Knowledge Bank

Over geologic timescales CO2 in the atmosphere is greatly affected by the weathering of silicate and phosphate rocks. Weathering of Ca-Mg phases is important because it results in precipitation of Ca Mg carbonates and removal to marine sediments, thereby affecting long term CO2 uptake. Weathering of iron or phosphate phases is important because these reactions release nutrients that promote plant growth and take up CO2 as organic carbon in the short term. This project investigates the dissolution kinetics of volcanic ash from five different eruptions (Mount St. Helens, USA eruption; Mt. Pinatubo, Philippines eruption; Eyjafjallajökull, Iceland; Mt. Pacaya, Guatemala; and Tungurahua, Ecuador eruptions) in synthetic sea water and in freshwater solutions, with and without the addition of iron oxidizing bacteria. Bulk ash composition determined by x-ray fluorescence ranged from basaltic andesite, to trachy-andesite, andesite and trachydacite. X-ray diffraction indicates the presence of plagioclase in all samples. Although the compositions both within ash and among samples could vary substantially. Over the course of the experiment, the solutions were sampled periodically via syringe, filtered, and analyzed with a Skalar San++ nutrient analyzer to determine concentrations of silica and phosphate. Marine experiments spanned 834 days. Freshwater experiments spanned 578 days. On the first day of the freshwater experiments, approximately 0.5 ml of Sideroxydans lithotrophicus in growth media was added to one set of the freshwater duplicates. Equal amounts of Mariprofundus ferrooxydans in growth media was added to one set of marine duplicates 260 days into dissolution experiments. Samples were also analyzed for iron by use of the ferrozine method; however, the concentrations of iron were too low to be detected (LOD=3 ppb). Phosphate concentrations were close to the detection limit and varied over time. The silica concentrations increased gradually over time and dissolution rates were estimated from a linear fit of the data. Samples with or without bacteria exhibited similar rates based on Si dissolution. When normalized for specific surface area, the highest dissolution rate was from the 2010 eruption of Pacaya, which had basaltic-andesitic composition. The initial silica release rate in freshwater without bacteria was 7.8•10-12 mole Si/m2/s, and in artificial seawater: 3.5•10-12 mole Si/m2/s. In general, marine dissolution rates were of the same order of magnitude as freshwater release rates, but slightly slower. The lowest release rate was 1.5•10-12 mol Si /m2 /s, from Icelandic ash, with the addition of iron oxidizing bacteria. Results are similar to those results of a similar experiments in which ash from the same eruptions was dissolved in deionized water at varying acidity.