青藏高原发现大型冻胀丘群

在青藏高原黄河源地区多格茸盆地内发现大型冻胀丘群,这些冻胀丘外观多呈穹窿状,有些呈脊状、新月状.盆地内的冻胀丘隆起高度多在3~6 m,最高不超过10 m,水平扩展范围数十米到上百米,最大超过300 m.冻胀丘在盆地内密集分布,众多小型湖塘嵌于其中,分布密度估计5~8个·km^-2,局部区域内可达10个·km^-2以上.从外观看,多数冻胀丘形态较完整,但有的已经部分塌陷,有的几乎完全塌陷,不同塌陷阶段的冻胀丘在盆地内均有发现.冻胀丘顶部钻孔岩芯揭示高含冰地层从丘顶以下5 m左右一直延续到20多米,局部发育厚层地下纯冰层,厚度可达2 m以上.根据外观形态、地下冰类型判断,这些多年生的冻胀丘属冰土丘(lithalsa),和常见的冰核丘(pingo)有所区别.初步判断,这些冻胀丘形成于全新世大暖期以后的新冰期时期.多格茸盆地冻胀丘群的发现对现行的工程建设及工程建筑物的运营安全、区域古环境、流域水文等方面的研究都具有重要意义.     

Segregated ice structures in various heaved permafrost landforms through CT Scan

The growth of segregated ice lenses in frost susceptible sediments in the discontinuous permafrost zone is the dominant mechanism for the formation of permafrost mounds, such as palsas, lithalsas and permafrost plateaus. Thawing of these mounds creates thermokarst lakes, which are particularly abundant in Nunavik, east of the Hudson Bay area. The inception of the permafrost in mounds and their growth are regulated by climate conditions, by local Quaternary geology and by environmental factors such as topography, vegetation, snow cover and surface humidity. Variable sizes and morphology of the permafrost mounds can be attributed to local factors that affect the ice segregation process, particularly the supply of water needed for ice‐lens growth and grain‐size composition of the soil into which aggradation takes place. Computer image analysis of CT scans on high quality cores obtained from permafrost mounds and plateaus of various shapes reveal that the ice layer sequences and permafrost internal structure vary with landform types. A relationship therefore exists between different morphological type within a family of landforms and their microscale internal structure. Copyright © 2007 John Wiley & Sons, Ltd.     

Composition and origin of a lithalsa related to lake‐level recession and Holocene terrestrial emergence,Northwest Territories,Canada

Lithalsas of the Great Slave Lowland, Northwest Territories, occur within fine‐grained glaciolacustrine, lacustrine, and alluvial deposits. Detailed investigations of a lithalsa revealed that it is composed of ice‐rich sediments with ice lenses up to 0.2 m thick below 4 m depth. The observed ice accounted for about 2 m of the 4 m between the top of the lithalsa and adjacent terrain. The ice is isotopically similar to modern surface water, but enriched in δ¹⁸O relative to local precipitation. Total soluble cation concentrations are low in the basal, Shield‐derived and unweathered glaciolacustrine sediments of the lithalsa. Higher concentrations in the overlying Holocene‐aged lacustrine and alluvial deposits may be due to greater ion availability in Holocene surface waters. Increasing Cl^‐ and Na⁺ concentrations in clays at depth likely relate to exclusion and migration of these dissolved ions in pore water during ice lens formation though total soluble cations remain comparatively low. The lithalsa developed 700 to 300 cal yr BP. A conceptual model of lithalsa formation and landscape evolution illustrates that this feature and more than 1800 other lithalsas in the region have developed in association with Holocene terrestrial emergence following lake‐level recession. Copyright © 2017 John Wiley & Sons, Ltd.