1312.