SOIL PROFILES AS INDICATORS OF MINERAL WEATHERING RATES IN BASALT

Two 4 m soil cores were collected from a basalt derived soil on a diabase dike in Central Pennsylvania to study the weathering process of basalt. Basalt weathering is an important contributor atmospheric carbon dioxide consumption from mineral weathering. A weathering front represents the zone wherein meteoric waters interact with parent material. The study of these weathering fronts will help explain the processes that control the weathering of basalt. X-Ray Diffraction, Scanning Electron Microscopy, Ion Chromatography, bulk density, and pore water analyses were performed on the soils to determine the chemical and mineral changes associated with the weathering of basalt. Quartz in the upper meter of the profile indicates aeolian deposition. Parent material consists of augite and plagioclase with trace amounts of magnetite and ilmenite. Much of the plagioclase is weathered from the profile below maximum sampling depth, indicating plagioclase weathers near the bedrock/saprolite interface. Augite persists throughout the weathering profile, but appears to be heavily altered towards the surface. Parent minerals weather to both kaolinite and smectite, but smectite appears to weather to kaolinite from 0 to 245 cm. We have identified two weathering fronts within the soil profile; one at 375 cm and one at 125 cm. Approximately 28% magnesium, 45% calcium, and 40% sodium are leached from the parent material across the lower front, and an additional 60% of the magnesium, 47% of the calcium, and 39% of the sodium are lost across the upper weathering front. In the upper meter less than 20% of the original parent composition of these three elements remains in the soil. Organic acids identified in the soils are formate, oxalate, acetate, and citrate. Organic acid concentrations are low in the top meter of the profile, reach maximum concentrations at 150-200 cm, then decrease with depth to bedrock. Maximum concentrations are located directly below the upper weathering front. This suggests that microbial activity in the upper 1.5 meters of the profile may be consuming organic acids and contributing to the weathering process in this zone. These soil profiles will help better understand basalt weathering, including relative rates of mineral weathering, clay formation, and the impact of organic acids on weathering throughout the profile.