A red clay layer in soils of the Yellow River Delta: Occurrence, properties and implications for elemental budgets and biogeochemical cycles.

Abstract Clay-enriched layers occurring in deltas are important as they reveal erosion, transportation and deposition of source materials from the basin to the coast. In the Yellow River Delta a red clay layer (RCL) with a thickness of 5–50 cm at 1 m depth in the soil profile occurs in the floodplain area dominated by fluvo-aquic soil derived from Yellow River sediments. The RCL typically has a median grain size of <20 μm, redness (a⁎) of >7, and relatively high contents of illite, calcite and iron oxides. In the Loess-Yellow River sediment-Yellow River Delta soil continuum a regime of weathering, mechanical sorting and dilution by carbonates and silica have been important factors altering the elemental geochemistry. The abundance of secondary minerals and deficiencies of silica and zircon in the RCL suggest that it was separated from a mixed source and was likely derived from highly weathered sediments (such as paleosol) from the basin. RCL samples show substantial accumulation of inorganic carbon with enhancement of the organic carbon content, indicating that increasing organic carbon in alkaline soils may lead to an increase in inorganic carbon in the subsoil. Large variations in elemental ratios have been found in the RCL characterized by excess nitrogen and trace metals and phosphorus deficiency due to adsorption and weathering effects. Implementation of the water-sediment regulation scheme (WSRS) may have altered the sedimentary environment and may have aggravated an elemental imbalance due to the occurrence of the yellow silt layer (YSL)-RCL sequence and thereby impacted biogeochemical cycling in the river-delta-coast continuum. Graphical abstract Unlabelled Image Highlights • The red clay layer (RCL) in the Yellow River Delta has unique characteristics. • Weathering, sorting and dilution are main factors altering elemental geochemistry. • Carbon, nitrogen and heavy metals are elevated but phosphorus is deficient in RCL. • Elemental imbalance may be aggravated by the RCL under anthropogenic perturbation. [ABSTRACT FROM AUTHOR]