Increased permafrost temperatures have been reported in the circum-Arctic, and widespread degradation of permafrost peatlands has occurred in recent decades. The timing of permafrost aggradation in these ecosystems could have implications for the soil carbon lability upon thawing, and an increased understanding of the permafrost history is therefore needed to better project future carbon feedbacks. In this study, we have conducted high-resolution plant macrofossil and geochemical analyses and accelerator mass spectrometry radiocarbon dating of active layer cores from four permafrost peatlands in northern Sweden and Norway. In the mid-Holocene, all four sites were wet fens, and at least three of them remained permafrost-free until a shift in vegetation toward bog species was recorded around 800 to 400 cal. BP, suggesting permafrost aggradation during the Little Ice Age. At one site, Karlebotn, the plant macrofossil record also indicated a period of dry bog conditions between 3300 and 2900 cal. BP, followed by a rapid shift toward species growing in waterlogged fens or open pools, suggesting that permafrost possibly was present around 3000 cal. BP but thawed and was replaced by thermokarst.

Aim We aim to use species attributes such as distributions and indicator values to reconstruct past biomes, environment, and temperatures from detailed plant-macrofossil data covering the late glacial to the early Holocene (ca. 14-9 ka). Location Krakenes, western Norway. Methods We applied attributes for present-day geographical distribution, optimal July and January temperatures, and Ellenberg indicator values for plants in the macrofossil data-set. We used assemblage weighted means (AWM) to reconstruct past biomes, changes in light (L), nitrogen (N), moisture (F), and soil reaction (R), and temperatures. We compared the temperature reconstructions with previous chironomid-inferred temperatures. Results After the start of the Holocene around 11.5 ka, the Arctic-montane biome, which was stable during the late-glacial period, shifted successively into the Boreo-arctic montane, Wide-boreal, Boreo-montane, Boreo-temperate, and Wide-temperate biomes by ca. 9.0 ka. Circumpolar and Eurasian floristic elements characteristic of the late-glacial decreased and the Eurosiberian element became prominent. Light demand (L), soil moisture (F), nitrogen (N), and soil reaction (R) show different, but complementary responses. Light-demanding plants decreased with time. Soil moisture was relatively stable until it increased during organic soil development during the early Holocene. Soil nitrogen increased during the early Holocene. Soil reaction (pH) decreased during the Allerod, but increased during the Younger Dryas. It decreased markedly after the start of the Holocene, reaching low but stable levels in the early Holocene. Mean July and January temperatures show similar patterns to the chironomid-inferred mean July temperature trends at Krakenes, but chironomids show larger fluctuations and interesting differences in timing. Conclusion Assigning attributes to macrofossil species is a useful new approach in palaeoecology. It can demonstrate changes in biomes, ecological conditions, and temperatures. The late-glacial to early-Holocene transition may form an analogue for changes observed in the modern arctic and in mountains, with melting glaciers, permafrost thaw, and shrub encroachment into tundra.

Ice-wedge polygon (IWP) peatlands in the Arctic and Subarctic are extremely vulnerable to climatic and environmental change. We present the results of a multidisciplinary paleoenvironmental study on IWPs in the northern Yukon, Canada. High-resolution laboratory analyses were carried out on a permafrost core and the overlying seasonally thawed (active) layer, from an IWP located in a drained lake basin on Herschel Island. In relation to 14 Accelerator Mass Spectrometry (AMS) radiocarbon dates spanning the last 5000 years, we report sedimentary data including grain size distribution and biogeochemical parameters (organic carbon, nitrogen, C/N ratio, delta C-13), stable water isotopes (delta O-18, delta D), as well as fossil pollen, plant macrofossil and diatom assemblages. Three sediment units (SUS) correspond to the main stages of deposition (1) in a thermokarst lake (SW : 4950 to 3950 cal yrs BP), (2) during transition from lacustrine to palustrine conditions after lake drainage (SU2: 3950 to 3120 cal yrs BP), and (3) in palustrine conditions of the IWP field that developed after drainage (SU3: 3120 cal yrs BP to 2012 CE). The lacustrine phase (pre 3950 cal yrs BP) is characterized by planktonic-benthic and pioneer diatom species indicating circumneutral waters, and very few plant macrofossils. The pollen record has captured a regional signal of relatively stable vegetation composition and climate for the lacustrine stage of the record until 3950 cal yrs BP. Palustrine conditions with benthic and acidophilic diatom species characterize the peaty shallow-water environments of the low-centered IWP. The transition from lacustrine to palustrine conditions was accompanied by acidification and rapid revegetation of the lake bottom within about 100 years. Since the palustrine phase we consider the pollen record as a local vegetation proxy dominated by the plant communities growing in the IWP. Ice-wedge cracking in water-saturated sediments started immediately after lake drainage at about 3950 cal yrs BP and led to the formation of an IWP mire. Permafrost aggradation through downward closed-system freezing of the lake talik is indicated by the stable water isotope record. The originally submerged IWP center underwent gradual drying during the past 2000 years. This study highlights the sensitivity of permafrost landscapes to climate and environmental change throughout the Holocene. (C) 2016 Elsevier Ltd. All rights reserved.

The response of peat-rich permafrost soils to human-induced climate change may be especially important in modifying the global C-flux. We examined the Holocene developmental record of a High Arctic peat-forming wetland to investigate its sensitivity to past climate change and aid understanding of the likely effects of future climate warming on high-latitude ecosystems. The microhabitat of mosses was quantified in the present-day polygon-complex at Bylot Island (73 degrees N, 80 degrees W) and used to interpret the radiocarbon-dated macrofossil record of three cores, comprising c. 3500 years of wetland development. Recurrent wet and dry phases in the reconstructed palaeohydrological record indicated pronounced temporal variability. Wet and dry phases were compared between cores and with palaeoclimatic proxy values, measured as percentage melt and delta O-18 in nearby ice cores. Periodic wet and dry phases appear unrelated to past climate over c. 50% of the combined stratigraphic records, and are attributable instead to geomorphological mechanisms. At other times, association of wet and dry phases with significantly lower and higher values of percentage melt and delta O-18 indicate a possible effect of past climate change on polygon hydrology and vegetation, although inconsistencies between cores suggest that local geomorphological processes continued to modify a regional climatic effect. However, during a period incorporating the Little Ice Age (c. 305-530 cal. years BP), reconstructed moisture and vegetation change is pronounced and consistent among all three cores. The results provide strong evidence for the sensitivity of a High Arctic terrestrial ecosystem to past climate change during the Holocene. The estimated magnitude of changes in soil moisture between wet and dry phases is sufficient to imply recurrent shifts in wetland function, periodically impacted upon by pronounced climatic variability, although controlled principally by autogenic processes. The structure and function of such wetlands may therefore be susceptible to predicted, human-induced climate warming.