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Climatic and limnological changes at Lake Karakul (Tajikistan) during the last similar to 29 cal ka
(2017)
We present results of analyses on a sediment core from Lake Karakul, located in the eastern Pamir Mountains, Tajikistan. The core spans the last similar to 29 cal ka. We investigated and assessed processes internal and external to the lake to infer changes in past moisture availability. Among the variables used to infer lake-external processes, high values of grain-size end-member (EM) 3 (wide grain-size distribution that reflects fluvial input) and high Sr/Rb and Zr/Rb ratios (coinciding with coarse grain sizes), are indicative of moister conditions. High values in EM1, EM2 (peaks of small grain sizes that reflect long-distance dust transport or fine, glacially derived clastic input) and TiO2 (terrigenous input) are thought to reflect greater influence of dry air masses, most likely of Westerly origin. High input of dust from distant sources, beginning before the Last Glacial Maximum (LGM) and continuing to the late glacial, reflects the influence of dry Westerlies, whereas peaks in fluvial input suggest increased moisture availability. The early to early-middle Holocene is characterised by coarse mean grain sizes, indicating constant, high fluvial input and moister conditions in the region. A steady increase in terrigenous dust and a decrease in fluvial input from 6.6 cal ka BP onwards points to the Westerlies as the predominant atmospheric circulation through to present, and marks a return to drier and even arid conditions in the area. Proxies for productivity (TOC, TOC/TN, TOCBr), redox potential (Fe/Mn) and changes in the endogenic carbonate precipitation (TIC, delta(18) OCarb) indicate changes within the lake. Low productivity characterised the lake from the late Pleistocene until 6.6 cal ka BP, and increased rapidly afterwards. Lake level remained low until the LGM, but water depth increased to a maximum during the late glacial and remained high into the early Holocene. Subsequently, the water level decreased to its present stage. Today the lake system is mainly climatically controlled, but the depositional regime is also driven by internal limnogeological processes.
Lakes are dominant and diverse landscape features in the Arctic, but conventional land cover classification schemes typically map them as a single uniform class. Here, we present a detailed lake-centric geospatial database for an Arctic watershed in northern Alaska. We developed a GIS dataset consisting of 4362 lakes that provides information on lake morphometry, hydrologic connectivity, surface area dynamics, surrounding terrestrial ecotypes, and other important conditions describing Arctic lakes. Analyzing the geospatial database relative to fish and bird survey data shows relations to lake depth and hydrologic connectivity, which are being used to guide research and aid in the management of aquatic resources in the National Petroleum Reserve in Alaska. Further development of similar geospatial databases is needed to better understand and plan for the impacts of ongoing climate and land-use changes occurring across lake-rich landscapes in the Arctic.
Lakes are a ubiquitous landscape feature in northern permafrost regions. They have a strong impact on carbon, energy and water fluxes and can be quite responsive to climate change. The monitoring of lake change in northern high latitudes, at a sufficiently accurate spatial and temporal resolution, is crucial for understanding the underlying processes driving lake change. To date, lake change studies in permafrost regions were based on a variety of different sources, image acquisition periods and single snapshots, and localized analysis, which hinders the comparison of different regions. Here, we present a methodology based on machine-learning based classification of robust trends of multi-spectral indices of Landsat data (TM, ETM+, OLI) and object-based lake detection, to analyze and compare the individual, local and regional lake dynamics of four different study sites (Alaska North Slope, Western Alaska, Central Yakutia, Kolyma Lowland) in the northern permafrost zone from 1999 to 2014. Regional patterns of lake area change on the Alaska North Slope (-0.69%), Western Alaska (-2.82%), and Kolyma Lowland (-0.51%) largely include increases due to thermokarst lake expansion, but more dominant lake area losses due to catastrophic lake drainage events. In contrast, Central Yakutia showed a remarkable increase in lake area of 48.48%, likely resulting from warmer and wetter climate conditions over the latter half of the study period. Within all study regions, variability in lake dynamics was associated with differences in permafrost characteristics, landscape position (i.e., upland vs. lowland), and surface geology. With the global availability of Landsat data and a consistent methodology for processing the input data derived from robust trends of multi-spectral indices, we demonstrate a transferability, scalability and consistency of lake change analysis within the northern permafrost region.
Tree stands in the boreal treeline ecotone are, in addition to climate change, impacted by disturbances such as fire, water-related disturbances and logging. We aim to understand how these disturbances affect growth, age structure, and spatial patterns of larch stands in the north-eastern Siberian treeline ecotone (lower Kolyma River region), an insufficiently researched region. Stand structure of Larix cajanderi Mayr was studied at seven sites impacted by disturbances. Maximum tree age ranged from 44 to 300 years. Young to medium-aged stands had, independent of disturbance type, the highest stand densities with over 4000 larch trees per ha. These sites also had the highest growth rates for tree height and stem diameter. Overall lowest stand densities were found in a polygonal field at the northern end of the study area, with larches growing in distinct " tree islands". At all sites, saplings are significantly clustered. Differences in fire severity led to contrasting stand structures with respect to tree, recruit, and overall stand densities. While a low severity fire resulted in low-density stands with high proportions of small and young larches, high severity fires resulted in high-density stands with high proportions of big trees. At waterdisturbed sites, stand structure varied between waterlogged and drained sites and latitude. These mixed effects of climate and disturbance make it difficult to predict future stand characteristics and the treeline position.