TEMPERATURE DEPENDENT DISSOLUTION EXPERIMENTS OF BASALT, DACITE AND PUMICE FROM THE LITTLE DESCHUTES RIVER VALLEY, OR

Alkalinity generation from chemical weathering of silicate minerals and the burial of that alkalinity as carbonate minerals in the ocean is hypothesized to be the primary negative feedback on the long-term carbon cycle. On shorter timescales, the abundance of poorly crystalline minerals (PCMs) in soils from the weathering of primary silicate minerals is associated with higher soil organic carbon (SOC) storage and more recalcitrant SOC. Due to its steep topography and young, volcanic soils, the Oregon Cascades are thought to be an important locus of alkalinity generation, PCM formation and SOC storage.

Previous work has hypothesized the effect of PCM abundance on basaltic soil carbon storage at the watershed scale, and several dissolution experiments have characterized the geochemistry of basalt rock weathering. Yet, the lithologic complexity of volcanic bedrock assemblages has been overlooked. The Oregon Cascades contains an array of extrusive igneous rocks ranging from mafic to felsic compositions resulting from fractional crystallization processes. Therefore, accurate assessments of watershed-scale PCM formation must consider the propensity and controls of PCM formation during the weathering of intermediate and felsic rocks.

We conducted dissolution experiments of basalt, dacite and pumice sampled from the Little Deschutes River Valley: a watershed on the eastern flank of the central Oregon Cascades. Each rock underwent replicate batch reaction experiments at 4º, 23º and 35ºC for 100 days, yielding a total of 9 experiments. To gauge how weathering reactions proceed over time, we measured pH, bulk cation and trace metal concentrations of water from batch reactions at semi-regular time intervals. With these data, we computed saturation indices for various PCM-forming reactions. By interrogating the controls of lithology and temperature on the abundance of PCMs, we further characterize the effects of chemical weathering on soil mineralogy and carbon storage.