Seismic Investigation of the El'gygytgyn Impact Crater Lake
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J Paleolimnol (2007) 37:49–63 DOI 10.1007/s10933-006-9022-9 ORIGINALPAPER Seismic investigation of the El’gygytgyn impact crater lake (Central Chukotka, NE Siberia): preliminary results F. Niessen Æ A. C. Gebhardt Æ C. Kopsch Æ B. Wagner Received: 11 August 2004 / Accepted: 1 May 2006 / Published online: 12 December 2006 Ó Springer Science+Business Media B.V. 2006 Abstract The 12 km wide and about 175 m deep 500–600 m is brecciated. The lake is filled with El’gygytgyn crater lake in Central Chukotka, NE two units of sediments, the upper one well strat- Siberia, is of special interest for investigation as it ified and the lower one massive. In the center of could provide the first undisturbed 3.6 Ma ter- the lake, the combined thickness of the two sed- restrial record from the Arctic realm, reaching imentary units is estimated to be 320–350 m. The back a million years before the first major glaci- upper unit is draped over the location of an ation of the Northern Hemisphere. A single- interpreted central peak and is locally interca- channel seismic survey was carried out on an lated with debris flows, mainly in the western part expedition to the lake in 2000, in which both high of the lake and at the lake margins. Most of the resolution and deep penetration data were ac- lower unit is obscured by multiples as a result of quired. Seismic data suggest an impact crater high reflection coefficients in the upper unit. As at structure in Cretaceous volcanic bedrock, indi- least the upper unit appears to be undisturbed by cated by velocities of >5000 m s–1, whose upper glaciation, the lake should yields unique infor- mation on the paleoclimatic development of the East Siberian Arctic. This is the fourth in a series of eleven papers published in this special issue dedicated to initial studies of El’gygytgyn Crater Lake and its catchment in NE Russia. Julie Keywords Siberia Æ Arctic Æ Lacustrine Brigham-Grette, Martin Melles, Pavel Minyuk were guest sediments Æ Impact crater Æ Seismic investigations Æ editors of this special issue. Paleoclimate F. Niessen (&) Æ A. C. Gebhardt Alfred-Wegener-Institute for Polar and Marine Research, Columbusstraße, D-27515 Bremerhaven, Introduction Germany e-mail: [email protected] With an average diameter of 18 km and an age of about 3.6 Ma (Layer 2000), the El’gygytgyn cra- C. Kopsch Alfred-Wegener-Institute for Polar and Marine ter in the remote Anadyr Mountains of Central Research, Telegrafenberg A43, D-14473 Potsdam, Chukotka in NE Siberia (Fig. 1), is one of the Germany largest young impact craters known on Earth (Hodge 1994). The surrounding hills are formed B. Wagner University Leipzig, Institute for Geophysics of nearly flat-lying Late Cretaceous volcanic and Geology, Talstr. 35, D-04103 Leipzig, Germany rocks, mainly andesites, rhyolites, trachyrhyolites 123 50 J Paleolimnol (2007) 37:49–63 Fig. 1 Location, crater rim and bathymetry of Lake El’gygytgyn. PG1351 marks the position of the core retrieved in 1998 (Melles et al. 2007). Bathymetry and topographic contours in m and their tuffs (Masaitis 1999, and references characterised by steep slopes down to depths of therein), into which the crater was excavated. The 150 m (locally as steep as 45°) beneath which the lake has a diameter of about 12 km and is slightly lake bottom slopes gently to the deepest point offset towards the south-eastern quarter of the north-east of the lake center. Most of the catch- crater by large sediment aprons that have filled ment is restricted to an area inside the crater rim parts of the basin from the west and north-west (Fig. 1) and comprises 293 km2 (Nolan et al. (Dietz and McHone 1976). The lake is almost 2003), which includes 50 small streams draining circular, encompasses an area of about 110 km2, into the lake (Nolan and Brigham-Grette 2007). and contains a water volume of about 14.1 km3 The Enmyvaan River at the southern end of the with a maximum depth of about 175 m (Nolan lake is the only outlet stream (Fig. 1). Mean and Brigham-Grette 2007). The flanks are January temperatures are around –32 to –36°C, 123 J Paleolimnol (2007) 37:49–63 51 and mean June temperatures range from +4 to vessel, had do be flown in by helicopter. The +8°C. Only from mid-July to mid-September, is expedition faced logistic problems due to a severe the lake totally ice-free. Russian investigations kerosene shortage in Pevek, a town on the coast of imply that the crater and surrounding hills have the East Siberian Sea and the closest helicopter never experienced glaciation (Glushkova et al. base to the lake. This fuel shortage limited heli- 1994), which is