Vostok tephra-An important englacial stratigraphic marker?

Mosley-Thompson, E., P.D. Kruss, and T. Bain. 1983. South Pole pit the South Pole. An example of an unusual meteorological event stratigraphic studies. Antarctic Journal of the U.S. 18(5), 116-118. recorded by the oxygen isotope ratios in the firn. Earth and Planetary Mosley-Thompson, E. and L.G. Thompson. 1983. South Pole ice core Science Letters, 1, 202-204. processing and microparticle analysis. Antarctic Journal of the U.S. Stauffer, B., and J. Schwander. 1983. Core processing and analyses of ice cores drilled at South Pole. Antarctic Journal of the U.S. 18(5), 18(5), 118-119. Picciotto, E., S. Deutsch, and L. Aldaz. 1966. The summer 1957-1958 at 114-116.

Vostok tephra-An important englacial stratigraphic marker? P. R. KYLE

Department of Geoscience New Mexico Institute of Mining and Technology Socorro, New Mexico 87801 J. PALAIS

Institute of Polar Studies Ohio State University Columbus, Ohio 43210

E. THOMAS Department of Geology Arizona State University Tempe, Arizona 85281

Tephra (volanic ash) layers, if they are widespread, have the potential to provide important stratigraphic markers in ice cores. If the source of the tephra can be identified and an age assigned to the eruption, then the tephra layer can also provide a valuable time plane (Kyle, Palais, and Delmas 1982). Hammer (1980) and Hammer, Clausen, and Dansgaard (1981) have demonstrated the value of tephra in ice cores from Greenland. Using surface conductivity measurements they located volcanicderived acid layers, which were in some cases correlated with known eruptions. Such work has not been conducted on antarctic ice cores and most tephra layers are located either by (1) visual inspection (Gow and Williamson 1971), (2) detailed chemical analyses of the ice (Delmas and Boutron 1980), or (3) continuous microparticle measurements (Mosley-Thompson 1980). We have been examining visible volcanic layers from several antarctic ice cores. The objectives being to determine the source of the eruptions, evaluate possible climatic impact of the eruptions and to establish the volcanic record for the Southern Hemisphere. A 0.05-meter thick tephra layer was discovered in the bottom of a 101-meter long ice core drilled at Vostok Station in December 1979 (Parker, Zeller, and Gow 1982). An age of 3,300 years was assigned to the tephra based on snow accumulation rates and the ice stratigraphy. The layer was informally called the Vostok tephra by Kyle et al. (1982). We have examined the Vostok tephra in more detail and have identified the source of the eruption. The ice associated with the tephra layer has concentrations of sulphate which exceed 550 milligrams per liter. This is an exceedingly high value and suggests the eruption 64

responsible for the tephra was large and had a significant aerosol (H 2 SO 4 ) component. The tephra is composed of glass shards and rounded cryptocrystalline lithic material. Petrographic and scanning electron microscope measurements show the grains to range up to about 40 micrometers in length. The mean grain size is between 15 and 30 micrometers. Semi-quantitative and quantitative analyses of the glass shards have been made using an energy-dispersive analyzer on a scanning electron microscope and by electron microprobe. Analyses are listed in the table. The Vostok tephra is andesitic and characterized by high iron concentrations. Enrichment of iron is a characteristic feature of tholeiitic suites. Taking the prevailing wind directions into account, three major sources for the Vostok tephra can be considered: the South Sandwich Islands, the South Shetland Islands, and the Southern Andes. Fortunately, the three provinces are easily distinguished, and we suggest the South Sandwich Islands is the source area. The Southern Andes volcanoes are calc-alkaline andesites and although only a few analyses are available they have lower iron contents than the Vostok tephra (Katsui 1982). The South Shetland Islands have a much higher total alkali content than the South Sandwich (Baker 1968; GonzalezFerran 1982). Based on the available analyses of the South SandAnalyses of the Vostok tephra Compound Electron microprobe Energy dispersive Wet chemical analysisa analysis' analysisc d Si02 60.56 (0 . 42)d 59.73 (1.79) 60.90 Ti02 0.74 (0.07) 0.90 (0.16) 0.95 Al203 14.92 (0.82) 14.84 (0.66) 14.80 FeOe 9.15 (0.57) 10.25 (1.23) 8.34 MnO 0.23 (0.04) 0.15 (0.12) 0.11 MgO 2.39 (0.34) 2.44 (0.48) 2.35 CaO 6.72 (0.26) 6.64 (0.76) 6.09 Na20 3.31 (0.38) 4.46 (0.57) 3.67 K20 0.44 (0.05) 0.63 (0.19) 0.39 -f - f Total 98.46 100.04 97.72 Number of 9 analyses

