Magnetic characterization of distinct soil layers and its implications for environmental changes in the coastal soils from the Yellow River Delta

The soils in the Yellow River Delta (YRD) were formed through the deposition of the Yellow River sediments under complicated hydrodynamic conditions, resulting in large variability in soil properties. In this study, environmental magnetic analyses were conducted on four typical soil profiles and one high resolution soil profile with distinct soil layers to examine the soil formation process and to track the environmental changes. The results showed that the red clay layer (RCL) in the YRD soil profiles had unique magnetic properties, which could be clearly separated from the yellow silt layer (YSL) at frequency dependent susceptibility (chi(fd)%) > 6%, susceptibility of anhysteretic remanent magnetization (chi(ARM)) > 250 x 10(-8) m(3) kg(-1), chi(ARM)/magnetic susceptibility (chi(lf)) > 5, chi(ARM)/saturated isothermal remanent magnetization (SIRM) > 45 x 10(-5) m A(-1) and SIRM/chi(lf) < 12 x 10(3) A m(-1). The magnetic enhancement of the RCL could be attributed to the presence of fine superparamagnetic (SP)/single domain (SD) ferrimagentic grains sourcing from hydrodynamic sorting of old sediments (e.g., paleosol in the Chinese Loess Plateau). It was hard to discriminate the RCL and YSL in the < 2 mu m fraction by magnetic bi-plots, suggesting that the abundance of clays with higher contents of secondary ferrimagnetic minerals contributed significantly to the magnetic variability of different soil layers in the YRD. Three units were identified in a high resolution soil profile based on the changes of magnetic curves, including a steady deposition process in the upper unit, a strong hydrodynamic process with varied provenance in the middle unit and a heterogeneous deposition process in the lower unit with the RCL. The three units were closely linked to the variation of nutrients (carbon and nitrogen) and metals (chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn), titanium (Ti) and zirconium (Zr)) in the soil profile, indicating the influence of provenance and sorting during sediment transportation and deposition. The magnetic method is sensitive to characterize soil formation and environmental changes in the coastal zone.