CARBONIFEROUS ENVIRONMENT INDICATED BY OXYGEN AND CARBON ISOTOPE RECORDS OF BRACHIOPOD SHELLS FROM YASHUI SECTION, GUIZHOU, SOUTH CHINA

This study constructed the Carboniferous tropical environment of Yashui section, Guizhou, South China by analyzing the stable isotope compositions of brachiopod shells. A total of 395 fossil brachiopod shells were collected. One hundred and thirty nine samples were cut, thin-sectioned, and examined under plain light and cathodoluminescence for shell preservation.

Only 89 out of 369 micro-drilled carbonate powder samples were determined from well preserved portions. In ascending order, mean δ¹⁸O values of well preserved brachiopod shells are -3.8 ± 0.7‰ (1σ, N = 52) and -3.0±0.5‰ (1σ, N = 37) for Shangsian Stage (Late Visean) and Dewaun Stage (Serpukhovian), respectively. Mean δ¹³C values of well preserved brachiopod shells are respectively 2.3 ± 2.4‰ for Shangsian Stage (Late Visean) and 4.1±2.0‰ for Dewaun Stage (Serpukhovian). Positive shift in δ¹³C value of 1.8‰ between Shangsian Stage and Dewaun Stage may correspond to the increased burial rate of organic matter as proposed by previous studies.

Assuming the δ¹⁸O of Early Carboniferous seawater was -1.0‰, the sea water temperature were between 24°C and 30°C (average 28°C) for Shangsian Stage and between 23°C and 26°C (average 25°C) for Dewaun Stage. The 3°C decrease in temperature from Shangsian Stage to Dewaun Stage was consistent with the estimated temperature change based on the Mg/Ca ratios. The Late Visean to Serpukhovian cooling may be caused by the decrease in atmospheric CO₂ concentration as indicated by the increase in carbon isotope value. Tropical seasurface tempetrature may have decreased 3°C prior to the expansion of continental ice volume from Late Visean to Serpukhovian. However, the overall warm tropical sea surface temperature (greater than 25°C ) of South China sea indicates that modern Western Pacific Warm Pool sea surface temperature distribution pattern may have existed in Early-Middle Carboniferous.