25 Paleo-climate and -ecosystems of the Southwest and High Plains: From the Paleozoic to the Quaternary (Posters)

Many Phanerozoic climates and ecosystems serve as potential analogues for the future; others highlight the Earth system response to tectonic and climatic forcings. We invite abstracts that probe the paleoclimate/ecosystems of the US Southwest

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

A dynamic, depth dependent record of soil water evaporation in pedogenic carbonate records

Paper number: 25-1
Program: Technical Sessions
Time: 8:15 AM – 8:30 AM

Authors

Abstract

The oxygen isotope composition of pedogenic carbonate (PC) records the isotopic composition of soil water at the time of formation. In dryland soils commonly associated with PC accumulation, soil water is susceptible to evaporation, which increases the calcium ion concentration and likely contributes to calcite saturation. For this reason, δ18OPC values (δ18O (‰) = Rsample/Rstandard -1 x 1000, where R is the 18O/16O abundance ratio) do not always record the δ18O of input water (precipitation) but rather are influenced to some degree by an isotopically evolved soil water. Triple oxygen isotopes combine the measurement of both δ'17O and δ'18O, where the Δ'17O value (Δ'17O  = δ'17O  – λ x δ'18O, λ = 0.528) quantifies minute offsets from a reference line induced by kinetic isotope effects. To understand the extent of kinetic isotope fractionation imparted on PC records, we developed a one-dimensional transient numerical soil evaporation model to trace the triple oxygen isotope evolution of soil water over time. We calibrated the numerical model by conducting a 100-day transient column evaporation experiment during which we monitored the triple oxygen isotope composition of soil pore water with depth over time. Our experimental and model results show a deepening of the evaporation front over time, with the depth and extent of isotopically evolved soil water related to both soil (porosity, tortuosity, soil temperature) and atmospheric (relative humidity, air temperature) parameters. Comparison to natural PC δ18O-depth records shows that carbonate does not track the maximum extent of isotopic fractionation due to evaporation, but rather reflects a dynamically changing soil water composition, with the degree of recorded evaporative enrichment decreasing linearly with depth. These results provide unique insight into how the timing of PC formation is depth-dependent, rather than static or steady state. Evaporatively evolved soil water persists in the modern PC records shown here up to 50cm depth, with deeper samples recording the isotopic composition of precipitation. Due to the lack of evaporative enrichment recorded by PC at depth, the primary mechanism of water loss resulting in PC formation below 50cm is likely transpiration.   

 

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410657

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