Reconstructing the Paleoenvironments of the Pliocene Barmur Group in the Tjörnes Basin, North Iceland

The Barmur Group of the Tjornes sequence in North Iceland contains data which are interpreted as up to 20 transgression-regression cycles between 5.3 and 3.2 Ma. The Barmur Group fauna is unique among Cainozoic marine faunas in the North Atlantic region north of the British Isles containing a Pliocene mollusc fauna. The Tjornes Basin was presumably an early stepping-stone for the invertebrate species migrating from the Pacific into the North Atlantic since the Miocene-Pliocene boundary. A marked faunal change, mainly visible in the lowermost part of the Serripes Zone at about 3.8 Ma, was not only connected to environmental changes in the Tjornes region but rather due to migration of at least 34 mollusc species of North Pacific origin into the North Atlantic. These species have since been among dominants in marine arctic, subarctic, and even boreal assemblages in the northern part of the Atlantic. We interpret our data as reflecting an overall change in sea-water temperature from summer temperatures between 10 and 15 °C during the deposition of the Tapes Zone and the lower part of the Mactra Zone to colder water conditions with summer temperatures between 5 and 10 °C at the top of the Serripes Zone.