In autumn 2008 and spring 2009, an international team of scientists successfully conducted drilling operations in Lake Elgygytgyn, a 170 m deep lake of 12 km diameter located in a 3.6 Myr old meteorite impact crater in NE Siberia. In the central part of the lake, 312 m of lake sediments were penetrated above a suevite layer and brecciated bedrock. Extrapolating the sedimentation rates known for the last 340 kyr from cores recovered during pre-site surveys, the drilled lacustrine sediment sequence likely represents the entire depositional history since the impact event. This record is unprecedented, opening new opportunities to investigate the natural climatic and environmental changes in the terrestrial Arctic since Pliocene times. The paleoenvironmental significance of the record, however, depends on its completeness. Erosion or interruption of deposition due to lake desiccation or glacial overriding can widely be excluded from the available seismic data (Niessen et al. 2007) and on-site core descriptions. Gravitational sediment mass movement events, in contrast, are frequently recorded in the seismic data. In order to investigate their impact on the pelagic sediment record in central Lake Elgygytgyn, a subrecent mass movement deposit (gravite) at the western lake slope, identified by 3.5 kHz echo sounding, was penetrated by two sediment cores. Sedimentological and chronological data from these cores, along with respective investigations on two sediment cores from the central lake, illustrate the processes associated with the subrecent mass movement event, how representative this event is for the depositional history during the past 340 kyr, and how often large mass movement events occur in Lake Elgygytgyn (Juschus et al. 2009). The lower part of the gravite at the western lake slope is stratified, probably reflecting an initial mass movement stage with a debris flow associated with limited sediment mixture. Massive sediments above and in front of the debrite indicate a second stage with liquefied sediment transport due to a dense flow. Both, the debris flow and the dense flow caused basal erosion of ca. 1 m thick unconsolidated sediments along parts of the flow path. The event produced a suspension cloud, whose deposition led to the formation of a turbidite. The occurrence of the turbidite throughout the lake, and limited erosion at the base base of the turbidite, suggest predominant deposition by pelagic rain following Stokes Law. Applying this depositional model to the Late Quaternary sediment record thus far investigated, the results indicate that 28 large mass movement events took place in Lake Elgygytgyn during the past 340 kyr, being widely independent from the climatic changes that have occurred during this period. The results obtained will support to identify gravites in the 3.6 Myr record recently recovered, and to estimate their impact on the pelagic sediments that will be used for paleoenvironmental reconstructions.
Helmholtz Research Programs > PACES I (2009-2013) > TOPIC 3: Lessons from the Past > WP 3.2: Tectonic, Climate and Biosphere Development from Greenhouse to Icehouse