PERMAFROST AND PERIGLACIAL PROCESSES Permafrost Periglac. Process. 14: 1 – 9 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ppp.435 Permafrost Aggradation followed by Brutal Degradation During the Upper Pleniglacial in Mongolia: the Probable Response to the H2 Heinrich Event at 21 kyr BP Pascal Bertran, 1 * Michel Fontugne 2 and Jacques Jaubert 3 1 INRAP et Institut de Pr´ ehistoire et de G´ eologie du Quaternaire, Talence, France 2 LSCE - CEA / CNRS, Gif-sur-Yvette, France 3 Service R´ egional de l’Arch´ eologie, Toulouse, France ABSTRACT Upper pleniglacial loess and slope deposits in Europe frequently show an intensely cryoturbated gley horizon used as a stratigraphic benchmark level (the Nagelbeek Tongue Horizon). An equivalent horizon has been found in Mongolia, suggesting that it may characterize the whole Eurasiatic continent between latitudes 45 ° and 50 ° north. This horizon reflects a very cold episode marked by the aggradation of permafrost, followed by a sudden warming and a probable increase in snowfall, that gave rise to permafrost degradation. The latter episode is indicated by cryoturbation on gentle slopes and plug-like solifluction or active-layer sliding on steeper slopes. We correlate these processes to the Heinrich H2 cold event at ca. 21 kyr 14 C BP and the following interstade that has been recognized in high-resolution ice and marine records. Copyright  2003 John Wiley & Sons, Ltd. KEY WORDS: palaeoclimatology; Upper Pleistocene; periglacial palaeosols; Heinrich event; micromorphology; permafrost INTRODUCTION Continental deposits of the last pleniglacial in Europe frequently show a gley horizon usually associated with a net of ice-wedge casts and strongly cryoturbated or deformed into tongues on steeper slopes. It was identified first in loess sections (e.g. Gullentops, 1954; Lautridou, 1985) and called the ‘Kesselt soil’. It was then renamed the ‘Nagelbeek soil’ because stratigraphic uncertainties arose from the original section at Kesselt (Haesaerts et al., 1981). The Nagelbeek soil is interpreted as a tundra gley (gelic gleysol, Food and Agricultural Organization (FAO), 1974) that underwent cryoturbation (i.e. periglacial load-casting) and deformation during * Correspondence to: P. Bertran, Institut de Pr´ ehistoire et de G´ eologie du Quaternaire, UMR 5808 CNRS, Bˆ atiment de G´ eologie, Avenue des Facult´ es, 33405 Talence, France. E-mail: pascalbertran@wanadoo.fr permafrost degradation (Van Vliet-Lano¨ e, 1992; Huijzer, 1993). It corresponds to a stratigraphic benchmark that can be traced from France to Poland. In this paper, we describe new examples of this soil from Mongolia (Figure 1) and suggest these phenomena may characterize the whole Eurasiatic continent between latitudes 45 ° and 50 ° north. Recent work in active periglacial environments allows renewed interpretation of this ‘tongue horizon’ with respect to cryopedology, sedimentary dynamics and palaeoclimates. NEW STRATIGRAPHIC DATA: THE D ¨ OR ¨ OLJ AND MOJLTYN AM SECTIONS (MONGOLIA) The D¨ or¨ olj and Mojltyn Am sections, located in central Mongolia, have been discovered during archaeological surveys (J. Jaubert et al., unpublished Received 10 September 2002 Revised 15 November 2002 Copyright  2003 John Wiley & Sons, Ltd. Accepted 21 November 2002