LATE EARLY CARBONIFEROUS SEASONALITY OF WESTERN PANTHALASSA BASED ON OXYGEN ISOTOPE RECORDS OF BRACHIOPODS FROM XINJIANG, CHINA

To further characterize the seasonality of western Panthalassa for late Early Carboniferous, we studied the isotope and element compositions of five Gigantoproductus brachiopod shells collected from Xinjiang, China. Samples were thin sectioned and examined under the petrographic and cathodoluminescence microscopes for evaluating shell preservation. Oxygen isotope values from non-luminescent and slightly luminescent portions of shells are between -7.0 and -1.7‰ (-4.0 ±1.1‰; N=128) whereas mean oxygen isotope values of associated luminescent matrix is -9.7 ±1.0‰ (N=15). Average oxygen isotope values of each specimen ranging from -3.2 to -4.5‰ (N=5) are comparable with those for South China (-3.1 to -3.8‰; N=2; Chen, 2001: NTNU M.S. thesis) but less than those of Russian Platform (-2.1 to -4.0‰; N=3; Wang, 1998: TAMU M.S. thesis). Some of the estimated seasonal isotope temperature difference for Xinjiang (3 – 19°C) was too large for tropical region. Therefore, the precipitation may have partly influenced the observed oxygen isotope records seasonally. Significant linear relationships between carbon and oxygen isotope values are observed for Xinjiang, South China, and Russian Platform. Although not intercepted one another, all the trends approach the same maximum oxygen isotope limit of -1.5‰. Assuming a tropical sea surface temperature of 25°C, Serpukhovian seawater oxygen isotope value was estimated as 0.5‰. Thus the environment was possibly under a glacial condition. Deducted 1.1‰ (5°C) for possible seasonal temperature variation, the remaining oxygen isotope values of Xinjiang (~4‰) was possibly due to different extent of mixing of meteoric water and seawater. The same effect of comparable magnitude was also observed in South China and Russian Platform records. Carbon isotope values of Xinjiang were between -4.3 and 3.2‰, consistently 1‰ less than those of Russian Platform with the same oxygen isotope values. This may indicate different regional hydrologic conditions and need further investigation.