RELATIONSHIP TO PREVIOUS NORTH ATLANTIC DRILLING
Prior to Expeditions 303 and 306, two 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, five sites (980984) were drilled on sediment drifts south of Iceland (Fig. F2). 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, Leg 172 in the northwest Atlantic between ~30° and 35°N recovered sequences 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 Expedition 303 and 306 sites augment those of Legs 162 and 172 in two fundamental ways. First, most of our sites are located in or close to the North Atlantic "IRD belt" (Fig. F2), where massive iceberg discharges are recorded in coarse layers of ice-rafted debris that are depleted in planktonic foraminifers and with oxygen isotope values indicative of reduced sea-surface salinities. Site 980 (from 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 are so well displayed to the south and west. Second, the depth distribution of these sites (22733884 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 water depths of 1650 to 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 12914595 mbsl. The Expedition 303 and 306 sites will 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 and 306 sites (Fig. F1) are distributed so that they monitor the major deepwater end-members of NADW: Norwegian-Greenland Sea Water (Site U1304) and Labrador Sea Water (Sites U1305U1307) 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. Together with the depth transects drilled during Legs 162 and 172, the Expedition 303 and 306 sites permit monitoring deep- and intermediate-water formation during all three circulation modes.
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