Deglacial and Holocene Evolution of Climate and Terrestrial Ecosystems in the Western Arctic.

The late- and post-glacial history of the Western Arctic land area is distinguished by the region being largely unglaciated (away from the coastal cordillera), and by a considerable reduction in areal extent due to sea-level rise. Between ca. 15 and 10 cal ka B.P. rising summer temperatures and increasing precipitation related to orbital forcing and adjustments in the hemispheric circulation resulted in widespread replacement of herbaceous vegetation with deciduous woody plants, rising lake levels, reduction of eolian activity, and the onset of thermokarst erosion. These changes had impacts at both global and regional levels, e.g., reduced atmospheric dust loading, increased carbon flux to the atmosphere, and feedbacks to the surface energy balance. The Younger Dryas event (ca. 12.9-11.6 cal ka B.P.) is not strongly expressed in terrestrial records. Some localities near the Pacific coast and in eastern Siberia record a weak to moderate climatic reversal, but central, northern, and eastern areas show no consistent signal. This suggests a muted response of the North Pacific and/or adjacent lands to events in the North Atlantic. However, the nearly coincident flooding and eventual breaching of the Bering land bridge probably reduced continentality in adjacent regions and may be linked to early-Holocene increases in effective moisture in both summer and winter. At ca. 10 cal ka B.P. evergreen conifers expanded to dominance or co-dominance much of the forest zone on both sides of the Bering Strait. This appears to be an example of a moisture-driven vegetation change in a region normally expected to respond largely to temperature. Particularly in Alaska and NW Canada, it likely led to two significant ecosystem changes: a vegetation-driven shift in the fire regime and enhanced paludification. The former could have influenced atmospheric chemistry and the latter nutrient availability, the status of permafrost, and the export of organic carbon and other nutrients to nearshore and shelf environments. Orbitally-driven mid-Holocene cooling is evidenced in the retreat of northern treeline in Siberia and North America; in North America further late-Holocene cooling is seen in the glacial record and the regrowth of epigenetic ice wedges. However, the boreal conifer forest has continued to expand westwards in Alaska until the present. Recent observations of the northward expansion of tundra shrubs, thawing of permafrost, increased severity of the fire season and outbreaks of pests and pathogens are linked to concerns about anthropogenic global warming and/or short-term modal changes in the Pacific Ocean. Key research topics linking the terrestrial system to other elements of the North Pacific system include i) the expression of THC-related oscillations in the North Pacific region, ii) feedbacks to the climate system at global and regional scales from land-cover changes and geomorphic and cryospheric processes ; and iii) links to the short-term (decadal-century) shifts in the behavior of the North Pacific that affect fire weather, plant growth, and pest and pathogen dynamics in the boreal forest.