ALBERT

Beschreibung: Beenchime Salaatinsky Crater (BSC) is a 8 km wide, multi-million-year-old ring structure located west of the Olenyok River in northern Yakutia. This is an area that has not been covered by Pleistocene time glaciers (Ehlers, J., Gibbard, P.L., 2007; Niessen et al., 2013). Short-term scientific goals of our study were (i) to reveal the origin of the crater (impact crater or volcanic crater) based on mineral analysis and (ii) to evaluate prospective Quaternary sediment records in the basin. Following earlier geomorphological surveys, it is assumed that the basin is the result of a volcanic explosion similar to Kimberlite Pipes elsewhere found in Yakutia (Pinchuk et al., 1971). Alternatively, a meteorite impact has been blamed, because suevitic breccias were identified (Mikhailov et al., 1979, Masaitis, 1999). According to geomorphological age estimates, the crater is believed to be between 65 and 40±20 Ma old (EarthImpactDatabase), but a robust physical dating is actually missing. We sampled several landforms of the basin interior after digging soil pits into the ground and extracting short cores from the underlying permafrost. Sample sites were a peat plateau and ancient river terraces. In addition, a modern lake depression in the central part, 300 m in diameter and 4 m deep at maximum, has been studied using 50 MHz ground penetrating radar profiles and short cores. Bedrock samples were taken from representative sites of outcropping Paleozoic formations inside and outside BSC. Thin sections from bedrock were analyzed using polarized light microscopy. In fact, shocked quartz grains with PDFs (planar deformation features) were found in samples taken from a Permian sandstone outcropping in the crater interior. The crystallographic orientations were measured using a U-stage microscope. Some other samples of the crater rim were found to be only slightly shocked. We sum up our results in a preliminary scenario, which suggests a Paleozoic meteoritic impact event, a Mesozoic overburdening of the area and a subsequent erosion in the course of the Olenyok Uplift. Finally, we propose late Quaternary landscape dynamics based on sediment dating using AMS 14C and sediment properties in the crater; fluvial sediment transport is documented for the MIS 3 and MIS 1 periods whereas mid to late Holocene lake formation results from thermokarst dynamics. A distinct grain size change in the fine silt fraction from coarser to finer indicates increasing aridity in the area with lake level lowering during late Holocene time. References EarthImpactDatabase, 2019. http://www.passc.net/EarthImpactDatabase/Beyenchimesalaatin.html. Ehlers, J., Gibbard, P.L., 2007. The extent and chronology of Cenozoic global glaciation. Quaternary International, 164, 6-20. Niessen, F. et al., 2013. Repeated Pleistocene glaciation of the East Siberian continental margin. Nature Geoscience, 6 (10), 842. Grieve, R.A., 1987. Terrestrial impact structures. Annual Review of Earth and Planetary Sciences, 15, 245-270. Masaitis, V.L., 1999. Impact structures of northeastern Eurasia: The territories of Russia and adjacent countries. Meteoritics & Planetary Science, 34, 5, 691-711. Mikhaylov, M.V. et al., 1979. The Beyenchime-Salaata meteorite crater. Doklady Akademii Nauk SSSR, 245, 76-78 [in Russian]. Pinchuk L.Y., 1971. Morphology and genesis of Beenchime-Salaatin depression - Kimberlite volcanism and prospects of primary diamond content of the north-eastern Siberian platform. Proceedings Arctic Geology Research Institute, Leningrad, 123-126 [in Russian].

Beschreibung: The Beenchime-Salaatinsky Crater (BSC) is located west of the Olenyok River in Northern Yakutia, ~ 260 km south-west of Tiksi and the Lena Delta. The age and origin (volcanic versus meteoritic) of this crater is poorly understood. The key scientific interest in re-visiting the BSC is the reappraisal of the Quaternary sedimentation dynamics for a better understanding of the sediment history and thickness in the basin. This aides for an assessment, if the site is prospective for a deeper drilling of a Quaternary (or Cenozoic) sediment archive. Soil pits and auger cores from slopes and lowland terrain in the basin were sampled and studied to infer sediment ages and transport dynamics. This also included a thermokarst lake placed in the centre of the basin. Studied properties include grain-size distribution, organic carbon and nitrogen contents (TOC and TN), heavy mineral compositions, δ13C of organic carbon, 14C ages from sediment, δ18O and δD from ground ice and waters, and lake bathymetry from GPR profiling, in addition. We conclude that the crater floor in the BSC is underlain by fluvial/alluvial sediments from the MIS 3 period. Thermokarst lake formation took place during the Holocene Thermal Maximum between 7600 and 6100 cal yr BP. The lake has been shrinking hereafter. Fluvial/alluvial sedimentation along the drainage pattern was active again between 5700 to 1500 cal yr BP, and it was flanked by the accumulation of peaty and organic-rich sediments and the formation of ice-wedge polygons.

