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| Titel |
Influence of terrain and highway construction on thermokarst distribution, North Slave region, NWT, Canada |
| VerfasserIn |
Peter Morse, Stephen Wolfe, Taylor McWade |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250145887
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| Publikation (Nr.) |
 EGU/EGU2017-9864.pdf |
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| Zusammenfassung |
| Permafrost degradation has been observed throughout the north and is expected to have broad
reaching effects on the land and its people. Nevertheless, for much of Northern Canada little
quantified information about thermokarst exists. For example, in the southern North Slave
region, NWT, thermokarst distribution has never been assessed though permafrost is in
thermal disequilibrium and modelling suggests permafrost extent will decline. Additionally,
differential subsidence has been observed in the subarctic city of Yellowknife and along
highway infrastructure connecting it to the south. To better understand present and future
permafrost conditions, we mapped the location and size of thermokarst ponding (a change
from forest cover to water) in the study area by comparing historic and modern remotely
sensed data sets available from 1945, 1961, and 2005. These data were used to evaluate
the dominant terrain controls on the distribution of thermokarst in the region, and
the potential influence that highway construction may have had on thermokarst
development.
Historically, discontinuous permafrost developed in a time-transgressive manner during
the Holocene as lake-level receded from glacial Lake McConnell to present-day Great Slave
Lake (5 mm⋅a−1 over the last 8000 years). As a consequence of inundation the upland areas
are characterized by extensive wave-washed bedrock outcrops with glaciolacustrine
(GL) sediments and glaciofluvial materials occurring between them, whereas the
lowland areas feature prominent GL deposits that cover nearly 70 % of the exposed
surface. Throughout much of the region ground ice accumulation likely accompanied
permafrost aggradation into fine-grained sediments, as is evident by lithalsa growth in
particular.
Highway 3, constructed during the mid-1960s, was preferentially aligned to crossed
terrain underlain by fine-grained sediments to avoid bedrock and waterbodies. Local silt and
clay used for highway embankment construction was sourced from shallow borrow pits
developed along the right-of-way. Following construction, many borrow pits developed into
ponds.
Thermokarst ponding is widespread in the study area (n = 3138). The transition of
approximately 3.57 km2 of land cover from forested permafrost terrain to ponds is different
than in the low subarctic where permafrost peatlands degrade to fens. Most ponds are small
(< 5000 m2), but range up to nearly 45 000 m2. Pond distribution relates to surficial
geology and elevation, with ponds dominantly constrained to GL deposits, and
decreased pond counts with increased elevation. Highway construction has substantially
affected thermokarst development. Compared to pond density within undisturbed GL
deposits, pond density is an order of magnitude greater in the vicinity of Highway 3,
where about half of the borrow pits have developed thermokarst ponds within them.
In contrast, only 6.5 % of ponds within 200 m of the highway developed before
1961.
Thermokarst is likely widespread throughout the region as GL deposits are extensive.
Reduced thermokarst ponding at higher elevation is likely related to reduced GL extent, but
may also be related to more time for past thermokarst development given the landscape
history. Regardless, future thermokarst development will continue to be associated with
permafrost in low lying forested GL deposits that should be avoided by new infrastructure
construction. |
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