MID-CRETACEOUS EVAPORATION RATES ESTIMATED FROM PEDOGENIC CARBONATE ISOTOPIC VALUES IN THE GLEN ROSE FORMATION, TEXAS

Exposure surfaces in the shallow marine carbonates of the Glen Rose Formation, Texas contain pedogenic carbonates that provide a record of meteoric-vadose evaporative enrichment. The isotopic values of the pedogenic calcites yield positive linear covariant trends (PLCTs) in δ¹⁸O vs. δ¹³C values. A 2-meter thick, sequence bounding paleosol developed at the top of the Glen Rose Formation near Kerrville, Texas yields a meteoric calcite line (MCL) with a mean δ¹⁸O value of –3.8 permil and δ¹³C values that range from –3.7 to –4.8 permil VPDB. Pedogenic carbonate nodules and micrite-dominated domains yield a PLCT that extends from the MCL to δ¹⁸O values of +1.1 permil and δ¹³C values of –2.7 permil VPDB. The PLCT results from pedogenic calcite precipitation from vadose fluids that were isotopically enriched through evaporation. The PLCT trends have been used to estimate zonally-averaged evaporation rates at 25°N paleolatitude during the Albian greenhouse warming. Terrestrial greenhouse world paleosol carbonate proxy records and quantitative reconstructions of the mid-Cretaceous hydrologic cycle show that heat transport through the atmosphere by water vapor may have played a greater role in cooling the tropics and warming the poles than it does today. A stable isotope mass-balance model of evaporative enrichment in the Glen Rose Formation suggests that evaporation rates at 25°N paleolatitude were 1.5 to 2 times present levels. The increased amount of heat removed from the tropics was transferred poleward and released through condensation and contributed to greater polar warmth.