WEATHERING OF VOLCANIC ASH AS A CARBON SINK

CO₂ in the atmosphere over geologic timescales is greatly affected by the weathering of silicate and phosphate rocks. Weathering of Ca-Mg phases is significant because it results in precipitation of Ca Mg carbonates and thereby storing CO₂ in marine sediments. Weathering of iron or phosphate phases is important because these reactions release nutrients that promote plant growth and eventual storage of CO₂ as organic carbon. My project investigated the dissolution kinetics of volcanic ash from five different eruptions (Mount St. Helens, USA; Mt. Pinatubo, Philippines; Eyjafjallajökull, Iceland; Mt. Pacaya, Guatemala; and Tungurahua, Ecuador) in synthetic sea water and in freshwater solutions, with and without the addition of cultured iron oxidizing bacteria. Bulk ash composition, determined by x-ray fluorescence, ranged from basaltic andesite, to trachy-andesite, andesite and trachydacite. Over the course of 9 months, the solutions were sampled periodically and analyzed with a Skalar San++ nutrient analyzer to determine concentrations of silica and phosphate. The samples were also analyzed for iron by 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 in all experiments while silica concentrations increased gradually over time and Si dissolution rates were estimated from a linear fit of the data. Samples with or without bacteria exhibited similar rates of Si dissolution. The highest dissolution rate was 47 ppb Si/day from Icelandic abiotic freshwater ash sample, a fine-grained trachy-andesite. Differences in dissolution rates of the ash samples were related to the differences in surface area and particle size of the ash. Faster dissolution rates are associated with greater surface area and smaller particle size. The lowest release rate determined was 2.4 ppb Si/day from the freshwater abiotic Tungurahua ash, a fine-grained andesite. Results are similar to previous dissolution studies conducted in low ionic strength solutions which suggest that the seawater matrix does not affect dissolution rate.