Leg 104

The Norwegian Sea

The Norwegian continental margin is a relatively young geological feature ideally suited for studying the formation and development of deep ocean basins. The primary features in this region are structural highs delimiting sedimentary basins whose formation culminated in the late Jurassic, pre-dating the time of initiation of seafloor spreading in the Norwegian Sea (56-58 Ma). The tops of the highs are characterized by a pronounced seismic marker of basaltic origin underlain by zones of seaward-dipping reflector sequences (SDRS's) resting on an acoustically homogeneous basement complex. Three sites (Sites 642, 643, and 644) were drilled during Leg 104.

Drilling at Site 642, located on the inner part of a prominent wedge of dipping reflectors, resulted in the recovery of pelagic to hemipelagic Cenozoic sediments and an Eocene volcanic sequence. The sequence is divided into an upper and lower series and the SDRS is interpreted to consist of the flows and interbedded volcaniclastic sediments of the upper volcanic series. The underlying rock unit is separated from the dipping sequence by a distinct reflector, K. Reflector K seems to represent a distinct volcaniclastic unit that defines the transition between the upper and lower series. The upper series comprises a succession of about 122 flows of transitional oceanic tholeiitic composition and volcaniclastic sediments, derived from mantle sources and possibly representing some of the first outpourings of MORB-type basalts associated with the opening of the Norwegian Sea. The lower series comprises a sequence of intermediate extrusive flow units and volcaniclastic sediments, erupted slightly prior to the onset of MORB-type magmatism and derived, at least partially, from continental source material.

A secondary objective of Leg 104 was a study of the evolution and variability of the Norwegian Current, a continuation of the North Atlantic Drift Current, transporting temperate North Atlantic water to the Arctic Ocean. Two additional sites (Sites 643 and 644) were drilled to address this objective and all three sites provided evidence that this temperate current began to deteriorate at 2.8-2.9 Ma (possibly as early as 4.5 Ma), being replaced by intermediate cold ice-covered polar water masses. At 0.73 Ma, a dramatic drop in surface water productivity occurred. Around 0.4-0.5 Ma, the Norwegian Sea became almost exclusively glacial. The transition from a pre-glacial to glacial environment occurs 0.5 m.y. earlier in the Norwegian Sea than in the North Atlantic Ocean to the south of the Greenland-Scotland Ridge, but is preceded by the 4-6 Ma glacial record in the Arctic Ocean. Cessation of pre-glacial conditions occurred almost simultaneously with a similar major northward expansion of Antarctic polar water.