Reactive transport modeling of depth to pedogenic carbonate in semi-arid silt loam soils of southeast Washington
Martin, Chase Thomas
May 14, 2014
Chemistry - Geology
Pedogenic carbonate deposition in arid and semi-arid soils can reveal important information about paleoclimate and paleoecology. However, the exact relationship between paleoprecipitation and depth to pedogenic carbonate is debated. Computer modeling of pedogenic carbonate formation is essential for analyzing and comparing the numerous variables involved. This study modeled depth-to-carbonate in a well-studied soil site in the Palouse region of southeast Washington, with a calcic horizon at 0.54 m depth. We used the Subsurface Transport Over Multiple Phases (STOMP) computer model, utilizing reactive transport in order to couple species transport and chemical reactions within the soil column. Four models were built based on soil and atmospheric properties of the published KP1 study site (Stevenson et al., 2005; Stevenson et al., 2010), and they were run for 1,000 years under a variety of calcium carbonate chemical conditions. The STOMP-WAE-R-B software did not recognize CO₂ gas as a reactive chemical species in our developed calcium carbonate reaction network.
No calcium carbonate precipitation occurred in the four models, but we made a number of additional observations. A seasonal aqueous saturation pattern of wetting and drying appeared at the surface of the soil column in each model, but the pattern dissipated at depths less than one meter from the surface. Near-surface Ca²⁺, CO₂ (aq), and H⁺ saturation patterns follow that of the seasonal aqueous saturation pattern, with limited fluctuation in species concentration at depth after reaching chemical equilibrium. Influxes of Ca²⁺ (dust) and CO₂ (aq) from the surface had little to no apparent effect on the species saturation patterns. Our results reveal that species saturation did not occur to the extent needed to form calcium carbonate. Additionally, initial species concentrations in the soil appear more important than minor fluxes from the surface, such as the Ca²⁺ dust. This study revealed that the interaction between STOMP and CO₂ v gas needs to be researched in the future STOMP modeling efforts due to the important role CO₂ plays in the formation of carbonic acid with its dissolution in water.
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