Development of cryogenic processes as a factor in the loess formation in the Lower Volga valley

The cyclic climate fluctuations in the Pleistocene are estimated by researchers to be reflected in the structureof loess-paleosol series: during warm interglacial and interstadial epochs, the role of biogenicsedimentation increased and soils were formed; during cold epochs, when there was a significantexpansion of the permafrost, loess horizons accumulated and frost fractures were formed. These judgmentsare based on the results of analysis of wedge-shaped structures, as well as numerous data from sporepollen,micro-faunal and other types of analysis. There are also notions, based mainly on the analysis of theparticle size distribution of loess, of a very cold environment during their formation and a significantdevelopment of cryogenic weathering at that time. Loess deposits in the Northern Caspian Sea region playa significant role in understanding continental scale palaeoclimate systems in Eurasia during thePleistocene. In contrast to the wellstudied loess areas of Europe, Central and East Asia, loess in theNorthern Caspian lowland has received very little research attention. Despite this lack of attention, itsgeographical position and the lack of other palaeoenvironmental data from continental deposits in theregion (Koltringer et al., ۲۰۲۰) make it an important ‘missing link’ for developing a full understanding ofEurasian palaeoclimate and wind systems. The authors conducted field studies in the Lower Volga regionin ۲۰۱۵-۲۰۲۱, which allowed identifying different-age post-cryogenic structures (pseudomorphosis,cryoturbation, involution, etc.) - traces of relict seasonal and perennial freezing. The loess-paleosol seriesof the Lower Volga region provide a unique opportunity to reconstruct the stages of landscapedevelopment between the Khazarian and Khvalynian transgressions of the Caspian Sea. The work is basedon the results of comprehensive study of the Srednyaya Akhtuba section located on the left bank of theVolga-Akhtuba valley, ۳ km south of the Srednyaya Akhtuba village (coordinates N ۴۸°۴۲'۰۱.۳"; E۴۴°۵۳'۳۷.۱"). To assess the effect of cryogenesis on the loess sediments, their mineralogy was analysed indetail for the main particle size fractions of loess - fine sand and coarse dust - and the cryogenic contrastcoefficient (CCC) was calculated. The CCC does not primarily reflect low negative temperatures, but ratherthe development of cryogenic fracture during ice formation in microcracks in particles under conditions ofsufficient ground humidity. The CCC value in Holocene stage sediments (MIS-۱, layers ۲ and ۱) decreasesfrom ۰.۹۸ to ۰.۷۶, corresponding to the transition from the cold phases of the Valdai glaciation to thewarmer Holocene period. The stage of alluvial sequence accumulation corresponding to the second half ofthe MIS-۳ stage and the early phase of the Early Khvalynian transgression of the Caspian Sea (strata ۱۱-۸)is characterized at its beginning and end by low values of the TCC (۰.۸۲), reaching a maximum in the sandaccumulated in the middle of the stage at the border of strata ۹ and ۱۰ (۱.۰۶). This value suggests a short,but very cold, time with the presence of permafrost, polygonal cracking of soils, as pseudomorphosis alongpermafrost wedges was encountered in the above layer. The Atelian regression stage is characterized bylow CCC values: layer ۱۲ has values of ۰.۸۶-۰.۸۷; layer ۱۳ reflects colder accumulation conditions. Thesevalues do not suggest permafrost, but indicate deep seasonal freezing of sediments. In the sediment seriescorresponding to the Late Khazarian transgressive-regressive stage in the Caspian Sea (MIS-۵), the effectof cryogenesis on the background of permafrost with the processes of frost bursting is clearly seen in thecomposition and structure of deposits. The most transformed strata are layers ۱۴ (MIS-۵a, KCC ۱.۰۷), ۱۶and ۱۷ (MIS-۵c, -۵d) with CCC values of ۱.۰۶ and ۱.۱۲. These horizons were subject to cryogenic changeduring cold MIS-۴ permafrost conditions in the study area. During the dry stages of loess accumulation(Atelian loess), lower CCC values can be explained by the existence of frozen (dry) rocks in conditions ofinsufficient moisture. The study was supported RFBR (project ۲۰-۵۵-۵۶۰۴۶).

کلیدواژه ها:

Cryogenesis ، Lower Volga loess ، Late Quaternary ، Lower Caspian Lowland