Fluctuations of Caspian Sea water due to climate change and the relict evidence in coastal soils

Coastal plains of large endorheic basins can preserve soil-sediment archives of shoreline mobility and groundwater reorganization under lake-level variability. We investigated 15 stratified soil profiles across mapped lagoon-barrier landforms on the southeastern Caspian coastal plain near the Gomishan Lagoon complex, Iran, to evaluate how depositional switching and hydrosalinity cycling are recorded in pedon architecture and pedofeatures. The study area is an ultra-low-relief coastal setting (elevations −22 to 0 m; slopes <2%) where modest changes in base level can translate into broad shifts in inundation, salinity, and shallow-groundwater behavior. Field observations show weak horizon development but pervasive stratification, including repeated buried C horizons with numeric prefixes (2C 4C, indicating changes in parent material), abrupt boundaries, laminated sandy-silty packages, and shell-bearing layers. Particle-size data and cumulative granulometric curves indicate interbedded fine- and coarse-textured sediment packages and support alternation among selective settling, event deposition, and barrier-related accumulation. Soils are uniformly alkaline and carbonate-buffered, yet show strong vertical and lateral heterogeneity in salinity and soluble-ion composition, consistent with shallow-groundwater influence, capillary rise, evaporation, and abrupt textural layering that creates contrasts in permeability, pore-size distribution, and water retention. pXRF major-element profiles exhibit stepwise down-profile shifts aligned with stratigraphy, supporting provenance and depositional contrasts rather than monotonic in-situ weathering. Micromorphology documents gypsum and carbonate pedofeatures together with redox features, indicating repeated wetting-drying and oxidation-reduction cycles. Collectively, these results show that the southeastern Caspian coastal soils function as polygenetic terrestrial archives of depositional switching and hydrosalinity cycling that are consistent with shoreline mobility and groundwater reorganization under Caspian base-level variability. However, independent chronology is required before individual sediment packages can be linked to specific dated lake-level events. • Coastal relict soils on the SE Caspian Sea record stacked depositional packages and hydromorphic overprinting under shoreline mobility. • Lithologic discontinuities are common and cluster at mid-profile depths, indicating repeated sedimentation rather than progressive horizon development. • Depth trends in soluble salts and redox features reflect oscillating shallow groundwater and episodic rainfall-driven leaching in a flat coastal plain. • Particle-size distributions and silt:clay shifts corroborate stratigraphy and distinguish selective vs poorly selective depositional regimes across landforms. • Weathering indices and micromorphology support predominantly source/depositional controls with localized pedogenic modification.