Only two sediment cores taken from ice island T-3 in the Arctic Basin have material older than Pliocene age. One of these cores includes a Late Cretaceous silicoflagellatc assemblage contained in tuffaceous sediment; the other is of Paleocene to Eocene age, containing abundant phytoplankton in tuffaceous sediment. The two cores represent displaced Cretaceous and early Cenozoic masses within a Pliocene-Pleistocene sequence. Both cores were taken from the Alpha Cordillera.

These cores provide the first firm evidence that the Arctic Ocean existed at least by Late Cretaceous time, a time during which it had no ice cover and was a much warmer ocean than it is now. Warm water conditions continued at least into Eocene time. No Oligocene or Miocene material has been identified, but, by Pliocene time, the Arctic was ice covered, and this condition has existed until the present.

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