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A sediment core from Lake BC01 (75"10.945?N, 111"55.181?W, 225"m asl) on south-central Melville Island, NWT, Canada, provides the first continuous postglacial environmental record for the region. Fossil pollen results indicate that the postglacial landscape was dominated by Poaceae andSalix, typical of a High Arctic plant community, whereas the Arctic herbOxyriaunderwent a gradual increase during the late Holocene. Pollen-based climate reconstructions suggests the presence of a cold and dry period ~12,000"cal yr BP, possibly representing the Younger Dryas, followed by warmer and wetter conditions from 11,000 to 5000"cal yr BP, likely reflective of the Holocene Thermal Maximum. The climate then underwent a gradual cooling and drying from 5000"cal yr BP to the present, suggesting a late Holocene neoglacial cooling. Diatom preservation was poor prior to 5000"cal yr BP, when conditions were warmest, suggesting that diatom dissolution may in part be climatically controlled. Diatom concentrations were highest ~4500"cal yr BP but then decreased substantially by 3500"cal yr BP and remained low before recovering slightly in the 20th century. An abrupt warming occurred during the past 70 yr at the site, although the magnitude of this warming did not exceed that of the early Holocene.

This study presents a diatom-based analysis of the post-glacial Holocene environmental history at Lake RS29 on Somerset Island in the Canadian High Arctic. Earliest post-glacial diatom assemblages (10 200-10 000 cal yr BP) consisted mainly of small, benthic fragilarioid taxa. Poor diatom preservation in the early Holocene (~10 000-6200 cal yr BP) is associated with warm conditions, as determined by pollen data from the same core and other paleoclimate estimates from the region. Analysis of this and other sites from across the Canadian Arctic suggest that zones of poor diatom preservation or diatom absence in lake sediment records may be associated with warm conditions. After 6200 cal yr BP, acidophilic assemblages consisting of Aulacoseira spp. and a suite of periphytic taxa indicate acidification since the mid-Holocene. During this time period, cooling causing changes in lake ice phenology was likely a major driver of the reconstructed mid-Holocene pH decline. Watershed processes, including reduced fluxes of base cations as the rate of sediment accumulation slowed, may also be contributors to long-term shifts in lake water pH and associated changes in diatom assemblages. The uppermost sediments in the Lake RS29 record were characterized by abrupt declines in Aulacoseira alpigena and increases in benthic diatom taxa Cyclotella sensu lato, suggesting an increase in lake water pH and longer ice-free seasons.Key words: paleoclimate, Holocene, Canadian Arctic Archipelago, lake sediments, diatoms, paleolimnology, pollen, diatom dissolution.Résumé : Cette étude présente une analyse basée sur les diatomées afin d'examiner l'histoire environnementale de l'Holocène postglaciaire au lac RS29 sur l'île Somerset dans le Haut-Arctique canadien. Les tout premiers assemblages de diatomées postglaciaires (10 200 -10 000 ans BP) étaient constitués principalement de petits taxons benthiques fragilarioid. La pauvre conservation des diatomées au cours de l'Holocène précoce (~10 000-6 200 ans BP) est associée aux conditions chaudes, comme déterminées par les données de pollen provenant de la même carotte et d'autres évaluations paléoclimatiques de la région. L'analyse de ce site et d'autres sites à travers l'Arctique canadien suggère que les zones de pauvre conservation de diatomées ou d'absence de diatomées dans les données de sédiments lacustres enregistrées pourraient être associées aux conditions chaudes. Après 6 200 ans BP, les assemblages d'acidiphiles constitués d'Aulacoseira spp. et d'une gamme de taxons periphytiques indiquent l'acidification depuis l'Holocène moyen. Pendant cette période, le refroidissement causant des changements phénologiques de la glace lacustre était probablement un facteur du milieu important causant la baisse du pH reconstruit de l'Holocène moyen. Les processus des bassins hydrographiques, y compris les flux réduits de cations basiques alors que le taux d'accumulation de sédiments a ralenti, peuvent aussi être des facteurs

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