Differing Subsurface Fracture Characteristics Inferred from Ground Penetrating Radar at Two Granitic Sites in the Southern Sierra Nevada, California

Lithology is a long-recognized regulator of chemical and physical weathering. However, the role of fracture density, in particular, is poorly understood because it is challenging to quantify, particularly in regolith-mantled landscapes, where subsurface fractures are hidden by soil. Here, we quantify vertical fracture density and length using ground penetrating radar (GPR) at two forested, non-glaciated sites in the Sierra Nevada, California: 1) Bald Mtn., underlain by leucogranite and 2) Dinkey Dome underlain by granite, which has more biotite and plant essential nutrients. Bald Mtn. has thinner regolith and less forest cover than Dinkey Dome, suggesting that the differences in lithology drive differences in weathering and ecosystem productivity. To explore this hypothesis, we collected 3400 m of GPR data with a 100 MHz antenna. At both sites, reflections are visible at depths greater than 25 m. Using diffraction hyperbola analysis, we show that two sites have different electromagnetic (EM) velocities (0.08 m/ns versus 0.11 m/ns). In addition we used an automated thresholding algorithm to quantify reflection density and reflection length of all 3400 m of GPR data. We observe long (>40 m) continuous reflections surrounded by distinct regions at both sites. However, at Dinkey Dome, the site with greater forest cover and thicker regolith, the GPR profile has a higher vertical reflection density and a slower EM velocity--suggestive of more weathered bedrock. Our results are consistent with a lithologically mediated feedback wherein initial fracture spacing promotes the flow and storage water, which ultimately encourages mineral dissolution and tree growth. Enhanced dissolution and tree growth both create more pore space, which in turn promotes more fractures. However, more work is needed to resolve the relative importance of fracture spacing and other lithologic controls on weathering, including bedrock nutrient and mafic mineral concentrations.