One of the central goals of Ocean Drilling Program (ODP) Leg 162 was to investigate the role of the Iceland-Faeroe Ridge as a critical gateway for the thermohaline circulation system in the North Atlantic. Today, warm saline surface water flows from the Atlantic via the Iceland-Faeroe Ridge into the Norwegian Sea. The advection of these temperate surface waters into the area provides a strong heat source for eastern subarctic Europe and causes the recent favorable warm climatic conditions. As a consequence of the cooling and sinking of the advected waters, deep-water formation takes place, especially in the Iceland and Greenland Seas. Much of the world's deep water is formed in this region. The northward inflow of temperate surface waters into the Norwegian Sea and southward outflow of deep, well-oxygenated waters from this area into the North Atlantic must pass the Iceland-Faeroe Ridge. Oceanographic as well as climatic changes on each side of the gateway are therefore closely related to each other.
The flow pattern via the Iceland-Faeroe Ridge as well as the surface-water circulation system in the northern North Atlantic has consistently changed over time with shifting climate regimes. Many recent studies have shown that variations in surface-water conditions and thermohaline circulation in the North Atlantic during the last glacial cycle are linked to the rapid and significant oscillations in air temperature (e.g., Lehman and Keigwin, 1992; McManus et al., 1994; Bond and Lotti, 1995; Oppo and Lehman, 1995). The present circulation pattern, which probably has been operating since ~10 ka (Baumann and Matthiessen, 1992; Koç Karpuz and Jansen, 1992; Sarnthein et al. 1995), is different from the unstable pattern of the glacials (e.g., Veum et al., 1992; Duplessy and Labeyrie, 1994; Sarnthein et al., 1995). The North Atlantic Drift changed its northward position considerably during the late Quaternary without extending far into the Norwegian Sea during glacials (CLIMAP, 1981). Although there is no evidence that the thermohaline circulation was totally hampered during cold stages, much of the heat loss occurred farther south in the North Atlantic. This is shown by sea-surface temperature reconstructions along south-north transects from the North Atlantic to the Norwegian Sea/northwest Europe (Sejrup and Larsen, 1991; Veum et al., 1992; Koç et al., 1996).
Direct comparisons between North Atlantic and Norwegian-Greenland Sea records, nevertheless, are usually not attempted. Evidence from both the Norwegian Sea and the North Atlantic suggests that the major intensification in Northern Hemisphere glaciation, which generally is defined by an abrupt increase in the amount of IRD, occurred at ~2.75 Ma (Raymo et al., 1989; Jansen et al., 1990; Jansen and Sjøholm, 1991; Fronval and Jansen, 1996). Climatic variations are characterized by a strong 41-k.y. obliquity frequency (Raymo et al., 1989; Ruddiman et al., 1989). Thermal gradients between the Norwegian Sea and the North Atlantic can certainly be observed by comparing the differences in carbonate records. Differences in carbonate content of the sediments have often been used to distinguish the surface-water masses in the Norwegian-Greenland Sea (Kellogg, 1975, 1976; Henrich et al., 1989; Henrich and Baumann, 1994). High carbonate contents reflect sediments underlying the warm inflowing Atlantic water, whereas low carbonate contents in sediments indicate cold, usually ice-covered surface-water masses. The North Atlantic record is, however, generally quite dissimilar to that of the Norwegian-Greenland Sea (Jansen et al., 1988). Especially in the interval 3.1-1.1 Ma, strong discrepancies are reflected by low-to-zero carbonate deposition in the Norwegian Sea compared with highly fluctuating but continuous carbonate deposition in the North Atlantic. Previous works (Jansen et al., 1988, 1989; Henrich, 1989) tried to interpret these differences in carbonate sedimentation as a result of low carbonate productivity and flux, combined with increased bottom-water pCO2 resulting from decreased deep-water ventilation rates in the Norwegian Sea. In the North Atlantic, on the other hand, carbonate distribution is primarily controlled by dilution by IRD during glacials. Here, cold extremes are most analogous to interglaciations (Substage 5d), whereas many warm extremes had more ice and/or (probably) colder surface-water temperatures than are observed today (Raymo, 1992). Thus, except for some low-carbonate spikes during extreme glacial episodes, most of the sediment has high carbonate content. Henrich and Baumann (1994) and Baumann et al. (1996) proposed at least some phases of warmer Atlantic water intrusions into the Norwegian Sea between 1.65 and 1.3 Ma, although the stronger influence of warmer surface waters should be restricted to the easternmost Norwegian Sea. These authors observed relatively high amplitudes in the percent of biogenic carbonate and in the abundance of warm-adapted species that indicate the short-term presence of relatively warm surface waters. In addition, the nannoplankton and foraminifer assemblages are characterized by well-preserved species during climatic optima between 1.65 and 1.3 Ma.
At ~1 Ma, a major shift toward more extensive glaciations of longer duration and warmer interglacials, most likely with less continental ice (Ruddiman et al., 1989; Raymo, 1992; Berger and Jansen, 1994; Henrich and Baumann, 1994; Fronval and Jansen, 1996), is also reflected by the transition from carbonate-free/-poor to carbonate-bearing sediments in the Norwegian Sea. In general, interglacials are associated with the enhanced influx of Atlantic surface water to the Norwegian-Greenland Sea. Therefore, high carbonate contents covaried with warm stages in the interglacial-glacial cycles in both the Norwegian-Greenland Sea and the North Atlantic.
In this paper, we present medium- to high-resolution records of bulk carbonate content, calcareous plankton assemblages (planktonic foraminifers and coccolithophores), and sedimentological proxy data. Continuous recovery and comparable sedimentation rates of Pliocene/Pleistocene sediments provided the opportunity to compare the records of Sites 982 and 985. We investigated both sites to reconstruct the interactions between the Norwegian Sea and the North Atlantic over the last 3 m.y. Our main objectives were to reconstruct the history of mid-term evolution of the Northern Hemisphere climate as well as the exchange of surface-water masses between the North Atlantic and the Norwegian Sea across the Iceland-Faeroe Ridge.