The first opening of Bering Strait is evidenced by the presence of the marine bivalve mollusk, Astarte, in the Miocene Bear Lake Formation on the Alaska Peninsula. Astarte lived only in the North Atlantic and Arctic Oceans in the Cenozoic until the opening of Bering Strait allowed it to migrate into the North Pacific. Astarte has previously been found in North Pacific Neogene faunas, but not with age-diagnostic microfossils. Astarte, with other abundant mollusk fauna, is present at two horizons in a 300-meter-thick, kilometer-long stratigraphic section of the Bear Lake Formation. One hundred diatom taxa from within mollusk shells at these two horizons represent Subzone B of the Neodenticula kamtschatica diatom zone. Subzone B of the Neodenticula kamtschatica zone has an age range of 4.8-5.5 million years ago (Ma) within a well-established North Pacific diatom biochronology and is significantly older than the approximate 3.0-4.0 Ma date for Bering Strait that is commonly cited by researchers. The stratigraphic position of Astarte in the Bear Lake Formation section is imprecisely known. It is likely that a significant portion of the section lies below the known occurrences of Astarte and will therefore contain older mollusks and diatoms. The first occurrence of Astarte in the North Pacific and, therefore, the first opening of Bering Strait was likely to have been in the late Miocene (more than 5.32 Ma). The proposed research is to be based on one month of fieldwork in southwestern Alaska, followed by two years of faunal and microfloral analysis and synthesis. The objectives of the study are to: (1) document a precise age for the first opening of Bering Strait, based on diatoms, magnetostratigraphy, and tuff ages; (2) understand the composition of the Bear Lake Formation molluscan fauna before Bering Strait opened and afterwards; and (3) place the opening of Bering Strait in a broader biogeographic and paleogeographic context. This project is expected to advance our knowledge of the northern oceans and marine faunas, including the opening of the Bering Strait. It should also give insights on the paleoecology of the marine invertebrate faunas of the North Pacific, Arctic, and North Atlantic oceans after Bering Strait opened.
