OXYGEN ISOTOPES AND MAJOR ELEMENTS IN CRETACEOUS HENSEL FORMATION PALEOSOL INDICATE ARIDITY AND RISING TEMPERATURES

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 dense micrite to 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 using a Gasbench II sample preparation device attached on-line with a Delta^Plus XP Isotope Ratio Mass Spectrometer. The δ¹⁸O_VPDB values increase, becoming heavier, up section and range -5.21 to -4.01‰. Temperature was estimated using the equation -0.498 T³ + (δ¹⁸O_{calcite(SMOW)} + 152.04) T² – 2.78 x 10⁶ = 0 which relates the MAT with the measured δ¹⁸O in the pedogenic calcite [1]. The MAT for the Hensel Paleosol ranges from 17-22 ºC increasing from bottom to top. Hensel Paleosol samples underwent X-Ray Fluorescence analyses to determine major element composition. The chemical index of alteration without potash, given by the molar ratio of Al₂O₃/(Al₂O₃+CaO+ Na₂O)*100, was used to compute mean annual precipitation (MAP) in millimeters using the correlation: MAP = 221×e^{0.0197(CIA-K)} (R²=0.73) [2]. The mean annual precipitation range for the Hensel Paleosol is 222-243 mm/yr.

The Hensel Paleosol data suggest warm, arid 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 Late Aptian was a relatively cool and stable period followed by a warming episode which occurs through the Albian. The deep root system and abundance of calcareous nodules is indicative of a dry climate where the roots must go deeper in order to find water. 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.

[1] Nordt, L., S. Atchley, and S. Dworkin, 2003. GSA Today Dec 03, 4-9. [2] Sheldon, N.D., G.J. Retallack, and S Tanaka, 2002. J. Geol 110, 687-696.