Timing and Consequences of Bering Strait Opening: New Insights From ⁴⁰Ar/³⁹Ar Dating of the Barmur Group (Tjörnes Beds), Northern Iceland

The Barmur Group (informally Tjörnes beds) sedimentary succession of northern Iceland is key to reconstructing the opening of the Bering Strait oceanic gateway because these rocks record migration of bivalve molluscs from the Pacific to the Atlantic via the Arctic. However, the timing of the migration event is poorly constrained owing to a lack of reliable absolute ages. To address this problem, we present the first Ar-Ar radiometric dates from four basaltic lavas that underlie, are intercalated with, and overlie the Barmur Group, and integrate them with existing paleomagnetic records. We show that the Barmur Group has a latest Miocene to early Pliocene age range (c. 6.0–4.4 Ma; C3r–C3n.2n), older than all previous age models. Thus, the Barmur Group does not record the mid-Piacenzian Warm Period, contra some previous suggestions. Abundant Pacific bivalve molluscs appeared in the Barmur Group during subchrons C3n.4n–C3n.3r at 5.235–4.896 Ma, over 1.3 million years earlier than previously suggested. Appearance of Pacific bivalves in the northern Atlantic occurred shortly after the 5.6–5.4 Ma age previously inferred for first appearance of Arctic bivalves in the Pacific. Thus, our data suggest that first opening of the Bering Strait gateway by the latest Miocene (c. 5.5 Ma) was soon followed by bidirectional trans-Arctic faunal exchange, and argue against a hypothesized two-stage faunal exchange process spanning c. 2 million years. Our results also confirm that first opening of the Bering Strait gateway was not directly associated with the growth of large northern hemisphere icesheets, which occurred several million years later.

Plain Language Summary: The extent to which global climate change is influenced by the flow of seawater between the world's oceans is an important question in Earth science. Sedimentary rocks from the Tjörnes Peninsula in northern Iceland record the opening of the Bering Strait oceanic gateway, which connects the Pacific and Atlantic Oceans via the Arctic. The signature of this gateway opening event is the sudden appearance in Iceland of bivalve mollusc fossils that previously lived only in the Pacific Ocean. However, the timing of their arrival is uncertain owing to the lack of reliable age measurements. To solve this problem, we applied radioisotope dating methods to a succession of rocks called the Barmur Group from the Tjörnes Peninsula. We found that the sudden appearance of Pacific fossils occurred over a million years earlier than previously thought. Our data show that first opening of the Bering Strait gateway was soon followed by migration of marine faunas in both directions, in contrast to the existing hypothesis that Pacific-to-Atlantic migration significantly post-dated Atlantic-to-Pacific migration. Our results also confirm the hypothesis that establishment of the Pacific-Arctic-Atlantic oceanic connection did not directly trigger development of large northern hemisphere icecaps, which occurred several million years later.