Active Layer Thaw - Using Field Observations to Determine the Physical

Chapter 2 Using Field Observations to Determine the Physical Drivers Behind Peat

2.5 Discussion

2.5.1 Active Layer Thaw

The timing of snowfall in the autumn and early winter, as well as the snow depth over the freeze-‐back season are both important factors in the timing and penetration depth of winter freezing. The effect of snow cover on the ground

thermal regime has the potential to dictate the depth of winter freeze-‐back due to its insulating properties. Snow arriving early in the season will insulate the ground

sooner, preventing deep frost penetration and potentially encouraging the

development of taliks, ultimately reducing winter freeze-‐back. Because snow cover was mostly consistent across both research sites, the timing of the first snowfall does not appear to be an important aspect in the variability of winter freeze-‐back seen between degraded and non-‐degraded areas of the sites. Though taliks were observed during the thaw season, the large spatial variability in their existence is not likely to have been caused by the generally uniform snowcover seen during the 2012 and 2013 winter seasons. However, the depth of accumulated snow at the end of the winter season just prior to thaw has the potential to promote large variability in near surface soil moisture come spring, especially if surface microtopography is conducive to redirecting meltwater into depressions.

In order for permafrost aggradation to occur, a general long-‐term trend must occur whereby the depth of freeze-‐back during the winter exceeds the depth of thaw the following thaw period. If the seasonally frozen active layer has completely

thawed before winter freeze-‐back commences, any further energy inputs to the ground will go towards warming the subsurface further and potentially thawing permafrost (Williams, 2012). Given the large differences in the thaw rates between hummocks and depressions, it appears as though depressions are not regenerating permafrost, while hummocks appear to be in a steady state. The main difference between steady-‐state hummocks and degrading depressions has been identified as variability in soil moisture and depression storage, with depressions accumulating water following the spring freshet. If snowmelt water were only transferred vertically through the soil profile, one would expect the pre-‐freeze soil moisture to

equal the post-‐thaw soil moisture, plus inputs from snowmelt. Given this scenario and an evenly distributed snowcover, the change in volumetric soil moisture between pre-‐freeze and post-‐thaw should be equal between hummocks and depressions. However, depressions had a significantly greater increase in

volumetric soil moisture as compared to hummocks following the spring freshet.

Given these results, it appears as though significant lateral flow is occurring from hummocks to depressions during and following the spring freshet.

The observed differences in frost table depth between the degraded and non-‐

degraded portions of the sites, as well as within individual hummocks and depressions, appears to be driven by two main factors; soil moisture and solar radiation penetration into the ground, which becomes intense under an open forest canopy in the summer months. The relationship between soil moisture conditions of the peat and thermal conductivity has been well documented to strongly influence the timing and rate of ground freeze and thaw. A high thermal conductivity at or near the ground surface during the summer months leads to increased thaw, and similarly an elevated thermal conductivity during the winter leads to increased freezing. The thermal conductivity of a dry peat is about 0.06 W m^-‐1 K^-‐1, while a saturated peat has a thermal conductivity approximately 8.3 times greater at 0.50 W m^-‐1 K^-‐1. During the winter, the thermal conductivity of a frozen saturated peat is approximately 3.9 times greater than an unfrozen saturated peat due to the fact that the thermal conductivity of ice is 2.24 W m^-‐1 K^-‐1versus the 0.57 W m^-‐1 K^-‐1 thermal conductivity of water (Oke, 1978; Williams, 2012). As a result, a frozen saturated peat will experience maximum thermal conductivity during the spring, leading to

accelerated thaw following the freshet. A dry site will experience much less

variation in thermal conductivity since the conductivities of an unsaturated thawed soil are not significantly different from that of an unsaturated frozen soil (Williams, 2012). This will result in similar thaw and freeze-‐back depths, establishing a stable permafrost table year after year.

The relationship between ground soil moisture and thermal conductivity has the potential to cause significant ground thaw where saturation exists, such as where the peat plateau meets the bog at the Airport site, and in depressions where water has pooled at both the Airport and Pontoon Lake sites. However, significant ground thaw has also been observed in unsaturated areas, particularly at Pontoon Lake where the forest canopy is open, allowing for uninhibited receipt of solar radiation. In the mid-‐section of the site, the forest canopy remains open, partially due to pockets of peat saturation and surface ponding that repeatedly occur during thaw seasons after rain events, inhibiting vegetation growth and in some cases causing vegetation death. Significantly greater ground thaw was observed in the middle of the site as compared to the adjacent forested areas. In the absence of a tree canopy, the incoming radiation is uninhibited and acts to warm the ground surface. This heat is then efficiently transferred downward due to a high thermal conductivity attributed to elevated soil moisture. Because the specific heat of water is approximately four times that of air, as the ground surface warms and heat is conducted downward, the water within the pore space of the peat is able to retain and thus conduct vertically for much longer than if the pore space were filled with

air. Thus, conditions conducive to deeper active layer thaw are created when soil moisture is elevated and incoming radiation is uninhibited by a forest canopy.

The relationships between surface microtopography, soil moisture, canopy cover, and thaw depth outlined above are reflected in the data collected from each of the logging sites. Based upon temperature and VWC data obtained between 2012-‐

2014, it appears as though the surface microtopography, and thus the physical structure of the peatland coupled with a changing climate, are the initial drivers behind the evident and perhaps accelerating (given future predicted climate) degradation of underlying permafrost. The redirection of meltwater, during and following the spring freshet, is the most significant source of water to low-‐lying depressions throughout the entire year, and has the potential to create a positive feedback mechanism for further thaw. During the 2013 field season, precipitation inputs during the summer were insufficient in affecting the soil moisture content below approximately 15 cm depth. Following the freshet, the accumulated water in depressions warms as a result of increasing air temperatures and in some cases, intense solar radiation receipt at the ground surface. Although the air space in a dry site such as Log2 (Figure 2.2) may react faster to increasing temperatures, the lowered thermal conductivity and heat capacity results in shallower total active layer thaw. When freeze-‐back occurs, heat dissipation from the saturated peat will occur more slowly than a dry site. If snowfall arrives early in the season, it will insulate the peat and the saturated active layer may not freeze back fully, creating an area of unfrozen peat between the permafrost and the seasonally frozen upper

active layer, enabling microbial breakdown of DOM throughout the entire winter season.

In document Seasonal Evolution of Active Layer Formation in Subarctic Peat Plateaux and Implications for Dissolved Organic Matter Composition and Transfer (Page 71-76)