The knowledge of past permafrost conditions is of importance to assess the potential magnitude of changes that periglacial environments may experience as a result of climate warming or disturbance. To assess if past thaw unconformities may be preserved from isotopic and geochemical discontinuities within permafrost, this study investigates the distribution of ground ice, stable water isotopes and major cations in two permafrost cores collected in a hummocky terrain site near Inuvik, Northwest Territories, Canada; a site where the evolution of the active layer during a recent period of permafrost degradation and subsequent aggradation was documented. Based on the high-resolution isotope geochemistry profiles, closed-system Rayleigh-type ionic segregation and isotope fractionation occurred during thermally-induced water migration into shallow permafrost and its freezing along a negative soil temperature gradient. Due to thermally-induced water migration into permafrost, delta O-18 may not always be able to identify thaw unconformities; however the calculation of the O-18 enrichment factors between ice and water (epsilon O-18(i-w)) may be used to determine position of thaw unconformities in permafrost, if thaw events are followed by permafrost aggradation. The approach of using epsilon O-18(i-w) provides additional information regarding past permafrost conditions that can complement change in cryostructures observed along natural exposures. Crown Copyright (C) 2014 Published by Elsevier B. V. All rights reserved.

The distribution of segregated ice and soluble ions in near-surface permafrost were investigated in hummocky terrain near Inuvik, Northwest Territories. Soil water content profiles from analyses of drill cores indicate that ice-poor permafrost developed beneath a permafrost table aggrading at approximately 4 cm/a, but an ice-rich zone, 10 to 20 cm thick, was observed beneath a permafrost table that had remained stable for about a decade. Ice-rich intervals 10 to 30 cm thick were observed immediately beneath both a thaw unconformity formed in 1981 and an older, deeper unconformity. In profile, the correspondence between zones of cation and ice enrichment suggests soluble materials were incorporated into permafrost during development of near-surface aggradational ice. Moisture enrichment below an experimentally degrading permafrost table was negligible. Similar ice contents beneath the present permafrost table and the deep thaw unconformity, and the preservation of ice-poor intervals immediately above the 1981 and deep thaw unconformities indicate limited vertical ice enrichment. The estimated rates of ice accumulation in two-decade-old permafrost are on the order of mm/a, but ice accumulation above older unconformities indicates that, in aggregate, these initial rates decrease with time. The ground ice and soluble cations sequestered in near-surface permafrost comprise an important pool of water and nutrients that may be released into the active layer during periods of deeper thaw. Copyright (C) 2003 John Wiley Sons, Ltd.