COMING TO GRIPS WITH THE EVAPORATION RATES RECORDED IN ANCIENT SOILS: INTERPRETING DIAGENETIC TRENDS FROM VADOSE & PHREATIC COMPONENTS IN CALCIC PALEOSOLS

Petrographic inventories of nodular carbonates in calcic paleosols routinely show that they consist of complex mixtures of calcite in microcrystalline, sparry, and microspar fabrics. Discounting the later overprinting noise introduced by vein-filling burial cements, the intrinsic δ¹³C and δ¹⁸O variabilities observed in the authigenic mineral phases preserve records of the paleohydrologic processes that were operant in ancient shallow groundwater systems. Microsampling campaigns in paleosols consistently are able to isolate diagenetic trends in carbon & oxygen isotope space that are unique to the authigenic components in each paleosol. These diagenetic trends include the vertical Meteoric Calcite Line (MCL) originally defined by Lohmann (1988, in the Choquette & James Paleokarst volume), and the more newly-defined Positive Covariant Linear Trend (PLCT), consisting of linear data arrays with positive slopes. The MCL trends record calcite precipitation in meteoric phreatic groundwater settings. The PLCT trends record calcite precipitation in meteoric vadose groundwater settings, with ¹⁸O enrichments produced by evaporation, and ¹³C enrichments produced by decreasing soil respiration rates and CO₂ degassing. We report on examples of mid-Cretaceous (Aptian-Albian) subtropical calcic paleosols from the Glen Rose (Texas), Antlers (Oklahoma), and Cedar Mountain (Utah) formations in which linear MCL and PLCT trends converge to a common point, phenomena that we interpret as records of the evolution of ancient groundwater systems from vadose to phreatic conditions during progressive burial. Paleosols with converging MCL & PLCT trends can permit quantitative estimates of ancient time-averaged terrestrial evaporation rates. We note that the δ¹⁸O offsets of Cretaceous PLCT trends (5 to 8 per mil) are larger than those reported in published Neogene examples (to 3 per mil), suggesting higher subtropical evaporation rates in the Cretaceous than in the Neogene.