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

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.