Soils in permafrost regions contain twice as much carbon as the atmosphere, and permafrost has an important influence on the natural and built environment at high northern latitudes. The response of permafrost to warming climate is uncertain and occurs on time scales longer than those assessed by direct observation. We dated periods of speleothem growth in a north-south transect of caves in Siberia to reconstruct the history of permafrost in past climate states. Speleothem growth is restricted to full interglacial conditions in all studied caves. In the northernmost cave (at 60°N), no growth has occurred since Marine Isotopic Stage (MIS) 11. Growth at that time indicates that global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost.

Climate change in the Arctic is occurring rapidly, and projections suggest the complete loss of summer sea-ice by the middle of this century 1 . The sensitivity of permanently frozen ground (permafrost) in the Northern Hemisphere to warming is less clear, and long-term trends are harder to monitor than those of sea-ice. Here we use paleoclimate data to indicate that Siberian permafrost is robust to warming when Arctic sea-ice is present, but vulnerable when it is absent. U-Pb chronology of carbonate deposits (speleothems) in a Siberian cave located at the southern edge of continuous permafrost, reveal periods when the overlying ground was not permanently frozen. The speleothem record starts 1.5 million years ago (Ma), a time when greater equator-to-pole heat transport led to a warmer northern hemisphere 2 . Speleothems' growth demonstrate that permafrost at the cave site was absent at this time, becoming more common from ≈1.35 Ma as the Northern Hemisphere cooled, and permanent after ≈0.4 Ma. This history mirrors that of year-round sea-ice in the Arctic Ocean, which was largely absent prior to ≈0.4 Ma 3 , but continuous since that date. The robustness of permafrost when sea-ice is present, and increased permafrost vulnerability when seaice is absent can be explained by changes in both heat and moisture transport. Reduced sea-ice may contribute to warming of Arctic air 4-6 that can lead to warming far inland 7 . Open Arctic waters also increase the source of moisture and increase autumn snowfall over Siberia, insulating the ground from cold winter temperatures 8-10 . These processes explain the relationship between an ice-free Arctic and permafrost thawing prior to 0.4 Ma. If these processes continue during modern climate change, future loss of summertime Arctic sea-ice will enhance thawing of Siberian permafrost.Arctic Ocean sea-ice declined increasingly rapidly in recent decades, with progressive ice thinning and increasing areas of open water during the summer-time 11 . Complete loss of summer sea-ice is expected by

New data on the geochemistry and isotopic composition of chloride brines of the Siberian Platform are presented. The distribution of stable isotopes (2H, 18O, and 37Cl) in brines of the Tunguska, Angara-Lena, western part of the Yakutian and Olenek artesian basins and 87Sr/86Sr in brines of the western part of the Olenek artesian basin was studied in the context of the problem of genesis of highly mineralized groundwaters. Results of the study and comparative analysis of the geochemical and isotopic peculiarities of the Siberian Platform brines conform to the theory of brine formation through the interaction of connate waters with enclosing rocks.

The Late Cenozoic alluvial deposits in the Tunka depression of the Baikal Rift Zone, Russia, contain wedge-shaped structures and involutions that provide valuable palaeogeographic information about the southern part of eastern Siberia. The structure of the unconsolidated host deposits, the shape and size of the deformation structures (wedge shaped, wave like or drop like) and the physical-mechanical properties of the sediments (particle size, density, porosity, water content and colour) point to a cryogenic origin of the deformation structures, as ice wedge pseudomorphs and cryoturbations, rather than an origin associated with seismic activity or sedimentation. The oldest pseudomorphs and cryoturbations developed during climate-warming periods of the subboreal climatic phase (3600-2600 years ago).

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