4-12 Lost in translation, found in triple oxygen: A temporal approach to soil water evaporation

Session: Recent Advances in Soil and Paleosol Science

Presenting Author:

Authors:

Abstract:

The triple oxygen isotope (TOI) composition of pedogenic carbonate (PC) is related to the isotopic composition of soil water from which it forms and can be used to constrain soil-forming processes in the context of Earth’s hydrological cycle. However, previous TOI studies applying steady-state soil evaporation models suggest isotopic deviation of calculated soil water from input precipitation is due to evaporative effects driven by atmospheric aridity. We sought to test this assumption through direct measurement of the TOI composition of soil pore water (rather than reconstructed soil water (RSW) from measured PC) throughout a 100-day soil-column evaporation experiment.

We examined the effects of evaporation on the TOI evolution of soil pore water with depth by constructing 1m tall, 50mm wide PVC pipe soil columns filled with a homogenous sand (>0.09mm) and tap water. We allowed the columns to drain before undergoing evaporation over a 2-, 20-, 60- and 100-day period, monitoring the temperature, humidity, and volumetric water contents throughout. Each column was subsampled at 5-10 cm increments and the TOI composition of soil pore water was measured via Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS). We adapted a non-steady state numerical soil evaporation model to calibrate the d'¹⁸O and D'¹⁷O (where D'¹⁷O = d'¹⁷O -0.528 x d'¹⁸O) depth profiles over time.

Our results suggest a co-variation of the D'¹⁷O value with atmospheric humidity and theta-diffusion (q_diff = (¹⁷a_A-B)/ ¹⁸a_A-B), where A and B are liquid water and water vapor, respectively). Our model fit is satisfied with a q_diff of 0.514, which is the minimum value previously assumed from steady-steady soil evaporation models applied towards RSW. Because the monitored humidity at laboratory conditions varied over the course of the experiment (~15% to 40%), we suggest that the fitted lower theta value is coupled to a variable atmospheric humidity and should be accounted for in the future. Our paired experimental and modeled non-steady state evaporation profiles characterize the time-dependent nature of soil water evaporation in coarse-grained soils and should be considered for future studies examining the timing and TOI composition of RSW from PC.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.