CALIBRATION OF CLUMPED ISOTOPES TEMPERATURES OF QUATERNARY CARBONATE CEMENT AND ITS APPLICATION TO PALEOELEVATION RECONSTRUCTION OF THE SOUTHERN ROCKY MOUNTAINS

Carbonate cement is abundant in geologic record and its stable and clumped isotope compositions have potential for paleoclimate and paleoelevation reconstructions. However, our limited understanding of cement clumped isotope temperatures (T_Δ47) hinders its use. Previous work has assumed carbonate cement formed under mean annual to mean summer temperatures. Here we present a preliminary calibration of T_Δ47 and δ¹⁸O values of Quaternary carbonate cement. Nine samples were collected from well-dated river terraces in Utah and Colorado and stream deposits associated with the Lava Creek B Ash (640 ka) in Kansas and Texas whose elevations vary between 60 and 1700 m and T_Δ47 values vary between 12.3 and 31.8°C. A comparison of the T_Δ47 values with modern temperatures reveals that with two exceptions, T_Δ47 values are biased to mean annual air temperatures at low elevations and to mean summer air temperatures at high elevations. It seems carbonate cement precipitates all year around at low elevations and only during warm seasons at high elevations. The calculated water δ¹⁸O values of the cement range from -8.3 to -0.2‰, similar to those of the mean annual river waters at low elevations, and higher than those at high elevations, suggesting formation of cement at high elevations in summer and evaporative condition.

T_Δ47 and δ¹⁸O values of eight late Eocene-Miocene carbonate-cemented sandstones from southern Colorado and western Kansas were also studied. Petrographic study shows that all cements are micritic and lack dissolution and recrystallization features, suggesting minimal late diagenesis. The preliminary data show that T_Δ47 values did not change significantly from the late Eocene to Miocene. T_Δ47 values in the southern Rockies are 4-10°C higher, and in the Great Plains are 2-4 °C higher than modern mean annual air temperatures. The calculated late Eocene- Miocene water δ¹⁸O values are of -7.4 to -5.7‰ in the Great Plains and of -17.6 to -11.4‰ in the southern Rockies, comparable to those of local mean annual river waters. More data are needed to constrain the influences of change in carbonate cement formation seasonality and global cooling in order to determine the paleoelevations.