Migration of Pacific Marine Mollusc Fauna into the North Atlantic Across the Arctic Ocean in Pliocene and Early Pleistocene Time

The Tjornes sequence documents an exchange of molluscs between the North Pacific and the Arctic and North Atlantic Oceans. Before the Pliocene, the main open passage between the Atlantic and the Pacific was through the Central American Seaway. A Beringian land bridge prevented exchange of marine biota in the northern region. The Bering Strait was probably opened towards the termination of the Miocene at 5.3 Ma, and the final closure of the Central American Seaway took place at about 2.7 Ma. At least 34 molluscan species of Pacific ancestry reached Iceland during the Pliocene and three species reached Iceland, while the Lower Pleistocene part of the Breiðavik Group on Tjornes was deposited. The major migration of Pacific species into the Tjornes area is manifested at the base of the Serripes biozone of the Barmur Group of the Tjornes sequence at about 3.8 Ma. This was preceded by earlier appearances of mollusc species close to the Miocene-Pliocene boundary. Mollusc migration into the Tjornes area was not a single, abrupt event, but occurred at various times during the Pliocene and Lower Pleistocene. Some of these species of Pacific origin have subsequently dominated boreal, subarctic, and arctic molluscan assemblages in the North Atlantic.