consistent with findings on the copter flights to the lake and thus transport glacial history of other parts of Chukotka (Brig- capacity. In order to carry out preliminary seismic ham-Grette et al. 2007). Therefore, the crater investigation under these circumstances, we fol- lake has the potential to provide an undisturbed lowed a Swiss approach for investigation of peri- paleoclimate record of the last 3.6 million years. alpine lakes (see Finckh et al. 1984 and references Previous analysis of sediments recovered in a therein) by using a small and portable compressor, 13 m long core from the deepest part of Lake air gun, single-channel streamer and sediment El’gygytgyn are consistent with this assumption. echosounding equipment. The core has a basal age of about 250,000 years One major goal of this study was to develop (Nowaczyk and Melles 2007) and contains a a preliminary two-dimensional velocity–depth complete and undisturbed lacustrine sequence of model based on sonobuoy seismic-refraction the last three climate cycles with no evidence of data. This model provides the first interpretation erosion during glacial periods (Melles et al. 2007). of the thickness of the sediment fill and of the There is various evidence for an impact origin presence and thickness of an impact breccia of the lake (Dietz and McHone 1976; Hodge layer overlying Cretaceous bedrock. The second 1994; Masaitis 1999). This includes a gravity goal was to analyse the geometry and seismic profile across the crater that exhibits a positive facies of the sediment fill from both air-gun anomaly of 15 to 20 mgals in the center of the reflection and sediment echosounding data. This crater. This anomaly is probably related to a allows a preliminary interpretation of the seismic central uplift of the type often seen in impact facies and sediment accumulation characteristics structures of the size of the El’gygytgyn crater, in the lake and thus an assessment of the and is located about 3 km to the NW of the center potential of the record for future deep drilling. of the present lake (Alyunin and Dabizha 1980; The seismic data obtained in 2000 were also used Dabizha and Feldmann 1982). Rocks and miner- to improve our acoustic acquisition techniques als from the crater show evidence of shock effects, on remote lakes which were applied during a ranging from planar features in quartz grains to second seismic survey of Lake El’gygytgyn in the presence of coesite, stishovite and lechatelie- 2003. At the time of acceptance of this paper, rite (Hodge 1994). Up to the time of our inves- the new results were still in the processing and tigation, no impact breccia has been reported interpretation stage but are now partly published from El’gygytgyn although numerous tectites are (Gebhardt et al. 2006). found in the catchment (e.g. Gurov 1986; Gurov and Gurova 1996). Tectites were dated by Layer (2000) to approx. 3.6 Ma ago, the likely time of Data acquisition, processing, and analysis impact. Belyi (1997) interpreted the El’gygytgyn depression largely as a product of the neotectonic For data acquisition, we used a 3 · 4 m alumin- evolution of Northeast Asia, which is not consis- ium platform (‘‘R/V Helga’’) equipped with 4 tent with any of this evidence. inflatable tubes for flotation and a 25 HP out- As part of a multidisciplinary expedition to Lake board engine (Fig. 2). The maximum total load- El’gygytgyn in August 2000 (THE-IMPACT ing capacity of the vessel for safe operation is Project, Terrestrial History of El’gygytgyn—Inter- about 1000 kg. A Bolt 600B airgun (5 cu inches) national Multidisciplinary Paleoclimate Project), supplied with a diving compressor (Bauer we carried out the first seismic investigations of the Oceanus, capacity 140 l min–1) was used as an crater lake. Due to the remote location of the lake acoustic energy source. The shot interval was 6 s (Fig. 1) all measurement equipment, including the with approximately 110 bar gun pressure. The 123 52 J Paleolimnol (2007) 37:49–63 Fig. 2 Research platform ‘‘Helga’’ on Lake El’gygytgyn in summer 2000 average speed of the vessel was 4–5 km h–1. For directions along lines crossing near the center of acquisition, we used a 20-element single channel the lake (Fig. 3). However, there is no overlap in hydrophone streamer (Geoacoustics AE5000), the resulting refraction data presented in this paper connected to a digital receiver (Octopus 360) via because the sonobuoys were placed 4.5 km apart interface box to create digitised data in SEG-Y but only up to 2.2 km of data directly adjacent to format. In addition to the Geoacoustics the two buoys were used due to very low signal to hydrophone streamer, a single hydrophone (OYO noise ratios at larger offsets.