Pollen Stratigraphy of Mackenzie Pingo Sediments, N.W.T., Canada

Abstract

Two eroded pingos, of the closed system type abundant in the uplands east of the Mackenzie Delta, yielded sections of pond sediment of Holocene age. Pollen stratigraphy of both sections confirms the general sequence established earlier by Ritchie (1972): Zone I dominated by Betula glandulosa associated with low frequencies of Salix and Artemisia; Zone II dominated by Picea (20 to 40%) and Betula (50%) and Zone III showing roughly equal representation of Alnus, Betula, and Picea. Zone IV is represented only at the Hendrickson Island site and is distinguished from Zone III by the increases in Betula, ericads, and sedges, and a decline of Alnus. Radiocarbon analyses suggest that the growth of the two pingos was initiated at least 2,500 years ago. The Zone I assemblage, which has been widely reported in the northwest Arctic-Subarctic of North America, was replaced by Zone II about 9,000 BP; at Eskimo Lakes it represents a migration of spruce into birch-dominated tundra, but it is unlikely that spruce actually grew at Hendrickson Island. The Zone III to IV changes confirm the proposition (Ritchie and Hare, 1972) that the tree line has retreated to its present position during the postglacial.