PLEISTOCENE PERMAFROST BOUNDARY SHIFTS AT THE EUROPE-ASIA BORDER

Permafrost is a very important player in the global climate system. Large amounts of organic carbon are stored in frozen grounds. When permafrost thaws in response to climate warming this carbon is released, in form of carbon dioxide and methane, into the atmosphere. Due to their greenhouse effect, these gases further enhance warming. Understanding this positive feedback between permafrost thawing and climate warming requires a sound understanding of how the spatial extent of permafrost has changed in response to changing climatic conditions in the past. The latter task is challenging. Delineation of the past extent of permafrost relies on field observations and dating of permafrost markers. Most of the available information dates back only to the Last Glacial Maximum (about 25,000 years ago) and knowledge about earlier times remains highly fragmentary. Variations in former permafrost extent related to shorter-scale and smaller-amplitude climate changes are even less well known. In this project we will study two types of cave deposits (speleothems) as a novel archive of permafrost. Common speleothems, such as stalagmites, form from water dripping into the cave; the very fact of the former presence of liquid water documents permafrost-free conditions. A specific speleothem type known as cryogenic cave carbonate forms upon freezing of water in caves; this speleothem indicates permafrost conditions. The major advantage of this novel archive is that speleothems can be precisely and accurately dated by the uranium-thorium method. Speleothems will be collected from 16 caves located in the Ural Mountains. Separating Europa and Asia, the Ural stretches for more than 1000 km and is ideal for tracing the north- south shifts of southern permafrost boundary in response to climate change. The collected samples of common and cryogenic speleothems will be dated using the U-Th method. In addition, the speleothems will be studied by means of stable isotope analyses; this will allow reconstruction of the paleoclimate during the permafrost-free times. We expect to obtain an unprecedented record of expansions and contractions of permafrost in northern Eurasia over the last 130,000 years.

Presently, permafrost occupies about 15 million sq. km of Earth land surface. It was distributed much wider (up to 26 million sq. km) during glacial periods. Permafrost ties up large amounts of greenhouse gases and thus exerts a strong control on the Earth's climate. To understand this control, and use this understanding for predicting future climate changes, it is important to know the precise areal distribution of permafrost during past climate cycles. In this project we used a novel natural marker of permafrost, cryogenic cave carbonates (CCC). These minerals precipitate when water freezes in caves. For this to happen, the cave must be located in permafrost. The time of CCC formation, and thus, the age of permafrost, can be precisely determined by means of U-Th dating. We explored 54 caves across Northern Eurasia and have found CCCs in 19 of them. With U-Th dating we determined that the boundary of permafrost in Northern Eurasia advanced southward for more than 1000 km during cold glacial climate stages and retreated during warm interglacial stages. The focus of this project, the Ural Mountains, is a 1000 km-long range that separates Europe and Asia. During the last glacial period, permafrost developed in Northern and Central Ural about 120,000 years ago, whereas in Southern Ural it developed later, around 76,000 years ago. Permafrost was persistent during relatively cold periods, between about 75,000-59,000 and 33,000-14,000 years ago. Outside these periods of stability, permafrost experienced episodes of thawing and freezing (retreats and advancements). The final demise of permafrost in the Urals occurred around 11,000 years ago. Importantly, we have found that the spatial configuration of permafrost changed in response to not only major climate swings (such as transitions from glacial to interglacial climate states) but also to much shorter and abrupt climate events (e.g., rapid, decade-long Dansgaard-Oeschger warming episodes). Another important finding relates to palaeoanthropology. Two caves in Southern Ural, Shulgan-Tash and Ignatievskaya, host rich Palaeolithic cave art on their walls. Caves were used by Palaeolithic man as sanctuaries at the end of and shortly after the Last Glacial Maximum. We have found that during times of artistic activity both caves were in conditions of permafrost and featured sub-zero temperatures. This created inhospitable conditions for visiting the caves, let alone creating parietal art in them. These findings prompt a re-evaluation by archaeologists and palaeoanthropologists of cultural practices of Palaeolithic man in this part of Eurasia.