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doi:10.2204/iodp.proc.303306.214.2010

Introduction

The sites occupied during Integrated Ocean Drilling Program (IODP) Expedition 303/306 (Fig. F1; Table T1) were chosen to recover Miocene to Quaternary records of North Atlantic environmental variability in terms of ice sheet–ocean interactions, deep circulation changes, or sea-surface conditions. The sites provide the requirements, including adequate sedimentation rates, for developing high-resolution stratigraphies through geomagnetic relative paleointensity (RPI), oxygen isotopes, and regional environmental patterns (see the “Expedition 303 summary” and “Expedition 306 summary” chapters; Channell et al., 2006).

The timing of the two drilling expeditions, separated by an interval of 5 months, allowed experience from the first expedition (Expedition 303: September–November 2004) to be incorporated in the plan for the second expedition (Expedition 306: March–May 2005). Results of the first expedition did indeed lead to a revised drilling plan for the second expedition, particularly for the Eirik Drift off southeast Greenland. In this region, cores recovered during Expedition 303, combined with the multichannel seismic data acquired in 2002, allowed a refined choice of sites. Unfortunately, poor weather conditions during Expedition 306 precluded the ship reaching the Eirik Drift.

Nonetheless, the sites occupied during Expeditions 303 and 306 provided a rich archive of Miocene to Quaternary environmental conditions from the North Atlantic. At the outset, the decision was made to limit drilling to depths achievable using the advanced piston corer (APC), the rationale being that the extended core barrel (XCB) does not provide adequate core quality for planned high-resolution studies. Recovery of complete composite sections through shipboard control of hole-to-hole offsets was given high priority.

The North Atlantic has been the focus of some of the most intriguing advances in paleoceanography in the last 20 y, including the recognition of Heinrich layers and Bond cycles and the proposed ~1500 y pacing in ice-rafted debris (IRD) proxies such as hematite-stained grains and Icelandic glass (Broecker et al., 1992; Bond et al., 1992, 1999, 2001). These observations have been largely restricted to the last glacial cycle because of the lack of high sedimentation–rate records with known continuity that extend beyond the last glacial cycle. A principal objective of Expedition 303/306 was to obtain sedimentary records that would extend our knowledge of millennial-scale paleoceanographic change beyond the last glacial cycle, through the so-called “100 k.y. world” and into the “41 k.y. world.” The critical role of age control in interpreting these paleoceanographic phenomena, particularly beyond the range of radiocarbon dating and the Greenland ice core record, requires that sites be appropriate not only for oxygen isotope stratigraphy but also for more recently developed stratigraphies based on RPI studies.

A summary of the polarity stratigraphies and biostratigraphies obtained from sites occupied during Expedition 303/306 are given in Figure F2. Sites from Orphan Knoll (Site U1302/U1303), Eirik Drift (Site U1305/U1306), and Gardar Drift (Site U1314) yielded records covering the last ~1–2 m.y., at mean sedimentation rates of 10–20 cm/k.y. Shipboard hole-to-hole correlations indicate that complete composite sections can be constructed for large parts of the sedimentary records recovered at these sites (Table T1). At Sites U1308 and U1313, two previously drilled North Atlantic sites (Deep Sea Drilling Project [DSDP] Sites 609 and 607, respectively) were reoccupied to obtain composite sections for sites that lie in the heart of the North Atlantic “Ruddiman” (1977) IRD belt (Site U1308) and at the southern edge of the belt (Site U1313). Sites 607 and 609 have provided key records of Pliocene to Quaternary North Atlantic paleoceanography (Ruddiman et al., 1986, 1989; Raymo et al., 1989), and the objective of reoccupation of these sites was to obtain demonstrably complete sections that would lend themselves to analytical methods developed since the sites were drilled over 25 y ago. At these two sites (U1308 and U1313), mean sedimentation rates are 4–10 cm/k.y., and the recovered sections extend back to 3.2 and 5 Ma, respectively. Site U1312 on the southern flank of the King’s Trough constituted a redrill of DSDP Site 608 with the principal target being the classic upper Miocene section (e.g., Miller et al., 1991). Unfortunately, severe weather conditions resulted in a high degree of drilling disturbance in the recovered cores that precluded the construction of a composite section at this site and resulted in its premature abandonment. Site U1315 was drilled at the site of Ocean Drilling Program (ODP) Site 642 to 179 meters below seafloor (mbsf) and cased to receive a 150 m thermistor string for monitoring bottom water temperatures and seafloor diffusion over a multiyear period.

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