VERTISOL GENESIS IN A SUBHUMID CLIMATE OF THE COASTAL PLAIN OF TEXAS

Globally, Vertisols constitute approximately 3200 km2 of land area, yet little is known about their genesis in relation to climate. The purpose of this study is to determine the effect of a subhumid climate on the formation of Vertisols (i.e. shrink-swell soils) to better understand biogeochemical constituent cycling, carbon sequestration, and the relation of paleosols to climate in the rock record. In the coastal plain of Texas, six pedons were described and characterized that formed under grasslands within a MAP range of 119 to 83 cm/yr. These soils formed in a clayey-calcareous facies of the Beaumont Formation, which was deposited as part of a coalescing fluvial-deltaic complex during the late Pleistocene. These pedons typically have A-Bss-Bssk-BC-C profiles with subsurface microhighs and microlows corresponding with surface gilgai relief. Minor parent material variability exists in all pedons, but does not prevent the interpretation of major pedogenic processes. The weathering zone of all pedons is greater than 3 m based on depth of structure development, biological activity, and iron redox features. Depth to slickensides begins at 30 to 40 cm and extends to the bottom of the sola, further demonstrating that wet-dry cycles penetrate to all depths. Strain calculations using zircon as the stable constituent reveal average collapse in the upper 150 cm between 25 and 50% and average dilation below 150 cm between 30 and 60%. After adjusting for density and volume changes, clay and diothionite extractable iron show slight losses in the upper sola and slight gains in the lower sola. Appreciable gains of organic carbon occur within the upper 100 cm of microlows, attesting to more favorable plant-soil moisture relations in this topographic position compared to the microhighs. Depth to carbonate ranges from 60 to 120 cm in microlows, but occurs at the surface in all microhighs because of less leaching. Results indicate that Vertisols under grasslands in a subhumid climate form primarily by organic carbon inputs to surface horizons, particlarly in microlows; moderate carbonate accumulation in subsurface horizons, particularly in microhighs; and development of slickensides to nearly all depths regardless of micro-relief.