Granulometry, mineralogy and organic matter content of permafrost core 5011-3

The combination of permafrost history and dynamics, lake level changes and the tectonical framework is considered to play a crucial role for sediment delivery to El'gygytgyn Crater Lake, NE Russian Arctic. The purpose of this study is to propose a depositional framework based on analyses of the core strata from the lake margin and historical reconstructions from various studies at the site. A sedimentological program has been conducted using frozen core samples from the 141.5 m long El'gygytgyn 5011-3 permafrost well. The drill site is located in sedimentary permafrost west of the lake that partly fills the El'gygytgyn Crater. The total core sequence is interpreted as strata building up a progradational alluvial fan delta. Four macroscopically distinct sedimentary units are identified. Unit 1 (141.5-117.0 m) is comprised of ice-cemented, matrix-supported sandy gravel and intercalated sandy layers. Sandy layers represent sediments which rained out as particles in the deeper part of the water column under highly energetic conditions. Unit 2 (117.0-24.25 m) is dominated by ice-cemented, matrix-supported sandy gravel with individual gravel layers. Most of the Unit 2 diamicton is understood to result from alluvial wash and subsequent gravitational sliding of coarse-grained (sandy gravel) material on the basin slope. Unit 3 (24.25-8.5 m) has ice-cemented, matrix-supported sandy gravel that is interrupted by sand beds. These sandy beds are associated with flooding events and represent near-shore sandy shoals. Unit 4 (8.5-0.0 m) is ice-cemented, matrix-supported sandy gravel with varying ice content, mostly higher than below. It consists of slope material and creek fill deposits. The uppermost metre is the active layer (i.e. the top layer of soil with seasonal freeze and thaw) into which modern soil organic matter has been incorporated. The nature of the progradational sediment transport taking place from the western and northern crater margins may be related to the complementary occurrence of frequent turbiditic layers in the central lake basin, as is known from the lake sediment record. Slope processes such as gravitational sliding and sheet flooding occur especially during spring melt and promote mass wasting into the basin. Tectonics are inferred to have initiated the fan accumulation in the first place and possibly the off-centre displacement of the crater lake.

研究认为，永久冻土（permafrost）的历史与动力学过程、湖泊水位变化以及构造格局，对俄罗斯北极东北部的埃尔吉特金陨石坑湖（El'gygytgyn Crater Lake）的沉积物输送起到关键作用。本研究旨在基于湖岸岩芯地层分析与该区域既往多项研究的历史重建结果，构建一套沉积学框架。研究团队使用取自总长141.5米的埃尔吉特金5011-3号永久冻土钻井的冷冻岩芯样品，开展了沉积学相关研究。该钻井点位位于湖泊西侧的沉积型永久冻土区，该区域部分充填了埃尔吉特金陨石坑。整套岩芯序列被解读为一套进积型冲积扇三角洲地层，共识别出4个宏观特征迥异的沉积单元。第1单元（141.5~117.0 m）由冰胶结、基质支撑的砂质砾岩及夹层砂层组成，其中砂层代表了在强水动力条件下以颗粒形式在水团深处沉降形成的沉积物。第2单元（117.0~24.25 m）以冰胶结、基质支撑的砂质砾岩为主，夹有单层砾石层；该单元的混杂堆积物（diamicton）主要由盆地斜坡上的粗粒（砂质砾石）物质经冲积冲刷及后续重力滑塌形成。第3单元（24.25~8.5 m）为冰胶结、基质支撑的砂质砾岩，其间发育砂岩层，这些砂层与洪水事件相关，代表了近岸砂滩沉积。第4单元（8.5~0.0 m）为冰胶结、基质支撑的砂质砾岩，冰含量自上而下存在变化且整体高于下部层位；该单元由斜坡物质与溪谷充填沉积物组成，其最顶部1米为活动层（active layer），即发生季节性冻融的表层土壤，其中已混入现代土壤有机质。从陨石坑西部与北部边缘发生的进积型沉积物搬运过程，或与湖盆中央区域频繁发育的浊积岩层（turbiditic layers）的互补产出存在关联，这一点可从该湖泊的沉积记录中得到验证。斜坡过程（如重力滑塌与片状洪水）多发生于春季融冰期，会促进物质失稳并搬运至湖盆中。研究推断，构造作用首先启动了该冲积扇的沉积，同时也可能造成了该陨石坑湖的偏心式分布。