Search

Abstract Anthropogenic climate change has been linked to the degradation of permafrost across northern ecosystems, with notable implications for regional to global carbon dynamics. However, our understanding of the spatial distribution, temporal trends, and seasonal timing of episodic landscape deformation events triggered by permafrost degradation is hampered by the limited spatial and temporal coverage of high-resolution optical, RADAR, LIDAR, and hyperspectral remote sensing products. Here we present an automated approach for detecting permafrost degradation (thermoerosion), using meso-scale high-frequency remote sensing products (i.e., Landsat image archive). This approach was developed, tested, and applied in the ice-rich lowlands of the Noatak National Preserve (NOAT; 12,369 km2) in northwestern Alaska. We identified thermoerosion (TE) by capturing the spectral signal associated with episodic sediment plumes in adjacent water bodies following TE. We characterized and extracted this episodic turbidity signal within lakes during the snow-free period (June 15–October 1) for 1986–2016 (continuous data limited to 1999–2016), using the cloud-based geospatial parallel processing platform, Google Earth Engine™. Thermoerosional detection accuracy was calculated using seven consecutive years of sub-meter high-resolution imagery (2009–2015) covering 798 (~33%) of the 2456 lakes in the NOAT lowlands. Our automated TE detection algorithm had an overall accuracy and kappa coefficient of 86% and 0.47 ± 0.043, indicating that episodic sediment pulses had a "moderate agreement" with landscape deformation associated with permafrost degradation. We estimate that lake shoreline erosion, thaw slumps, catastrophic lake drainage, and gully formation accounted for 62, 23, 13, and 2%, respectively, of active TE across the NOAT lowlands. TE was identified in ~5% of all lakes annually in the lowlands between 1999 and 2016, with a wide range of inter-annual variation (ranging from 0.2% in 2001 to 22% in 2004). Inter-annual variability in TE occurrence and spatial patterns of TE probability were correlated with annual snow cover duration and snow persistence, respectively, suggesting that earlier snowmelt accelerates permafrost degradation (e.g. TE) in this region. This work improves our ability to detect and attribute change in permafrost degradation across space and time. Highlights • New automated approach for detecting permafrost degradation. • Overall accuracy of the detection of thermoerosion (TE) was ~86%. • TE was identified in ~5% of all lakes per year (1999–2016) in the Noatak lowlands. • Temporal variability in TE was correlated with the annual snow-free period. • Spatial heterogeneity in TE was correlated with snow persistence. [ABSTRACT FROM AUTHOR]

Despite recent progress in understanding high-latitude climate variability, paleoclimate records are scarce from the Alaskan Arctic. We conducted isotopic and sediment analyses at Wahoo Lake to infer Holocene climate variability in northeastern Alaska. Water δ18O and δD values from the lake and its inlet/outlet streams suggest that winter precipitation dominates modern water inputs and that evaporation has limited influence on the lake's hydrological budget. The isotopic composition of Pisidium exhibits marked variations during the past 11,500 years, with δ18O ranging between -18.7‰ and -16.2‰ and δ13C between -7.1‰ and -2.3‰ (Vienna Pee Dee Belemnite (VPDB)). Elevated δ18O and sediment composition from 11.5 to 8.9 kcal. BP suggest evaporative 18O enrichment and arid conditions. Rising lake levels are evidenced by the disappearance of Pisidium and a transition to low-carbonate gyttja ca. 6.3 kcal. BP and by the onset of sediment deposition on an adjacent shelf by 5.3 kcal. BP. These changes coincided with enhanced effective moisture in interior and southern Alaska as inferred from lake-level records and may be related to broad-scale atmospheric circulation changes. In the shelf sediments, carbonate abundance increases markedly at 3.5 kcal. BP and δ18O increases from -18.0‰ to -16.5‰ at 2.1 kcal. BP, possibly resulting from increased temperature and/or summer precipitation. After 2.1 kcal. BP, δ18O fluctuates with an overall decreasing trend to -17.2‰ at 0.9 kcal. BP. Late-Holocene variations in our δ18O record display coherent patterns with regional glacier fluctuations at centennial to millennial scales, suggesting that δ18O minima were related to a combination of low temperatures and enhanced winter snowfall. Holocene variations in organic matter abundance at Wahoo Lake also show broad similarities to total solar irradiance, implying that suborbital solar variability played a role in modulating regional climate and aquatic productivity. [ABSTRACT FROM AUTHOR]

