PETROGRAPHY AND STABLE AND CLUMPED ISOTOPE GEOCHEMISTRY OF OLIGOCENE-MIOCENE CONTINENTAL CARBONATES IN SOUTH TEXAS: IMPLICATIONS FOR PALEOCLIMATE AND PALEOENVIRONMENT VARIATIONS NEAR SEA-LEVEL (Invited Presentation)
Measured Δ47 values vary between 0.659-0.670 ‰, with the exception of one upper Miocene groundwater cement which has a Δ47 value of 0.617 ‰. Using the stipulated Δ47 values, the calculated carbonate precipitation temperatures (T(Δ47)), following the calibration of Tripati et al. (2015), are between 27-30 ºC, while the cement yields a reconstructed temperature of 42 ºC. The T(Δ47) of different types of pedogenic carbonate and of two pairs of pedogenic and groundwater carbonates overlap within error. Using pedogenic carbonate d18O values, meteoric water d18O values were calculated in the range of -2.9 to 0.9 ‰ (VSMOW). Both the T(Δ47) and meteoric water δ18O values do not exhibit any obvious trends during the Oligocene-Miocene. However, the variability of the carbonate δ18O values increased in the middle Miocene, possibly due to an increase in seasonality. Mean pedogenic carbonate δ13C values increased from -9.2 ‰ to -4.6 ‰ in the middle Miocene, alluding to an early expansion of C4 grassland in south Texas, consistent with a response to enhanced seasonality.