Paper No. 209-9
Presentation Time: 10:30 AM
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)
FAN, Majie1, GODFREY, Conan1, JESMOK, Greg2, UPADHYAY, Deepshikha3 and TRIPATI, Aradhna4, (1)Department of Earth and Environmental Sciences, University of Texas at Arlington, 500 Yates Street, Arlington, TX 76019, (2)Departments of Earth, Planetary, and Space Sciences and Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095, (3)Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, 595 Charles E. Young Dr. East, Los Angeles, CA 90095, (4)Department of Earth, Planetary, and Space Sciences, Departments of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095, mfan@uta.edu
This study provides a continental record of paleoclimate and paleoecology from clumped and stable isotope compositions of pedogenic carbonate for a low-latitude, near sea-level region in North America. We examine Cenozoic sedimentary rocks in the southern Texas Gulf Coastal Plain region that contain abundant continental carbonates that are useful archives of terrestrial conditions. Our field observations and thin section characterizations of Oligocene and Miocene carbonates indicate three types of pedogenic carbonates, including rhizoliths, carbonate nodules, and platy horizons, and two types of groundwater carbonates, including carbonate-cemented beds and carbonate concretions, with distinctive macromorphologic and micromorphologic features. Based on the preservation of authigenic microfabrics and variations of stable isotopic compositions, we suggest that these carbonates experienced minimal diagenesis, and reflect past paleoclimate and paleoenvironmental conditions.
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.
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