Depositional dynamics in the El'gygytgyn Crater margin: implications for the 3.6 Ma old sediment archive

Abstract

Abstract. 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 icecemented, 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.

Past freeze and thaw cycling in the margin of the El'gygytgyn crater deduced from a 141 m long permafrost record

“…The remaining sample parts were freeze-dried and then the absolute ice content was determined and is expressed as total water content equivalent in weight percentage (wt%) of the moist sample. The solid portion of the sample underwent a standard suite of sediment analyses, which is described elsewhere (Schwamborn et al, 2012a). Precipitation (snow and rain) and surface water from the lake and tundra ponds were sampled between early May and late August 2003.…”

“…2). The sediment interpretation of core 5011-3 concluded that the strata belong to different portions of a prograding fan delta that enters Lake El'gygytgyn from the west (Schwamborn et al, 2012a). Between 141.0 and 117.0 m core depth several thin (< 5 cm thick) sand layers intercalate with sandy gravel and are interpreted to be the bottomset facies of prodelta sediments, which were deposited in deeper water.…”

“…Creeks are intermittent and ponds do not persist. The coring site is located at the distal end of a sediment fan that is the most distinctive sediment body on the western-tonorthern braid plain; several fans in a row cover this area (Schwamborn et al, 2012a).…”

“…Interpretation of the permafrost core sediments suggests that there is a close linkage between alluvial fan growth on the slopes and the sediment delivery to the lake; in modern times the seasonal (i.e. snow-melt related) sediment transport removes sediment basinward from the subaerial plain into the subaqueous fan and basin floor environments and this is also interpreted for much of the geologic past in this Arctic setting even though core 5011-3 is poorly dated (Schwamborn et al, 2012a). Age control exists only for the upper 9.6 m of core where an Allerød (AD)-Younger Dryas (YD)-Holocene sequence is preserved based on the pollen record .…”

“…stream migration) and permafrost processes that have shaped the unique Lake El'gygytgyn archive. This paper complements the sedimentological approach of Schwamborn et al (2012a) by focusing on the hydrochemical aspects (i.e. changes in stable water isotopes and in ion concentration) of the 5011-3 permafrost record.…”