Preliminary Results from the Bering Land Bridge Coring Expedition

Over the course of the Pleistocene, the Bering Sea has alternated many times between a marine gateway connecting the Arctic and Pacific Oceans and a land bridge connecting North America to Asia. However, very few sediment cores for paleoceanographic or paleoenvironmental reconstructions of this region have been collected. Some have proposed that the marine transgression deposited thick layers of sand and gravel across the shelf, making Pleistocene sediments difficult to recover. In 2023, we conducted a cruise with the goal of identifying and coring lakes that were present on the land bridge. We expected to find a few large lakes. Instead, Chirp images revealed networks of filled channels with small basins in between. Integration of sub-bottom profiles from prior expeditions confirms that swaths of Central Beringia were transected by anastomosing river floodplains dotted with freshwater ponds or bogs. These represent the former courses of the Yukon and Kuskokwim rivers on the low-gradient land bridge. Cores collected from 30 sites in the Bering Sea at water depths less than ~100 m all contain a basal freshwater sedimentary unit overlain by marine mud or sand. A freshwater origin for these basal deposits is confirmed by plant macrofossils, diatoms, and stable isotope analyses. Preliminary dates indicate that the ponds or bogs were present prior to ~25ka or after ~22ka but may have dried up during the last glacial maximum. In this talk we’ll also look at what the preliminary pollen, diatom, and sedimentary ancient DNA data can tell us about ecosystems on the land bridge and sea level rise.

Speaker: Beth Caissie, U.S. Geological Survey