Connecting the deep and shallow critical zone: Quantifying inherited heterogeneity of the CZ structure

The critical zone is the region near earths surface where water, rock, and life all interact. These interactions power a variety of chemical and physical processes that transform bedrock into soil. During rocks transformation into soil, it is unclear how much physical and chemical heterogeneity present in the bedrock survives the weathering processes and reaches the shallowest regions of the CZ. Here we present a preliminary multi-scale picture of the critical zone from the South Carolina Piedmont (known as the Calhoun Critical Zone Observatory). Our observations range from the deepest regions of the CZ (>30 m) to the surface. We use ground penetrating radar (GPR), seismic refraction, structure from motion (SFM), and X-ray fluorescence (XRF) to create a unique 3D and interactive view of the CZ. The chemical index of alteration (CIA) identifies the mobile regolith and highlights the argillic Bt horizon then shows variations laterally over 1-2 m. The lateral variations in CIA correspond to visible changes in color, hardness, and grain size. Furthermore, the mobile regolith identified by XRF also corresponds to the slowest refraction velocities (< 300 m/s), suggesting high-resolution (0.5 m geophone spacing) seismic refraction surveys could map the depth of mobile regolith. In addition, the high-resolution seismic refraction survey shows the depth to saprolite, identified by 1100 m/s that varies in thickness from less than 2 m to greater than 4 m across only 23 m laterally. Our preliminary results suggest that heterogeneity inherited from deep in the CZ propagates through the weathering engine but is all but lost in the surficial mobile regolith due to pedoturbation and particle mixing. However, residual structures in the saprolite, including dikes, veins, corestones, and textural changes exist across multiple scales and multiple geophysical and XRF measurements.