Climate and Geomorphology of a Periglacial Area

(a)  Study the climate graphs, Figures 5 and 6, for two weather stations which are located on Figure 7. Compare the climates of the two weather stations, with special references to seasonal differences. Suggest reasons for the differences you have observed. [9]

The data shows the mean monthly temperature and precipitation, and the number of days per month above and below 0°C. Figure 5 shows the climate graph for Green Harbour in Spitsbergen. Figure 6 shows the data for Yakutsk, Central Siberia.

The precipitation values for the two areas are relatively comparable. Yakutsk has an annual precipitation input of 247 mm, in comparison to the 298 mm of Spitsbergen. This is due to the similar results of the weather pressure systems which act on these areas. Spitsbergen has a slightly higher input due to the Polar Maritime predominance, which contains more water in comparison to the Polar Continental systems which dominate Central Siberia.

The mean monthly temperature of Spitsbergen ranges from a minimum of -20°C to a summer maximum of approximately 7°C, a variation of 27°C, as shown by Fig. 5 (a). The minimum occurs in late February/early May, and the maximum temperature occurs mid-August. In comparison, Figure 6 (a) shows us that the mean monthly temperature of Yakutsk ranges from -45°C to approximately 20°C, an annual range of 65°C. The minimum temperature occurs December/January, while the maximum occurs in July. The moderating effect of the North Atlantic Drift, the warm ocean current shown on Figure 7, is clearly illustrated from these temperature graphs. In addition, Central Siberia suffers the effect of “continentality”, the difference in specific heat capacity of earth and water; which means that inland areas warm and cool slower that the coastal regions, like Spitsbergen.

The remaining data shows the number of days per month with a temperature of above and below 0°C. The chart shows the days over zero, fluctuating around zero and below zero. In Yakutsk, the number of days with temperatures permanently above freezing is shown in Figure 6 (b) as 126. The period of such temperatures begins in late May, and ends in early September, with a strong peak mid June, with the values suggesting that the majority of June is spent above zero degrees.  The number of days with temperatures permanently below zero is shown as 197. This period begins late August, and ends mid-May. Overlapping these two periods is the time when the temperature fluctuates around the freezing point diurnally, that is to say, on a day to day basis. The distribution of such days is bi-modal, with the peaks occurring  mid April and mid September. This could be associated with the “turning” of the seasons. The central minimum of the distribution occurs mid June, and coincides with the peak of the number of days >30°C. This is perhaps due to the continentality mentioned earlier, and the typical high pressure weather pattern of a Siberian summer.

In comparison, Figure 5 (b) shows the number of days per month below 0°C in Spitsbergen to be considerably greater. The value of 260 indicates that the moderating effect of the Gulfstream does not compensate for the proximity to the Arctic Ocean, and over 15° of increased latitude compared to Yakutsk. Despite the coastal location, and the warming of the sea, the temperature is below zero for longer, with zero degree temperatures found throughout the entire year. The effect of the moderation results in a milder winter, of -20 compared to -45, but the winter is of a longer duration.  Consequently, the “summer” climate is far shorter. The graph shows that there are just 35 days with temperatures permanently over 0°C, in comparison to the 126 of Yakutsk. The “summer” climate begins in late May, and ends abruptly in late September. However, there are more days where the temperature fluctuates around the freezing point. The 70 days where this occurs results in every month except January having some fluctuation about 0°C. This is again due to the comparison of specific heat capacities. The coastal regions heat and cools quicker, giving greater diurnal range, but the land masses respond to thermal change much slower, but also retain the heat for longer. The permafrost will therefore take longer to thaw, but it will thaw to a greater depth in Siberia than in Spitsbergen. The diurnal variations in Spitsbergen will result in a far greater frost shattering net product, since the amount of days where the temperature fluctuates around the freezing point is far higher.

(b)  Explain the position and formation of four of the periglacial landforms identified in Figure 8. [12]

The first periglacial landform identified in Figure 8 is the frost shattered slope with scree. The slope exists because of geomorphological process. It may be the valley sides, or a particularly large outcrop of base rock. Assuming it is a steep slope, there will be very little substantial vegetation cover. This will remove any insulation and moisture removing abilities. The water passing the slope may be stored in cracks, caused by chemical or exfoliation weathering. This water freezes during the diurnal temperature variation, and expands by 10%. This widens the crack. The daily repetition of this cycle, with fresh input each day results in a frost shattered rock face. When the crack widens, and the rock can no longer support its own weight, it falls under gravity to the base of the slope, where it forms scree, or talus. Since the entire rock face is being acted upon in this way, then a sizeable amount of jagged, angular material of assorted size forms a scree deposit, resulting the landform shown.

The block field is located near to the frost shattered slope. It forms when frost shattered material from a nunatak or peak under periglacial influence deposits material in an unsorted fashion, by glacial process. The material is usually of large size, ranging from 0.5 to 10m in diameter. It is unsorted, and usually randomly arranged. The size and angularity of the material, combined with the unnatural position make fluvial deposition very unlikely. Thus, a block field is formed.

The stone polygons, garlands and stripes form when ice wedges create patterns in the ground. Ice wedges work on a very similar principle to frost shattering. The main difference is that the ground does not crack, so the sediment around the wedge is pushed up, forming a small cone. The stone polygons form as stones roll away from the ice wedge centre, to the periphery of the wedge cone, and to the periphery of each wedge cone. Stripes and garlands occur when the wedges form close to an incline, in which case the stones roll away down the incline, and stretch out in to a garland. Stripes occur when the stone polygons are subjected to consequent movement through solifluction, which elongates them in one axis, usually parallel to the downslope direction.

Solifluction lobes are usually found downslope, with the centre in an axis such that it is parallel to the downslope direction. When the upper layer, the active layer, of permafrost thaws in the summer, the resulting mass is a viscous semi-liquid semi-solid unsorted ooze. This can move downhill under the influence of gravity, since it has a low friction coefficient and a potential energy gradient, enabling it to move on slopes as shallow as 2°. Thus, it moves at a relatively slow rate in a process known as creep, downhill. Any soil or vegetation which has developed above the active layer will be taken along on the surface, and it will be transported downslope. When the ooze ceases to move, either due to it running out of potential energy, or the refreezing of the active layer, then a solifluction lobe is formed. This is very similar in appearance to a small cliff, up to 5m high. It extends up valley, and may present a “tongue” layout in plan view.

(c)  Study Figure 9. Suggest reasons for both the increased depth of the active layer and the ground level subsidence after the clearance of the vegetation. [4]

As the vegetation is cleared, the shade it provided from the insolation is removed. Thus the sunlight can act on the active layer for a greater period of time. This melts more of the active layer, and the resulting warmth reaches lower in to the soil. Since the active layer was previously super saturated, on draining it will lose 10% of its volume through reduction to water, and a further 25% as the excess drains away. This explains the 5 m ground level subsidence apparent from the diagram. The increase in active layer is due to the increased solar penetration which thaws more upper permafrost, creating a larger active layer, by approximately 25 m.