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

Calcitic speleothems in caves can form through abiogenic or biogenic processes, or through a combination of both. Many issues conspire to make the assessment of biogenicity difficult, especially when focusing on old speleothem deposits. This study reports on a multiproxy analysis of a Siberian stalactite, combining high-resolution microscopy, isotope geochemistry and microbially enhanced mineral precipitation laboratory experiments. The contact between growth layers in a stalactite exhibits a biogenic isotopic signature; coupled with morphological evidence, this supports a microbial origin of calcite crystals. SIMS δ¹³C data suggest that microbially mediated speleothem formation occurred repeatedly at short intervals before abiotic precipitation took over. The studied stalactite also contains iron and manganese oxides that have been mediated by microbial activity through extracellular polymeric substance (EPS)-influenced organomineralization processes. The latter reflect paleoenvironmental changes that occurred more than 500 000 yr ago, possibly related to the presence of a peat bog above the cave at that time. Microbial activity can initiate calcite deposition in the aphotic zone of caves before inorganic precipitation of speleothem carbonates. This study highlights the importance of microbially induced fractionation that can result in large negative δ¹³C excursions. The microscale biogeochemical processes imply that microbial activity has only negligible effects on the bulk δ¹³C signature in speleothems, which is more strongly affected by CO₂ degassing and the host rock signature

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Abstract. Calcitic speleothems in caves can form through abiogenic, biogenic, or a combination of both processes. Many issues conspire to make the assessment of biogenicity difficult, especially when focusing on old speleothem deposits. This study reports a multiproxy analysis of a Siberian stalactite, combining high-resolution microscopy, isotope geochemistry and microbially enhanced mineral precipitation laboratory experiments. The contact between growth layers in a stalactite exhibits a biogenic isotopic signature; coupled with morphological evidence this supports a microbial origin of calcite crystals. SIMS δ13C data suggest that microbially mediated speleothem formation occurred repeatedly for short intervals before abiotic precipitation took over. The studied stalactite also contains iron and manganese oxides that have been mediated by microbial activity through extracellular polymeric substances (EPS)-influenced organomineralization processes. The latter reflect palaeoenvironmental changes that occurred more than 500 000 yr ago, possibly related to the presence of a peat bog above the cave at that time. Microbial activity can initiate calcite deposition in the aphotic zone of caves before inorganic precipitation of speleothem carbonates. This study highlights the importance of microbially induced fractionation that can result in large negative δ13C excursions. The micro-scale biogeochemical processes imply that microbial activity has only negligible effects on the bulk δ13C signature in speleothems, which is more strongly affected by CO2 degassing and the hostrock signature.