|Abstract||Natural gas discoveries in the Mackenzie Delta spurred development of the Mackenzie Gas Pipeline to deliver gas to southern markets via a route up the Mackenzie Valley to Alberta. Construction of the
pipeline would enable production from the Taglu and Niglintgak gas fields in the delta and other discoveries. This study was undertaken in response to concern about sources and rates of subsidence in the delta and implications for flooding and
erosion hazards, including impacts on nesting bird habitat. The project addresses design constraints and environmental impacts of development for the information of regulators, industry, and Inuvialuit communities. |
Little is known about sources
of subsidence in major Arctic deltas such as the Mackenzie. Permafrost with varying conditions of ice-bonding extends to >600 m beneath the margins of the delta and to lesser depths (<100 m) beneath the Holocene delta plain. A vast network of lakes
and channels covers the delta plain: many are <2 m deep and freeze to the bottom in winter, maintaining sub-zero temperatures in underlying deposits; others are deeper, do not freeze to the bottom, and create taliks in underlying sediments. The
result is a frozen surface layer punctuated by numerous thaw bulbs and pipes in which sediment compaction can proceed unimpeded by ice bonding and through which gas venting can occur. Other sources of subsidence include postglacial isostatic
adjustment, crustal response to long-term delta loading, tectonics, and deepening of the surface active layer inducing thaw of shallow excess ice. Several linear features (channels, lakes, and the eastern edge of the delta along the Caribou Hills
escarpment) may be the surface expression of underlying faults.
Rates of subsidence in the Mackenzie Delta are being determined using a range of techniques, including geophysical models, the tide-gauge record at Tuktoyaktuk, continuous and
episodic GPS, and InSAR. The low-relief delta plain topography is being mapped using airborne LiDAR to create a digital elevation model with vertical resolution of ±0.2 m. Coastal erosion across the region has been measured by repetitive surveys and
digital photogrammetry with QuickBird imagery. Preliminary results indicate variable rates of subsidence reaching 11 mm/yr or more. With regional isostatic subsidence of ~2 mm/yr, this implies delta compaction+loading at rates as high as 9 mm/yr,
which seems high for an ice-bonded delta, perhaps pointing to a tectonic component. In addition to subsidence, other factors relevant to flood risk in the outer delta include relative sea level rise (3.5±1.2 mm/yr at Tuktoyaktuk), storm surges,
changes in spring freshet affecting breakup flooding, other climate factors in the Mackenzie drainage basin, and any differential tilting across the delta.
This work has been supported by Natural Resources Canada (PERD, GSC, PCSP), Indian and
Northern Affairs Canada (Northern Oil and Gas Research Initiative and IPY funding), Environment Canada, ArcticNet and the Networks of Centres of Excellence, Aurora Research Institute, Chevron Canada Resources, and MGM Energy Corporation, among
others, and guided through annual consultation with Inuvialuit communities. Field support from JC Lavergne has been critical to the success of this project.