ИСТИНА

Bottom sediments of the Lake Donguz-Orun (Central Caucasus, Elbrus region), first retrieved in 2012, revealed regular laminae typical for proglacial varved lakes. In order to confirm the annual lamination we used several geochemical markers. The sediment core was scanned for elemental analysis using XRF-SR (synchrotron) technology in the Institute of Nuclear Physics SB RAS at the resolution of 200 µm and ultra-high resolution of 30 µm for the upper 20 mm. The ratio Rb/Sr was selected as an indicator of an annual cycle of sedimentation due to different mobility of the two elements depending on the sediment grain-size. Indeed, the Rb/Sr curve revealed a strong pattern of annual variability with lower values corresponding to the spring snowmelt (with a near-perfect fit with the visible layers). Subsequent use of the other indicators, namely Zr/Rb and others, provided an independent calculation of the annual layers. Together with the Cs-137 dating, all the results agree on the sedimentation rate around 1.7 mm/yr for the period 1910-2010. Distinguishing the separate annual layers enabled the comparison of the sedimentary geochemical data with the annually resolved data, such as the local meteorological observations and tree-ring series. The concentration of bromine – often considered as a temperature-sensitive biophile element – revealed a significant correlation with the local annual temperature (r2=+0.48). The concentrations of terrigenous elements – K, Ti, Fe, Zn, As, Rb – show a correspondence with the annual atmospheric precipitation, which seems logical considering that the lake is glacier-fed. On a shorter timescale, certain elements were able to record precisely abrupt changes in the instrumental records. Thus, varved sediments of the lake Donguz-Orun revealed a great potential in terms of creating regional reconstructions of climatic parameters, which are still scarce in the Caucasus. Currently, the work on a longer sediment core is undertaken with an estimated timespan of 350-400 years.