Triple oxygen isotopes of soil water and soil carbonates: evaporation experiment, numerical modeling, and natural carbonate samples
Description
We investigated how the triple oxygen isotope composition of soil water evolves due to transient evaporation and how the isotopic composition of soil water is recorded by soil carbonates. We used these data to calibrate a numerical soil evaporation model. Our model-data comparison shows that the isotopic composition of soil water evolves as the evaporation front migrates deeper below the surface over time. The evaporation front is recognized by the maximum isotopic offset from meteoric water, recorded by both oxygen-18 and oxygen-17 (triple oxygen). Model application to soil carbonate profiles reveal that soil carbonates to do record the full extent of isotopic fractionation due to evaporation.
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Steps to reproduce
For the sand column experiment we established four separate 1m long (50mm diameter) PVC pipes filled with sand and tap water (at field capacity) and allowed the columns to dry from the top surface for 2, 20, 60, and 100 days in a controlled, ventilated environment. We monitored the relative humidity and temperature during the experiment. For excavation, we cut each column in 5cm segments from the top to 30 cm, and 10cm segments to 1 meter. We analyzed the triple oxygen isotope composition of the pore water using CO2 equilibration at 25°C and analysis via Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS). Water samples were corrected from daily analysis of an internal water standard, NM2. For soil carbonates, we collected recently formed soil carbonate from the undersides of granitic coasts in the Sandia Foothills of Albuquerque, New Mexico. An unpublished 18O depth profile was collected by Dan Breecker alongside his dissertation. See Breecker et al. (2009) for methods of analysis. We include two carbonates from 15 and 80 cm depth that were analyzed for their triple oxygen isotope composition via phosphoric acid digestion at 50°C and analysis of the purified CO2 via TILDAS. All oxygen isotope data are relative to VSMOW.
Institutions
- University of New MexicoNew Mexico, Albuquerque
- The University of Texas at AustinTexas, Austin