Mid- to Late Holocene environmental dynamics on the Yukon Coastal Plain and Herschel Island (Canada) - envidence from polygonal peatlands and lake sediment

The North American Arctic witnessed high-amplitude climatic change during the Early Holocene that resulted in regional-scale environmental change. These changes are well documented in the literature. The environmental impacts of moderate climatic oscillations during the Mid- to Late Holocene are less well understood, especially on the Yukon Coastal Plain, which is geographically and topographically isolated from the rest of the western Canadian Arctic. The region is currently experiencing increased thaw of ice-rich permafrost, alterations in landscape water balance, and shrub expansion. These processes are connected to severe transformations in a landscape that is overwhelmingly composed of periglacial landforms. Especially the widespread thaw lakes and ice-wedge polygons are known to be vulnerable to climatic and geomorphic change because of their direct dependence on permafrost conditions, and hence on air temperatures. Tundra vegetation dynamics are linked to permafrost conditions and geomorphology, yet the interplay between vegetation, permafrost, geomorphology and climate is not well articulated in Low Arctic tundra. Finally, the temporal and spatial scales at which climatic change and geomorphic processes may affect periglacial landforms on the one hand and tundra vegetation on the other hand are not clearly constrained. Yet, these scale-dependent relationships are crucial components of the adaptation and resilience potential of high-latitude environments. \nThis thesis identified long-term as well as short-term trends in the development of thaw lakes, ice-wedge polygons and tundra vegetation during the Mid- to Late Holocene. This was done by studying modern, sub-decadal, and centennial- to millennial-scale records from ice-wedge polygons and lake sediment in different landscape units on the Yukon Coastal Plain. Additionally, drivers of change to these systems and possible causes of environmental stability were assessed. \nTo address and constrain the wide range of spatial and temporal dimensions involved in landscape development, at first the modern state of ice-wedge polygons and a thaw lake were examined. The following analyses characterized organic matter (organic carbon contents, nitrogen contents, stable carbon isotopes), biological proxies (pollen, plant macrofossils, diatoms), and abiotic sediment (grain size composition, pore water hydrochemistry) in multiple short cores. The age-depth relationship was determined by Accelerator Mass Spectrometry radiocarbon dating in all cores and additional 210Pb/137Cs dating in the younger lake sediment core. These records encompassed the environmental history at four sites dispersed along the Yukon Coastal Plain in the Western Canadian Low Artic. \nLong-term thaw lake decline was observed at all sites now occupied by ice-wedge polygons. These lakes drained gradually or abruptly, leaving behind wet to shallow submerged areas, which prevailed for up to 1000 years and subsequently provided waterlogged terrestrial conditions with impeded drainage. The investigations have shown that coastal erosion contributed to thaw lake drainage. The newly exposed lake floors were then rapidly invaded by pioneer vegetation, and ice-wedge polygon development began immediately after drainage. Subsequently, low-centred ice-wedge polygons grew and peat accumulation persisted in a relatively stable state for millennia, before ice-wedge degradation and drying of the ground surfaces set in, likely during the twentieth century. At two sites, the emergence of intermediate- and high-centred polygons ensued. This rapid change was reflected by the vascular plant taxa composition at the studied sites, which shifted from a graminoid-dominated to a shrub-dominated pattern. At the same time, however, the overall regional vegetation, which was reconstructed from pollen in lake sediment, remained largely stable even across the transition from cooler conditions of the Little Ice Age to twentieth century warming. \nDegradation of ice-rich permafrost is increasingly causing geomorphic disturbances on the Yukon Coastal Plain and on Herschel Island. The widespread polygon degradation might lead to changes in microtopography and landscape hydrology that are irreversible on decadal to centennial time-scales and decoupled from climate-driven vegetation change alone. \nThe sensitivity of permafrost and vegetation to climatic change depends on amplitude and duration of change. While permafrost responds rapidly to climatic change, the response of tundra vegetation may lag behind climate forcing. Tundra vegetation resilience and small-scale landscape heterogeneity may also buffer a certain amount of stress. Warming-induced change to permafrost may, however, trigger geomorphic change, which would affect tundra vegetation at much shorter time-scales. \nDuring the Early Holocene, high-amplitude climatic forcing was the dominant driver of environmental change. The Late Holocene experienced moderate climatic oscillations, and geomorphic and biological processes complicated the response of vegetation and permafrost to climatic forcing. This facilitated localized environmental variability. The modern warming trend is, however, currently causing extensive permafrost degradation and shrub expansion that could trigger a strong and irreversible environmental response.

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