A naturally-occurring ‘cold earth’ spot in Northern China

Permafrost is determined to a large extent by the Earth’s surface temperature, therefore it distributes mainly in high altitude and latitude regions. However, stable, warm (about −1 °C) permafrost occurs within a scree slope in northern China that is more than 600 km south of the southernmost limit of latitudinal permafrost on the Eurasian Continent. It is at an elevation of only 900 m above sea level (ASL). The area has a mean annual air temperature (MAAT) of 6 to 8 °C. Thermal processes of the scree slope, investigated through field monitoring and numerical simulation, showed that the permafrost is caused by winter air convection within the porous rock deposits and is stable as air convection does not occur in summer time. The deposit is covered by a 30-cm-thick peaty soil layer dated (carbon C-14) to between 1,000 to 1,600 years ago. The layer also contributes to the permafrost’s existence due to the peat’s thermal conductivity offset when frozen and thawed. The existence of permafrost under such warm climatic conditions confirms the effectiveness of using crushed rock layer as basement or slope cover to protect the warm permafrost subgrade of the recently-constructed Qinghai-Tibet Railway (QTR), even under the predicted climate warming conditions.


Results
The site was investigated by borehole drilling and ground penetrating radar (GPR) in 2011 (Fig. 2). Five boreholes (No. [1][2][3][4][5] were drilled within the slope and used to construct the site stratigraphy. Thermistors, installed within the boreholes, monitored ground temperatures between the ground surface and a depth of 11 m. The GPR soundings (GPR #1 to GPR #5), shown as Fig. 2a-e, show a layer of peaty soil (Fig. 2f), usually around 30 cm thick which overlies a coarse block layer ( Fig. 2g) with varying thickness. This is well illustrated in boreholes from No. 1 to No. 5 and the GPR soundings, indicating that the blocky layer has a maximum thickness up to 11 m. The blocky layer is underlain by basalt bedrock. Figure 3 shows temporal and spatial distributions of the ground temperatures measured in the five boreholes. The ground temperatures between 1.5 m to 4 m in borehole No. 1 were − 4.5 °C to 0 °C during the monitoring period (Fig. 3a), which was more than 3 years. While in borehole No. 4, temperatures ranged from − 8.1 °C to 0 °C at depths between 2.5 m to 5.0 m (Fig. 3d). Figure 4 shows the averaged ground temperature profiles in the five boreholes within the period from 2011 to 2014. The lowest averaged ground temperature at a depth of 0.8 m and 1.5 m in borehole No. 1 and No. 4 was − 1.7 °C and − 3.4 °C, respectively. While the averaged ground temperatures at every monitored depths in the other borehole were not lower than 5 °C. Therefore, permafrost exists in the slope in an area where the MAAT is 6 to 8 °C, detected from the borehole No. 1 and No. 4. There were no frozen layers in summer in boreholes No. 2, No. 3 and No. 5 (Fig. 3b,c,e). Accordingly, the ground temperatures monitored in the five boreholes show differences that reflect the varied thicknesses of the coarse blocky layer. For example, in borehole No. 1, the coarse blocky layer is 8.4 m in thickness and the permafrost thickness of 2.5 m was measured. In comparison, at borehole No. 4, the 11 m thick coarse blocky layer has permafrost thicker than 2.5 m.
It can be also seen from Fig. 3 that temperature within the coarse blocky layer is very sensitive to external (surface) temperatures during the freezing period. When air temperature drops sharply in mid-November, the cold, dense air penetrates rapidly into the voids between the blocks, resulting in a near-heterogeneous temperature distribution in the blocky layer from top to bottom due to air convection within the porous material. During the early thaw period, the isotherms are laterally parallel indicating that the air in the voids is stable and there is no convective heat transfer between the warmer (lighter) air above and the colder (denser) air below. Such a thermal regime indicates the heat exchange between the coarse blocky layer and the ambient environment is relatively instantaneous and intense in winter, while prolonged and feeble in summer. In summary, the permafrost results from the seasonal thermal diode. The major differences between boreholes No. 1 and No. 4, and boreholes No. 2, No. 3 and No. 5 is the thickness of the coarse blocky layer, and fine particles filling in the pores, thus resulting in the observed ground temperature deference. According to drilling and GPR investigations, the Maps were generated using GIS software (ArcGIS 8.0, http://resources.arcgis.com/), and the permafrost distribution data was from the International Permafrost Association 27 . Many 'cold earth' , i.e. low ground temperature anomaly (LGTA, marked as L-number for short) have been reported to occur on scree or talus slopes spreading away from the present southern or lower limit of permafrost ( Table 1). The studied robust permafrost site (marked as L-15) in North China is the southernmost in the Northern Hemisphere, except the one marked L-13, which is on the Qinghai-Tibet Plateau with much higher elevation (4,700 m ASL.). thickness in boreholes No. 1 and No. 4 is over 8.0 m, while it is less than 4.0 m in the rest boreholes. In areas where the coarse blocky layer is either thinner than 1.5 m or absent, the ground thermal regime is characteristic of seasonally-frozen ground.

