Single-channel seismic data acquired on Mohn's Ridge, in the Norwegian-Greenland Sea, near 72-degrees-N, show two intra-sedimentary horizons, at respectively, 140 to 160 ms two-way travel time beneath the sediment surface. Using a sedimentation rate of about 50 to 70 m/Ma, the age of the first horizon is estimated to be between 2 and 3 Ma; the age of the second one, between 4.5 and 6.5 Ma. The first horizon may thus mark the beginning of the Pliocene glaciations (2.5 Ma) in the Northern Hemisphere. The second one is likely to be the regional seismic reflector "O", identified at numerous DSDP and ODP deep drilling sites. Reflector "O" dates from the Pliocene-Miocene boundary, which is known to be associated with a very large increase of the detritic sedimentation production originating at the Spitzbergen continental margin. According to the interpretation of our magnetic data [1], Mohn's ridge near 72-degrees-N was affected by a phase of dramatic decrease of sea-floor spreading rate between 9 and 6 Ma. Following a phase of "tectonic release", sea-floor spreading starts again between about 6 and 5 Ma. If we assume that this latter phase corresponds to a regional tectonic reorganization which also affected the Spitzbergen margin, a straightforward correlation appears between enhanced tectonic activity and increase of detritic sedimentation at the Miocene-Pliocene transition time.
