194-18 Reconstruction of Late Permian-Mesozoic pCO2 from δC13 Composition of Pedogenic Carbonate, Ordos and Sichuan Basins of Central China

Poster Number 98

See more from this Division: General Discipline Sessions
See more from this Session: Paleoclimatology/Paleoceanography (Posters)

Monday, 6 October 2008
George R. Brown Convention Center, Exhibit Hall E

Amy Weislogel, Geological Sciences, University of Alabamam, Tuscaloosa, AL, Stephan Graham, Geological and Evironmental Sciences, Stanford University, Stanford, CA and C. Page Chamberlain, Earth and Environmental Sciences, Stanford University, Stanford, CA
Abstract:

Pedogenic carbonate samples from 28 locations in the Ordos and Sichuan basins were analyzed for C and O isotopic composition in the Stanford University Stable Isotope Biogeochemistry Laboratory using a Finnigan Gasbench II and Delta+XL mass spectrometer. Samples yield d18O values typical of pedogenic carbonate (20-27‰ V-SMOW), suggesting little diagenetic alteration.  Upper Permian-Lower Triassic samples exhibit d 13C values between -3 to -4‰ (V-PDB). Upper Lower Triassic-Middle Triassic samples show an increase to -7 and -10‰ d 13C. Middle Jurassic values range from around -5 to -7.5‰ up to -1.5‰ d 13C, followed by a decrease to between -4 and -6 ‰ d 13C in Upper Jurassic-Lower Cretaceous samples.  Atmospheric pCO2 (Ca) is modeled following the Cerling method, which assumes isotopic equilibrium of the soil-atmosphere system:

Ca = Sz ((d 13Cs-(1.0044*d 13Cr)-4.4)/ d 13Catm-d 13Cs))

where Sz is soil CO2 concentration from diffusive mixing of atmospheric and plant-respired CO2, and d 13Cs,d 13Cr, d 13Catm are the d13C composition of soil CO2, plant-respired CO2, and atmospheric CO2, respectively. Values of d 13Cs are determined by adjusting sample d13C for temperature-dependent fractionation. An average Sz value of 4500 ppmV is based on diffusion rates of modern soils. Values for d13Cs and d13Catm are based on d13C composition of coeval terrestrial organic matter from other studies. Modeling results reveal Late Permian-Early Triassic pCO2 of 2400-6300 ppmV. Subsequently, pCO2 decreases during late Early Triassic time (~1240 ppmV) and then increases in Early Middle Triassic time (~2040 ppmV). Late Middle Triassic pCO2 rises from ~990 ppmV to ~3240 ppmV. Jurassic and Cretaceous results show moderate variability, with an average around ~4660 ppmV. The pCO2 estimates reported here are similar to other estimates from age-equivalent pedogenic carbonates, and support climate models that depict latest Permian warming as a consequence of elevated pCO2 and not paleogeography.

See more from this Division: General Discipline Sessions
See more from this Session: Paleoclimatology/Paleoceanography (Posters)

<< Previous Abstract | Next Abstract >>