Sedimentology & geochemistry of upper Permian & lower Triassic paleosols, NW China: Implications for environmental change across the end-Permian life crisis.

摘要

The effects of climate change on Late Permian and Early Triassic environments and ecosystems have been understated recently in favor of more complex, catastrophic hypotheses of mass extinction. Terrestrial (fluvial and lacustrine) strata from the Bogda Mountains, NW China, provide insight into mid-latitude environmental response to global climate change from the Wuchiapingian (∼260 Ma) through the Olenekian (∼245 Ma). Upper Permian paleosol morphologies are characterized by intense redoximorphy, accumulation of vascular plant matter, accumulation of clay minerals and Fe-oxides, slickensides, and clastic dikes, indicating a perennially wet to distinctly seasonal in soil moisture budget. Uppermost Permian paleosols are characterized by subsurface accumulations of clay and carbonate as well as surficial accumulations of organic matter, indicating sub-humid to semi-arid soil moisture. Lower Triassic paleosol morphologies indicate a drier environment characterized by net soil moisture deficiency based on pedogenic CaCO3 accumulations and gypsum pseudomorphs. Therefore, paleosol morphologies indicate a long-term decline in soil moisture availability associated with aridification.;Organic matter delta13C values range from -19.1‰ to -31.9‰, and are terrestrial vascular plant in origin. The delta 13C values overall show a significant decrease (6‰ on average) from the more humid Late Permian time to the more arid Early Triassic. Major delta 13C anomalies occur within strata directly associated with the Permian-Triassic boundary. The variability in a13C values may reflect changes in the plant groups associated with aridification in conjunction with a global negative trend in Earth's nearsurface carbon reservoir.;Calcite nodules were examined petrographically, and observed calcite textures include radial fibrous calcite interpreted as Microcodium, microspar, micrite, and spar cements. Micritic and microsparitic calcite is considered here to be pedogenic, and used to reconstruct atmospheric CO2 concentrations. Previous models indicate an increase in atmospheric PCO2 from near present day values at 270 Ma to as much as 13x modern day values at 250 Ma. When using atmospheric PCO 2 estimates provided by the GEOCARBSULF model, soil CO 2 concentrations are overestimated, and therefore reflect an unrealistic estimate of atmospheric PCO2. The use of soil CO2 concentrations of 5000 ppmV for Permian paleosols and between 1000 and 2000 ppmV for Triassic paleosols yields atmospheric P CO2 that remained at or near present day values until the latest Permian (∼251.3 Ma), and increased into the Induan to 930 to 1860 ppmV, before reaching a maximum value of 1100 to 2200 ppmV at ∼248.7 Ma, which indicates atmospheric PCO2 levels during the Olenekian could have been as great as 6x present day levels.