Chronology, cryostratigraphy, and palaeoenvironmental evidence from the CRREL Permafrost Tunnel, Alaska

The Cold Regions Research and Engineering Laboratory Permafrost Tunnel (CRREL Tunnel) in Alaska is one of the best studied Yedoma sequences in North America and is an exceptional site for understanding ice-rich, syngenetic permafrost. Here, we present cryostratigraphy, chronology, and stable isotopic analyses from eight exposures within the tunnel. These data are combined with previous studies to develop a composite sedimentary history and palaeoenvironmental record. At the sequence base, Fox Gravels were deposited ca. 45,000 calibrated years before C.E. 1950 (cal yr BP) and are overlain by silt that began accumulating ca. 43,000 cal yr BP. A slowing of silt accumulation (and possible hiatus) occurred ca. 36,000 cal yr BP and is associated with higher pore ice δ18O values and thermokarst cave ice. Previous radiocarbon dates on ice wedge CO2 from this unit are shown to be inaccurate, highlighting the difficulty in dating these features. Silt accumulation reactivated between 35,000–31,000 cal yr BP, with higher δ15N values, suggesting more arid conditions. Published palaeoenvironmental data, placed in our new chronology, show shrub tundra was present between 43,000–41,000 cal yr. By 36,000 cal yr BP herbs and sedges replaced shrub tundra as the Bering Land Bridge became established.