BACKGROUND

Geological Setting

The North Atlantic Ocean is undoubtedly one of the most climatically sensitive regions on Earth because the ocean-atmosphere-cryosphere system is prone to mode jumps that are triggered by changes in freshwater delivery to source areas of deepwater formation. During the last glaciation, these abrupt jumps in climate state are manifest by Dansgaard/Oeschger (D/O) cycles and Heinrich Events in ice and marine sediment cores, respectively. Given the paramount importance of the North Atlantic as a driver of global climate change, we propose to drill at nine key locations to extend the study of millennial-scale climate variability over the last few million years. What is the rationale for studying millennial-scale variability in the North Atlantic over the last few million years rather than just the last glacial cycle (recoverable by conventional piston cores)? Determining the long-term evolution of millennial-scale variability in surface temperature, ice sheet dynamics, and thermohaline circulation can provide clues to the mechanisms responsible for abrupt climate change. For example, the average climate state evolved toward generally colder conditions with larger ice sheets during the Pliocene–Pleistocene. This shift was accompanied by a change in the spectral character of climate proxies, from dominantly 41- to 100-k.y. periods between ~920 and 640 ka (Schmieder et al., 2000). Among the numerous questions to be answered are the following:

Seismic data for positioning the Eirik Drift (LAB) sites and southern Gardar Drift (GAR) sites were collected during the Knorr KN166-14 cruise (principal investigator: Greg Mountain) in summer 2002. Seismic reflection profiles were collected with Lamont-Doherty's portable high-resolution acquisition system. This comprised a Price 135 standard cubic feet per minute compressor, two 45 in³ generator injector air guns depth-controlled at 2.5 m and fired every 12.5 m along track, a 48-channel, 600 m solid Innovative Transducers Inc. streamer, also depth-controlled at 2.5 m, and on-board recording based on an OYO Instruments Data Acquisition System-1 and additional software for gunfire control and 1 ms sampling. Onboard as well as more elaborate onshore seismic processing was performed using Landmark's Promax software. A cruise of the Hudson in August 2001 (principal investigator: David Piper) obtained seismic reflection lines for Orphan Knoll (ORPH) proposed Sites ORPH2A and ORPH3A using a Huntec deep-towed sparker system augmented by single-channel seismic data (Toews and Piper, 2002).