A terrestrial sediment sequence exposed in an eroding pingo provides insights into the late-Quaternary environmental history of the northern Seward Peninsula, Alaska. We have obtained the first radiocarbon-dated evidence for a mid-Wisconsin thermokarst lake, demonstrating that complex landscape dynamics involving cyclic permafrost aggradation and thermokarst lake formation occurred over stadial-interstadial as well as glacial-interglacial time periods. High values of Picea pollen and the presence of Larix pollen in sediments dated to 50-40 ka BP strongly suggest the presence of forest or woodland early in MIS 3; the trees grew within a vegetation matrix dominated by grass and sedge, and there is indirect evidence of grazing animals. Thus the interstadial ecosystem was different in structure and composition from the Holocene or from the preceding Last Interglacial period. An early Holocene warm period is indicated by renewed thermokarst lake formation and a range of fossil taxa. Multiple extralimital plant taxa suggest mean July temperatures above modern values. The local presence of spruce during the early Holocene warm interval is evident from a radiocarbon-dated spruce macrofossil remain and indicates significant range extension far beyond the modern tree line. The first direct evidence of spruce in Northwest Alaska during the early Holocene has implications for the presence of forest refugia in Central Beringia and previously assumed routes and timing of post-glacial forest expansion in Alaska.
Bibliographical noteFunding Information:
The fieldwork was financially supported by the National Science Foundation (NSF grant № 0732735 ) and the Deutsche Forschungsgemeinschaft (DFG grants № SCHI 530/7-1 , WE 4390/2-1 and KI 849/2-1 ). Additional support was provided from NASA Carbon Cycle Sciences grant NNX08AJ37G . We thank Hanno Meyer and Georg Schwamborn for help in the field, and Helga Kemnitz (GeoForschungsZentrum Potsdam, Germany) who helped with SEM photography. We thank Katey Walter Anthony (WERC, University of Alaska, Fairbanks, USA), the CH2M Hill Polar Services Fairbanks Office staff as well as Curtis Nayokpuk and Fred Goodhope from Shishmaref for logistical support. We acknowledge US National Park Service permitting to work in Bering Land Bridge National Preserve. Two reviewers of the paper (Anonymous and Thomas D. Hamilton, USGS Alaska Science Center, Anchorage, USA) greatly helped by their suggestions to improve the final publication of this study.
Copyright 2012 Elsevier B.V., All rights reserved.
- Central Beringia
- Holocene Thermal Maximum
- Picea sp.