LEG 105

Baffin Bay and The Labrador Sea

Baffin Island and Greenland, are separated by the sill at Davis Strait, which is no deeper than 700 m. Today these basins form the western branch of the North Atlantic Ocean. The eastern branch is composed of the Reykjanes Basin and Norwegian-Greenland seas. Before the formation of the Greenland-Norwegian seas in the late Paleogene, the Labrador Sea and Baffin Bay perhaps formed the main conduit between the Arctic and Atlantic oceans, at least for surface-water mass exchange.

During Leg 105, three sites (Sites 645, 646, and 647) were drilled on a north-south transect to investigate the tectonic evolution and climatic and paleoceanographic changes of this region. Dates associated with oceanic crust recovered at Site 647 in the southern Labrador Sea confirm that a major change in seafloor spreading direction occurred at about 55 Ma. Results from Site 645, near Baffin Island, indicate that Baffin Bay began opening at about the same time. Extrapolation of sedimentation rates at Site 645 to a major deep regional reflector suggests an Eocene-Oligocene age for the reflector. Because the reflector extends across the entire basin, Baffin Bay must have stopped forming by 36 Ma, the same time that spreading ceased in the Labrador Sea.

Drilling conducted at all three sites recovered continuous, high-sedimentation rate Plio-Pleistocene sequences, recording high-frequency climatic oscillations and documented that major glaciation, defined at the first occurrence of ice-rafted debris, began at 2.5 Ma. At Site 647, the earliest ice- rafted material rests unconformably above a highly condensed upper Miocene sequence which, in turn, unconformably overlies lower Miocene and older strata. At Site 646, on the northern flank of the Eirik Ridge drift, the onset of ice-rafted sedimentation closely coincides with the end of a major episode of erosion and depositional modification of the drift. At Site 645, isolated pebbles in sediments as old as 3.5 Ma and a few clasts in strata as old as 8 Ma indicate that glaciation in the Baffin Bay region may have begun earlier than in the northern Atlantic.

Drilling results and seismic data also provide critical information on the older history of bottom- water circulation. The most significant intensification in deep circulation occurred after the mid- Miocene. An unexpected result was the mid-Miocene record of increased southerly transport of Arctic water masses and the intensification of deep circulation; seismic reflection profiles show that drift deposits of this age occur off the Baffin Island slope.