Assessing pedogenic calcite stable-isotope values: Can positive linear covariant trends be used to quantify palaeo-evaporation rates?

Palaeoclimate models suggest enhanced evaporation rates in subtropical regions (15-30° latitude) during greenhouse-world conditions, however, there are no empirical data to support these estimates. Laboratory evaporation experiments have shown that calcites precipitated from variably saturated solutions yield a positive linear covariant trend (PLCT) in δ¹⁸O_calcite versus δ¹³C_calcite values. The goal of the present controlled laboratory experiments is to develop a method to quantify regional palaeo-evaporation rates from palaeosol calcrete PLCTs. Samples of powdered CaCO₃ were dissolved in de-ionized water in pressure sealed containers with a range of elevated atmospheric pCO₂ concentrations for 24 h. The solution was then allowed to evaporate completely from an open container within an incubator with the time of calcite crystallization noted, and aliquots removed for analysis every 24 h. The precipitated calcite produced an array of δ¹⁸O_calcite versus δ¹³C_calcite values that fall upon a PLCT projected from a theoretical meteoric calcite line (MCL). Water analyses yielded δ¹⁸O_water enrichments from an initial value of - 4.75‰ VSMOW ranging up to between + 10.0‰ and + 14.8‰. The experimental results show that solutions formed under higher pCO₂ conditions precipitated calcite very early on during evaporation, and thus have δ¹⁸O_calcite and δ¹³C_calcite values that are slightly enriched relative to the MCL. The solutions that formed under low pCO₂ conditions precipitated calcite after much of the fluid had evaporated, and thus yield more enriched δ¹⁸O_calcite and δ¹³C_calcite values. Repeat trials with varying temperature and relative humidity show that the PLCT is steeper under both higher temperature and low relative humidity. The wide range of pCO₂, temperature and relative humidity conditions used simulate meteoric phreatic and meteoric vadose conditions that may affect a calcrete horizon over time during numerous dissolution/precipitation reactions. The results of these experiments show that a dominant factor in the precipitation of vadose calcite is the saturation state of the fluid with respect to CaCO₃, while the primary factors affecting the magnitude and steepness of the PLCT are vadose zone temperature, relative humidity, the starting δ¹⁸O_water value and saturation state of the fluid with respect to CaCO₃. Since the pCO₂ of the rooting zone is cross-controlled by the local soil and surface ecology, the magnitude of the PLCT enrichment may not be a direct proxy for palaeo-evaporation rates.