| Description |
Soil carbonate is accumulated calcite (CaCO3) in a soil profile. It has been widely used as a paleoenvironmental proxy because its stable isotope composition records information about soil conditions at the time of mineral formation. Carbon isotope ratios record the proportion of biomass type (C3, C4, and CAM) and activity. Oxygen isotope ratios record that of meteoric water as modulated by soil processes and temperature. Soil carbonate can form as laminated rinds on the bottoms of clasts, presenting the possibility of their use as high-resolution paleoenvironmental records. Producing such a record requires precisely analyzing sample laminations as well as establishing a chronology and the conditions of mineral formation required to interpret the record. Working at two field sites in southern Utah, United States, I demonstrated the utility of laminated soil carbonate as a paleoenvironmental archive. On a debris flow surface near Torrey, UT, I used integrated soil monitoring and modeling to identify summer bias in soil carbonate formation seasonality. After establishing a radiocarbon chronology for 35-3 ka, I developed C- and O-isotope transects using secondary ion mass spectrometry. The last glacial period had a mixed C3-C4 summer biomass. However, from 8-3 ka, a large ecologic change resulted in a landscape nearly devoid of C3 biomass. Inferred changes in soil moisture and biomass composition were not always coincident, suggesting multiple ecologic controls. I then explored how the proxy could be applied in a different geologic setting. At a field site near Moab, UT, suitable soil carbonate rinds were only available on an eroded river terrace, which precluded characterizing in situ soil carbonate dynamics. Instead, I studied a younger, intact terrace to discern spring to early summer soil carbonate formation. Laser ablation-derived isotope profiles were interpreted based on a 40-13 ka radiocarbon chronology. C-isotopes reflected a mixed C3-C4 environment and may have correlated with winter insolation while gradually increasing O-isotope profiles were affected by changing climate and erosion. Demonstration that the laminated soil carbonate proxy can provide useful paleoenvironmental information opens a new tool for paleoclimatologists. The proxy provides a local perspective, permits flexible site selection, and yields continuous information over 10s-100s ka. |