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Los Alamos National Laboratory (Analyst: J. Palais). Using a scanning microscope, Arizona State University (Analyst: E. Thomas). Aphyric andesite, Cauldron Lake lava flow, northern Candelmas Island (Tomblin 1979). d One standard deviation. Total iron as ferrous oxide. Not analyzed. ANTARCTIC JOURNAL

wich Islands (Baker 1978), we find that an aphyric andesite from northern Candlemas Island is almost identical to the Vostok tephra (table). Lavas from Candlemas Island are exceptionally low in potash (K20) (Tomblin 1979), a characteristic feature of the Vostok tephra. All available data at this time strongly suggest that the South Sandwich Islands-and in particular Candlemas Island-is the source of the Vostok tephra. Vostok Station is over 4,000 kilometers in a direct line from Candlemas Island, and yet the Vostok tephra is clearly visible and 0.05 meter thick. It is highly prob able that the Vostok tephra is widespread over the east antarctic ice sheet, and has the potential to act as an important stratigraphic marker horizon. We wish to thank Crank Heikan and the Los Alamos National Laboratory for their assistance. This work is supported by National Science Foundation grant DPP 80-21402. References Baker, P.E. 1968. Comparative volcanology and petrology of the Atlantic island-arcs. Bulletin Volcanologique, 32, 189-206. Baker, P.E. 1978. The South Sandwich Islands: III Petrology of the volcanic rocks. (British Antarctic Survey Scientific Reports No. 93.) Cambridge, England: British Antarctic Survey. Delmas, R., and C. Boutron. 1980. Are the past variations of the stratospheric sulfate burden recorded in central Antarctic snow and ice

Thermomechanical behavior of large ice masses D. A. YUEN and M. R. SAARI Department of Geology Arizona State University Tempe, Arizona 85287 C. SCHUBERT

Department of Earth and Space Sciences University of California Los Angeles, California 90024

The simulation of the thermomechanical structure of large ice sheets is complicated by the coupling between the temperature and the deformation due to the strong temperature-dependence of ice rheology. We have proposed (Yuen and Schubert 1979; Schubert and Yuen 1982) that instabilities may arise from viscous dissipation, leading to basal melting and foundering of the antarctic ice sheets. We have continued our work on this problem by developing: (1) a faster way of obtaining one-dimensional steady-state velocity and temperature profiles of large ice masses, (2) a numerical code that can be used to

1984 REVIEW

layers? Journal of Geophysical Research, 85, 5645-5649. Gonzalez-Ferran, 0. 1982. The Antarctic Cenozoic volcanic provinces and their implications in plate tectonic processes. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Cow, A.J., and T. Williamson. 1971. Volcanic ash in the Antarctic ice sheet and its possible climatic implications. Earth and Planetary Science Letters, 13, 210-213. Hammer, C.U. 1980. Acidity of polar ice cores in relation to absolute dating, past volcanism, and radio-echoes. Journal of Glaciology, 25, 359-372. Hammer, C. U., H. B. Clausen, and W. Dansgaard. 1981. Past volcanism and climate revealed by Greenland ice cores. Journal of Volcanology and Geothermal Research, 11, 3-10. Katsui, Y. 1982. Late Cenozoic petrographic provinces of the volcanic rocks from the Andes to Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Kyle, P.R., J.M. Palais, and R. Delmas. 1982. The volcanic record of Antarctic ice cores: Preliminary results and potential for future investigations. Annals of Glaciology, 3, 172-177. Mosley-Thompson, E. 1980. 911 years of microparticle deposition at the South Pole: A climatic interpretation. (Institute of Polar Studies, Report 73.) Columbus: Ohio State University Press. Parker, B.C., E.J. Zeller, and A.J. Cow. 1982. Nitrate fluctuations in Antarctic snow and firn: Potential sources and mechanisms of formation. Annals of Glaciology, 3, 243-248. Tomblin, J.F. 1979. The South Sandwich Islands: II The geology of Candlemas Island. (British Antarctic Survey Scientific Reports No. 92.) British Antarctic Survey, Cambridge, England.

monitor the time history of shear heating instabilities of ice flows, and (3) an essentially analytical model to account for the role played by the thinning of the ice sheet in glacial surges. In the course of constructing steady-state profiles, we have found a new class of solutions associated with large accumulation rates. The character of the new solutions arises physically from a thermal blanketing effect produced by advection of cold material toward the base of the ice sheet. For representative values of ice rheological parameters and typical surface conditions, the new solutions are obtained whenever the accumulation rate exceeds a few tenths of a meter per year. This solution branch is characterized by its relative stability to finite-amplitude perturbations. For accumulation rates smaller than a few tenths of a meter per year, the solutions are unstable to finite-amplitude perturbations. Because accumulation rates are on the order of 0.1 meter per year, it is important to delineate better the boundary separating these two families of solutions. This research was supported by National Science Foundation grant DPP 82-15015. References Schubert, C., and D.A. Yuen. 1982. Initiation of ice ages by creep instability and surging of the East Antarctic ice sheet. Nature, 296, 127-130. Yuen, D.A., and C. Schubert. 1979. The role of shear heating in the dynamics of large ice masses. Journal of Glaciology, 24, 195-212.

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