The stability of arctic permafrost and the carbon it contains are currently threatened by a rapidly warming climate. Burial Lake, situated in northwestern arctic Alaska, is underlain by continuous permafrost and has a uniquely rich set of paleoclimate proxy data that comprise a 40-ka record of climate and environmental change extending well into Marine Isotope Stage (MIS) 3. Here, we examine the relationship between erosion, subsurface hydrology, and primary productivity from the Burial Lake sediments to improve our understanding of the links between climate, hydrology, sediment transport, and carbon mobility. The record is developed with radiocarbon (14C) age-offsets from two independent methods used to date the lake sediments: 1) 14 C measurements on paired bulk sediment and plant macrofossils from the same stratigraphic layer of lake sediment and 2) ramped pyrolysis- oxidation (RPO) 14 C analysis that separates fractions of organic carbon (OC) from a single bulk sediment sample based on thermochemical differences through continuous heating. As lakes capture and archive OC transported from the watershed, changes in the amount and relative age of permafrost-derived OC mobilized during past climatic variations can be documented by examining how age-offsets change over time. The Burial Lake sediment revealed higher age-offsets during the cold Last Glacial Maximum (LGM; 29-17 ka) than the comparatively warmer post-glacial ( 17 ka-present) and the MIS 3 interstadial ( 40-29 ka) periods. The relatively warm, wet climate of the post-glacial period promoted both terrestrial and aquatic productivity, resulting in increased OC deposition, and it likely favored transport via subsurface flow of dissolved OC (DOC) sourced from soils. This resulted in a greater flux of contemporary OC relative to ancient OC into the lake sediment, lowering the average age offset to 2 ka. In contrast, the low-productivity conditions of the LGM resulted in slow soil accumulation rates, leaving ancient OC in a shallower position in the soil profile and allowing it to be easily eroded in the form of particulate OC (POC). Although the amount of total OC deposited in the lakebed during the LGM is small relative to post-glacial deposition, the majority is ancient, which leads to a relatively high average age offset of 9 ka. Finally, climate and environmental conditions of the MIS 3 interstadial were intermediate between those of the post-glacial and the LGM. As with post-glacial sediments, a relatively large amount of OC is present; however, the vast majority of it is ancient (more similar to the LGM), and it produces an average age offset of 6 ka. The Burial Lake radiocarbon record demonstrates the complexities of the thaw and mobilization of permafrost OC in arctic Alaska, including the balance between production, transport, deposition, remobilization, and preservation. This record highlights the importance of considering factors that both enhance and inhibit erosion (i.e. vegetation cover, lake level, precipitation) and the mechanisms of OC transport (i.e. subsurface flow or erosion) in predictions of future permafrost response to changes in climate.

Wildfire is a major source of biomass burning aerosols, which greatly impact Earth climate. Tree species in North America (NA) boreal forests can support high-intensity crown fires, resulting in elevated injection height and longer lifetime (on the order of months) of the wildfire aerosols. Given the long lifetime, the properties of aged NA wildfire aerosols are required to understand and quantify their effects on radiation and climate. Here we present comprehensive characterization of climatically relevant properties, including optical properties and cloud condensation nuclei (CCN) activities of aged NA wildfire aerosols, emitted from the record-breaking Canadian wildfires in August 2017. Despite the extreme injection height of similar to 12 km, some of the wildfire plumes descended into the marine boundary layer in the eastern North Atlantic over a period of similar to 2 weeks, owing to the dry intrusions behind mid-latitude cyclones. The aged wildfire aerosols have high single scattering albedos at 529 nm (omega(529); 0.92-0.95) while low absorption Angstrom exponents (angstrom(abs)) at 464 nm/648 nm (0.7-0.9). In comparison, angstrom(abs) of fresh/slightly aged ones are typically 1.4-3.5. This low angstrom(abs) indicates a nearly complete loss of brown carbon, likely due to bleaching and/or evaporation, during the long-range transport. The nearly complete loss suggests that on global average, direct radiative forcing of BrC may be minor. Combining Mie calculations and the measured aerosol hygroscopicity, volatility and size distributions, we show that the high omega(529) and low angstrom(abs) values are best explained by an external mixture of non-absorbing organic particles and absorbing particles of large BC cores (> similar to 110 nm diameter) with thick non-absorbing coatings. The accelerated descent of the wildfire plume also led to strong increase of CCN concentration at the supersaturation levels representative of marine low clouds. The hygroscopicity parameter, kappa(CCN), of the aged wildfire aerosols varies from 0.2 to 0.4, substantially lower than that of background marine boundary layer aerosols. However, the high fraction of particles with large diameter (i.e., within accumulation size ranges, similar to 100-250 nm) compensates for the low values of., and as a result, the aged NA wildfire aerosols contribute more efficiently to CCN population. These results provide direct evidence that the long-range transported NA wildfires can strongly influence CCN concentration in remote marine boundary layer, therefore the radiative properties of marine low clouds. Given the expected increases of NA wildfire intensity and frequency and regular occurrence of dry intrusion following mid-latitude cyclones, the influence of NA wildfire aerosols on CCN and clouds in remote marine environment need to be further examined.

