Paleotopography continues to drive surface to deep-layer interactions in a subtropical Critical Zone Observatory

Subsurface critical zone structures (SCZS) refer to the spatial variation in the interactive layers underground. Although SCZS greatly affect terrestrial biogeochemical and hydrological cycles, underpinning mechanisms are poorly documented. Herein, we characterized the SCZS of a typical red soil in subtropical China, a type of soil with vast global distribution. The thickness information of three layers was derived from hand augers, boreholes and ground-penetrating radar (GPR) radargrams and incorporated into geographically weighted regression (GWR) models for the reconstruction of paleotopography (Cretaceous sandstone). The interpreted GPR results in terms of thicknesses and interfaces for the three layers were consistent with the borehole logs. The trained GWR models accounted for 43%–77% of the spatial variations in the three layers. The paleotopographic elevations were highly correlated with those of the current land surface (r = 0.85). Spatial analysis showed that the rougher paleotopography was inherited by the current landform. The SCZS evolution involving mainly the mantling covered by Quaternary red clay (QRC) was primarily driven by terrain attributes. These findings may enhance our understanding of the interaction between the paleoclimate and paleoenvironment. The combination of geophysical techniques, geochemical indicators and spatial prediction techniques provides an effective tool for understanding QRC landform evolution.