Oxygen isotopes, trace elements, and sedimentology of the Cretaceous-aged Hensel Paleosol indicate aridity and rising temperatures in Central Texas

An Early Aptian to Late Albian Cretaceous aged paleosol (~112 MA) in the Hensel Formation in Kimble County, Texas, was used as a climate proxy. Abundant, downward tapering rhizoliths, along with columnar peds and carbonate nodules, are dominant macroscopic features. Petrographic analyses reveal primary soil features such as dense microspar plasmic fabric with quartz and calcite filled root traces. Stable oxygen isotope ratios (δ18O) in the carbonate material within the paleosol were used as a proxy to determine the Mean Annual Temperature (MAT). Isotopic analyses of 36 samples from the Hensel Paleosol were performed with a DeltaPlus XP Isotope Ratio Mass Spectrometer. The δ18O_VPDB values increase, becoming heavier, up section and range -5.21 to -4.01 %_o. Temperature was estimated using an equation which relates the MAT with the measured δ18O in the pedogenic calcite. The MAT for the Hensel Paleosol ranges from 17-22 ºC increasing from bottom to top. Hensel Paleosol samples were analyzed by X-Ray Fluorescence to determine major element composition and Mean Annual Precipitation (MAP).

The Hensel Paleosol data suggest warm, arid subaerial conditions existed during the mid-Cretaceous in Central Texas. The warming trend apparent from the Hensel Paleosol section is in agreement with global warming conditions noted from Sea Surface Temperatures during this period. The MAP estimate for the Hensel Paleosol data suggest arid conditions at the Aptian/Albian boundary in Central Texas which corresponds to the arid belt at the paleolatitude at 25°N. The deep root system and abundance of calcareous nodules is indicative of a dry climate.