Seasonal Bias in Soil Carbonate Formation and Its Implications for Interpreting High‐Resolution Paleoarchives: Evidence From Southern Utah

Abstract

Abstract Pedogenic carbonate is commonly used as a paleoarchive, but its interpretation is limited by our understanding of its formation conditions. We investigated laminated soil carbonate rinds as a high‐resolution paleoarchive in Torrey, Utah, USA, by characterizing and modeling their formation conditions. We compared late Holocene (<5 ka) soil carbonate conventional (C and O) and “clumped” isotopes to modern soil environment and isotope measurements: soil CO 2 partial pressure, soil temperature, soil moisture, δ 13 C‐soil CO 2 , δ 18 O precipitation, and δ 18 O‐soil water. Data unambiguously identified a strong summer seasonality bias, but modeling suggested soil carbonate formed several times throughout the year during infiltration events causing dissolution‐formation reactions. This apparent discrepancy resulted from preferential preservation of calcite formed from the largest annual infiltration events (summer) overprinting previously formed calcite. Soil carbonate therefore formed predominantly due to changes in soil water content. As soil CO 2 was at its annual maximum during soil carbonate formation, assuming uniformly low soil CO 2 formation conditions for soil carbonate in estimating paleoatmospheric CO 2 is likely not viable. Additionally, we showed modern summer δ 13 C‐soil CO 2 and soil CO 2 measurements could not produce a modeled δ 13 C‐soil carbonate consistent with late Holocene observations. We suggest using multiple lines of evidence to identify nonanalogous modern conditions. Finally, a nearly linear radiocarbon age model from a laminated rind showed that rinds can be used as a high‐resolution paleoarchive if samples are from a single depth and the timing and conditions of soil carbonate formation can be constrained through time.