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Subsurface temperature transients resulting from past climatic changes are often still present at depth within ice sheets and permafrost. Once measured in deep boreholes, these temperature transients can be used to reconstruct past climatic changes using geophysical inverse methods. This paleoclimate reconstruction technique, known as "borehole paleothermometry," can provide the most accurate estimates of past temperatures on the Earth's surface. Borehole temperature (BT) data can also be used to calibrate the isotopic oxygen-18 (δ18O) proxy-temperature method for an ice core site as was recently done by Cuffey et al. (1995) for the Greenland Ice Sheet Project 2 (GISP2).
During the 1994-1995 and 1995-1996 field seasons, we acquired high-precision temperature measurements in the 554-meter (m) deep borehole (TD-D) recently drilled through the ice at Taylor Dome (77°50'S 159°00'E). This work is one component of the multifaceted Taylor Dome Ice Core project (see Grootes, Steig, and Stuiver 1994; Waddington, Morse, and Clow 1994). The BT data will be used to reconstruct paleoclimate through borehole paleothermometry and calibrate the δ18O paleothermometer for this site. During the last two field seasons, we also measured temperatures in boreholes on the Taylor Glacier and in Taylor Valley. The Taylor Dome/Taylor Valley transect lies in the climatic transition between the cold polar plateau and the maritime Ross Sea.
Temperatures below the firn/ice transition (74 m) were successfully measured in the TD-D borehole both before and after the installation of a "convective-damping" system; this system reduced the thermal noise associated with convective eddies in the borehole fluid to less than our system's sensitivity, 0.14 millikelvins (mK). To obtain temperatures in the firn layer, we monitored temperatures at selected depths in the 130-m dry corehole (TD-C) located 50 m from TD-D. While monitoring at each depth, small temperature fluctuations (<30 mK) were detected that correlate very well with atmospheric pressure changes recorded on the surface ( figure 1; Clow, Saltus, and Waddington in press). Since the TD-C corehole had been carefully sealed from air intrusion at the surface, the observed temperature fluctuations are due to the influence of atmospheric pressure variations on the firn layer. Temperature fluctuations in the 100-m corehole (TD-B) on the Taylor Glacier (77°38'S 159°39'E) are much smaller than at TD-C due to the reduced firn permeability at TD-B. The BT data indicate the present mean-annual surface temperature is -41.23°C at Taylor Dome and -36.37°C on the Taylor Glacier at TD-B.
During the early 1970s, several deep boreholes were drilled in the McMurdo Dry Valleys under the Dry Valleys Drilling Project (DVDP). Unfortunately, only one of the DVDP boreholes (11, near the Commonwealth Glacier) remains open below 75 m at this time. A comparison of our DVDP 11 temperature measurements with those obtained 20 years ago by Decker and Bucher (1977) shows that mean-annual temperatures in lower Taylor Valley have increased about 1 K over the last several decades ( figure 2). This is consistent with the increase in summer temperatures suggested by rising lake levels in the McMurdo Dry Valleys (Wharton et al. 1992).
To interpret the BT data from Taylor Dome accurately in terms of past climatic changes, it is essential to understand the local ice dynamics. To measure the vertical component of the ice flow, metal bands were emplaced at 5-m intervals along the TD-C corehole. A down-hole video camera was then used to determine the position of the bands during January 1995 and January 1996. Vertical velocities and strain rates have been determined from the change in band positions during the intervening 12 months.
The primary goal of the BT logging program is to determine past temperatures on the surface of Taylor Dome. Mean-annual surface temperatures presently vary by at least 5 K near Taylor Dome, however, because of microclimatic effects (Waddington and Morse 1994). This occurs between sites differing in elevation by only 65 m. Since 5 K represents 30 percent of the accepted ice-age/interglacial temperature change, it is essential to understand whether these microclimate zones are stable or whether they move over time. If the microclimate zones drift with time, they would leave a "trail" in the temperatures recorded in the boreholes, even though the shifts might not represent real shifts in regional climate.
We currently have four automatic weather stations recording wind speed, wind direction, and air and snow temperatures at several levels. Preliminary data indicate that the temperature microclimates are associated with winter-time differences in the near-surface thermal inversion layer. We plan to examine advanced very-high-resolution radiometer (AVHRR) thermal infrared imagery to learn more about these microclimates and their possible impact on paleoclimate interpretations.
We thank many individuals in the U.S. Antarctic Program and also our invaluable field assistants, Jason Bailey, Susan Douglass, and Bob Hawley. This research was supported by National Science Foundation grant OPP 92-21261 and by the U.S. Geological Survey's Climate History Program.
References
Clow, G.D., R.W. Saltus, and E.D. Waddington. In press. A new high-precision borehole temperature logging system used at GISP2, Greenland, and Taylor Dome, Antarctica. Journal of Glaciology.
Cuffey, K.M., G.D. Clow, R.B. Alley, M. Stuiver, E.D. Waddington, and R.W. Saltus. 1995. Large arctic temperature change at the Wisconsin-Holocene glacial transition. Science, 270, 455-458.
Decker, E.R., and G.J. Bucher. 1977. Geothermal studies in Antarctica. Antarctic Journal of the U.S., 12(4), 102-104.
Grootes, P.M., E.J. Steig, and M. Stuiver. 1994. Taylor Ice Dome study 1993-1994: An ice core to bedrock. Antarctic Journal of the U.S., 29(5), 79-81.
Waddington, E.D., and D.L. Morse. 1994. Spatial variations of local climate at Taylor Dome, Antarctica: Implications for paleoclimate from ice cores. Annals of Glaciology, 20, 219-225.
Waddington, E.D., D.L. Morse, and G.D. Clow. 1994. Glacier geophysical studies at Taylor Dome: Year 4. Antarctic Journal of the U.S., 29(5), 82-84.
Wharton, R.A., Jr., C.P. McKay, G.D. Clow, D.T. Andersen, G.M. Simmons, Jr., and F.G. Love. 1992. Changes in ice cover thickness and lake level of Lake Hoare, Antarctica: Implications for local climatic change. Journal of Geophysical Research, 97, 3505-3513.