Ninth International Conference on Permafrost ... - IARC Research

Rock Glacier Response to Post-Little Ice Age Warming: Spruce Creek Rock Glacier,Ten Mile Range, Colorado, USAEric M. LeonardDepartment of Geology, Colorado College, Colorado Springs, CO, USAStephen G. WeaverDepartment of Geology, Colorado College, Colorado Springs, CO, USAJames A. BradburyDepartment of Geosciences, University of Massachusetts, Amherst, MA, USAErica E. LangbeckerDepartment of Geology, Colorado College, Colorado Springs, USAJeffrey A. WollenbergDepartment of Geology, Colorado College, Colorado Springs, USAIntroductionIn this study, we use multiple methods to document changesin flow velocity, internal deformation, and surface geometryof the Spruce Creek rock glacier in the Ten Mile Rangeof central Colorado. Our methods include lichenometry,photogrammetry, and detailed ground surveying. The dataprovide evidence of strong rock glacier response to bothLittle Ice Age cooling and post-Little Ice Age warming.Response to warming over the last century appears to haveoccurred with little lag time.MethodsLong-term flow variations are evaluated throughmeasurement of lichens (Rhizocarpon s.l.) on the rockglacier surface, which provide a record of surface ages,and thus flow rates, spanning the last 2500–3000 years.Lichen measurements are calibrated using a modifiedversion of Benedict’s (1993) Rhizocarpon growth curvefor the Colorado Front Range. Aerial photographs taken atseveral intervals since 1938 provide a more detailed recordof changing flow rates over the last seven decades. Repeatedground surveys begun in the 1985 provide information onchanging velocities, strain rates, and strain patterns, andthinning of the rock glacier over the last two decades.ResultsLichenometry provides only approximate surface agesand thus only approximate flow rates. During the 2000-year interval before about 1600 A.D., mean centerline flowrate at Spruce Creek was approximately 6 cm/yr. The flowrate increased dramatically after about 1600 A.D. For thefollowing three centuries, until the late 19 th century, a timeinterval corresponding to the global peak of the Little IceAge, mean flow rates were on the order of 40–55 cm /yr. Bythe mid 20 th century, flow rate had declined again to abouthalf of this peak rate, and from the mid 1980s through 2000A.D., centerline flow rates had declined further to 5.9–10.2cm/yr on three surveyed transects. Mean flow rates along thethree transects over this interval ranged from 4.5 to 6.7 cm/yr.The relatively fast-moving upper portion of the rock glacier,which formed initially during the Little Ice Age, sloweddown by 25% between the late 1980s and the late 1990s,and strain rates measured at 12 strain diamonds distributedacross the rock glacier declined by an average of 19% overthe same interval. Lowering of the rock glacier surface wasgeneral, but spatially highly variable, over the period from1985 to 2000 A.D. Mean surface lowering exceeded 1 mduring this period.Records from the nearest continuously operatingmeteorological station, at Climax, Colorado, 8–9 kmsouthwest of the Spruce Creek rock glacier, and the morecomprehensive records from Niwot Ridge in the ColoradoFront Range about 75 km to the northeast, both indicatestrong summer warming after the mid 1970s. The velocityand strain rate decreases and surface lowering measured atthe rock glacier since the mid 1980s appear to be a responseto that warming.Ongoing ResearchDuring summer 2008, we will resurvey the rock glacierto examine its response to continued warming. Since 2000,summer (June through August) temperatures at Climax haveaveraged 0.96°C higher than during the initial 15 years ofour survey.ReferenceBenedict, J.B. 1993. A 2000-year lichen-snowkill chronologyfor the Colorado Front Range, USA. The Holocene 3:27-33.179