PERMIAN PALEOENVIRONMENT INDICATED BY STABLE CARBON AND OXYGEN ISOTOPE RECORDS OF BRACHIOPOD SHELLS FROM TIBET AND MONGOLIA

We have studied the paleoenvironmental records for Permian Tibet (Selong) and southeastern Mongolia (Hovsugol Section and Dzhirem- Ula Section) region by analyzing the stable carbon and oxygen isotope records of fossil brachiopod shells collected from Tubet (N = 29) and Mongolia (N = 40). All brachiopod samples were thin sectioned and examined under the petrographic and cathodoluminescence microscopes for evaluation of shell preservation. A total of 130 and 90 isotopic analyses were performed on Tibet and Mongolia samples, respectively.

According to the characteristic observed under the cathodoluminescent microscope, average δ¹³C values of the well preserved samples (NL) are 4.7 ± 0.7‰ (N = 69; with the maximum value of 5.9‰) for Tibet and 4.3 ± 0.5‰ (N = 37; with the maximum value of 5.6‰) for Mongolia. These δ¹³C values are comparable to those of North America and Europe and reflecting the partial pressure of carbon dioxide and regional ocean circulation pattern for Permian.

The average δ¹⁸O values of the well preserved samples are -10.3 ± 3.2‰ for Tibet and -11.9 ± 5.2‰ for Mongolia. The maxima oxygen isotope values are respectively -2.2‰ and -3.9‰ for Tibet and Mongolia, whereas the minimum oxygen isotope values are -14.5‰ and -19.7‰, respectively. These maximum values are most likely reflecting the original environmental signals whereas these minimum oxygen isotope values may be attributed to diagenesis or melted glacial water input in this region.

Using the maximum oxygen isotope values, the estimated seawater temperature were 21°C and 29°C (assuming ice free δ¹⁸O_seawater of -1‰) or 26°C and 34°C (assuming modern δ¹⁸O_seawater of 0‰) for Tibet and Mongolia region, respectively. Cool temperature in Tibet whereas warm temperature in Mongolia is consistent with the Permian geography and climate model simulation. However, more well preserved samples are necessary to confirm preliminary result from this study.