Comparison of modern and ancient Vertisols developed on limestone in terms of their geochemistry and parent material

Abstract

Identification of paleosol parent material can be difficult; a common simplistic assumption is that nonpedogenic strata directly beneath paleosols are representative of the parent material. Field relationships, petrography, and geochemical mass-balance were used to test the simple hypothesis that the parent material of both an Early Carboniferous (Late Mississippian) vertic (Vertisol-like) paleosol and a modern analog soil (Vertisol) was immediately subjacent marine carbonate rock. An alternative hypothesis also tested was that the paleosol formed primarily from siliciclastic sediment deposited on top of the carbonate. The modern-ancient analog comparison is valid because both the paleosol and the modern soil have high clay contents, extensive shrink–swell features, and rest directly on top of marine carbonate substrates. Field and thin-section observations indicate minor assimilation of marine carbonate into both the paleosol and soil matrices. Mass-balance of both the paleosol and soil, calculated assuming that the subjacent carbonate was the parent material and that Ti was immobile during weathering, indicated 25–75% net volume loss during weathering and 25–99% net losses of many alkali, alkaline earth, and redox-sensitive elements. However, calculations based on an alternative hypothesis of clayshale parent material for the paleosol indicated only 10–25% volume loss and lower translocation losses for weathering of this material. The insoluble residue content of the dolostone would require that a fourfold volume reduction during weathering occurred, if the dolostone were the only parent material. If the paleosol formed from clayshale, then significantly less volume loss characterised weathering. These results collectively illustrate that the simplest interpretation of paleosol parent material may not always be correct, and that paleosols can be derived from weathering of more than one parent material.