Pedogenic Relationships in a Texas Vertisol Climosequence Defined by Geochemical Mass Balance of Whole Soil and Chemistry of Iron-Manganese Nodules

Climosequence Vertisol profiles derived from the Upper Beaumont Formation in the Gulf Coastal Prairie physiographic province of Texas were examined for geochemical trends ascribed to different precipitation regimes. Mass-balance relationships were utilized for the comparisons. Compositional differences in zirconium (Zr) content between the solum and lower sub-soil precluded it use an immobile strain (volumetric change) indicator. This difference was correlated to sand weight percent (r² = 0.65**). Titanium (Ti) content did not shift correlatively with depth, thus making Ti the preferred strain index element for mass-balance calculations. Depths at which the Zr compositions shifted were not directly related to mean annual precipitation (MAP) regime and are considered to be functional boundaries between open-system pedogenesis and more closed system hydrogeochemical weathering.

Mass-balance relationships of ten elements examined within the climosequence showed trends ascribed to precipitation intensity. Mass-balance relationships varied not only between locations but also between microtopographic positions within each Vertisol pedon. Basic mass-balance trends with depth in the climosequence profiles fall into four broad categories depending on the relative mobility and geochemical reactivity of the individual elements: 1) framework, 2) clay interaction, 3) leachable/biocycle, and 4) climatic/redox-sensitive. Net mass flux percentages indicate a relative steady-state of around -16% (±3%) in climosequence Vertisol pedons with MAP > 900 mm.

The total Fe (Fe_TOT) content of pedogenic iron-manganese (Fe-Mn) nodules from Vertisol profiles correlates with mean annual precipitation (MAP, r² = 0.92***). No significant trend of Fe_TOT with depth was noted in profiles. Using the regression developed from modem Vertisol data, Feror contents of Paleozoic paleo-Vertisol Fe-Mn nodules yielded MAP regimes comparable to previously inferred paleoenvironmental interpretations. Paleoprecipitation estimates derived from Fe-Mn nodules for an uneroded, Late Mississippian paleo-Vertisol are very close to estimates made from a depth to pedogenic carbonate horizon (DCH) proxy determined from the modem Vertisol climosequence. Because the Fe-Mn nodule proxy is independent of depth, consistent paleoprecipitation estimates can be made even in eroded paleo-Vertisols and, in combination with the DCH, may be useful in determining original paleosol thickness.