1,350,000 Year History of Siberian Permafrost Based on U-Pb Chronology of Speleothems
Abstract:Permafrost history of the last ~1.35 millions of years (Ma) was reconstructed using chronology of speleothems from a Siberian cave. Rain and snowmelt waters can penetrate into caves only when soil and subsoil temperatures are above 0°C and permafrost above the cave is discontinuous or absent. Speleothems in regions currently affected by permafrost therefore provide a tracer of past permafrost thawing events.
Ledyanaya Lenskaya Cave is located at 60°22’N-116°57’E, on the southern boundary of modern continuous permafrost, with no present-day water seepage and a mean annual temperature of -5°C ‒ -6°C. U-Th dating of speleothems from this cave in a previous study  showed that the youngest speleothem growth period occurred at 427±23 thousand years ago (ka), during early Marine Isotope Stage (MIS) 11. In this study we dated several horizons of older speleothems from this cave using the U-Pb method . Two high precision ages indicate growth at 1074.2 +6.6/-5.2 ka and 947.8 +3.3/-3.4 ka, while new preliminary data provide strong evidence of an older growth period around 1.35-1.30 Ma. Other preliminary data mostly overlap the high precision ages, but also hint at limited growth at ~860 ka, with one data point suggesting younger, but minor growth at ~560 ka. The timing of these permafrost thawing events apparently correlates with interglacial episodes of exceptionally high Pacific Warm Pool sea surface temperature (~30°C) . During these warm episodes the average global temperature was 1.1-1.5°C higher than pre-industrial temperatures. These findings put the threshold of thawing of continuous permafrost at its southern boundary at slightly more than 1.0°C above preindustrial level.
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