講演情報
[ACC45-P07]Landform associated with excess ground ices in Mongolia – distribution morphology, internal structure, age, and formation processes
*石川 守1 (1.北海道大学)
キーワード：
過剰地下氷、ピンゴ、サーモカルスト、構造土、モンゴル、永久凍土
The excess ground ice (EGI), occupying more volume than soil porosity, greatly influence landform development in permafrost regions. Its accumulation forms dome-shaped hilly terrains (pingos), while disappearance subsidence terrains (thermokarst). Repeated cycles of precipitation infiltration into permafrost and subsequent freezing develop wedge-like EGI, which form polygonal features (non-sorted patterned ground). Understanding the morphology, structure and distribution of these landforms would provide new insight into permafrost dynamics, which have been conventionally evaluated only from thermal characteristics. In Mongolia, the southern limit of the Eurasian permafrost, the effects of permafrost thawing on surface environments would appear most rapidly and significantly in the globe. Permafrost distribute continuously in higher latitudes and altitudes, discontinuously and sporadically in mid latitudes. Over the lower latitudes, seasonally frozen ground are dominantly widespread. Pingos, thermokarst and polygons are well developed in water-rich environments and their form and dimension are highly diverse along with transition of ground thermal states. We have conducted geomorphic researches in the Darkhad, Mongmot, Jaramtai, Chuluut, and Galuut areas, using satellite-based analysis, geophysical survey, high-resolution topographic modelling by unmanned aerial vehicle system. This presentation describes the findings regarding the distribution, geomorphic and limnological settings, morphology, internal structure, and chronology of these landforms.
In the Darkhad, numerous pingos occur in former lakes and river channels. Their morphology is diverse with dome-shaped mounds and obvious craters at their summits. These pingos are mostly formed by pressurized closed-system ground water, as InSAR analysis identified continuous uplift areas (several cm/year), which correspond to former lake bottom. Mean annual ground temperatures (MAGT) are relatively low, ranging from -2.1 to -2.4°C, and internal resistivity values are generally high (>20kΩm). Indeed, drilling surveys found thick pure ice layer of several meters in thickness. Analysis of stable isotope and C14 indicated that ice formed slowly over several millennia in a closed environments. The pingos in the Jaramtai area have subsided top and low resistivity (8–15kΩm). While most pingos generally have gentle slopes, one exceptionally has steep slopes and contains high-resistivity core exceeding 300kΩm. Obvious tension cracks on the tops and side slopes indicate this pingo is currently in growing stage. In the Mongmot area, pingo are generally large with relative height of 10–20 m developed on floodplains and former lake bottoms. Drilling survey revealed the presence of an EGI layer and pressurized groundwater beneath it (MAGT: -1.2°C). A pingo far from rivers and lakes show subsidence morphology, and no high resistivity zones suggesting excess ground ice were found. In the Chuluut area, four pingos are scattered across a alluvial terraces composed of somewhat dry soil deposits, all exhibiting typical dome-shaped profiles. Resistivity near the surface are generally low (0.3–1.2kΩm), but some include high-resistivity layers exceeding 20kΩm at depth, suggesting ice cores (MAGT: -0.5°C). In this area, InSAR analysis observed that the toe of alluvial cones are gradually uplifting (several cm/year). This may capture the embryo stage of an open-system pingo. The Galuut area locates within sporadic permafrost zone, and pingos and thermokarst concentrate in the wet environments around spring at the toe of alluvial fans. The most prominent pingo suggests the presence of an ice-rich layer exhibiting high resistivity exceeding 20kΩm (MAGT: -0.8°C). This pingo, with height of several meters, is surrounded by embankment-like mounds, and possibly regenerated within the crater formed by collapsing of a former pingo.
In the Darkhad, numerous pingos occur in former lakes and river channels. Their morphology is diverse with dome-shaped mounds and obvious craters at their summits. These pingos are mostly formed by pressurized closed-system ground water, as InSAR analysis identified continuous uplift areas (several cm/year), which correspond to former lake bottom. Mean annual ground temperatures (MAGT) are relatively low, ranging from -2.1 to -2.4°C, and internal resistivity values are generally high (>20kΩm). Indeed, drilling surveys found thick pure ice layer of several meters in thickness. Analysis of stable isotope and C14 indicated that ice formed slowly over several millennia in a closed environments. The pingos in the Jaramtai area have subsided top and low resistivity (8–15kΩm). While most pingos generally have gentle slopes, one exceptionally has steep slopes and contains high-resistivity core exceeding 300kΩm. Obvious tension cracks on the tops and side slopes indicate this pingo is currently in growing stage. In the Mongmot area, pingo are generally large with relative height of 10–20 m developed on floodplains and former lake bottoms. Drilling survey revealed the presence of an EGI layer and pressurized groundwater beneath it (MAGT: -1.2°C). A pingo far from rivers and lakes show subsidence morphology, and no high resistivity zones suggesting excess ground ice were found. In the Chuluut area, four pingos are scattered across a alluvial terraces composed of somewhat dry soil deposits, all exhibiting typical dome-shaped profiles. Resistivity near the surface are generally low (0.3–1.2kΩm), but some include high-resistivity layers exceeding 20kΩm at depth, suggesting ice cores (MAGT: -0.5°C). In this area, InSAR analysis observed that the toe of alluvial cones are gradually uplifting (several cm/year). This may capture the embryo stage of an open-system pingo. The Galuut area locates within sporadic permafrost zone, and pingos and thermokarst concentrate in the wet environments around spring at the toe of alluvial fans. The most prominent pingo suggests the presence of an ice-rich layer exhibiting high resistivity exceeding 20kΩm (MAGT: -0.8°C). This pingo, with height of several meters, is surrounded by embankment-like mounds, and possibly regenerated within the crater formed by collapsing of a former pingo.