GEOCHEMICAL MASS-BALANCE RELATIONSHIPS IN A TEXAS VERTISOL CLIMO-/CHRONOSEQUENCE: READING TIME AND CLIMATE IN THE SOIL RECORD

Paleosols are useful tools for making interpretations of paleoenvironments prevalent during their formation. To quantitatively exploit the climatological and environmental significance of paleosols, we must first understand how long it takes for characteristic physico-chemical features to form in equivalent modern soils, and how climatic regime influences these same features. Quaternary Vertisols (clay-rich soils) occur in many climatic settings, and have distinctive pedogenic characteristics, including prominent bowl-like structures and microtography (gilgai), randomly oriented slickenside planes, and pedogenic carbonate and oxide glaebules. Their ancient counterparts, paleo-Vertisols, have been widely identified in the geologic record and effectively preserve morphologic and geochemical characteristics formed within their paleopedogenic settings. We have assessed mass-balance relationships within modern Vertisols from the Texan Gulf Coastal Plain, including both a climosequence transect with a mean annual precipitation (MAP) range of 800-1500 mm (parent alluvium age ca. 35 kA) and a chronosequence transect aged from 0.4-35 kA (MAP ca. 1000 mm). Both the precipitation regime and duration of exposure strongly influence the major geochemical characteristics of the Vertisols, and have induced differences distinguishable within mass-balance relationships. Four statistically-grouped element categories were identified, depending on their gross geochemical behavior: 1) primary framework lithology; 2) clay-enrichment; 3) leached/biocycled; and 4) redox-responsive. Characteristic translocation trends are rapidly established. The climosequence relationships show that the 35 kA Vertisols have achieved a relative net mass-flux equilibrium of ca. -16% (±3%) of total profile elemental content when MAP > 900 mm. Chronosequence data show that this net mass-flux equilibration value is approached within 3 - 7 kA in gilgai microlows. Geochemical patterns in drier Vertisols (MAP < 900 mm) indicate positive net mass-flux, suggesting that characteristics in these settings are controlled by other pedogenic mechanisms than their wetter counterparts and require significantly longer equilibration times.