REACTIVE TRANSPORT MODELING OF WEATHERING, WITH APPLICATIONS TO PALEOSOLS

We have developed a model to simulate weathering on a wide variety of terrestrial planetary surfaces, including the Archean Earth. This model functions independently but will also function as a module of the Virtual Planetary Laboratory (VPL). The present 1D model is an extension of KINFLOW, a reactive-transport model to simulate mineralogic changes in the crust. We can simulate weathering involving a growing list of characteristics including several dozen minerals, aqueous species and equilibrium reactions, as well as a limited number of kinetically controlled redox and water/gas exchange reactions. The kinetic rates of dissolution and precipitation of minerals in our simulations are based upon experimentally derived kinetics that depend on the deviation from equilibrium. During each simulation we allow the surface layer to erode, thus continuously exhuming less weathered rocks. Soil profile evolution is naturally our primary output. We also characterize solute chemistry and the composition of eroded material for input into the fluvial / sedimentary system of subsequent VPL modules. Simulations are performed for various rates of infiltration and erosion, initial rock mineralogy, surface temperature, and atmospheric composition. Water/air volatile exchange is currently implemented at the eroding surface with a water saturated soil. However, extensions are underway to allow full vadose zone vapor / aqueous phase exchange, with diffusion of gases in the vapor phase.

Simulation results will be presented for several scenarios of Archean paleosol development. Surface temperature has a profound influence on weathering rates and the mineralogical profiles that develop. By comparison of model profiles that develop under different thermal conditions to profiles deduced from paleosols, the general thermal regime extant during paleosol development can be distinguished, among the cold-, temperate-, or hot-Archean conditions that have been hypothesized.