RELATIONSHIP TO PREVIOUS NORTH ATLANTIC DRILLING

Two previous ODP legs to the North Atlantic recovered sequences that are continuous and have sedimentation rates high enough to study oceanic variability on sub-Milankovitch timescales. During Leg 162, four sites were drilled on sediment drifts south of Iceland. These sequences are yielding invaluable insight into the nature of millennial-scale climate variability in the North Atlantic (Raymo et al., 1998, 2004; McManus et al., 1999; Raymo, 1999; Flower et al., 2000; Kleiven et al., 2003). Similarly, during Leg 172 in the northwest Atlantic between ~30° and 35°N, sequences were recovered with high deposition rates that are suitable for millennial- and perhaps centennial-scale studies (Keigwin, Rio, Acton, et al., 1998). Given the successes of Legs 162 and 172, why are additional sites needed in the North Atlantic? The sites drilled during Expedition 303 will augment those of Legs 162 and 172 in two fundamental ways. First, most of our sites are located in the North Atlantic "IRD belt" where massive iceberg discharges are recorded by coarse layers of ice-rafted detritus that are depleted in planktonic foraminifers and have oxygen isotopic values indicative of reduced sea-surface salinities. Site 980 (from ODP Leg 162) does lie within the IRD belt, but it is located on its distal northeastern edge and, consequently, lacks the strong sea-surface response to millennial-scale IRD events that is so well displayed to the south and west. Second, the depth distribution of the Expedition 303 sites (2273–3884 meters below sea level [mbsl]) is ideal for monitoring millennial-scale changes in the production of North Atlantic Deep Water (NADW). Leg 162 sites span 1650–2170 mbsl and provide the intermediate-depth end-member for studies of the formation of Glacial North Atlantic Intermediate Water (GNAIW). Leg 172 drift sites provide a relatively complete depth transect spanning 1291–4595 mbsl. The Expedition 303 sites unify the record of millennial-scale variability in the North Atlantic by bridging the "gap" between Legs 162 and 172. The sites will also expand the geographic range of sites needed to distinguish between latitudinal changes in the mixing zone between southern and northern source waters and changes due to vertical migration of water mass boundaries (Flower et al., 2000).

Data and modeling studies point to changes in the modes of NADW formation as one of the principal factors driving millennial-scale climate change in the high-latitude North Atlantic and Europe (for review, see Alley et al., 1999). Expedition 303 sites (Fig. F1) are distributed so that they monitor the major deepwater end-members of NADW: Norwegian-Greenland Seawater (Site U1304) and Labrador Seawater (Sites U1305–U1307) as well as the final NADW mixture (Sites U1302 and U1303). Alley et al. (1999) discussed three distinct modes of thermohaline circulation in the North Atlantic: modern (M), glacial (G), and Heinrich (H). The modern mode is marked by deepwater formation in the Nordic Seas and North Atlantic where the three end-members mix to form NADW. In the glacial mode, deepwater formation is suppressed in the Nordic Seas and GNAIW forms farther south in the North Atlantic. In the Heinrich mode, both deep- and intermediate-water formation is greatly reduced. The modern mode may be a relatively rare feature of the Pleistocene (Raymo et al., 2004). Together with the depth transects drilled during Legs 162 and 172, the Expedition 303 sites will permit monitoring deep- and intermediate-water formation during all three modes of formation.