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

• Published in: Journal of geophysical research. Biogeosciences Vol. 124; no. 3; pp. 616 - 632
• Main Authors: Huth, T. E., Cerling, T. E., Marchetti, D. W., Bowling, D. R., Ellwein, A. L., Passey, B. H.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2019
• Subjects: Bias; Calcite; Carbon dioxide; carbon isotopes; Carbonates; Colorado Plateau; Holocene; Infiltration; Isotopes; Modelling; Moisture content; oxygen isotopes; Partial pressure; pedogenic carbonate; Preservation; Radiocarbon dating; Radiometric dating; Resolution; Seasonal variations; Seasonality; Soil; Soil conditions; Soil environment; Soil investigations; Soil moisture; soil monitoring; Soil temperature; Soil water; Summer; Water content; United States > US; Utah
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2018JG004496

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 CO2 partial pressure, soil temperature, soil moisture, δ13C‐soil CO2, δ18O precipitation, and δ18O‐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 CO2 was at its annual maximum during soil carbonate formation, assuming uniformly low soil CO2 formation conditions for soil carbonate in estimating paleoatmospheric CO2 is likely not viable. Additionally, we showed modern summer δ13C‐soil CO2 and soil CO2 measurements could not produce a modeled δ13C‐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.
Key Points
At Torrey, UT, comparison between modern soil and late Holocene soil carbonate isotopes shows soil carbonate forms during the summer
Summer formation seasonality occurs because calcite dissolution‐formation reactions during infiltration events overprint prior material
Torrey soil carbonate rinds are suitable material for high‐resolution paleorecords as proxies of summer soil and vegetation conditions