NEW INSIGHTS INTO PHANEROZOIC TERRESTRIAL PALEOCLIMATE USING PLANT AND ANIMAL TRACES AND ELEMENT TRANSLOCATIONS OBSERVED IN A MODERN VERTISOL CLIMOSEQUENCE

Vertisols, clay-rich soils dominated by shrink-swell processes, are represented in the geologic record by widespread vertic claystone paleosols. We sampled a Vertisol climatic transect extending from SW to NE across the Coast Prairie of Texas, where mean annual precipitation (MAP) ranges from 65-140 cm/yr, in order to examine climate-sensitive soil properties with preservation potential in the rock record. Unique to Vertisols is lateral displacement of large volumes of soil along slickenside (shear) planes, which effects root growth, and also produces soil microtopographic relief (gilgai microhighs and microlows), thereby influencing soil hydrology, leaching and organic matter. Microhighs are drier and support more drought-tolerant forbs; microlows are wetter and contain more deep-rooted grasses. Crayfish (Fallicambarus devastator) burrows, 1.5 to 2.5+ m deep and 3-5 cm in diameter, are abundant for MAP >120 cm/yr; crayfish transport oxidized and reduced sediments from depth to the soil surface and oxidized, organic C-rich surface soil to the deeper subsoil zones, as well as provide macropores for soil water and CO₂ movement. Crayfish burrow distribution ranges from 1-2 to >10 burrows m^-2, increasing with higher MAP. Hydrologic processes influence distributions of alkaline earth elements (Mg, Ca, and Sr), K, Fe, and P, which show significant depletions in microlows and accumulations in microhighs. Element leaching and biocycling also decrease with lower MAP. P losses are partly related to biological productivity of the soil; losses in microlows approach 100%. By analogy with our modern analog study, we conclude that analysis of trace fossil distributions in vertic claystone paleosols, combined with analysis of biocycled and hydrologically sensitive elements, should lead to better refined estimates of Phanerozoic paleoclimates. We present examples from Paleozoic and Mesosoic paleosols to support these contentions.