OXYGEN ISOTOPE RECORDS IN BRACHIOPOD SHELLS FROM SOUTH CHINA¡XA PALEOENVIRONMENTAL COMPARISON BETWEEN EAST AND WEST PANGEA IN EARLY CARBONIFEROUS

To further unravel the environmental difference between East and West Pangea in late Early Carboniferous, we studied the isotope and element compositions of four Gigantoproductus and one Delepinea brachiopod shells collected from South China. All samples were thin sectioned and examined under the petrographic and cathodoluminescence microscope for evaluating shell preservation.

The average Mg/Ca ratio of the four Gigantoproductus shells (17.0 ± 3.5 mmol/mol; 1s) is similar to that of European Gigantoproductus shells (Popp et al., 1986: Geol. Soc. America Bull., v. 97). Average Na and S contents, respectively 5.9 ± 1.5 and 3.1 ± 0.6 mmol/mol, are similar to those of Pennsylvanian Composita specimens of North America (Mii et al., 1999: Geol. Soc. America Bull., v. 111).

Average oxygen isotope value of non-luminescence (NL) portions of all specimens is -3.5 ± 1.2 permil (N=54) whereas that of luminescence portions is -6.3 ± - 1.8 permil (N=28). Mean oxygen isotope values of NL portion of each specimen range from -2.8 to -3.9 permil. Assuming the seawater oxygen isotope value was -1.0 permil, average seawater temperatures of South China were between 24¢XC and 29¢XC. These results are in accordant with model prediction for tropic sea surface temperature of Visean (25¢XC~30¢XC; T. Crowley, 1996, personal communication).

Average oxygen isotope records of South China consistently keep around -3.4 permil from Late Visean (-3.4 ± 0.6 permil; N=3) to Early Serpukhovian (-3.4 ± 0.5 permil; N=2). Whereas those of North American spiriferid brachiopods shifted from -3.0 ± 0.5 permil (Late Visean; N=15) to -2.5 ± 0.5 permil (Early Serpukhovian; N=10) for the roughly comparable stratigraphic intervals (Mii et al., 1999: Geol. Soc. America Bull., v. 111). Because the oxygen isotope values positively correlate to carbon isotope values within single specimens for South China, more regional influence of fresh water/precipitation in South China (East Pangea) than in North America (West Pangea) was possible in Early Carboniferous.