USGS

Chronological Refinement of an Ice Core Record at Upper Fremont Glacier in South-Central North America

Paul F. Schuster1, David E. White2, David L. Naftz1, L. DeWayne Cecill



Abstract

The potential to use ice cores from alpine glaciers in the mid latitudes to reconstruct paleoclimate records has not been widely recognized. Although excellent paleoclimatic records exist for the high latitudes of Northern and Southern hemispheres, paleoclimatic ice core records are not common from the mid-latitude locations of the continental United States. An ice core removed from the Upper Fremont Glacier (UFG) in Wyoming provides evidence for abrupt climate Volcanic events (Krakatau and Tambora) were identified from recent electrical conductivity measurements (ECM). The ECM were correlated with existing isotopic and chemical data from the UFG and reexamined to confirm and refine previous chronological estimates of the ice core. At a depth of 152 m, the refined age-depth profile shows good agreement (1736±10 AD) with the 14C age date (1729±95 AD).

Naftz and others (1996) described a shift in Del 18O signal and increase in variance as an indication of a climate change known as the Little Ice Age (LIA). However, the sampling interval forDel 18O is sufficiently large that it is difficult to pinpoint the LIA termination based onDel 18O data alone. Other research has shown an increase in the Del 18O variance is generally coincident with a decrease in ECM variance. The ECM data set contains over 125,000 data points. A 999-point running average of the ECM data set and results from f-tests indicated the variance of the ECM data decreases significantly at about 108 m. At this depth, the age-depth profile predicts an age of 1845 AD. These data indicate the termination of the LIA was abrupt; a major climatic shift to warmer temperatures occurred around 1845 AD and continues to present day. Prediction limits (error bars) calculated for the profile ages were ±10 years (90% confidence level). Thus a conservative estimate for the time taken to complete the LIA climatic shift to present-day climate is about 10 years, suggesting the LIA termination in alpine regions of central North America may have occurred on a relatively short (decadal) time scale.

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