Ninth International Conference on Permafrost ... - IARC Research

Importance of Changes in Moisture for Geomorphic Responses to Rapid ClimaticWarming in the Western Brooks Range and the Arctic Foothills, Northern Alaska:Lessons from the PastDaniel MannInstitute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, USAPamela GrovesInstitute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, USAMichael KunzArctic Field Office, Bureau of Land Management, Fairbanks, AK 99709, USAClimate changes between 12,500 and 8,000 14 C yr BPtriggered sweeping changes in vegetation cover, slopestability, and floodplain dynamics in the Brooks Range andArctic Foothills of northern Alaska. Some of these climatechanges involved rapid warming, so they provide analogsto the warming predicted for the coming century. Usingpalynology and radiocarbon-dated basal peats, Mann et al.(2002a) inferred that peat deposition (paludification) began,and shrub vegetation became widespread, ca. 12,500 14 C yrBP, probably in response to a warmer and wetter climate.Stream-bank stratigraphy reveals that increased slope erosioncaused rapid alluviation in valleys at the same time thatPopulus trees spread northward along braided floodplainsbefore ca. 11,000 14 C yr BP (Bockheim et al. 2003). Duringthe Younger Dryas (YD) Chronozone (11,000–10,000 14 C yrBP), lake levels fell and streams incised, probably in responseto a drier, cooler climate that caused active layers to thin andthe erosion of slopes to slow. A hiatus in records of Populussuggest that its geographic range contracted during the YD,and pollen records of other species suggest a cooler and drierclimate during this interval. Basal peats dating to the YDare rare, suggesting that paludification slowed. Starting ca.10,000 14 C yr BP, lake levels rose, streams aggraded rapidlyagain, intense solifluction seems to have occurred, andPopulus re-invaded the region (Mann et al. 2002a).Paleoindian people occupied the Arctic Foothills brieflyat the close of the YD, though there is increasingly goodevidence that they were also present several centuries beforethe YD began ca. 11,000 14 C yr BP (Kunz & Reanier 1994,Rasic 2000, Kunz, unpubl. data). We speculate that thespread of moist acidic tundra between 10,000 and 8,500 14 Cyr BP, along with the wet, organic-rich soils characteristicof the present landscape, caused the Paleoindians and theirprey species to disappear from the region.Floodplain dynamics are of particular interest forunderstanding the responses of arctic landscapes to climatechanges, because they are centers of primary productivityand biodiversity (Walker et al. 2001). Floodplains aggradedat rates of meters/century just prior to 11,000 14 C yr BP andimmediately after 10,000 14 C yr BP. Aggradation at suchrapid rates must have been accompanied by widespread slopeerosion, which suggests intense and widespread thermokarstformations.Dating of relict alluvial fans in the western BrooksRange suggests several periods of increased depositionfrom headwater streams over the course of the Holocene.This work is in progress, but currently we have evidencefor fan building ca. 4,000 14 C yr BP and ca. 2,000 14 C yr BP.Systematic observations begun in 2005 of floodplains in thewestern Brooks Range reveal that rapid aggradation is nowunderway in the headwater reaches of some stream systems.This aggradation is associated with mass movements ofseveral different kinds occurring on hill slopes and in streamchannels. Our observations suggest that we are at the cusp ofanother major ecosystem transition in the Brooks Range.Most of the landscape-scale changes in the Arctic Foothillsduring the Pleistocene-Holocene transition involved changesin moisture balance, many imply changes in active-layerthickness, and some occurred very rapidly. The vulnerabilityof ecosystems in northern Alaska to changes in moisturebalance is still evident today in the sensitive thresholdexisting between sand dunes and the vegetation surroundingthem (Galloway & Carter 1993, Mann et al. 2002c). Duringthe Late Pleistocene and early Holocene, the encroachment ofmarine water caused by rising global sea level was probablya major driver of increasing effective moisture on Alaska’sNorth Slope (Mann et al. 2002b). Today, the moisturebalance, which is still poorly constrained by predictiveclimate models, may be changing under the influences ofincreased winter snowfalls that accompany warmer wintertemperatures, by northward shifts in the summer positionof the Polar Front over the western Brooks Range and bynet thawing of ice-rich permafrost as the result of warmingin both summer and winter. Just as it was during the earlyHolocene, we suspect that moisture balance will be the key,proximal driver of ecosystem change in northern Alaska andin other arctic regions during the coming century.ReferencesBockheim, J.G., O’Brien, J.D., Munroe, J.S. & Hinkel, K.M.2003. Factors affecting the distribution of Populusbalsamifera on the North Slope of Alaska, U.S.A.Arctic, Antarctic, and Alpine Research 35: 331-340.Galloway, J.P. & Carter, L. D. 1993. Late holocene longitudinaland parabolic dunes in northern Alaska: Preliminaryinterpretations of age and paleoclimatic significance.U.S. Geological Survey Bulletin 2068: 3-11.201