Quartz Weathering in Freeze-Thaw Cycles; Experiment and Application to Elgygytgyn Crater Lake Record for Tracing Siberian Permafrost History

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Schwamborn, G. , Diekmann, B. , Schirrmeister, L. and Foerster, A. (2010): Quartz Weathering in Freeze-Thaw Cycles; Experiment and Application to Elgygytgyn Crater Lake Record for Tracing Siberian Permafrost History , Third European Conference on Permafrost, 13-17 June 2010, Longyearbyen, Svalbard, Norway .
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Brittle and fragile sand and silt is produced in near surface frozen ground. It makes up much of the re-golith in permafrost areas such as Arctic Siberia. Microscopical grain features (e.g. angular outlines, surficial microcracks) illustrate grooves of cryogenic destruction in the course of numerous seasonal freezing and thawing events. Even after a grain is transported off place (i.e. in mobile slope material, in seasonal melt water run-off, into a lake basin), it still keeps the particular weathering traces.This is also valid for a mineralogical peculiarity; quartz is more susceptible to frost weathering than e.g. feldspar, another ubiquitous mineral. Quartz quickly reacts to cryogenic break-up and small grains disintegrate due to thermal fluctuations and the explosive power of expanding ice in micro-meter scale fissures. Quartz enriches in the fines and this anomaly when compared to low latitude weathering is linked to cryogenic weathering. This is demon-strated in an experimental set-up, where after more than 100 freeze and thaw cycles quartz-rich silt is produced in initially fine sandy samples of perma-frost and non-permafrost origins (Figure 1 A). The preferential crack of quartz grains (with reference to feldspar) can be expressed using the so-called Cryo-genic Weathering Index (CWI). This quartz-to-feldspar ratio highlights quartz enrichment in the fines when values are >1 (Konishchev and Rogov, 1993) and marks a zone, which is indicative for ma-terial resulting from cryogenic break-up.The combination of silt abundance, quartz grain micromorphology, and quartz enrichment is used as a proxy data set for frost weathering history in sedi-mentary archives. It has been examined in a 5 m core composed of frozen slope deposits and weath-ered bedrock (P2), in surface samples and along a lake sediment core from Elgygytgyn Impact Crater, Central Chukotka (Figure 1 B+C). This site provides the longest continuous terrestrial archive available for the continental Arctic. The basin was non-glaciated in Quaternary times and studied layers are dating back 220.000 years according to the available age model (Juschus et al., 2007). All frozen ground samples and also the upper 12 meters of the lake sediment core are characterised by silt abundance, cryogenic grain micromorphology, and quartz en-richment in the silt fraction. This argues for persis-tent permafrost conditions in the area as reflected by the continuous input of cryogenic weathering detri-tus into the basin. Even when periods were as warm as or warmer than today (i.e. during the Eemian In-terglacial) the permafrost signal does not disappear according to the lake record.

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