SOIL CARBONATE PENDANTS FROM SOUTHERN UTAH AS A NEW PALEORECORD: DEMONSTRATING PENDANT FORMATION DURING SUMMER

Understanding the magnitude and duration of past climate events is essential to make accurate predictions about how global climate will vary over the next century. Quantitative paleoclimate records from arid regions are difficult to obtain, often present complex growth histories, and have poor chronologic dating relative to marine sediment archives. Pendants, which are several centimeter thick coatings of soil carbonate forming on the bottoms of large (>1m diameter) boulders, represent a novel source for soil and climate records in arid areas. We investigated modern soil conditions and developed several coarse resolution paleorecords from a pendant collected near Capitol Reef National Park, UT, USA to identify the timing of pendant formation. The collected pendant is dense, finely laminated, and formed on the bottom of a large boulder, likely at a >25 cm depth. A cosmogenic ³He date on the stable, armored surface in which the pendant was precipitated give an age of 256.3 ± 10.9 ka, but it is likely that the soil carbonate is younger. For example, temperature estimates derived from clumped isotope measurements mimic solar insolation curves for the past 160 ka. Coarse resolution δ¹³C and δ¹⁸O records from the carbonate show ‘jigsaw’ patterns in that they are enriched at the start of the record (near the penultimate glacial maximum?), become progressively depleted towards the present, and then show a rapid enrichment in the most modern material.

Modern soil monitoring encompassed soil CO₂ concentration, temperature, and moisture measurements at a field site near Capitol Reef National Park. In addition, we collected late summer stable isotope profiles from soil CO₂ three adjacent field sites. Modern soil CO₂ isotopic composition at 25-45 cm depth demonstrates all locations are dominantly influenced by plants using the C4 photosynthetic pathway. This matches C isotopic compositions from the most recent part of the paleorecord given a CO₂-calcite fractionation. Furthermore, the most recent clumped isotope temperature estimates are consistent only with summer soil temperatures. These findings support interpreting pendant carbonate records from this area as indicative of summer soil conditions.