Range of Fragipan Expression in Some Michigan Soils:
II. A Model for Fragipan Evolution
Beth N. Weisenborn and Randall J. Schaetzl
The evolution of Michigan’s fragipans is largely unexplored. To help resolve this problem, three fragipan soils in northern Michigan were studied to explore the pedogenic pathways for fragipan evolution, and the results extended to soils forming under similar conditions. To accomplish this, the physical, chemical, and micromorphological properties of these soils were first characterized. Then, these data were evaluated in terms of the major models of fragipan genesis, to identify those that may apply. A combination of pedogenic models explains many of the fragipan properties of our soils, while also allowing for their variable expression. In that light, a new model - one that integrates and synthesizes existing models and our findings - was developed to explain fragipan evolution for similar soils in Michigan and the Great Lakes region. The Michigan Model of Fragipan Evolution is therefore a hybrid, synthesizing model that involves, first, the self-weight collapse of a soil or parent material, followed by physical ripening of the collapsed zone. Ripening helps to retain the closely packed fabric and intergrain bridging in the collapsed zone, creating a protofragipan. Later, amorphous bonding agents precipitate in the protofragipan due to its position near a weathering-front discontinuity. The resulting fragipan develops progressively and becomes better expressed with further pedogenesis. Fragipan degradation is eventually initiated by an increasingly prominent perched zone of saturation that forms seasonally above the fragipan. With time, processes associated with fragipan degradation and translocation of materials to lower parts of the profile exceed processes associated with progressive development, and the fragipan is destroyed. The “end point” of fragipan evolution in Great Lakes’ fragipan soils may therefore be a well-developed bisequal solum that lacks a fragipan. Subsequent fragipan genesis is not likely to reinitiate in the residual profile.