Under favourable conditions, more robust than previously assumed
Accordingly, the findings that the team has now published in the magazine Quaternary Research are of major importance. They show that, at consistently low ground temperatures, extremely old and deeply buried permafrost can survive naturally occurring warmer periods, whereas the permafrost elsewhere undergoes massive thawing, especially near the surface. At the same time, however, the situation near Batagay demonstrates how sensitive permafrost is to anthropogenic disturbances. The 650,000-year-old permafrost is located on a hillslope at a depth of ca. 50 metres, where the temperature remains stable at minus 10 degrees Celsius. But, from the 1940s to the 1960s, the slope was partly cleared, and used by heavy tracked vehicles as a road to reach a nearby mine. In the process, it lost its protective and insulating plant cover. As a result, the younger permafrost began thawing near the surface in summer, until the ground finally began sliding away, uncovering the older permafrost. For years, meltwater has transported the thawed material downhill, forming a large crater that is now up to 50 metres deep. In addition, the cliff continues to erode at a rate of up to 30 metres per year.
Combining a range of analytical methods
The team, which included German, Russian and English researchers and was led by Prof Julian Murton from the University of Sussex, explored the permafrost from the upper end of the cliff to its base, using a range of methods to precisely determine the age of the permafrost at various depths. For example, luminescence dating was used to determine when the quartz and feldspar grains found in sand at various depths were covered by subsequent layers, making it the last time they were exposed to sunlight. In contrast, experts from the Helmholtz-Zentrum Dresden-Rossendorf measured the concentration of radioactive and stable chlorine in the ice samples using highly sensitive accelerator mass spectrometry. This approach makes it possible to directly measure the age of the ice, which forms long wedges in the permafrost in the course of millennia.
In addition, isotopes of certain chemical elements were measured. Isotopes are atoms that contain different numbers of neutrons and therefore have different weights. There are, for example, three naturally occurring isotopes of oxygen – 16O, 17O and 18O – two of which Thomas Opel and his team used for their analyses. 18O has two more neutrons than 16O, making it heavier. In the course of interglacial and glacial periods, the relative concentrations of 16O and 18O isotopes in the ice of glaciers and permafrost change, making it possible to deduce the prevailing climatic conditions and, indirectly, the ages of the respective permafrost layers and the ice they contain.