We present chironomid species assemblage data from 402 lakes across northern North America, Greenland, Iceland, and Svalbard to inform interpretations of Holocene subfossil chironomid assemblages used in paleolimnological reconstruction. This calibration-set was developed by re-identifying and taxonomically harmonizing chironomids in previously described surface sediment samples, with identifications made at finer taxonomic resolution than in original publications. The large geographic coverage of this dataset is intended to provide climatic analogues for a wide range of Holocene climates in the northwest North Atlantic region and North American Arctic, including Greenland. For many of these regions, modern calibration data are sparse despite keen interest in paleoclimate reconstructions from high latitudes. A suite of chironomid-based temperature models based upon this training set are evaluated here and the best statistical model is used to reconstruct late glacial (Allerød and Younger Dryas) and Holocene paleotemperatures at five non-glacial lakes representing a wide range of climate zones across Greenland. The new continent-scale training set offers more analogues for the majority of Greenland subfossil assemblages than existing smaller training sets, with many in Iceland and northern Canada. We find strong agreement between chironomid-based reconstructions derived from the new model and independent glacier-based evidence for multi-millennial Holocene temperature trends. Some of the new Holocene reconstructions are very similar to published data, but at a subset of sites and time periods we find improved paleotemperature reconstructions attributable both to the new model's finer taxonomic resolution and to its expanded geographic/climatic coverage, which resulted in improved characterization of species optima. In the late glacial, the new model's finer taxonomic resolution yields a unique ability to resolve temperatures of the Allerød from colder temperatures of the Younger Dryas, although the magnitude of that temperature difference may be underestimated. This study demonstrates the value of geographically and climatically broad paleoecological training sets. The large, taxonomically harmonized dataset presented here should be useful for a wide range of future investigations, including but not limited to paleotemperature reconstructions across the Arctic. • Chironomid assemblages were identified from 402 lakes for a paleotemperature model. • The new model includes lakes in Canada, Greenland, Iceland, and Svalbard. • We increase taxonomic resolution and present a broader climate gradient. • The model improves reconstructions for some Greenland subfossil records. • We demonstrate the value of geographically broad paleoecological training sets. [ABSTRACT FROM AUTHOR]

Wildfire activity in boreal forests is anticipated to increase dramatically, with far-reaching ecological and socioeconomic consequences. Paleorecords are indispensible for elucidating boreal fire regime dynamics under changing climate, because fire return intervals and successional cycles in these ecosystems occur over decadal to centennial timescales. We present charcoal records from 14 lakes in the Yukon Flats of interior Alaska, one of the most flammable ecoregions of the boreal forest biome, to infer causes and consequences of fire regime change over the past 10,000 y. Strong correspondence between charcoal-inferred and observational fire records shows the fidelity of sedimentary charcoal records as archives of past fire regimes. Fire frequency and area burned increased ~6,000-3,000 y ago, probably as a result of elevated landscape flammability associated with increased Picea mariana in the regional vegetation. During the Medieval Climate Anomaly (MCA; ~1,000-500 cal B.P.), the period most similar to recent decades, warm and dry climatic conditions resulted in peak biomass burning, but severe fires favored less-flammable deciduous vegetation, such that fire frequency remained relatively stationary. These results suggest that boreal forests can sustain high-severity fire regimes for centuries under warm and dry conditions, with vegetation feedbacks modulating climate-fire linkages. The apparent limit to MCA burning has been surpassed by the regional fire regime of recent decades, which is characterized by exceptionally high fire frequency and biomass burning. This extreme combination suggests a transition to a unique regime of unprecedented fire activity. However, vegetation dynamics similar to feedbacks that occurred during the MCA may stabilize the fire regime, despite additional warming. [ABSTRACT FROM AUTHOR]

The article discusses a study on the variability of tundra fire regimes in Alaska. It mentions the significant ecological impacts of tundra fires on vegetation, wildlife and carbon cycling. It notes that various histories of the fire within the state indicate a wide change in ecological effects along with important implications for wildlife-habitat maintenance and processes to climatic changes.