Geological Society of America

T209 Advances in Terrestrial Paleoclimate Reconstructions Using Paleosols

Tuesday, 24 October 2017
8:00 AM - 12:00 PM
Skagit 3

PETROGRAPHY AND STABLE AND CLUMPED ISOTOPE GEOCHEMISTRY OF OLIGOCENE-MIOCENE CONTINENTAL CARBONATES IN SOUTH TEXAS: IMPLICATIONS FOR PALEOCLIMATE AND PALEOENVIRONMENT VARIATIONS NEAR SEA-LEVEL

Paper number: T209-11
Program: Technical Sessions
Time: 10:30 AM – 10:50 AM

Authors

Majie Fan Presenting Author
University of Texas at Arlington, Arlington, TX
Conan Godfrey
University of Texas at Arlington, Arlington, TX
Greg Jesmok
University of California, Los Angeles, Los Angeles, CA
Deepshikha Upadhyay
University of California, Los Angeles, Los Angeles, CA
Aradhna Tripati
University of California, Los Angeles, Los Angeles, CA

Abstract

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 Δ₄₇values, 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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-302557

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