Neighboring Ridgetops With Distinct Weathering Drivers Have Strikingly Similar Saprolite Chemical Weathering

Chemical weathering, and the subsequent porosity it generates controls soil development, water cycling, and nutrient availability. However, predicting spatial variability in chemical weathering remains challenging due to complex interactions between dominant drivers including climate, topography, water cycling, and vegetation dynamics. Here, we leverage neighboring ridgetops with the same lithology but distinct topographic relief, vegetation, water cycling, and bedrock fracturing to investigate if and how compound weathering drivers reflect the observed chemical weathering profiles. Specifically, within a central coast California site underlain by metagreywacke sandstone, we drilled four boreholes (6–40 m deep), collected rock cores for geochemical and physical analyses, and conducted hydrological monitoring during the water year 2023 on a ridgetop with oak trees and one with only grasses. Despite these distinct environmental and geomorphic conditions, we observed strikingly similar chemical weathering magnitudes within the shallow subsurface (<5 m) between ridgetops. These near‐surface similarities likely reflect legacy effects of widespread tree cover during the cooler and wetter Pleistocene climate. Chemical weathering differences emerged most strongly within the fractured bedrock (5–25 m), emphasizing the importance of fracture density for deep chemical reactions. Additionally, chemical depletion extended to the stream channel elevation within both ridgetops, coinciding with the present‐day water table and indicating dominant groundwater control on chemical weathering initiation. These findings have widespread implications for predicting subsurface weathering and associated ecosystem processes across water‐limited landscapes, where paleoenvironmental legacies may overprint shallow chemical weathering patterns while fracture density mediates deep weathering extent.