National, Eel, Luquillo, Shale Hills, INVESTIGATOR, COLLABORATOR
Carbon dioxide and oxygen concentrations in soils within regolith affect weathering and its formation, which is a unit that is a significant component of Earth’s life-sustaining Critical Zone. Here, we examine CO2 and O2 gas concentrations in the soil atmosphere and study the atmospheric and biotic affects that influence gas concentrations and distributions throughout the soil profile. Bulk geochemistry, particle size, pH, and sulfur content is also measured and related to gas. This data was collected by installing gas sampling wells and taking samples throughout the Fall and Winter. Three sites were chosen, two of which were on diabase and one on granite in Pennsylvania (diabase) and Virginia (diabase and granite). Soil samples were collected from three sites via hand-auger or Geoprobe and analyzed in the laboratory. Soil CO2 and O2 were inversely related with depth. Clay content increased towards the surface displaying a more rapid increase in the profile where CO2 concentrations began to deplete via consumption or more diffusive loss towards the surface. All soils became more acidic towards the surface. The shallowest profile (Virginia diabase) had the largest pH range, while the deepest profile (Virginia granite) had the smallest pH range. There was a decrease in sulfur content also with depth, however, the Pennsylvania diabase showed a significant increase in sulfur in its deepest sample. An interesting discovery was made when plotting O2 versus CO2. The Virginia granite and Pennsylvania granite had evidence of similar respiration and consumption rates of the gases, as indicated by the slope values of their trendlines, which were -0.9644 and -0.9817, respectively. The Virginia diabase had a much steeper slope of -2.2644. A slope of -1.0 means when one mole of O2 is consumed, one mole of CO2 is produced, so the Virginia diabase must have a lack of biotic activity at depth due to its slope, which translates to less O2 consumption with less CO2 production. Weathering can be visualized by the gas slope gradient, affecting particle size and oxide depletion. Soil pH decreases with the depletion of base oxides as the combination of gas and acidic rainwater weather regolith. Soil gas concentrations in regolith and characteristics of the soil on granite and diabase have broad similarities, but interesting differences even in a small region.
Davis, Reese (2014): Reading Atmospheric and Biotic Influences in Soils: The relationship between soil gas and weathering properties in soils on granite and diabase. Bachelor of Science, The Pennsylvania State University.
(2 MB pdf)