Abstract

Five offshore drill holes northeast of Richards Island reveal permafrost conditions, which are interpreted in terms of the sea-level and paleoenvironmental history of the Canadian Beaufort Shelf. The top of ice-bonded permafrost lies 76–88 m below the seabed within 12 km of the shore, 11 m below the seabed 20 km offshore, and at 60 m some 50 km offshore. The base of permafrost is 500 m to over 700 m deep. Temperature–depth gradients are negative or nearly isothermal, and permafrost is degrading. Geothermal modelling of the temperatures and permafrost observations indicates that the sites were subaerially exposed throughout most of the Wisconsinan, and during the early Holocene. Three sites 2–10 km from the present shoreline experienced inundation at 3.5–4.0 ka. These times support dates of marine transgression predicted by the sea-level curve, but the sites may have been occupied by thermokarst lakes prior to transgression. In contrast, the site 20 km offshore with shallow ice bonding was inundated about 0.4 ka; this area may have persisted as an island until recently. Shoals observed nearby may be analogues of this process, being outliers that are now being eroded below sea level. Marine transgression occurred about 6 ka some 50 km offshore.

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