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doi:10.2204/iodp.proc.303306.206.2009

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 306, three sites were drilled in the North Atlantic (Fig. F1) to obtain expanded Pliocene–Pleistocene sections for generation of integrated millennial-scale stratigraphies incorporating geomagnetic paleointensity, stable isotopes, and detrital carbonate layers (see the “Expedition 306 summary” chapter). Two sites, U1313 and U1314, yielded sections with suitably high sedimentation rates to achieve this objective.

Site U1313 represents a reoccupation of Deep Sea Drilling Project (DSDP) Site 607, a classic locality for studying changes in ocean circulation and ice sheet variability during the Pliocene and Pleistocene (see the “Site U1313” chapter). Four holes were drilled in ~3412 m of water at Site U1313 (41°0.0′N, 32°57.4′W), located at the base of the upper western flank of the Mid-Atlantic Ridge northwest of the Azores. These holes yielded two complete composite sections to 174 meters composite depth (mcd) (mid-Pliocene). Site U1313 includes a complete and relatively expanded Pliocene–Pleistocene section, with sedimentation rates based on shipboard biostratigraphy of 4.1–4.5 cm/k.y.

Site U1314 is located on the southern Gardar Drift in the North Atlantic (see the “Site U1314” chapter). This site is situated south of DSDP Sites 983 and 984, where high sedimentation rates (10–15 cm/k.y.) during the Pliocene and Pleistocene were recorded. However, Sites 983 and 984 are outside the main ice-rafted debris (IRD) belt and located at water depths too shallow to monitor North Atlantic Deep Water (NADW). Site U1314 was selected in a location more proximal to the IRD belt and at depths deep enough to record NADW (see the “Site U1314” chapter). Three holes were drilled in ~2800 m of water at Site U1314 (56°21.9′N, 27°53.3′W), recovering a section spanning the upper Pliocene and Pleistocene. Sedimentation rates based on shipboard biostratigraphy indicate higher rates than those found at Site U1313, with rates during the late Pliocene close to 11 cm/k.y., decreasing to 7.0–7.5 cm/k.y. during the Pleistocene.

Calcareous nannofossils are abundant and moderately to well preserved throughout the Pliocene and Pleistocene sections of Sites U1313 and U1314. Shipboard biostratigraphic analysis pinpointed datums to core catcher samples and more rarely to a section within a core based on a limited number of “toothpick” samples taken to refine the biostratigraphy. Because the primary objective of Expedition 306 was to produce a paleointensity-assisted chronology suitable for correlation at sub-Milankovitch scales, it is vital to further refine the biostratigraphy to fit into the chronologic framework. This study refines the shipboard calcareous nannofossil biostratigraphy by examining one sample per section (or one sample every ~1.5 mcd) spanning the last 3 m.y. at Site U1313 and 1 m.y. at Site U1314.

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