Modern geochemical evolution of lagoon-marshy landscapes in the western Caspian Sea region

The landscape-geochemical consequences of the full cycle of Caspian Sea fluctuations (regression-transgression-stabilization) that has taken place in the recent decades are analyzed. The regressive and lagoon-transgressive sets of landscape-geochemical processes (LGPs) are characterized. The impact of the regressive stage on the environment is seen in the development of the regressive LGPs in the soils and sediments of the coastal landscapes with a predominance of salinization and weak humus accumulation. The transgression of the sea was accompanied by the formation of specific lagoon-marsh landscapes, in which the accumulation of sulfides, gleyzation, iron oxidation, and humus formation were clearly pronounced. The general scheme of the evolution of the LGPs within the past 30 years is discussed. The rates of the humus accumulation and salinization during the stabilization stage are estimated. The organic matter reserves in the coastal soils increased by 1.5-2 times in a decade. The transformation of the particular forms of chemical elements in the lagoon-marsh landscapes upon fluctuations in the sea level is shown. The rise in the Caspian sea level was accompanied by the enhanced migration of elements with varying valences, such as Fe and Mn (as well as Zn, Cu, Pb, Ni, and Co) due to their mobilization in the slightly alkaline and neutral reducing conditions and their precipitation at the geochemical barriers.