Landscape Dynamics in the Caspian Lowlands Since the Last Deglaciation Reconstructed From the Pedosedimentary Sequence of Srednaya Akhtuba, Southern Russia

Abstract

Surface Kastanozem of the Lower Volga area was first studied as a part of the pedocomplex, with the lower part (148–160 cm) formed in Early Khvalynian Chocolate clays (13–15 ka), the middle part (100–148 cm) in a mixed clay-loess sediment sand, and the upper part (0–100 cm) in loess. This resulted from local aeolian transport, with the source material derived from the rewinding of marine sediments. They are enriched in aggregates of Chocolate clays and glauconitic grains of a fine sand-course silt size and have similar contents of clay minerals. The high salinity of similar types evidences marine genesis for both Chocolate clays and source material for loess sediments. Clay fragments of a sand and silt size are responsible for the heavy texture and high gypsum content of loess. The study of soils with the focus on micromorphology and clay mineralogy allows the identification of the complex character of a shift from marine to sub-areal sedimentation. This shift was accompanied by short breaks in sedimentation, allowing the development of synlithogenic soil horizons of Late Khvalynian, after-Khvanynian, and Boreal time. The features of shallowly buried soil horizons confirm increased aridity after the last deglaciation. Surface Calcic Kastanozem is a full Holocene soil reflecting the present environment. However, it is deeply influenced by shallow buried soil horizons and Chocolate clays.

Enviromagnetic study of Late Quaternary environmental evolution in Lower Volga loess sequences, Russia

“…Notably, there is a lack of paleoclimate information during regressive phases, when much of the Caspian region was sub-aerially exposed. In addition to the well-studied marine sediments that represent Caspian transgressive phases, considerable thicknesses of terrestrial deposits also comprise the Lower Volga sequences (e.g., Lebedeva et al, 2018). Periods of continental sedimentation under regressive phases are represented by fluvial sands as well as more silty, apparently sub-aerial deposits in the Lower Volga region.…”

“…Sediments from transgressive stages are described primarily in Russian literature, where past climate conditions are reconstructed based on marine fauna and sedimentological analyses (Fedorov, 1978; Svitoch and Yanina, 1997). However, recent pedological investigations have begun to shed light on the environmental conditions during stages of continental sedimentation in the Lower Volga region, suggesting a cool and arid climate (Lebedeva et al, 2018). This investigation also points to local sources for the loess deposits, notably from marine sediments exposed during regression (Lebedeva et al, 2018).…”

“…The origin and transport of these silty sediments, as well as their post-depositional modification, are controversial. While apparently displaying many characteristics of loess (in the general sense of accumulation of windblown terrestrial silts, sensu Muhs, 2013) (Lebedeva et al, 2018), the term "loess-like material" is used in many studies (Kolomiytsev, 1985;Shakhovets, 1987;Svitoch and Yanina, 1997;Lavrushin et al, 2014). This careful way of addressing the material might derive from the common support for an in-situ loess formation theory by Russian scientists, one of the main paradigms of Russian loess research (e.g., Smalley et al, 2010Smalley et al, , 2011.…”

“…This careful way of addressing the material might derive from the common support for an in-situ loess formation theory by Russian scientists, one of the main paradigms of Russian loess research (e.g., Smalley et al, 2010Smalley et al, , 2011. However, some authors are increasingly referring to these sediments as true wind-blown loess (Tudryn, 2013;Lebedeva et al, 2018), although others still argue for an alternative genesis not involving aeolian dust transport, namely in situ silt particle formation from underlying sediments. In addition, Lavrushin et al (2014) suggest re-working processes of slope sediments while instead Goretskiy (1958) favours the idea of cryogenesis for the origin of this silt.…”

“…However, recent pedological investigations have begun to shed light on the environmental conditions during stages of continental sedimentation in the Lower Volga region, suggesting a cool and arid climate (Lebedeva et al, 2018). This investigation also points to local sources for the loess deposits, notably from marine sediments exposed during regression (Lebedeva et al, 2018). Presuming this silt material is indeed true loess then it will allow testing of some of the hypotheses of the Caspian sea-level fluctuations noted above, especially the importance of local versus regional climate factors.…”