Major Holocene Climatic Cycle Recorded in Snow Stratigraphy of the East Antarctic Plateau

Satellite images show near-surface snows of the East Antarctic Plateau can be divided into four previously unrecognized stratigraphic units, each separated by unconformities. The sequence records an apparent cycle of hot to cold conditions followed by slow return to the modern warm climate. It began with megadune deposition during hot conditions with strong winds and heavy snowfalls from local sources in open marine waters, most likely during the Holocene hypsothermal climatic excursion. Megadunes are unique to this plateau as upwind-migrating, upper flow regime features, characterized by alternating glazed and unglazed snow pseudo-beds with 2 - 4 km wavelength but only 1 - 5 m amplitude. They form a zebra-striped pattern with exposure across ~ 106 km2 or ~ 15 - 20% of the plateau surface. As the hypsothermal excursion ended, wind velocity decreased, snow supplies declined, Froude numbers approached unity, and deposition shifted abruptly from upper flow regime megadunes into lower flow regime downwind-migrating, ephemeral, high topographic relief dunes, a type previously undescribed and like megadunes unique to the plateau. Soon these were replaced by ordinary, downwind migrating, parabolic dunes to complete a trio of dune types comprising the basal or unit #1 member of the stratigraphic sequence. All were abandoned with development of unit #2, characterized by pervasive snow-starvation and overlapping "tiger claw" wind lineations that sweep downslope for distances of 1000 to 1500 km. These two units record the first part of the cycle, a long-term monotonic decrease in snow availability, best explained by a steadily cooling climate that expanded ice cover across marine moisture sources. Ultimately much of the adjacent Southern Ocean froze to allow both units to develop a strongly lineated, wind-burned, recrystallized surface. When climates finally began to warm and marine ice cover began to recede, unit #3's snows began to expand with basal unconformity across most of that old surface. Just a few centuries ago, unit #4 began a new style of unconformable deposition as patches of very parallel, short wavelength, sastrugi-related, longitudinal dunes with distribution and wind patterns similar to those of megadune times. The change probably represents global warming adding to the final warming phase of the cycle. This unconformity of unit #4 is obvious in satellite images but presently unrecognized in critical megadune ice cores. It may account for the generally accepted but probably incorrect paradigm that megadunes are currently active, long-term features of a very stable, later Holocene environment. Deep radar profiles of the ice sheet record many other relic megadune layers as evidence that this multi-millennial cycle has been repeated many times, probably playing a fundamental role in ice sheet development. At present, global warming and marine ice retreat are just about ready to start the next cycle. Upwind-migrating megadunes will soon return to the plateau, possibly within this century.