CLUMPED ISOTOPE RECORD OF PRE-LGM, LAST GLACIAL PERIOD PALEOCLIMATE IN PALOUSE LOESS SOIL CARBONATES

The Palouse loess of the Pacific Northwest United States is a valuable pedosedimentary proxy archive of last glacial period regional paleoclimate. Here, we present new carbonate clumped isotope thermometry and conventional carbonate stable isotope (δ¹³C, δ¹⁸O) data for pre-Last Glacial Maximum (LGM) loess soil carbonates collected from the Palouse region of eastern Washington state to provide measures of last glacial period surface and soil conditions in the region. Studied Palouse sequences are characterized by a strong correlation between carbonate δ¹³C values and temperatures calculated from measured clumped isotope Δ₄₇ ratios (T(∆₄₇)). Minimum measured carbonate δ¹³C_VPDB values of -4.7 to -6.3 ‰ correspond to average T(∆₄₇) values of 25-40°C. In comparison to loess carbonate T(∆₄₇) for Holocene and LGM-aged Palouse intervals reported in previous study, the pre-LGM T(∆₄₇) range overlaps with Holocene carbonate T(∆₄₇) (16-31°C) but is significantly warmer than T(∆₄₇) for samples dated to the LGM (8-19°C). Accordingly, T(∆₄₇) values obtained for this study suggest pre-LGM soil temperatures across the Palouse were similar to those of the Holocene but not as cold of those during the LGM. Carbonates with measured δ¹³C_VPDB values > -4.7‰ correspond to apparent clumped isotope temperatures of 57 to 126°C. These high δ¹³C and T(∆₄₇) values are unreasonable for near-surface soil conditions in the Palouse region, indicating that certain Palouse loess intervals preserve carbonate that is either detrital and did not from in situ in Palouse soils or was subject to kinetic isotope effects during formation. In either case, samples with such extreme δ¹³C and T(∆₄₇) values cannot be used for paleoclimate reconstruction. The correlation between carbonate δ¹³C and T(∆₄₇) observed in records presented here suggests that carbonate δ¹³C alone may be valuable in determining depth intervals in Palouse sequences that most reliably record equilibrium surface conditions during the last glacial period.