VARIATIONS BETWEEN δ¹⁸O IN RECENTLY DEPOSITED SNOW AND ON-SITE AIR TEMPERATURE, UPPER FREMONT GLACIER, WYOMING

David L. N aftz, David D. Susong, L. DeW ayne Cecil, and Paul F. Schuster

1. INTRODUCTION

Oxygen isotopic ratios (δ¹⁸O) in ice cores have been used extensively to reconstruct past climate trends (Covey and Haagenson, 1984; Charles et al.; 1994; Lorius et al., 1990; Cuffey et al., 1994; and Jouzei et al., 1997).  Recent investigations have documented positive correlations between site- specific 3]80 values in snow and ice to on-site average air temperature (T _A) during snow accumulation events in polar regions (Thompson., 1994; Shuman et al., 1995).

To date (2003), only limited observations link the δ¹⁸O values in ice and snow samples to on-site air temperature at high-altitude, mid- and low- latitude ice-coring sites (Davis et al., 1995; Yao et al., 1996; Yao et al., 1999; and Naftz et al., 2002). Work by Naftz et al., (2002) developed a series of site-specific transfer functions between on-site air temperature and δ¹⁸O, values in snow deposited at a mid-latitude ice coring site established at over 4,000 m above sea level (masl) on Upper Fremont Glacier (UFG), Wyoming. The site-specific transfer functions developed from this site were used in conjunction with δ¹⁸O values from ice cores obtained from UFG to reconstruct changes in air temperature since the early 1700s.

Because UFG is a remote, high-altitude site, it is not possible to physically collect discrete snow samples from individual storm events for δ¹⁸O analysis.  Instead, the site was visited one to three times per year and snow pits were excavated and used to sample the accumulated snowpack. The timing and amount of each snow accumulation event in the excavated snow pits on UFG was determined from snow pillow data from Cold Springs SNOTEL site, approximately 22 km northeast from UFG, and used in the development of site-specific transfer functions (Naftz et al., 2002). This method of determing the timing and relative amounts of accumulation events on UFO relied on the following assumptions: (1) snow redeposition from wind events and melting was minimal after each accumulation event; (2) the same storms impact both sites with the same relative intensities; and; (3) all precipitation on the UFG is in the form of snow and, therefore, accumulates and can be used as a proxy for cumulative precipitation. None of these assumptions were valid 100 percept of the time.

To better record the on-site snow accumulation and redeposition on UFG during 1999-2000, an hourly record of snow depth was obtained using an ultrasonic sensor (Figure 1).  Instead of relying on the SNOTEL data as a proxy record for snow accumulation on UFG, on-site data were possible. The objectives of this chapter are to: (1) investigate and model the transfer function between δ¹⁸O values in snow and the corresponding air temperature using the continuous, on-site snow-depth and air temperature monitoring equipment installed on UFG (Figure 2); (2) compare the transfer function developed from the on-site snow-depth sensor to transfer functions developed using off-site snow accumulation data; and (3) reconstruct and compare air temperatures from δ¹⁸O values in UFG ice cores using both transfer functions.