Understanding the causes and consequences of global climatic and environmental change is an important challenge for society. The northern polar oceans are of great relevance to this task, because they directly influence the global environment through the formation of permanent and seasonal ice-cover, transfer of sensible and latent heat to the atmosphere, and by deep-water formation and deep-ocean ventilation which control or influence both oceanic and atmospheric carbon content. Thus, any serious attempt to model and understand the Cenozoic variability of global climate must take into account the climatic processes occurring in this region.
Leg 162 represents the second in a two-leg program designed to investigate three geographic locations in the high northern latitudes (the Northern Gateway region, the Greenland-Norway transect, and the Southern Gateway region; Fig. 1). Our goal is to reconstruct the temporal and spatial variability of the oceanic heat budget, the history of intermediate- and deep-water formation, and the history of glaciation on the surrounding land masses. Ultimately, we want to understand the role played by the high northern latitude seas in the global climate system on time scales ranging from decades (Heinrich/Dansgaard-Oeschger events) to millions of years.
Overall, the choice of sites for Leg 162 was guided by two primary scientific objectives. First, we wanted to recover sequences with sedimentation rates high enough to delineate millennial-scale variability in lithologic, biologic, and geochemical characteristics. This goal was attained by recovering sedimentary sequences at five sites located on sediment drifts in the North Atlantic (Fig. 2) and on rapidly accumulating continental slope regions in the Nordic seas. Both of these areas have average accumulation rates greater than 10 cm/k.y. Continuous sediment recovery was documented over millions of years at almost all the sites, and clear evidence was found for variability of sediment physical properties on millennial time scales over many different time periods (Fig. 3). In addition, the drift sites (980, 981, 983, and 984) open a new window of exploration in the pelagic realm of the deep sea, and will allow us, for the first time, to study the evolution of millennial-scale climate variability in the North Atlantic over millions of years. In particular, we will be able to evaluate the amplitude and frequency of millennial-scale variability during the mid-Pliocene, a time period warmer than today.
The second objective of Leg 162 was to recover sequences in a spatial array suitable for examining the evolution of vertical and horizontal gradients in water-mass properties. The North Atlantic sites form a depth transect in the northeastern basins spanning the depth interval of glacial intermediate water-mass formation (specific depths of sites: Site 982-1150 m, Site 984-1660 m, Site 983-1995 m, Site 980-2180 m, and Site 981-2184 m). Likewise, two of the sites (983 and 984) are located just south of waters spilling over the Iceland-Faeroe Ridge while the other sites (981 and 982) are just south of the Wyville-Thompson Ridge Overflow (Figs. 4, 5). These sites will allow us to examine the history of North Atlantic thermohaline circulation both on millennial and Milankovitch time scales. In addition, the sediment records of two sites, 981 at 2157 m and 982 at 1150 m, extend back to the upper Miocene and lower middle Miocene, respectively. These long sediment sequences will allow us to examine how North Atlantic thermohaline circulation responded to tectonic changes in the sill depth of the Greenland-Scotland Ridge, as well as other tectonic (e.g., the Isthmus of Panama) and climatic events (e.g., middle Miocene glaciation of Antarctica) that may have influenced the physical oceanography in the source areas of bottom-water formation.
The sites in the North Atlantic also form a northwest-southeast surface-water transect (Fig. 2), which crosses the major area of polar front movement on glacial-interglacial (G-I) cycles. Thus, we should be able to compare east-west gradients in surface-water temperature and iceberg trajectories on suborbital time scales, and further improve our understanding of the dynamics of Heinrich events and the even shorter duration Dansgaard-Oeschger events (e.g., Broecker, 1994; Bond and Lotti, 1995).
Likewise in the Nordic sea Sites 987, 907, and 985 complete an east-west transect originally begun with Leg 104 Sites 642, 643, and 644 (Fig. 6). With these sites we will be able to reconstruct the history of the strong climatic gradients in the Nordic seas caused by warm-water inflow ("the Nordic heat pump") in the east. This warm inflow is compensated by outflow of cold, polar waters in the west and cold deep-water outflow across the bottom of the Southern Gateway ridge.
Lastly, with the addition of Site 986 on the Svalbard Margin and Site 987 on the Greenland Margin, the Nordic sea sites are situated to determine the long-term initiation and growth history of the three major regional ice sheets: the Barents Sea Ice Sheet, the Scandinavian Ice Sheet, and the Greenland Ice Sheet.