Beschreibung: Muostakh Island (N 71°36’; E 129°57’) is a spectacular site in the Siberian Arctic due to its very high coastal erosion rates of up to 20 m/year as well as substantial permafrost subsidence (Günther et al., 2015). The island extends about 7.5 km in the N-S direction and up to 500m in E-W direction and has been connected to the mainland until a few thousand years ago. Sediments and ground ice of the Yedoma type (the so called Ice Complex; Schirrmeister et al., 2011) form up to 20 m high coastal cliffs on Muostakh Island. The main aim of this multi-proxy study is to cover the complete sedimentological and geocryological sequence for a detailed palaeoenvironmental interpretation of the island. In general, the sedimentary sequence from Muostakh Island is divided into three stratigraphic units and consists of sediments of late Pleistocene to Holocene age. The lowermost unit A is approximately 8 m thick, rich in ground ice and comprises mostly sandy silt alternating with thin peat layers. At the top of this unit, a prominent 1 m thick peat layer is found in many sections on the island. In unit A, the ice wedges may reach widths of up to 5 m. At ca. 10m a.s.l., a hiatus from ca. 41.6 kyr BP to ca. 19.7 kyr BP is indicated by an erosional plane sharply intersecting ice wedges and sedimentary structures as well as by geocryological observations, i.e. distinctly narrower ice wedges above the discordance. Above this erosional plane, unit B, which is composed of 8-9 m of coarse-grained material, is indicative for fast and highly-energetic deposition, absent in many other sites of Ice Complex exposures in the Laptev Sea region (i.e. at nearby Mamontovy Khayata outcrop on Bykovsky Peninsula; Meyer et al., 2002). The upper (and youngest) sedimentary unit C reaches a maximum thickness of about 4-5 m and is laterally discontinuous. About 10 m wide peaty patches of Holocene organic-rich and ice-rich sandy silts cover the underlying deposits. Peat patches are intersected by ice wedges of generally less than 1 m wide, but in exceptions also reaching 3-5 m in width, penetrating downwards into the older layers. Here we present the complete sedimentological sequence of Muostakh Island including new AMS 14C radiocarbon ages both from sediment and ice wedges. Sedimentological, geochemical and geocryological data include the coarse-grained layer for the first time, which highlights an episode of erosion and rapid deposition in Siberian Ice Complex during the Last Glacial Maximum. Stable isotope data from ice wedges complement this study with winter season paleoclimate information. This multidisciplinary study will re-evaluate the Late Quaternary depositional and palaeo-environmental history of the region.

Beschreibung: This Bachelor's thesis focuses on the heavy mineral analysis of sediment samples taken in 2012 by an expedition team lead by Hanno Meyer from the Ice Complex formation on Muostakh Island in the Laptev Sea (NE Siberia). The heavy mineral analysis was used to investigate the provenance and transportation and sedimentation processes of the settled material. As influencing effects on the heavy mineral composition the origin of the material, further transportation energy and chemical weathering could be identified, which was also confirmed by a principal component analysis in combination with grain-size distribution data, provided by Hanno Meyer. The heavy mineral composition is dominated by amphibole, followed by pyroxene, garnet and opaque minerals. The origin indicating mineral leucoxene, appearing regional augmented, could not be identified. As provenance the Lena river could be identified by the use of comparative heavy mineral data. The sedimentation occurred in three phases. Between the first and second phase a hiatus in the stratigraphic record exist. 14C-dating (Meyer (unpublished)) confirm this indicating a gap between ca. 41.6 kyr BP and ca. 19.7 kyr BP. This disconformity is caused by an erosional event. After this event chemical weathering took place at the top of the deposited layers of the first phase producing significant red aggregates. The second phase is characterized by higher transportation energy compared to the first and third phase, which is reflected by the appearing of rutile almost just in the corresponding unit and a more coarse grain-size distribution.