Stratigraphy, chronology and paleoenvironmental significance of perennially frozen interglacial sediments in eastern Beringia.

摘要

Stratigraphic observations from sites in eastern Beringia - Ch'ijee's Bluff and nearby exposures in northern Yukon, the Palisades on the Yukon River in Alaska, and placer mining exposures at Thistle Creek in west-central Yukon -- provide insight into the chronostratigraphic significance of Old Crow tephra (124 +/- 10 ka) and permafrost response to warming during the last interglaciation. Pollen and insect, bryophyte, and vascular plant macrofossils from a vegetated surface buried by Old Crow tephra suggest the local presence of mesic tundra, wet meadow, and aquatic habitat when the tephra was deposited. A revised glass fission-track age for Old Crow tephra places its deposition closer to the time of the last interglaciation than earlier age determinations, but stratigraphy and paleoecology of sediments associated with the tephra at the Palisades, and elsewhere in eastern Beringia, indicate a late Marine Isotope Stage 6 age. Multiple late Pleistocene and Holocene 14C dates from an organic silt bed at the Palisades, which was tentatively assigned a last interglacial age based on tephrostratigraphy and paleoecology, illustrate how permafrost thaw may facilitate reworking of last interglacial and younger sediments into cryptic new stratigraphic positions with respect to Old Crow tephra. Prominent deposits of last interglacial wood-rich organic silt are present across eastern Beringia, representing accumulations of reworked forest vegetation due to thaw slumping or thermokarst processes. Consistent stratigraphic relations between these deposits, Old Crow tephra, and ice wedge pseudomorphs suggest that thaw of shallow permafrost was widespread during the last interglaciation. However, Old Crow tephra overlies several relict ice wedges at the Palisades and Thistle Creek, indicating that permafrost at those sites did not thaw completely during late Pleistocene warm intervals. The persistence of relict pre-last interglacial ice wedges highlights the potential for regional antiquity of discontinuous permafrost, and provides compelling field evidence for the long-term resilience of deep permafrost during sustained periods of warmer-than-present climate. A revised chronology of peatland initiation, based on summed probability distributions of calibrated 14C dates, clarifies the interaction between peatland dynamics, permafrost degradation, and changes in atmospheric CH4 concentration during the most recent glacial-interglacial transition.