Discussion
The dominant controlling factor over ground freezing at the site is the HRLC effect induced by the closed coarse block layer on the slope. Because cold air is denser than warm air, it tends to displace warmer air in the pore spaces between the blocks and causes air convection in winter time; while in summer time, no convective heat transfer takes place and heat exchange is mainly in the form of conduction. Essentially, the imbalance of heat losses in winter and heat absorbs in summer produces a net heat loss around a year and thus lowers the ground temperature.
The ground thermal data at the site suggests that the near-surface peaty soil layer serves as a kind of thermal diode. Because site precipitation is concentrated in the summer months, this results in a high moisture content of the surface peaty soil prior to winter freezing. It is well known that the thermal conductivity of icy peaty soil increases by about 1-2 times over dry and unfrozen peat which, in the summer months, serves as an excellent thermal insulator 20,21 . Therefore, in a year, the peat layer has the thermal properties of a thermal diode or semi-conductor.
To further investigate the heat exchange process, the site was modeled with a numerical simulation using the field monitored boundary conditions and measured material parameters to define the processes. Figure 5 shows the heat exchange was greatest in the blocky layer in winter; as indicated by the circular pattern of isotherms and air flow circles on February 1 st (Fig. 5a,c) and the parallel isotherms on August 1 st (Fig. 5b). The heat flux in the main part of the blocky layer was around 6 W/m 2 in winter and 4 W/m 2 in summer. This difference was reflected on the slope by holes (Fig. 5a) in the snow cover in winter and the sparse vegetation in the low ground temperature region in summer (Fig. 5b).
The surface peaty soil was AMS-14 C dated at 1,040 years B. P. (top) and 1,590 years B. P. (base, at depth of 35 cm) by Bata Analytic Company in USA (sample numbers Beta − 352496, 352496, and error is + /− 30 a). Therefore, the blocky scree is around 1,600 years old at least. Combining the data from field monitoring and laboratory modeling, the cooling effect of the blocky layer might have persisted during the climate warming periods of the Holocene Megathermal, medieval and latest decades. Whether ground-ice was developed or not in the shallow part was related to the local climate, since the area was involved in the global semi-arid climate changing process in last 60 years 22,23 . The frozen ground also survived a forest fire in 2008. Taking into account the local MAAT of approximately 6.3 to 7.8 °C and the mean annual ground surface temperature (MAGST) of approximately 5.3 to 6.8 °C, the blocky permafrost layer will probably survive through a predicted climate warming of approximately 2.3 to 4.2 °C in the 21 st century.
Such naturally occurring 'cold spot' phenomena have practical implications for engineering design and construction in cold regions. In China, Cheng proposed a principle of 'proactive cooling' for designing and building roadbeds above ice-rich permafrost 24 . In engineering applications, crushed rock layers can effectively protect underlying warm and ice-rich permafrost. This is a technique that has been used for the Qinghai-Tibet Railway (QTR). Monitoring data for 10 years shows that such a roadbed structure is successful in cooling the foundation

Methods
Site description and monitoring measurements. Peat soil and rock samples for this study were collected at the described site (41°13′ 10″ N, 118°51′ 26″ E), located in Pingquan County, Hebei Province, China, with a distance of around 300 km from Beijing. The ground temperatures were measured with thermistors, with a precision of ± 0.02 °C, manufactured and assembled by the State Key Laboratory of Frozen Soil Engineering. The air temperature data of the Pingquan County was collected from the Pingquan County Weather Station, and  Lower than in nearby till and bedrock. *** Lower than in nearby mineral soils.
the air temperature on the studied slope was collected by a simple weather station installed in 2011. The ground temperature data were collected automatically by a CR3000 data logger four times per day.
Numerical simulation. An air convective heat transfer model with non-Darcy flow for porous media material was used to simulate the cooling process and effect of the natural rock layer located at the site. Temperature and velocity fields were obtained based on the in-situ thermal boundaries and geo-material characteristics. The temperature distributions were in accordance with the observed results, suggesting that the simulated results were reasonable and valuable.