A FLEXIBLE MONTHLY EQUILIBRIUM MODEL OF SOIL CARBONATE PRESERVATION APPLIED TO SOUTHERN UTAH

Soil carbonate isotopic records are commonly used in paleoclimate studies. However, the conditions and timing of carbonate formation are still widely debated, which limits the power of this paleorecord. Some of the variability in formation timing is likely real, but questions remain as to the validity of study and model designs. Here, we use a calibration study and developed a new model at a site near Torrey, southern Utah to answer the following questions: (a) Are carbonate saturation conditions driven by one dominant parameter? (b) Do models of soil carbonate preservation and formation agree? (c) Do modern δ¹³C-soil gas conditions accurately reflect Holocene conditions?

For the modern calibration, we constrained environmental (pCO₂, temperature, and soil moisture), and isotopic parameters (δ¹³C-soil CO₂, δ¹⁸O-rainwater, δ¹⁸O-soil water) relevant to carbonate chemistry over a 12 month interval. We analyzed a soil carbonate rind for δ¹³C, δ¹⁸O, and “clumped” isotope temperatures to allow a modeled modern-paleo comparison. The model is a monthly equilibrium evaluation of soil carbonate chemistry driven by the observed modern conditions and incorporates dissolution.

The data and model each suggest soil carbonate at Torrey is strongly biased towards the summer growing season. The model predicts significant carbonate formation during spring snowmelt (FM), the mid- to late summer (JAS), and early fall (O). Soil carbonate in unimodal precipitation regimes should therefore preserve a signal from the season of maximum precipitation timing. This prediction should be tested at other sites, as the model is adaptable to other locations.

Comparing the model results for the modern and paleo systems at the Torrey site reveals that modern δ¹³C-vegetation has a stronger C3 component than all data except the early Holocene for the interval from 10 kyr to present. This indicates a recent change in conditions due to either natural or anthropogenic causes. Low insolation is correlated with dominantly C4 vegetation, making it an unlikely driver for this observation. This leaves the possibility that ecological conditions have been impacted by grazing or invasive species quite recently. Future calibration studies should assess human impact at their sites and be aware that this can produce false positives in isotopic matching.