Carbon Oxygen Isotope Analysis and Its Significance of Carbonate in the Zhaogezhuang Section of Early Ordovician in Tangshan, North China

In the Early Ordovician Zhaogezhuang ((sic)) Section of Tangshan ((sic)), North China, the Yeli ((sic)) Formation is composed of an entire third-order sequence, with facies ranging from the inner ramp restricted platform and open marine to the middle and even outer ramps. The Liangjiashan ((sic)) Formation is dominated by highstand system tracts (HST) with predominantly inner ramp grain-shoal and lagoon facies. Analyzing the carbon and oxygen isotope during the whole-rock carbonate reveals the delta C-13 values in the Yeli Formation range from -7.11 parts per thousand-0.76 parts per thousand (PDB), with the mean value at -2.98 parts per thousand, while the delta O-18 values range of -9.09 parts per thousand- -4.65 parts per thousand with the mean value at -6.12 parts per thousand. The delta C-13 values in the Liangjiashan Formation range of -1.15 parts per thousand-0.3 parts per thousand, and the mean value of -0.57 parts per thousand; the delta O-18 values are -8.76 parts per thousand- -7.48 parts per thousand, and the mean value is -8.06 parts per thousand. The delta C-13 values in the Yell Formation decrease, but at the bottom of the Liangjiashan Formation the values increase steadily. In the middle-upper formation, there is an extended fluctuation between 0- -1.00 parts per thousand. The delta C-13 trend in the studied section is similar to that of the contemporary sections, except that it has much lower delta C-13 values and a more negative excursion. The correlation between the delta C-13 changes and the eustatic events, as well as the sedimentary facies, indicates that in the Tangshan area, the carbon isotope evolution can be attributabled to the processes of the eutrophic sea/oligotrophic sea, the seafloor organism-mediated oxidation in shallow water and the organic reduction after maximum flooding. The changes in the carbon isotope contents were primarily affected by the regional relative sea level changes. Compared to the other coeval data, the Early Ordovician of the Tang shan area is also severely depleted in O-18, with all of the delta O-18 sample values being below -5 parts per thousand, except for one sample with a value of -4.65 parts per thousand. With the ancient seawater having a delta O-18 value of -5.5 parts per thousand (SMOW), it is reasonable to delineate a temperature of less than 37 degrees C.