Alaska's North Slope is especially vulnerable to climatic change because higher latitudes are subject to positive snow- and sea ice-atmosphere feedbacks under warming conditions and because the dynamics of frozen seascapes and landscapes are tightly determined by thermal regime. Shifts in timing and magnitude of freeze-thaw processes are observed to have or expected to have non-linear, threshold-crossing impacts on sea ice, landforms, and biota. Observed changes in North Slope surface air temperatures and precipitation were non-monotonic over the last century, but have trended upward for the last several decades. These changes are linked to hemispheric climate dynamics, reflected in North Pacific and Arctic Oscillation circulation indices. Projected anthropogenic climate changes-with the possibility of continued warming, increased storm frequency and intensity, and decreased insulating snow cover-portend an uncertain future for this domain. Current or foreseen physical system shifts include: (1) declining seasonal and permanent sea ice extent and character, (2) rapid coastal erosion due to storm exposure over a longer near-shore ice-free season, (3) deeper soil active layer over warmer permafrost, along with altered thermokarst processes-contributing to thaw lake expansion, surface drainage re-organization, and hillslope instability. Biogeophysical responses encompass (1) modified surface-atmosphere energy balance from snow cover, vegetation, and hydrologic change and (2) shifted soil and wetland biogeochemical dynamics, including accelerated carbon efflux. Climate-driven plant community shifts on the North Slope result from the interplay of climate, vegetation response, and landscape processes. Some transitions involve stabilizing, others destabilizing plant-permafrost feedbacks. Impacts on caribou, migratory avifauna, and freshwater biota are through direct effects of climate on organism physiology and reproductive biology and indirectly through disruption of habitat mosaics (including along migratory routes) and shifts in competition and trophic linkages. The North Slope's physical and biological vulnerabilities to shifting climate and observed leading indicators of change are compelling reasons for land managers to consider climatic instability as a threat in conjunction with other known stressors while seeking strategies for protection of this domain's natural heritage and ecosystem services.

A snapshot of the thermal state of permafrost in northern North America during the international Polar Year (IPY) was developed using ground temperature data collected from 350 boreholes. More than half these were established during IPY to enhance the network in sparsely monitored regions. The measurement sites span a diverse range of ecoclimatic and geological conditions across the continent and are at various elevations within the Cordillera. The ground temperatures within the discontinuous permafrost zone are generally above -3 degrees C, and range down to -15 degrees C in the continuous zone. Ground temperature envelopes vary according to substrate, with shallow depths of zero annual amplitude for peat and mineral soils, and much greater depths for bedrock. New monitoring sites in the mountains of southern and central Yukon suggest that permafrost may be limited in extent. In concert with regional air temperatures, permafrost has generally been warming across North America for the past several decades, as indicated by measurements from the western Arctic since the 1970s and from parts of eastern Canada since the early 1990s. The rates of ground warming have been variable, but are generally greater north of the treeline. Latent heat effects in the southern discontinuous zone dominate the permafrost thermal regime close to 0 degrees C and allow permafrost to persist under a warming climate. Consequently, the spatial diversity of permafrost thermal conditions is decreasing over time. Copyright (C) 2010 Crown in the right of Canada and John Wiley & Sons, Ltd.