CRITICAL ZONE STUDIES OF TEXAS VERTISOLS: IMPLICATIONS FOR INTERPRETING CLIMATE, LANDSCAPES, AND BIOTA OF THE PAST

The National Academy of Sciences identified integrative studies of the "Critical Zone" (CZ) as one of the six compelling opportunities for earth scientists in the next decades. The CZ is defined as the near-surface environment in which complex interactions involving rock, soil, water, air and living organisms regulate the natural habitat. Integrative studies of the surface- and near-surface environment are recognized as an important area of focus by scientific communities including geology, biology, pedology, and hydrology. Modern-ancient analog studies of Texas Coast Prairie and Blackland CZs relate the morphology, microscopy and chemistry of surface soils to interpreting the genesis and climate record of equivalent paleosols. Modern Vertisols are clay-rich soils characterized by macro- and micromorphological characteristics formed in response to shrink-swell behavior. Paleo-Vertisols have provided important information on paleoenvironments, including Phanerozoic estimates of atmospheric pCO₂ and rainfall and changes in root morphology accompanying the evolution and diversification of terrestrial land plants. Many features, including slickensides, rhombohedral and wedge-shaped peds, shrinkage cracks and sepic-plasmic microfabrics are well-preserved in the ancient rock record and can be easily identified in the field and in thin section. Stable carbon isotope analysis of organic matter in paleo-Vertisols may help to interpret the soil ecosystem (dominance of C₃ vs. C₄ flora) and changes in soil water availability that may indicate climate change. Similar information may be preserved in the isotopic compositions of pedogenic carbonates, as well as provide estimates of paleoatmospheric pCO₂ using the soil carbonate paleobarometer. Bulk geochemistry provides estimates of MAP and pedotransfer functions provide estimates of colloidal properties. The wide spatial and temporal distribution of paleo-Vertisols reflects the potential for formation under wide ranges of moisture regimes (from arid to semi-wet tropics); the only strict requirements are a parent material weatherable to fine clay, and an environment in which rainfall is seasonally distributed, or in which soils experience seasonal soil-moisture deficits. Terrestrial facies intervals within Phanerozoic sedimentary deposits in North America contain abundant paleo-Vertisols. Paleo-Vertisols appear over-represented in the deep-time geological record as compared with their present global distribution of 2-3% of modern soils.