MODELING ENHANCED ARIDITY FROM PEDOGENIC CARBONATES IN THE MID-CRETACEOUS DRY-BELT: THE UPPER GLEN ROSE FORMATION, TEXAS

The oxygen isotopic values of pedogenic carbonates developed in paleosols of the upper Glen Rose Formation provide a proxy record of meteoric-vadose evaporative enrichment. The isotopic values of the pedogenic calcites yield positive linear covariant trends (PLCTs) in d¹⁸O vs. d¹³C space. 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 d¹⁸O value of -3.6‰, and d¹³C values that range from -3.7 to -4.8‰ VPDB. Pedogenic carbonate nodules and micrite-dominated domains yield a PLCT that extends from the MCL to d¹⁸O values of +1.0‰ and d¹³C values of -2.7‰ VPDB. The PCLT results from pedogenic calcite precipitation from vadose fluids that were isotopically-enriched through evaporation. The PLCT trend has been used to estimate zonally-averaged evapotranspiration (ET) rates at 25°N paleolatitude during the Albian greenhouse warming. Using a mass-balance modeling approach that incorporates the MCL, PLCT, and paleotemperature values, the ET rates for the Glen Rose at 25°N paleolatitude are estimated to have ranged from 1360 to 2100 mm/yr. These values may be compared to modern 30 yr average ET rates of 1592 mm/yr for south Texas (Fipps, G., 2005, Texas Evapotranspiration Network; http://texaset.tamu.edu/). These new data will be used to better constrain quantitative reconstructions of the mid-Cretaceous hydrologic cycle which have been based upon the sphaerosiderite proxy records between 34-75°N paleolatitude. Earlier mass-balance modeling experiments (Ufnar et al., 2002, Palaeo3 188) estimated that Albian moisture deficits between 7.5 and 30°N paleolatitude were up to 3.6 x greater than present. These estimates however, were not constrained by empirical data from the subtropics. The estimated ET rates determined from PLCT trends from multiple sites in the Glen Rose Formation and other units will better constrain estimates of ET and latent heat flux from the subtropics during the mid-Cretaceous greenhouse warming. The increased amount of heat removed from the tropics was transferred poleward and released via condensation, and may have contributed to increased polar warmth and reduced equator-to-pole temperature gradients.