Agenda

Prof. David B. McWethy, Montana State University

Titolo: Ice Patch Discoveries: New paleoclimate proxies from permanent ice patches document rapid Holocene climate variability paralleling human activity in the Rocky Mountains, USA.

Abstract:
Permanent ice patches from mountain environments in North America, Europe, and Japan are emerging as valuable archaeological and paleoenvironmental archives. These alpine ice patches have survived perennially for thousands of years, preserving fragile archaeological and biological artifacts and documenting the history of ancient hunters in alpine environments. Permanent ice patches also have the potential to preserve paleoclimate information in the chemical records recorded in the ice. Here, we present a highly resolved 10,000-year ice-core record of northern Rocky Mountain, wintertime climate recovered from an ice patch located on the Beartooth Plateau, Wyoming and compare it with a record of past vegetation and disturbance derived from analysis of plant macrofossils, pollen and charcoal from 29 organic layers in the core. Plant material from each of the 29 organic layers was radiocarbon dated to develop a robust chronology for the 5.5 m-long ice core and pollen and macrofossils were analyzed from each of the organic layers. Water isotopes (δ18O, δ2H) and ice accumulation rates from an ice core from the ice patch suggest a pronounced cooling in the region during the mid-Holocene, consistent with climate inferred from stable isotopes from a speleothem record from the Wasatch Mountains, Idaho and population declines in the nearby Bighorn Mountains. Additionally, radiocarbon dates and a floating chronology of cross-sections of 27 large diameter whitebark pine (Pinus albicaulis) trees found entombed in an adjacent ice patch reveal the trees established over 250 meters above current treeline during a warm and dry interval then died as conditions abruptly cooled and became wetter c. 5.3 kcal yrs BP, documenting a period of rapid climatic change. Our findings suggest that these small alpine ice patches preserve regionally-representative climate records that document a sustained 800-year period of peak Holocene warmth centered at 4,100 cal BP followed by a rapid cooling leading to a prolonged era of cooler and wetter winters. The ice-patch records suggest rates of winter temperature changed at the onset and termination of this 1,500-year cold period were as high as observed rates of modern warming. The well-dated ice-patch climate records are consistent with reconstructions of environmental change and closely parallel archeological indices of human activity in western Wyoming throughout the Holocene, with activity suppressed during both especially warm and cold climate periods. The chemical record and diverse assemblages of plant, geologic, and archaeological material contained within ice patches provides a wealth of information about past climate and environmental conditions and archeological resources, presenting an unprecedented opportunity to document and evaluate human use of the alpine during periods of rapid climate and environmental change.