RICHARD B. ALLEY, MATTHEW K. SPENCER, and DONALD E. VOIGT, Earth System Science Center and Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802
Nine shallow cores were collected in a transect across Siple Dome during the 1996-1997 field season by the Polar Ice Coring Office, Lincoln, Nebraska, and the WAISCORES (West Antarctic Ice Sheet Cores) Science Coordination Office, Reno, Nevada. These cores were analyzed by many WAISCORES investigators at the National Ice Core Laboratory, Denver, Colorado, during the summer of 1997. As part of this analysis, all cores were examined visually, and many were thin sectioned by investigators in our team from Penn State. Optical analyses were also conducted by Kendrick Taylor, Desert Research Institute, Reno, Nevada, and some cores were examined visually by Tony Gow and Debra Meese of the U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire. Thin-section analyses and some visible examinations were also conducted by Joan Fitzpatrick, U.S. Geological Survey, Denver, Colorado.
The "typical" visible annual signal of summertime depth hoar and finer-grained, denser wintertime accumulation is present ( see Alley and Bentley 1988). The relatively low accumulation rate (approximately 0.12 meter of ice per year, based on our preliminary dating) complicates interpretation somewhat; accumulation from a prominent storm is potentially a significant fraction of a year, so storm-related features may mimic annual features, and vice versa. It is also possible that "depositional depth hoars" are present owing to odd wintertime weather patterns. Repeated counting of layers by one investigator and comparisons among investigators indicate that our identification of annual layers is reproducible to better than 10 percent but not at the 1 percent level found in optimal cores. Comparison to preliminary data graciously advanced by other WAISCORES investigators indicates that our reproducibility may be close to our accuracy, although further work remains.
Among the features visible in the cores are melt layers. Occurrence of melt layers at similar depths and ages in many different cores probably indicates that the records of major melt events are sufficiently widespread and continuous to provide time horizons, although some discontinuity is indicated by failure of prominent layers to appear in certain cores. Improved dating should allow further testing of these results. The occurrence of melt layers in the cores is shown in figure 1, arranged as a transect across Siple Dome from south to north; cores near the crest of the dome are plotted in the middle of the figure. Data also are included from a core farther south at Upstream C on ice stream C (82°26'S 135°58'W, altitude 506.25 meters). Only the upper 30 meters of the Upstream C core are shown, because deeper layers in that core appear disturbed. Clear variation in melt frequency is apparent and quantified in figure 2.
Over the most recent 150 years, based on our preliminary dating, melt occurs in between 0.7 percent and 6.7 percent of years in the Siple Dome cores and in 4.7 percent of the years at Upstream C. Notice that most of the accumulation from years containing melt features is ordinary, unmelted firn. This occurrence is not so frequent that it is likely to compromise other paleoclimatic measurements, especially because the melt layers are easily recognized and avoided in sampling for proxies that might be affected by melt. The lower frequency of melt near the crest of the dome almost certainly is related to its higher elevation.
Further statistical analyses are required (cf. Alley and Anandakrishnan 1995), but it appears that melt occurence has time trends. Interesting patterns, including a possible biannual signal at some ages in the lower elevation cores, also occur. Based on previous work relating melt occurrence to summertime warmth (Zwally and Fiegles 1994), we expect that the melt frequency can be used as a paleothermometer for summertime conditions.
This research was supported by National Science Foundation grants OPP 95-26374 and OPP 94-17848. We thank Todd Dupont and Byron Parizek for "field" work, at the National Ice Core Laboratory, and the WAISCORES Science Coordination Office, the National Ice Core Laboratory, the Polar Ice Coring Office, and WAISCORES colleagues.
Alley, R.B., and S. Anandakrishnan. 1995. Variations in melt-layer frequency in the GISP2 ice core: Implications for Holocene summer temperatures in central Greenland. Annals of Glaciology , 21, 64-70.
Alley, R.B., and C.R. Bentley. 1988. Ice-core analysis on the Siple Coast of West Antarctica. Annals of Glaciology , 11, 1-7.
Zwally, H.J., and S. Fiegles. 1994. Extent and duration of antarctic surface melting. Journal of Glaciology , 40(136), 463-476.