Disturbances, Resilience, And The Role Of Calcium Bearing Minerals Weathering In Northeastern Forests

The Critical Zone (CZ) is a complex and dynamic system where rock, water, and organisms interact to sustain essential life-supporting services. Disturbances can threaten this capacity if the CZ cannot resist or recover (i.e., if it lacks resilience), but CZ responses vary depending on both its structure and the type of disturbance. For example, subsurface weathering provides critical nutrients such as calcium for plant health; however, disturbances such as logging and past acid deposition can deplete calcium reserves. This study systematically investigated how varying levels of disturbance influence weathering through a combined field and laboratory approach across four forested sites in Vermont and New Hampshire. These sites differ in subsurface calcium availability due to differences in bedrock and glacial till composition, as well as in logging history, though all have been impacted by historic acid deposition. Building on a previous study that employed mesh-bag experiments—where powdered mineral samples were buried in mineral soils to weather under present-day conditions—I analyzed a subset of samples left in-situ for two years using scanning electron microscopy (SEM). My results show differences in visible weathering features in minerals among sites and minerals, with the most pronounced weathering indicators for calcite grains at a site with intermediate levels of logging and lower amounts of calcium minerals. SEM images showed presence of fungal hyphae, indicating a possible bioweathering contribution. To complement these in-situ experiments, I conducted soil core experiments to analyze soil solution chemistry characteristics of these sites. My findings suggest that upper soil layers buffer pH while also introducing substantial amounts of organic material into the subsurface. I used a geochemical model for speciation and determination of saturation indices to investigate the propensity for weathering. Saturation indices were generally below zero, indicating undersaturation and thermodynamically favorable weathering for most mineral phases, a finding that aligned with the observed weathering features of select samples from mesh-bag experiments with the SEM. However, certain minerals, such as the aluminum hydroxide gibbsite, occasionally reached supersaturation. I further extended the soil core experiments to simulate historical conditions by using a laboratory mixed acid rain simulant as inflow solution to my soil cores. Results indicated upper soil layers buffering pH, possibly protecting the subsurface from the most extreme impacts. Overall, my results demonstrate that responses to disturbances are coupled and complex. Bioweathering played a key role in the mesh-bag experiments, modulating expected responses in ways that the geochemical model could not fully predict. Generally, sites with low logging disturbance and high subsurface calcium exhibited the least weathering response.