ABSTRACT: An assessment of ground ice volume near Eureka, Ellesmere Island, N.W.T.

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Abstract

Significant amounts of ground ice have been observed on Ellesmere Island in the Canadian Arctic Archipelago (Hodgson and Edlund 1977; Pollard 1991; Robinson 1994) and the objective of this study is to provide a first approximation of the total volume of such ice in the Eureka area of Ellesmere Island (Figure 1). Determining the location and volume of ground ice within permafrost is of value in reconstructing geomorphic history and in assessing terrain sensitivity to anthropogenic or natural thermal disturbance. Instability caused by disruptions to the ground thermal regime often result in terrain subsidence, or thermokarst. The extent of disturbance is dependent on the volume of ground ice, its vertical distribution, and the porosity of the enclosing sediments. Three types of ground ice are considered in our calculations, including: 1) pore ice and thin segregated ice lenses, 2) wedge ice, and 3) beds of massive ground ice. Ground ice volume is calculated by estimating the areal extent of ice-rich terrain in the study area and establishing percentages of ice content by volume with depth. We selected a study area large enough to encompass the representative terrain units for the region. Once we excluded zones which were assumed to have no ground ice or to contain amounts which are insignificant for the purposes of this study, the total area where ground ice might occur was 1456.8 km2. We considered the top 6.5 m of soil , but since we assumed an average active layer thickness of 0.6 m, the thickness of materials used in calculations as 5.9 m. The total volume of frozen materials in the study area is therefore 8.60 km3. The ice content for various terrain units was determined from published data and field measurements. Percentages of ground ice for the terrain units in the study area were calculated and the results are summarized in Table 1. The highest ice volume (70.5%) is, as expected, in areas underlain by beds of massive ground ice. Pore ice and thin segregated ice lenses make up 48.6% of the volume of frozen materials, except in areas of bedrock mantle, where they make up 1.1%. In areas of ice wedge polygons, wedge ice comprises 3.8% of the total volume. Overall, ground ice of all types comprises 47% of the upper 5.9 m of permafrost in the study region. Table 2 shows the contributions of the different ice types to the total ice volume and the amount of excess ice associated with each ice type in the study region. Pore and segregated ice make up the bulk of the ice in the Eureka area, comprising 94.7% of the total. Massive ice accounts for 4.6%, and wedge ice for less than 1%. When excess ice is present, soil instability is usually greater upon melting. In our study region, excess ice accounts for 0.6% of the total volume of frozen materials in areas of pore ice and thin segregated ice lenses. Where massive ice is present, 19.1% is excess ice. In areas of ice wedge polygons, excess ice is contributed not only by the ice wedges themselves, but also by pore ice and the thin segregated ice lenses in the sediment surrounding the wedges. In such areas, excess ice accounts for 2.6% of the permafrost volume. In conclusion, ground ice is shown to be a significant component of the surficial materials near Eureka, Northwest Territories. The total volume of ground ice in the study area is similar to that reported for other parts of the Arctic (Pollard and French 1980; Brown 1967; French et al. 1986). It is slightly less than the 53% from an earlier report for Ellesmere Island (Hodgson and Nixon 1994), but these researchers had intentionally focussed on areas suspected of being ice-rich. In areas of pore ice and thin segregated ice lenses, relatively low value of excess ice suggest that ground subsidence would not be dramatic should surface disturbance increase the active layer. However, the high volumes of excess ice in areas underlain by beds of massive ground ice can have a significant effect, as demonstrated by the intense thermokarst processes for such areas (Robinson, in press). Ice wedge ice comprises only a small percentage of the total ground ice, but because of its concentrated nature, the thaw of this type of ground ice becomes locally important in assessing terrain disturbance. Therefore, massive ice and, to a lesser extent, wedge ice are expected to have an important localized effect on terrain response to disturbance.

Bibliography of Canadian Geomorphology