183 Paleoclimatology/Paleoceanography I

Wednesday, 31 October 2007
8:00 AM - 12:00 PM

EVALUATION OF PEDOGENIC CALCITE NODULES FROM THE JIUYUCUAN FM, NW CHINA: IMPLICATIONS FOR EARLIEST TRIASSIC ATMOSPHERIC PCO2

Paper number: 183-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

The earliest Triassic Jiuyucuan fm, Turpan basin, NW China, hosts >200 m of fluvio-lacustrine strata, including abundant paleosols. Paleosol types include Gypsisols and Calcisols, both of which are indicative of relatively arid (~<250 and 750 mm/yr, respectively) and non-seasonal soil moisture regimes. Petrographic analysis of carbonate nodules identified five major textural components: brown micrite, green micrite, pink micrite, microspar, and blocky spar. Micritic calcite is an early cement, with δ13C values ranging from -14.0 to -10.6‰. Collectively, we consider micritic calcite from these nodules to be primary, earliest Triassic, pedogenic cement. Sparry calcite textures are later cement, with δ13C values near -2‰.

Proximal fossil vascular plant organic matter in the Jiuyucuan fm has a δ13C composition of -31‰. Considering the range of (1) micritic calcite and (2) organic matter δ13C values, and assuming (3) partial pressure of CO2 in the Jiuyucuan fm soil profiles was 5000 ppmV, earliest Triassic atmospheric pCO2 is estimated to be no less than 2.5 times Pre-industrial Atmospheric Levels (xPAL), and no greater than 7xPAL. To our knowledge, this is the first terrestrial geochemical proxy record for earliest Triassic atmospheric pCO2. Nevertheless, these earliest Triassic atmospheric pCO2 estimates are analytically indistinguishable from latest Permian (251 Ma) pCO2 estimates.